1 | =head1 NAME |
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2 | |
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3 | perlfunc - Perl builtin functions |
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4 | |
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5 | =head1 DESCRIPTION |
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6 | |
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7 | The functions in this section can serve as terms in an expression. |
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8 | They fall into two major categories: list operators and named unary |
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9 | operators. These differ in their precedence relationship with a |
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10 | following comma. (See the precedence table in L<perlop>.) List |
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11 | operators take more than one argument, while unary operators can never |
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12 | take more than one argument. Thus, a comma terminates the argument of |
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13 | a unary operator, but merely separates the arguments of a list |
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14 | operator. A unary operator generally provides a scalar context to its |
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15 | argument, while a list operator may provide either scalar or list |
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16 | contexts for its arguments. If it does both, the scalar arguments will |
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17 | be first, and the list argument will follow. (Note that there can ever |
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18 | be only one such list argument.) For instance, splice() has three scalar |
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19 | arguments followed by a list, whereas gethostbyname() has four scalar |
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20 | arguments. |
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21 | |
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22 | In the syntax descriptions that follow, list operators that expect a |
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23 | list (and provide list context for the elements of the list) are shown |
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24 | with LIST as an argument. Such a list may consist of any combination |
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25 | of scalar arguments or list values; the list values will be included |
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26 | in the list as if each individual element were interpolated at that |
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27 | point in the list, forming a longer single-dimensional list value. |
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28 | Elements of the LIST should be separated by commas. |
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29 | |
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30 | Any function in the list below may be used either with or without |
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31 | parentheses around its arguments. (The syntax descriptions omit the |
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32 | parentheses.) If you use the parentheses, the simple (but occasionally |
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33 | surprising) rule is this: It I<looks> like a function, therefore it I<is> a |
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34 | function, and precedence doesn't matter. Otherwise it's a list |
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35 | operator or unary operator, and precedence does matter. And whitespace |
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36 | between the function and left parenthesis doesn't count--so you need to |
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37 | be careful sometimes: |
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38 | |
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39 | print 1+2+4; # Prints 7. |
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40 | print(1+2) + 4; # Prints 3. |
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41 | print (1+2)+4; # Also prints 3! |
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42 | print +(1+2)+4; # Prints 7. |
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43 | print ((1+2)+4); # Prints 7. |
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44 | |
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45 | If you run Perl with the B<-w> switch it can warn you about this. For |
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46 | example, the third line above produces: |
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47 | |
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48 | print (...) interpreted as function at - line 1. |
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49 | Useless use of integer addition in void context at - line 1. |
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50 | |
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51 | A few functions take no arguments at all, and therefore work as neither |
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52 | unary nor list operators. These include such functions as C<time> |
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53 | and C<endpwent>. For example, C<time+86_400> always means |
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54 | C<time() + 86_400>. |
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55 | |
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56 | For functions that can be used in either a scalar or list context, |
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57 | nonabortive failure is generally indicated in a scalar context by |
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58 | returning the undefined value, and in a list context by returning the |
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59 | null list. |
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60 | |
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61 | Remember the following important rule: There is B<no rule> that relates |
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62 | the behavior of an expression in list context to its behavior in scalar |
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63 | context, or vice versa. It might do two totally different things. |
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64 | Each operator and function decides which sort of value it would be most |
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65 | appropriate to return in scalar context. Some operators return the |
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66 | length of the list that would have been returned in list context. Some |
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67 | operators return the first value in the list. Some operators return the |
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68 | last value in the list. Some operators return a count of successful |
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69 | operations. In general, they do what you want, unless you want |
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70 | consistency. |
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71 | |
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72 | An named array in scalar context is quite different from what would at |
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73 | first glance appear to be a list in scalar context. You can't get a list |
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74 | like C<(1,2,3)> into being in scalar context, because the compiler knows |
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75 | the context at compile time. It would generate the scalar comma operator |
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76 | there, not the list construction version of the comma. That means it |
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77 | was never a list to start with. |
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78 | |
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79 | In general, functions in Perl that serve as wrappers for system calls |
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80 | of the same name (like chown(2), fork(2), closedir(2), etc.) all return |
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81 | true when they succeed and C<undef> otherwise, as is usually mentioned |
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82 | in the descriptions below. This is different from the C interfaces, |
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83 | which return C<-1> on failure. Exceptions to this rule are C<wait>, |
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84 | C<waitpid>, and C<syscall>. System calls also set the special C<$!> |
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85 | variable on failure. Other functions do not, except accidentally. |
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86 | |
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87 | =head2 Perl Functions by Category |
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88 | |
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89 | Here are Perl's functions (including things that look like |
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90 | functions, like some keywords and named operators) |
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91 | arranged by category. Some functions appear in more |
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92 | than one place. |
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93 | |
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94 | =over 4 |
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95 | |
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96 | =item Functions for SCALARs or strings |
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97 | |
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98 | C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>, |
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99 | C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>, |
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100 | C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///> |
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101 | |
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102 | =item Regular expressions and pattern matching |
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103 | |
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104 | C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//> |
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105 | |
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106 | =item Numeric functions |
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107 | |
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108 | C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>, |
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109 | C<sin>, C<sqrt>, C<srand> |
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110 | |
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111 | =item Functions for real @ARRAYs |
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112 | |
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113 | C<pop>, C<push>, C<shift>, C<splice>, C<unshift> |
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114 | |
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115 | =item Functions for list data |
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116 | |
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117 | C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack> |
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118 | |
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119 | =item Functions for real %HASHes |
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120 | |
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121 | C<delete>, C<each>, C<exists>, C<keys>, C<values> |
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122 | |
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123 | =item Input and output functions |
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124 | |
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125 | C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>, |
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126 | C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>, |
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127 | C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>, |
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128 | C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>, |
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129 | C<warn>, C<write> |
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130 | |
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131 | =item Functions for fixed length data or records |
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132 | |
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133 | C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec> |
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134 | |
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135 | =item Functions for filehandles, files, or directories |
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136 | |
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137 | C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>, |
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138 | C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>, |
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139 | C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>, |
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140 | C<unlink>, C<utime> |
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141 | |
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142 | =item Keywords related to the control flow of your perl program |
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143 | |
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144 | C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>, |
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145 | C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray> |
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146 | |
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147 | =item Keywords related to scoping |
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148 | |
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149 | C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use> |
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150 | |
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151 | =item Miscellaneous functions |
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152 | |
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153 | C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>, |
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154 | C<scalar>, C<undef>, C<wantarray> |
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155 | |
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156 | =item Functions for processes and process groups |
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157 | |
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158 | C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>, |
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159 | C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>, |
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160 | C<times>, C<wait>, C<waitpid> |
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161 | |
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162 | =item Keywords related to perl modules |
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163 | |
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164 | C<do>, C<import>, C<no>, C<package>, C<require>, C<use> |
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165 | |
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166 | =item Keywords related to classes and object-orientedness |
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167 | |
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168 | C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>, |
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169 | C<untie>, C<use> |
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170 | |
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171 | =item Low-level socket functions |
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172 | |
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173 | C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>, |
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174 | C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>, |
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175 | C<socket>, C<socketpair> |
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176 | |
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177 | =item System V interprocess communication functions |
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178 | |
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179 | C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>, |
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180 | C<shmctl>, C<shmget>, C<shmread>, C<shmwrite> |
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181 | |
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182 | =item Fetching user and group info |
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183 | |
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184 | C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>, |
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185 | C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>, |
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186 | C<getpwuid>, C<setgrent>, C<setpwent> |
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187 | |
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188 | =item Fetching network info |
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189 | |
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190 | C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>, |
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191 | C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, |
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192 | C<getprotobyname>, C<getprotobynumber>, C<getprotoent>, |
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193 | C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>, |
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194 | C<setnetent>, C<setprotoent>, C<setservent> |
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195 | |
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196 | =item Time-related functions |
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197 | |
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198 | C<gmtime>, C<localtime>, C<time>, C<times> |
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199 | |
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200 | =item Functions new in perl5 |
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201 | |
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202 | C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>, |
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203 | C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>, |
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204 | C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>, |
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205 | C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use> |
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206 | |
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207 | * - C<sub> was a keyword in perl4, but in perl5 it is an |
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208 | operator, which can be used in expressions. |
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209 | |
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210 | =item Functions obsoleted in perl5 |
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211 | |
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212 | C<dbmclose>, C<dbmopen> |
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213 | |
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214 | =back |
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215 | |
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216 | =head2 Portability |
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217 | |
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218 | Perl was born in Unix and can therefore access all common Unix |
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219 | system calls. In non-Unix environments, the functionality of some |
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220 | Unix system calls may not be available, or details of the available |
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221 | functionality may differ slightly. The Perl functions affected |
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222 | by this are: |
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223 | |
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224 | C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>, |
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225 | C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>, |
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226 | C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>, |
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227 | C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>, |
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228 | C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>, |
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229 | C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>, |
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230 | C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>, |
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231 | C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>, |
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232 | C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>, |
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233 | C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>, |
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234 | C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>, |
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235 | C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>, |
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236 | C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>, |
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237 | C<shmwrite>, C<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>, |
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238 | C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>, |
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239 | C<utime>, C<wait>, C<waitpid> |
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240 | |
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241 | For more information about the portability of these functions, see |
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242 | L<perlport> and other available platform-specific documentation. |
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243 | |
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244 | =head2 Alphabetical Listing of Perl Functions |
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245 | |
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246 | =over 8 |
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247 | |
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248 | =item I<-X> FILEHANDLE |
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249 | |
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250 | =item I<-X> EXPR |
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251 | |
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252 | =item I<-X> |
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253 | |
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254 | A file test, where X is one of the letters listed below. This unary |
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255 | operator takes one argument, either a filename or a filehandle, and |
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256 | tests the associated file to see if something is true about it. If the |
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257 | argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN. |
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258 | Unless otherwise documented, it returns C<1> for true and C<''> for false, or |
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259 | the undefined value if the file doesn't exist. Despite the funny |
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260 | names, precedence is the same as any other named unary operator, and |
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261 | the argument may be parenthesized like any other unary operator. The |
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262 | operator may be any of: |
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263 | X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p> |
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264 | X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C> |
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265 | |
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266 | -r File is readable by effective uid/gid. |
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267 | -w File is writable by effective uid/gid. |
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268 | -x File is executable by effective uid/gid. |
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269 | -o File is owned by effective uid. |
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270 | |
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271 | -R File is readable by real uid/gid. |
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272 | -W File is writable by real uid/gid. |
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273 | -X File is executable by real uid/gid. |
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274 | -O File is owned by real uid. |
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275 | |
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276 | -e File exists. |
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277 | -z File has zero size (is empty). |
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278 | -s File has nonzero size (returns size in bytes). |
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279 | |
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280 | -f File is a plain file. |
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281 | -d File is a directory. |
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282 | -l File is a symbolic link. |
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283 | -p File is a named pipe (FIFO), or Filehandle is a pipe. |
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284 | -S File is a socket. |
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285 | -b File is a block special file. |
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286 | -c File is a character special file. |
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287 | -t Filehandle is opened to a tty. |
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288 | |
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289 | -u File has setuid bit set. |
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290 | -g File has setgid bit set. |
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291 | -k File has sticky bit set. |
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292 | |
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293 | -T File is an ASCII text file. |
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294 | -B File is a "binary" file (opposite of -T). |
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295 | |
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296 | -M Age of file in days when script started. |
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297 | -A Same for access time. |
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298 | -C Same for inode change time. |
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299 | |
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300 | Example: |
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301 | |
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302 | while (<>) { |
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303 | chomp; |
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304 | next unless -f $_; # ignore specials |
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305 | #... |
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306 | } |
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307 | |
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308 | The interpretation of the file permission operators C<-r>, C<-R>, |
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309 | C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode |
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310 | of the file and the uids and gids of the user. There may be other |
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311 | reasons you can't actually read, write, or execute the file. Such |
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312 | reasons may be for example network filesystem access controls, ACLs |
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313 | (access control lists), read-only filesystems, and unrecognized |
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314 | executable formats. |
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315 | |
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316 | Also note that, for the superuser on the local filesystems, the C<-r>, |
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317 | C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1 |
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318 | if any execute bit is set in the mode. Scripts run by the superuser |
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319 | may thus need to do a stat() to determine the actual mode of the file, |
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320 | or temporarily set their effective uid to something else. |
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321 | |
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322 | If you are using ACLs, there is a pragma called C<filetest> that may |
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323 | produce more accurate results than the bare stat() mode bits. |
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324 | When under the C<use filetest 'access'> the above-mentioned filetests |
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325 | will test whether the permission can (not) be granted using the |
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326 | access() family of system calls. Also note that the C<-x> and C<-X> may |
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327 | under this pragma return true even if there are no execute permission |
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328 | bits set (nor any extra execute permission ACLs). This strangeness is |
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329 | due to the underlying system calls' definitions. Read the |
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330 | documentation for the C<filetest> pragma for more information. |
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331 | |
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332 | Note that C<-s/a/b/> does not do a negated substitution. Saying |
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333 | C<-exp($foo)> still works as expected, however--only single letters |
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334 | following a minus are interpreted as file tests. |
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335 | |
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336 | The C<-T> and C<-B> switches work as follows. The first block or so of the |
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337 | file is examined for odd characters such as strange control codes or |
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338 | characters with the high bit set. If too many strange characters (>30%) |
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339 | are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file |
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340 | containing null in the first block is considered a binary file. If C<-T> |
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341 | or C<-B> is used on a filehandle, the current stdio buffer is examined |
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342 | rather than the first block. Both C<-T> and C<-B> return true on a null |
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343 | file, or a file at EOF when testing a filehandle. Because you have to |
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344 | read a file to do the C<-T> test, on most occasions you want to use a C<-f> |
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345 | against the file first, as in C<next unless -f $file && -T $file>. |
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346 | |
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347 | If any of the file tests (or either the C<stat> or C<lstat> operators) are given |
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348 | the special filehandle consisting of a solitary underline, then the stat |
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349 | structure of the previous file test (or stat operator) is used, saving |
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350 | a system call. (This doesn't work with C<-t>, and you need to remember |
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351 | that lstat() and C<-l> will leave values in the stat structure for the |
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352 | symbolic link, not the real file.) Example: |
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353 | |
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354 | print "Can do.\n" if -r $a || -w _ || -x _; |
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355 | |
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356 | stat($filename); |
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357 | print "Readable\n" if -r _; |
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358 | print "Writable\n" if -w _; |
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359 | print "Executable\n" if -x _; |
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360 | print "Setuid\n" if -u _; |
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361 | print "Setgid\n" if -g _; |
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362 | print "Sticky\n" if -k _; |
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363 | print "Text\n" if -T _; |
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364 | print "Binary\n" if -B _; |
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365 | |
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366 | =item abs VALUE |
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367 | |
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368 | =item abs |
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369 | |
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370 | Returns the absolute value of its argument. |
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371 | If VALUE is omitted, uses C<$_>. |
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372 | |
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373 | =item accept NEWSOCKET,GENERICSOCKET |
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374 | |
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375 | Accepts an incoming socket connect, just as the accept(2) system call |
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376 | does. Returns the packed address if it succeeded, false otherwise. |
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377 | See the example in L<perlipc/"Sockets: Client/Server Communication">. |
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378 | |
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379 | On systems that support a close-on-exec flag on files, the flag will |
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380 | be set for the newly opened file descriptor, as determined by the |
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381 | value of $^F. See L<perlvar/$^F>. |
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382 | |
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383 | =item alarm SECONDS |
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384 | |
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385 | =item alarm |
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386 | |
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387 | Arranges to have a SIGALRM delivered to this process after the |
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388 | specified number of seconds have elapsed. If SECONDS is not specified, |
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389 | the value stored in C<$_> is used. (On some machines, |
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390 | unfortunately, the elapsed time may be up to one second less than you |
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391 | specified because of how seconds are counted.) Only one timer may be |
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392 | counting at once. Each call disables the previous timer, and an |
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393 | argument of C<0> may be supplied to cancel the previous timer without |
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394 | starting a new one. The returned value is the amount of time remaining |
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395 | on the previous timer. |
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396 | |
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397 | For delays of finer granularity than one second, you may use Perl's |
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398 | four-argument version of select() leaving the first three arguments |
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399 | undefined, or you might be able to use the C<syscall> interface to |
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400 | access setitimer(2) if your system supports it. The Time::HiRes module |
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401 | from CPAN may also prove useful. |
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402 | |
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403 | It is usually a mistake to intermix C<alarm> and C<sleep> calls. |
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404 | (C<sleep> may be internally implemented in your system with C<alarm>) |
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405 | |
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406 | If you want to use C<alarm> to time out a system call you need to use an |
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407 | C<eval>/C<die> pair. You can't rely on the alarm causing the system call to |
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408 | fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to |
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409 | restart system calls on some systems. Using C<eval>/C<die> always works, |
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410 | modulo the caveats given in L<perlipc/"Signals">. |
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411 | |
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412 | eval { |
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413 | local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required |
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414 | alarm $timeout; |
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415 | $nread = sysread SOCKET, $buffer, $size; |
---|
416 | alarm 0; |
---|
417 | }; |
---|
418 | if ($@) { |
---|
419 | die unless $@ eq "alarm\n"; # propagate unexpected errors |
---|
420 | # timed out |
---|
421 | } |
---|
422 | else { |
---|
423 | # didn't |
---|
424 | } |
---|
425 | |
---|
426 | =item atan2 Y,X |
---|
427 | |
---|
428 | Returns the arctangent of Y/X in the range -PI to PI. |
---|
429 | |
---|
430 | For the tangent operation, you may use the C<Math::Trig::tan> |
---|
431 | function, or use the familiar relation: |
---|
432 | |
---|
433 | sub tan { sin($_[0]) / cos($_[0]) } |
---|
434 | |
---|
435 | =item bind SOCKET,NAME |
---|
436 | |
---|
437 | Binds a network address to a socket, just as the bind system call |
---|
438 | does. Returns true if it succeeded, false otherwise. NAME should be a |
---|
439 | packed address of the appropriate type for the socket. See the examples in |
---|
440 | L<perlipc/"Sockets: Client/Server Communication">. |
---|
441 | |
---|
442 | =item binmode FILEHANDLE, DISCIPLINE |
---|
443 | |
---|
444 | =item binmode FILEHANDLE |
---|
445 | |
---|
446 | Arranges for FILEHANDLE to be read or written in "binary" or "text" mode |
---|
447 | on systems where the run-time libraries distinguish between binary and |
---|
448 | text files. If FILEHANDLE is an expression, the value is taken as the |
---|
449 | name of the filehandle. DISCIPLINE can be either of C<":raw"> for |
---|
450 | binary mode or C<":crlf"> for "text" mode. If the DISCIPLINE is |
---|
451 | omitted, it defaults to C<":raw">. |
---|
452 | |
---|
453 | binmode() should be called after open() but before any I/O is done on |
---|
454 | the filehandle. |
---|
455 | |
---|
456 | On many systems binmode() currently has no effect, but in future, it |
---|
457 | will be extended to support user-defined input and output disciplines. |
---|
458 | On some systems binmode() is necessary when you're not working with a |
---|
459 | text file. For the sake of portability it is a good idea to always use |
---|
460 | it when appropriate, and to never use it when it isn't appropriate. |
---|
461 | |
---|
462 | In other words: Regardless of platform, use binmode() on binary |
---|
463 | files, and do not use binmode() on text files. |
---|
464 | |
---|
465 | The C<open> pragma can be used to establish default disciplines. |
---|
466 | See L<open>. |
---|
467 | |
---|
468 | The operating system, device drivers, C libraries, and Perl run-time |
---|
469 | system all work together to let the programmer treat a single |
---|
470 | character (C<\n>) as the line terminator, irrespective of the external |
---|
471 | representation. On many operating systems, the native text file |
---|
472 | representation matches the internal representation, but on some |
---|
473 | platforms the external representation of C<\n> is made up of more than |
---|
474 | one character. |
---|
475 | |
---|
476 | Mac OS and all variants of Unix use a single character to end each line |
---|
477 | in the external representation of text (even though that single |
---|
478 | character is not necessarily the same across these platforms). |
---|
479 | Consequently binmode() has no effect on these operating systems. In |
---|
480 | other systems like VMS, MS-DOS and the various flavors of MS-Windows |
---|
481 | your program sees a C<\n> as a simple C<\cJ>, but what's stored in text |
---|
482 | files are the two characters C<\cM\cJ>. That means that, if you don't |
---|
483 | use binmode() on these systems, C<\cM\cJ> sequences on disk will be |
---|
484 | converted to C<\n> on input, and any C<\n> in your program will be |
---|
485 | converted back to C<\cM\cJ> on output. This is what you want for text |
---|
486 | files, but it can be disastrous for binary files. |
---|
487 | |
---|
488 | Another consequence of using binmode() (on some systems) is that |
---|
489 | special end-of-file markers will be seen as part of the data stream. |
---|
490 | For systems from the Microsoft family this means that if your binary |
---|
491 | data contains C<\cZ>, the I/O subsystem will regard it as the end of |
---|
492 | the file, unless you use binmode(). |
---|
493 | |
---|
494 | binmode() is not only important for readline() and print() operations, |
---|
495 | but also when using read(), seek(), sysread(), syswrite() and tell() |
---|
496 | (see L<perlport> for more details). See the C<$/> and C<$\> variables |
---|
497 | in L<perlvar> for how to manually set your input and output |
---|
498 | line-termination sequences. |
---|
499 | |
---|
500 | =item bless REF,CLASSNAME |
---|
501 | |
---|
502 | =item bless REF |
---|
503 | |
---|
504 | This function tells the thingy referenced by REF that it is now an object |
---|
505 | in the CLASSNAME package. If CLASSNAME is omitted, the current package |
---|
506 | is used. Because a C<bless> is often the last thing in a constructor, |
---|
507 | it returns the reference for convenience. Always use the two-argument |
---|
508 | version if the function doing the blessing might be inherited by a |
---|
509 | derived class. See L<perltoot> and L<perlobj> for more about the blessing |
---|
510 | (and blessings) of objects. |
---|
511 | |
---|
512 | Consider always blessing objects in CLASSNAMEs that are mixed case. |
---|
513 | Namespaces with all lowercase names are considered reserved for |
---|
514 | Perl pragmata. Builtin types have all uppercase names, so to prevent |
---|
515 | confusion, you may wish to avoid such package names as well. Make sure |
---|
516 | that CLASSNAME is a true value. |
---|
517 | |
---|
518 | See L<perlmod/"Perl Modules">. |
---|
519 | |
---|
520 | =item caller EXPR |
---|
521 | |
---|
522 | =item caller |
---|
523 | |
---|
524 | Returns the context of the current subroutine call. In scalar context, |
---|
525 | returns the caller's package name if there is a caller, that is, if |
---|
526 | we're in a subroutine or C<eval> or C<require>, and the undefined value |
---|
527 | otherwise. In list context, returns |
---|
528 | |
---|
529 | ($package, $filename, $line) = caller; |
---|
530 | |
---|
531 | With EXPR, it returns some extra information that the debugger uses to |
---|
532 | print a stack trace. The value of EXPR indicates how many call frames |
---|
533 | to go back before the current one. |
---|
534 | |
---|
535 | ($package, $filename, $line, $subroutine, $hasargs, |
---|
536 | $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i); |
---|
537 | |
---|
538 | Here $subroutine may be C<(eval)> if the frame is not a subroutine |
---|
539 | call, but an C<eval>. In such a case additional elements $evaltext and |
---|
540 | C<$is_require> are set: C<$is_require> is true if the frame is created by a |
---|
541 | C<require> or C<use> statement, $evaltext contains the text of the |
---|
542 | C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement, |
---|
543 | $filename is C<(eval)>, but $evaltext is undefined. (Note also that |
---|
544 | each C<use> statement creates a C<require> frame inside an C<eval EXPR>) |
---|
545 | frame. C<$hasargs> is true if a new instance of C<@_> was set up for the |
---|
546 | frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller |
---|
547 | was compiled with. The C<$hints> and C<$bitmask> values are subject to |
---|
548 | change between versions of Perl, and are not meant for external use. |
---|
549 | |
---|
550 | Furthermore, when called from within the DB package, caller returns more |
---|
551 | detailed information: it sets the list variable C<@DB::args> to be the |
---|
552 | arguments with which the subroutine was invoked. |
---|
553 | |
---|
554 | Be aware that the optimizer might have optimized call frames away before |
---|
555 | C<caller> had a chance to get the information. That means that C<caller(N)> |
---|
556 | might not return information about the call frame you expect it do, for |
---|
557 | C<< N > 1 >>. In particular, C<@DB::args> might have information from the |
---|
558 | previous time C<caller> was called. |
---|
559 | |
---|
560 | =item chdir EXPR |
---|
561 | |
---|
562 | Changes the working directory to EXPR, if possible. If EXPR is omitted, |
---|
563 | changes to the directory specified by C<$ENV{HOME}>, if set; if not, |
---|
564 | changes to the directory specified by C<$ENV{LOGDIR}>. If neither is |
---|
565 | set, C<chdir> does nothing. It returns true upon success, false |
---|
566 | otherwise. See the example under C<die>. |
---|
567 | |
---|
568 | =item chmod LIST |
---|
569 | |
---|
570 | Changes the permissions of a list of files. The first element of the |
---|
571 | list must be the numerical mode, which should probably be an octal |
---|
572 | number, and which definitely should I<not> a string of octal digits: |
---|
573 | C<0644> is okay, C<'0644'> is not. Returns the number of files |
---|
574 | successfully changed. See also L</oct>, if all you have is a string. |
---|
575 | |
---|
576 | $cnt = chmod 0755, 'foo', 'bar'; |
---|
577 | chmod 0755, @executables; |
---|
578 | $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to |
---|
579 | # --w----r-T |
---|
580 | $mode = '0644'; chmod oct($mode), 'foo'; # this is better |
---|
581 | $mode = 0644; chmod $mode, 'foo'; # this is best |
---|
582 | |
---|
583 | You can also import the symbolic C<S_I*> constants from the Fcntl |
---|
584 | module: |
---|
585 | |
---|
586 | use Fcntl ':mode'; |
---|
587 | |
---|
588 | chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables; |
---|
589 | # This is identical to the chmod 0755 of the above example. |
---|
590 | |
---|
591 | =item chomp VARIABLE |
---|
592 | |
---|
593 | =item chomp LIST |
---|
594 | |
---|
595 | =item chomp |
---|
596 | |
---|
597 | This safer version of L</chop> removes any trailing string |
---|
598 | that corresponds to the current value of C<$/> (also known as |
---|
599 | $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total |
---|
600 | number of characters removed from all its arguments. It's often used to |
---|
601 | remove the newline from the end of an input record when you're worried |
---|
602 | that the final record may be missing its newline. When in paragraph |
---|
603 | mode (C<$/ = "">), it removes all trailing newlines from the string. |
---|
604 | When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is |
---|
605 | a reference to an integer or the like, see L<perlvar>) chomp() won't |
---|
606 | remove anything. |
---|
607 | If VARIABLE is omitted, it chomps C<$_>. Example: |
---|
608 | |
---|
609 | while (<>) { |
---|
610 | chomp; # avoid \n on last field |
---|
611 | @array = split(/:/); |
---|
612 | # ... |
---|
613 | } |
---|
614 | |
---|
615 | If VARIABLE is a hash, it chomps the hash's values, but not its keys. |
---|
616 | |
---|
617 | You can actually chomp anything that's an lvalue, including an assignment: |
---|
618 | |
---|
619 | chomp($cwd = `pwd`); |
---|
620 | chomp($answer = <STDIN>); |
---|
621 | |
---|
622 | If you chomp a list, each element is chomped, and the total number of |
---|
623 | characters removed is returned. |
---|
624 | |
---|
625 | =item chop VARIABLE |
---|
626 | |
---|
627 | =item chop LIST |
---|
628 | |
---|
629 | =item chop |
---|
630 | |
---|
631 | Chops off the last character of a string and returns the character |
---|
632 | chopped. It is much more efficient than C<s/.$//s> because it neither |
---|
633 | scans nor copies the string. If VARIABLE is omitted, chops C<$_>. |
---|
634 | If VARIABLE is a hash, it chops the hash's values, but not its keys. |
---|
635 | |
---|
636 | You can actually chop anything that's an lvalue, including an assignment. |
---|
637 | |
---|
638 | If you chop a list, each element is chopped. Only the value of the |
---|
639 | last C<chop> is returned. |
---|
640 | |
---|
641 | Note that C<chop> returns the last character. To return all but the last |
---|
642 | character, use C<substr($string, 0, -1)>. |
---|
643 | |
---|
644 | =item chown LIST |
---|
645 | |
---|
646 | Changes the owner (and group) of a list of files. The first two |
---|
647 | elements of the list must be the I<numeric> uid and gid, in that |
---|
648 | order. A value of -1 in either position is interpreted by most |
---|
649 | systems to leave that value unchanged. Returns the number of files |
---|
650 | successfully changed. |
---|
651 | |
---|
652 | $cnt = chown $uid, $gid, 'foo', 'bar'; |
---|
653 | chown $uid, $gid, @filenames; |
---|
654 | |
---|
655 | Here's an example that looks up nonnumeric uids in the passwd file: |
---|
656 | |
---|
657 | print "User: "; |
---|
658 | chomp($user = <STDIN>); |
---|
659 | print "Files: "; |
---|
660 | chomp($pattern = <STDIN>); |
---|
661 | |
---|
662 | ($login,$pass,$uid,$gid) = getpwnam($user) |
---|
663 | or die "$user not in passwd file"; |
---|
664 | |
---|
665 | @ary = glob($pattern); # expand filenames |
---|
666 | chown $uid, $gid, @ary; |
---|
667 | |
---|
668 | On most systems, you are not allowed to change the ownership of the |
---|
669 | file unless you're the superuser, although you should be able to change |
---|
670 | the group to any of your secondary groups. On insecure systems, these |
---|
671 | restrictions may be relaxed, but this is not a portable assumption. |
---|
672 | On POSIX systems, you can detect this condition this way: |
---|
673 | |
---|
674 | use POSIX qw(sysconf _PC_CHOWN_RESTRICTED); |
---|
675 | $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED); |
---|
676 | |
---|
677 | =item chr NUMBER |
---|
678 | |
---|
679 | =item chr |
---|
680 | |
---|
681 | Returns the character represented by that NUMBER in the character set. |
---|
682 | For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and |
---|
683 | chr(0x263a) is a Unicode smiley face (but only within the scope of |
---|
684 | a C<use utf8>). For the reverse, use L</ord>. |
---|
685 | See L<utf8> for more about Unicode. |
---|
686 | |
---|
687 | If NUMBER is omitted, uses C<$_>. |
---|
688 | |
---|
689 | =item chroot FILENAME |
---|
690 | |
---|
691 | =item chroot |
---|
692 | |
---|
693 | This function works like the system call by the same name: it makes the |
---|
694 | named directory the new root directory for all further pathnames that |
---|
695 | begin with a C</> by your process and all its children. (It doesn't |
---|
696 | change your current working directory, which is unaffected.) For security |
---|
697 | reasons, this call is restricted to the superuser. If FILENAME is |
---|
698 | omitted, does a C<chroot> to C<$_>. |
---|
699 | |
---|
700 | =item close FILEHANDLE |
---|
701 | |
---|
702 | =item close |
---|
703 | |
---|
704 | Closes the file or pipe associated with the file handle, returning true |
---|
705 | only if stdio successfully flushes buffers and closes the system file |
---|
706 | descriptor. Closes the currently selected filehandle if the argument |
---|
707 | is omitted. |
---|
708 | |
---|
709 | You don't have to close FILEHANDLE if you are immediately going to do |
---|
710 | another C<open> on it, because C<open> will close it for you. (See |
---|
711 | C<open>.) However, an explicit C<close> on an input file resets the line |
---|
712 | counter (C<$.>), while the implicit close done by C<open> does not. |
---|
713 | |
---|
714 | If the file handle came from a piped open C<close> will additionally |
---|
715 | return false if one of the other system calls involved fails or if the |
---|
716 | program exits with non-zero status. (If the only problem was that the |
---|
717 | program exited non-zero C<$!> will be set to C<0>.) Closing a pipe |
---|
718 | also waits for the process executing on the pipe to complete, in case you |
---|
719 | want to look at the output of the pipe afterwards, and |
---|
720 | implicitly puts the exit status value of that command into C<$?>. |
---|
721 | |
---|
722 | Prematurely closing the read end of a pipe (i.e. before the process |
---|
723 | writing to it at the other end has closed it) will result in a |
---|
724 | SIGPIPE being delivered to the writer. If the other end can't |
---|
725 | handle that, be sure to read all the data before closing the pipe. |
---|
726 | |
---|
727 | Example: |
---|
728 | |
---|
729 | open(OUTPUT, '|sort >foo') # pipe to sort |
---|
730 | or die "Can't start sort: $!"; |
---|
731 | #... # print stuff to output |
---|
732 | close OUTPUT # wait for sort to finish |
---|
733 | or warn $! ? "Error closing sort pipe: $!" |
---|
734 | : "Exit status $? from sort"; |
---|
735 | open(INPUT, 'foo') # get sort's results |
---|
736 | or die "Can't open 'foo' for input: $!"; |
---|
737 | |
---|
738 | FILEHANDLE may be an expression whose value can be used as an indirect |
---|
739 | filehandle, usually the real filehandle name. |
---|
740 | |
---|
741 | =item closedir DIRHANDLE |
---|
742 | |
---|
743 | Closes a directory opened by C<opendir> and returns the success of that |
---|
744 | system call. |
---|
745 | |
---|
746 | DIRHANDLE may be an expression whose value can be used as an indirect |
---|
747 | dirhandle, usually the real dirhandle name. |
---|
748 | |
---|
749 | =item connect SOCKET,NAME |
---|
750 | |
---|
751 | Attempts to connect to a remote socket, just as the connect system call |
---|
752 | does. Returns true if it succeeded, false otherwise. NAME should be a |
---|
753 | packed address of the appropriate type for the socket. See the examples in |
---|
754 | L<perlipc/"Sockets: Client/Server Communication">. |
---|
755 | |
---|
756 | =item continue BLOCK |
---|
757 | |
---|
758 | Actually a flow control statement rather than a function. If there is a |
---|
759 | C<continue> BLOCK attached to a BLOCK (typically in a C<while> or |
---|
760 | C<foreach>), it is always executed just before the conditional is about to |
---|
761 | be evaluated again, just like the third part of a C<for> loop in C. Thus |
---|
762 | it can be used to increment a loop variable, even when the loop has been |
---|
763 | continued via the C<next> statement (which is similar to the C C<continue> |
---|
764 | statement). |
---|
765 | |
---|
766 | C<last>, C<next>, or C<redo> may appear within a C<continue> |
---|
767 | block. C<last> and C<redo> will behave as if they had been executed within |
---|
768 | the main block. So will C<next>, but since it will execute a C<continue> |
---|
769 | block, it may be more entertaining. |
---|
770 | |
---|
771 | while (EXPR) { |
---|
772 | ### redo always comes here |
---|
773 | do_something; |
---|
774 | } continue { |
---|
775 | ### next always comes here |
---|
776 | do_something_else; |
---|
777 | # then back the top to re-check EXPR |
---|
778 | } |
---|
779 | ### last always comes here |
---|
780 | |
---|
781 | Omitting the C<continue> section is semantically equivalent to using an |
---|
782 | empty one, logically enough. In that case, C<next> goes directly back |
---|
783 | to check the condition at the top of the loop. |
---|
784 | |
---|
785 | =item cos EXPR |
---|
786 | |
---|
787 | =item cos |
---|
788 | |
---|
789 | Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, |
---|
790 | takes cosine of C<$_>. |
---|
791 | |
---|
792 | For the inverse cosine operation, you may use the C<Math::Trig::acos()> |
---|
793 | function, or use this relation: |
---|
794 | |
---|
795 | sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) } |
---|
796 | |
---|
797 | =item crypt PLAINTEXT,SALT |
---|
798 | |
---|
799 | Encrypts a string exactly like the crypt(3) function in the C library |
---|
800 | (assuming that you actually have a version there that has not been |
---|
801 | extirpated as a potential munition). This can prove useful for checking |
---|
802 | the password file for lousy passwords, amongst other things. Only the |
---|
803 | guys wearing white hats should do this. |
---|
804 | |
---|
805 | Note that C<crypt> is intended to be a one-way function, much like breaking |
---|
806 | eggs to make an omelette. There is no (known) corresponding decrypt |
---|
807 | function. As a result, this function isn't all that useful for |
---|
808 | cryptography. (For that, see your nearby CPAN mirror.) |
---|
809 | |
---|
810 | When verifying an existing encrypted string you should use the encrypted |
---|
811 | text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This |
---|
812 | allows your code to work with the standard C<crypt> and with more |
---|
813 | exotic implementations. When choosing a new salt create a random two |
---|
814 | character string whose characters come from the set C<[./0-9A-Za-z]> |
---|
815 | (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). |
---|
816 | |
---|
817 | Here's an example that makes sure that whoever runs this program knows |
---|
818 | their own password: |
---|
819 | |
---|
820 | $pwd = (getpwuid($<))[1]; |
---|
821 | |
---|
822 | system "stty -echo"; |
---|
823 | print "Password: "; |
---|
824 | chomp($word = <STDIN>); |
---|
825 | print "\n"; |
---|
826 | system "stty echo"; |
---|
827 | |
---|
828 | if (crypt($word, $pwd) ne $pwd) { |
---|
829 | die "Sorry...\n"; |
---|
830 | } else { |
---|
831 | print "ok\n"; |
---|
832 | } |
---|
833 | |
---|
834 | Of course, typing in your own password to whoever asks you |
---|
835 | for it is unwise. |
---|
836 | |
---|
837 | The L<crypt> function is unsuitable for encrypting large quantities |
---|
838 | of data, not least of all because you can't get the information |
---|
839 | back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories |
---|
840 | on your favorite CPAN mirror for a slew of potentially useful |
---|
841 | modules. |
---|
842 | |
---|
843 | =item dbmclose HASH |
---|
844 | |
---|
845 | [This function has been largely superseded by the C<untie> function.] |
---|
846 | |
---|
847 | Breaks the binding between a DBM file and a hash. |
---|
848 | |
---|
849 | =item dbmopen HASH,DBNAME,MASK |
---|
850 | |
---|
851 | [This function has been largely superseded by the C<tie> function.] |
---|
852 | |
---|
853 | This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a |
---|
854 | hash. HASH is the name of the hash. (Unlike normal C<open>, the first |
---|
855 | argument is I<not> a filehandle, even though it looks like one). DBNAME |
---|
856 | is the name of the database (without the F<.dir> or F<.pag> extension if |
---|
857 | any). If the database does not exist, it is created with protection |
---|
858 | specified by MASK (as modified by the C<umask>). If your system supports |
---|
859 | only the older DBM functions, you may perform only one C<dbmopen> in your |
---|
860 | program. In older versions of Perl, if your system had neither DBM nor |
---|
861 | ndbm, calling C<dbmopen> produced a fatal error; it now falls back to |
---|
862 | sdbm(3). |
---|
863 | |
---|
864 | If you don't have write access to the DBM file, you can only read hash |
---|
865 | variables, not set them. If you want to test whether you can write, |
---|
866 | either use file tests or try setting a dummy hash entry inside an C<eval>, |
---|
867 | which will trap the error. |
---|
868 | |
---|
869 | Note that functions such as C<keys> and C<values> may return huge lists |
---|
870 | when used on large DBM files. You may prefer to use the C<each> |
---|
871 | function to iterate over large DBM files. Example: |
---|
872 | |
---|
873 | # print out history file offsets |
---|
874 | dbmopen(%HIST,'/usr/lib/news/history',0666); |
---|
875 | while (($key,$val) = each %HIST) { |
---|
876 | print $key, ' = ', unpack('L',$val), "\n"; |
---|
877 | } |
---|
878 | dbmclose(%HIST); |
---|
879 | |
---|
880 | See also L<AnyDBM_File> for a more general description of the pros and |
---|
881 | cons of the various dbm approaches, as well as L<DB_File> for a particularly |
---|
882 | rich implementation. |
---|
883 | |
---|
884 | You can control which DBM library you use by loading that library |
---|
885 | before you call dbmopen(): |
---|
886 | |
---|
887 | use DB_File; |
---|
888 | dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db") |
---|
889 | or die "Can't open netscape history file: $!"; |
---|
890 | |
---|
891 | =item defined EXPR |
---|
892 | |
---|
893 | =item defined |
---|
894 | |
---|
895 | Returns a Boolean value telling whether EXPR has a value other than |
---|
896 | the undefined value C<undef>. If EXPR is not present, C<$_> will be |
---|
897 | checked. |
---|
898 | |
---|
899 | Many operations return C<undef> to indicate failure, end of file, |
---|
900 | system error, uninitialized variable, and other exceptional |
---|
901 | conditions. This function allows you to distinguish C<undef> from |
---|
902 | other values. (A simple Boolean test will not distinguish among |
---|
903 | C<undef>, zero, the empty string, and C<"0">, which are all equally |
---|
904 | false.) Note that since C<undef> is a valid scalar, its presence |
---|
905 | doesn't I<necessarily> indicate an exceptional condition: C<pop> |
---|
906 | returns C<undef> when its argument is an empty array, I<or> when the |
---|
907 | element to return happens to be C<undef>. |
---|
908 | |
---|
909 | You may also use C<defined(&func)> to check whether subroutine C<&func> |
---|
910 | has ever been defined. The return value is unaffected by any forward |
---|
911 | declarations of C<&foo>. Note that a subroutine which is not defined |
---|
912 | may still be callable: its package may have an C<AUTOLOAD> method that |
---|
913 | makes it spring into existence the first time that it is called -- see |
---|
914 | L<perlsub>. |
---|
915 | |
---|
916 | Use of C<defined> on aggregates (hashes and arrays) is deprecated. It |
---|
917 | used to report whether memory for that aggregate has ever been |
---|
918 | allocated. This behavior may disappear in future versions of Perl. |
---|
919 | You should instead use a simple test for size: |
---|
920 | |
---|
921 | if (@an_array) { print "has array elements\n" } |
---|
922 | if (%a_hash) { print "has hash members\n" } |
---|
923 | |
---|
924 | When used on a hash element, it tells you whether the value is defined, |
---|
925 | not whether the key exists in the hash. Use L</exists> for the latter |
---|
926 | purpose. |
---|
927 | |
---|
928 | Examples: |
---|
929 | |
---|
930 | print if defined $switch{'D'}; |
---|
931 | print "$val\n" while defined($val = pop(@ary)); |
---|
932 | die "Can't readlink $sym: $!" |
---|
933 | unless defined($value = readlink $sym); |
---|
934 | sub foo { defined &$bar ? &$bar(@_) : die "No bar"; } |
---|
935 | $debugging = 0 unless defined $debugging; |
---|
936 | |
---|
937 | Note: Many folks tend to overuse C<defined>, and then are surprised to |
---|
938 | discover that the number C<0> and C<""> (the zero-length string) are, in fact, |
---|
939 | defined values. For example, if you say |
---|
940 | |
---|
941 | "ab" =~ /a(.*)b/; |
---|
942 | |
---|
943 | The pattern match succeeds, and C<$1> is defined, despite the fact that it |
---|
944 | matched "nothing". But it didn't really match nothing--rather, it |
---|
945 | matched something that happened to be zero characters long. This is all |
---|
946 | very above-board and honest. When a function returns an undefined value, |
---|
947 | it's an admission that it couldn't give you an honest answer. So you |
---|
948 | should use C<defined> only when you're questioning the integrity of what |
---|
949 | you're trying to do. At other times, a simple comparison to C<0> or C<""> is |
---|
950 | what you want. |
---|
951 | |
---|
952 | See also L</undef>, L</exists>, L</ref>. |
---|
953 | |
---|
954 | =item delete EXPR |
---|
955 | |
---|
956 | Given an expression that specifies a hash element, array element, hash slice, |
---|
957 | or array slice, deletes the specified element(s) from the hash or array. |
---|
958 | In the case of an array, if the array elements happen to be at the end, |
---|
959 | the size of the array will shrink to the highest element that tests |
---|
960 | true for exists() (or 0 if no such element exists). |
---|
961 | |
---|
962 | Returns each element so deleted or the undefined value if there was no such |
---|
963 | element. Deleting from C<$ENV{}> modifies the environment. Deleting from |
---|
964 | a hash tied to a DBM file deletes the entry from the DBM file. Deleting |
---|
965 | from a C<tie>d hash or array may not necessarily return anything. |
---|
966 | |
---|
967 | Deleting an array element effectively returns that position of the array |
---|
968 | to its initial, uninitialized state. Subsequently testing for the same |
---|
969 | element with exists() will return false. Note that deleting array |
---|
970 | elements in the middle of an array will not shift the index of the ones |
---|
971 | after them down--use splice() for that. See L</exists>. |
---|
972 | |
---|
973 | The following (inefficiently) deletes all the values of %HASH and @ARRAY: |
---|
974 | |
---|
975 | foreach $key (keys %HASH) { |
---|
976 | delete $HASH{$key}; |
---|
977 | } |
---|
978 | |
---|
979 | foreach $index (0 .. $#ARRAY) { |
---|
980 | delete $ARRAY[$index]; |
---|
981 | } |
---|
982 | |
---|
983 | And so do these: |
---|
984 | |
---|
985 | delete @HASH{keys %HASH}; |
---|
986 | |
---|
987 | delete @ARRAY[0 .. $#ARRAY]; |
---|
988 | |
---|
989 | But both of these are slower than just assigning the empty list |
---|
990 | or undefining %HASH or @ARRAY: |
---|
991 | |
---|
992 | %HASH = (); # completely empty %HASH |
---|
993 | undef %HASH; # forget %HASH ever existed |
---|
994 | |
---|
995 | @ARRAY = (); # completely empty @ARRAY |
---|
996 | undef @ARRAY; # forget @ARRAY ever existed |
---|
997 | |
---|
998 | Note that the EXPR can be arbitrarily complicated as long as the final |
---|
999 | operation is a hash element, array element, hash slice, or array slice |
---|
1000 | lookup: |
---|
1001 | |
---|
1002 | delete $ref->[$x][$y]{$key}; |
---|
1003 | delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys}; |
---|
1004 | |
---|
1005 | delete $ref->[$x][$y][$index]; |
---|
1006 | delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices]; |
---|
1007 | |
---|
1008 | =item die LIST |
---|
1009 | |
---|
1010 | Outside an C<eval>, prints the value of LIST to C<STDERR> and |
---|
1011 | exits with the current value of C<$!> (errno). If C<$!> is C<0>, |
---|
1012 | exits with the value of C<<< ($? >> 8) >>> (backtick `command` |
---|
1013 | status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside |
---|
1014 | an C<eval(),> the error message is stuffed into C<$@> and the |
---|
1015 | C<eval> is terminated with the undefined value. This makes |
---|
1016 | C<die> the way to raise an exception. |
---|
1017 | |
---|
1018 | Equivalent examples: |
---|
1019 | |
---|
1020 | die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news'; |
---|
1021 | chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" |
---|
1022 | |
---|
1023 | If the value of EXPR does not end in a newline, the current script line |
---|
1024 | number and input line number (if any) are also printed, and a newline |
---|
1025 | is supplied. Note that the "input line number" (also known as "chunk") |
---|
1026 | is subject to whatever notion of "line" happens to be currently in |
---|
1027 | effect, and is also available as the special variable C<$.>. |
---|
1028 | See L<perlvar/"$/"> and L<perlvar/"$.">. |
---|
1029 | |
---|
1030 | Hint: sometimes appending C<", stopped"> to your message |
---|
1031 | will cause it to make better sense when the string C<"at foo line 123"> is |
---|
1032 | appended. Suppose you are running script "canasta". |
---|
1033 | |
---|
1034 | die "/etc/games is no good"; |
---|
1035 | die "/etc/games is no good, stopped"; |
---|
1036 | |
---|
1037 | produce, respectively |
---|
1038 | |
---|
1039 | /etc/games is no good at canasta line 123. |
---|
1040 | /etc/games is no good, stopped at canasta line 123. |
---|
1041 | |
---|
1042 | See also exit(), warn(), and the Carp module. |
---|
1043 | |
---|
1044 | If LIST is empty and C<$@> already contains a value (typically from a |
---|
1045 | previous eval) that value is reused after appending C<"\t...propagated">. |
---|
1046 | This is useful for propagating exceptions: |
---|
1047 | |
---|
1048 | eval { ... }; |
---|
1049 | die unless $@ =~ /Expected exception/; |
---|
1050 | |
---|
1051 | If C<$@> is empty then the string C<"Died"> is used. |
---|
1052 | |
---|
1053 | die() can also be called with a reference argument. If this happens to be |
---|
1054 | trapped within an eval(), $@ contains the reference. This behavior permits |
---|
1055 | a more elaborate exception handling implementation using objects that |
---|
1056 | maintain arbitrary state about the nature of the exception. Such a scheme |
---|
1057 | is sometimes preferable to matching particular string values of $@ using |
---|
1058 | regular expressions. Here's an example: |
---|
1059 | |
---|
1060 | eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) }; |
---|
1061 | if ($@) { |
---|
1062 | if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) { |
---|
1063 | # handle Some::Module::Exception |
---|
1064 | } |
---|
1065 | else { |
---|
1066 | # handle all other possible exceptions |
---|
1067 | } |
---|
1068 | } |
---|
1069 | |
---|
1070 | Because perl will stringify uncaught exception messages before displaying |
---|
1071 | them, you may want to overload stringification operations on such custom |
---|
1072 | exception objects. See L<overload> for details about that. |
---|
1073 | |
---|
1074 | You can arrange for a callback to be run just before the C<die> |
---|
1075 | does its deed, by setting the C<$SIG{__DIE__}> hook. The associated |
---|
1076 | handler will be called with the error text and can change the error |
---|
1077 | message, if it sees fit, by calling C<die> again. See |
---|
1078 | L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and |
---|
1079 | L<"eval BLOCK"> for some examples. Although this feature was meant |
---|
1080 | to be run only right before your program was to exit, this is not |
---|
1081 | currently the case--the C<$SIG{__DIE__}> hook is currently called |
---|
1082 | even inside eval()ed blocks/strings! If one wants the hook to do |
---|
1083 | nothing in such situations, put |
---|
1084 | |
---|
1085 | die @_ if $^S; |
---|
1086 | |
---|
1087 | as the first line of the handler (see L<perlvar/$^S>). Because |
---|
1088 | this promotes strange action at a distance, this counterintuitive |
---|
1089 | behavior may be fixed in a future release. |
---|
1090 | |
---|
1091 | =item do BLOCK |
---|
1092 | |
---|
1093 | Not really a function. Returns the value of the last command in the |
---|
1094 | sequence of commands indicated by BLOCK. When modified by a loop |
---|
1095 | modifier, executes the BLOCK once before testing the loop condition. |
---|
1096 | (On other statements the loop modifiers test the conditional first.) |
---|
1097 | |
---|
1098 | C<do BLOCK> does I<not> count as a loop, so the loop control statements |
---|
1099 | C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. |
---|
1100 | See L<perlsyn> for alternative strategies. |
---|
1101 | |
---|
1102 | =item do SUBROUTINE(LIST) |
---|
1103 | |
---|
1104 | A deprecated form of subroutine call. See L<perlsub>. |
---|
1105 | |
---|
1106 | =item do EXPR |
---|
1107 | |
---|
1108 | Uses the value of EXPR as a filename and executes the contents of the |
---|
1109 | file as a Perl script. Its primary use is to include subroutines |
---|
1110 | from a Perl subroutine library. |
---|
1111 | |
---|
1112 | do 'stat.pl'; |
---|
1113 | |
---|
1114 | is just like |
---|
1115 | |
---|
1116 | scalar eval `cat stat.pl`; |
---|
1117 | |
---|
1118 | except that it's more efficient and concise, keeps track of the current |
---|
1119 | filename for error messages, searches the @INC libraries, and updates |
---|
1120 | C<%INC> if the file is found. See L<perlvar/Predefined Names> for these |
---|
1121 | variables. It also differs in that code evaluated with C<do FILENAME> |
---|
1122 | cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the |
---|
1123 | same, however, in that it does reparse the file every time you call it, |
---|
1124 | so you probably don't want to do this inside a loop. |
---|
1125 | |
---|
1126 | If C<do> cannot read the file, it returns undef and sets C<$!> to the |
---|
1127 | error. If C<do> can read the file but cannot compile it, it |
---|
1128 | returns undef and sets an error message in C<$@>. If the file is |
---|
1129 | successfully compiled, C<do> returns the value of the last expression |
---|
1130 | evaluated. |
---|
1131 | |
---|
1132 | Note that inclusion of library modules is better done with the |
---|
1133 | C<use> and C<require> operators, which also do automatic error checking |
---|
1134 | and raise an exception if there's a problem. |
---|
1135 | |
---|
1136 | You might like to use C<do> to read in a program configuration |
---|
1137 | file. Manual error checking can be done this way: |
---|
1138 | |
---|
1139 | # read in config files: system first, then user |
---|
1140 | for $file ("/share/prog/defaults.rc", |
---|
1141 | "$ENV{HOME}/.someprogrc") |
---|
1142 | { |
---|
1143 | unless ($return = do $file) { |
---|
1144 | warn "couldn't parse $file: $@" if $@; |
---|
1145 | warn "couldn't do $file: $!" unless defined $return; |
---|
1146 | warn "couldn't run $file" unless $return; |
---|
1147 | } |
---|
1148 | } |
---|
1149 | |
---|
1150 | =item dump LABEL |
---|
1151 | |
---|
1152 | =item dump |
---|
1153 | |
---|
1154 | This function causes an immediate core dump. See also the B<-u> |
---|
1155 | command-line switch in L<perlrun>, which does the same thing. |
---|
1156 | Primarily this is so that you can use the B<undump> program (not |
---|
1157 | supplied) to turn your core dump into an executable binary after |
---|
1158 | having initialized all your variables at the beginning of the |
---|
1159 | program. When the new binary is executed it will begin by executing |
---|
1160 | a C<goto LABEL> (with all the restrictions that C<goto> suffers). |
---|
1161 | Think of it as a goto with an intervening core dump and reincarnation. |
---|
1162 | If C<LABEL> is omitted, restarts the program from the top. |
---|
1163 | |
---|
1164 | B<WARNING>: Any files opened at the time of the dump will I<not> |
---|
1165 | be open any more when the program is reincarnated, with possible |
---|
1166 | resulting confusion on the part of Perl. |
---|
1167 | |
---|
1168 | This function is now largely obsolete, partly because it's very |
---|
1169 | hard to convert a core file into an executable, and because the |
---|
1170 | real compiler backends for generating portable bytecode and compilable |
---|
1171 | C code have superseded it. |
---|
1172 | |
---|
1173 | If you're looking to use L<dump> to speed up your program, consider |
---|
1174 | generating bytecode or native C code as described in L<perlcc>. If |
---|
1175 | you're just trying to accelerate a CGI script, consider using the |
---|
1176 | C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI. |
---|
1177 | You might also consider autoloading or selfloading, which at least |
---|
1178 | make your program I<appear> to run faster. |
---|
1179 | |
---|
1180 | =item each HASH |
---|
1181 | |
---|
1182 | When called in list context, returns a 2-element list consisting of the |
---|
1183 | key and value for the next element of a hash, so that you can iterate over |
---|
1184 | it. When called in scalar context, returns only the key for the next |
---|
1185 | element in the hash. |
---|
1186 | |
---|
1187 | Entries are returned in an apparently random order. The actual random |
---|
1188 | order is subject to change in future versions of perl, but it is guaranteed |
---|
1189 | to be in the same order as either the C<keys> or C<values> function |
---|
1190 | would produce on the same (unmodified) hash. |
---|
1191 | |
---|
1192 | When the hash is entirely read, a null array is returned in list context |
---|
1193 | (which when assigned produces a false (C<0>) value), and C<undef> in |
---|
1194 | scalar context. The next call to C<each> after that will start iterating |
---|
1195 | again. There is a single iterator for each hash, shared by all C<each>, |
---|
1196 | C<keys>, and C<values> function calls in the program; it can be reset by |
---|
1197 | reading all the elements from the hash, or by evaluating C<keys HASH> or |
---|
1198 | C<values HASH>. If you add or delete elements of a hash while you're |
---|
1199 | iterating over it, you may get entries skipped or duplicated, so |
---|
1200 | don't. Exception: It is always safe to delete the item most recently |
---|
1201 | returned by C<each()>, which means that the following code will work: |
---|
1202 | |
---|
1203 | while (($key, $value) = each %hash) { |
---|
1204 | print $key, "\n"; |
---|
1205 | delete $hash{$key}; # This is safe |
---|
1206 | } |
---|
1207 | |
---|
1208 | The following prints out your environment like the printenv(1) program, |
---|
1209 | only in a different order: |
---|
1210 | |
---|
1211 | while (($key,$value) = each %ENV) { |
---|
1212 | print "$key=$value\n"; |
---|
1213 | } |
---|
1214 | |
---|
1215 | See also C<keys>, C<values> and C<sort>. |
---|
1216 | |
---|
1217 | =item eof FILEHANDLE |
---|
1218 | |
---|
1219 | =item eof () |
---|
1220 | |
---|
1221 | =item eof |
---|
1222 | |
---|
1223 | Returns 1 if the next read on FILEHANDLE will return end of file, or if |
---|
1224 | FILEHANDLE is not open. FILEHANDLE may be an expression whose value |
---|
1225 | gives the real filehandle. (Note that this function actually |
---|
1226 | reads a character and then C<ungetc>s it, so isn't very useful in an |
---|
1227 | interactive context.) Do not read from a terminal file (or call |
---|
1228 | C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such |
---|
1229 | as terminals may lose the end-of-file condition if you do. |
---|
1230 | |
---|
1231 | An C<eof> without an argument uses the last file read. Using C<eof()> |
---|
1232 | with empty parentheses is very different. It refers to the pseudo file |
---|
1233 | formed from the files listed on the command line and accessed via the |
---|
1234 | C<< <> >> operator. Since C<< <> >> isn't explicitly opened, |
---|
1235 | as a normal filehandle is, an C<eof()> before C<< <> >> has been |
---|
1236 | used will cause C<@ARGV> to be examined to determine if input is |
---|
1237 | available. |
---|
1238 | |
---|
1239 | In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to |
---|
1240 | detect the end of each file, C<eof()> will only detect the end of the |
---|
1241 | last file. Examples: |
---|
1242 | |
---|
1243 | # reset line numbering on each input file |
---|
1244 | while (<>) { |
---|
1245 | next if /^\s*#/; # skip comments |
---|
1246 | print "$.\t$_"; |
---|
1247 | } continue { |
---|
1248 | close ARGV if eof; # Not eof()! |
---|
1249 | } |
---|
1250 | |
---|
1251 | # insert dashes just before last line of last file |
---|
1252 | while (<>) { |
---|
1253 | if (eof()) { # check for end of current file |
---|
1254 | print "--------------\n"; |
---|
1255 | close(ARGV); # close or last; is needed if we |
---|
1256 | # are reading from the terminal |
---|
1257 | } |
---|
1258 | print; |
---|
1259 | } |
---|
1260 | |
---|
1261 | Practical hint: you almost never need to use C<eof> in Perl, because the |
---|
1262 | input operators typically return C<undef> when they run out of data, or if |
---|
1263 | there was an error. |
---|
1264 | |
---|
1265 | =item eval EXPR |
---|
1266 | |
---|
1267 | =item eval BLOCK |
---|
1268 | |
---|
1269 | In the first form, the return value of EXPR is parsed and executed as if it |
---|
1270 | were a little Perl program. The value of the expression (which is itself |
---|
1271 | determined within scalar context) is first parsed, and if there weren't any |
---|
1272 | errors, executed in the lexical context of the current Perl program, so |
---|
1273 | that any variable settings or subroutine and format definitions remain |
---|
1274 | afterwards. Note that the value is parsed every time the eval executes. |
---|
1275 | If EXPR is omitted, evaluates C<$_>. This form is typically used to |
---|
1276 | delay parsing and subsequent execution of the text of EXPR until run time. |
---|
1277 | |
---|
1278 | In the second form, the code within the BLOCK is parsed only once--at the |
---|
1279 | same time the code surrounding the eval itself was parsed--and executed |
---|
1280 | within the context of the current Perl program. This form is typically |
---|
1281 | used to trap exceptions more efficiently than the first (see below), while |
---|
1282 | also providing the benefit of checking the code within BLOCK at compile |
---|
1283 | time. |
---|
1284 | |
---|
1285 | The final semicolon, if any, may be omitted from the value of EXPR or within |
---|
1286 | the BLOCK. |
---|
1287 | |
---|
1288 | In both forms, the value returned is the value of the last expression |
---|
1289 | evaluated inside the mini-program; a return statement may be also used, just |
---|
1290 | as with subroutines. The expression providing the return value is evaluated |
---|
1291 | in void, scalar, or list context, depending on the context of the eval itself. |
---|
1292 | See L</wantarray> for more on how the evaluation context can be determined. |
---|
1293 | |
---|
1294 | If there is a syntax error or runtime error, or a C<die> statement is |
---|
1295 | executed, an undefined value is returned by C<eval>, and C<$@> is set to the |
---|
1296 | error message. If there was no error, C<$@> is guaranteed to be a null |
---|
1297 | string. Beware that using C<eval> neither silences perl from printing |
---|
1298 | warnings to STDERR, nor does it stuff the text of warning messages into C<$@>. |
---|
1299 | To do either of those, you have to use the C<$SIG{__WARN__}> facility. See |
---|
1300 | L</warn> and L<perlvar>. |
---|
1301 | |
---|
1302 | Note that, because C<eval> traps otherwise-fatal errors, it is useful for |
---|
1303 | determining whether a particular feature (such as C<socket> or C<symlink>) |
---|
1304 | is implemented. It is also Perl's exception trapping mechanism, where |
---|
1305 | the die operator is used to raise exceptions. |
---|
1306 | |
---|
1307 | If the code to be executed doesn't vary, you may use the eval-BLOCK |
---|
1308 | form to trap run-time errors without incurring the penalty of |
---|
1309 | recompiling each time. The error, if any, is still returned in C<$@>. |
---|
1310 | Examples: |
---|
1311 | |
---|
1312 | # make divide-by-zero nonfatal |
---|
1313 | eval { $answer = $a / $b; }; warn $@ if $@; |
---|
1314 | |
---|
1315 | # same thing, but less efficient |
---|
1316 | eval '$answer = $a / $b'; warn $@ if $@; |
---|
1317 | |
---|
1318 | # a compile-time error |
---|
1319 | eval { $answer = }; # WRONG |
---|
1320 | |
---|
1321 | # a run-time error |
---|
1322 | eval '$answer ='; # sets $@ |
---|
1323 | |
---|
1324 | Due to the current arguably broken state of C<__DIE__> hooks, when using |
---|
1325 | the C<eval{}> form as an exception trap in libraries, you may wish not |
---|
1326 | to trigger any C<__DIE__> hooks that user code may have installed. |
---|
1327 | You can use the C<local $SIG{__DIE__}> construct for this purpose, |
---|
1328 | as shown in this example: |
---|
1329 | |
---|
1330 | # a very private exception trap for divide-by-zero |
---|
1331 | eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; |
---|
1332 | warn $@ if $@; |
---|
1333 | |
---|
1334 | This is especially significant, given that C<__DIE__> hooks can call |
---|
1335 | C<die> again, which has the effect of changing their error messages: |
---|
1336 | |
---|
1337 | # __DIE__ hooks may modify error messages |
---|
1338 | { |
---|
1339 | local $SIG{'__DIE__'} = |
---|
1340 | sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x }; |
---|
1341 | eval { die "foo lives here" }; |
---|
1342 | print $@ if $@; # prints "bar lives here" |
---|
1343 | } |
---|
1344 | |
---|
1345 | Because this promotes action at a distance, this counterintuitive behavior |
---|
1346 | may be fixed in a future release. |
---|
1347 | |
---|
1348 | With an C<eval>, you should be especially careful to remember what's |
---|
1349 | being looked at when: |
---|
1350 | |
---|
1351 | eval $x; # CASE 1 |
---|
1352 | eval "$x"; # CASE 2 |
---|
1353 | |
---|
1354 | eval '$x'; # CASE 3 |
---|
1355 | eval { $x }; # CASE 4 |
---|
1356 | |
---|
1357 | eval "\$$x++"; # CASE 5 |
---|
1358 | $$x++; # CASE 6 |
---|
1359 | |
---|
1360 | Cases 1 and 2 above behave identically: they run the code contained in |
---|
1361 | the variable $x. (Although case 2 has misleading double quotes making |
---|
1362 | the reader wonder what else might be happening (nothing is).) Cases 3 |
---|
1363 | and 4 likewise behave in the same way: they run the code C<'$x'>, which |
---|
1364 | does nothing but return the value of $x. (Case 4 is preferred for |
---|
1365 | purely visual reasons, but it also has the advantage of compiling at |
---|
1366 | compile-time instead of at run-time.) Case 5 is a place where |
---|
1367 | normally you I<would> like to use double quotes, except that in this |
---|
1368 | particular situation, you can just use symbolic references instead, as |
---|
1369 | in case 6. |
---|
1370 | |
---|
1371 | C<eval BLOCK> does I<not> count as a loop, so the loop control statements |
---|
1372 | C<next>, C<last>, or C<redo> cannot be used to leave or restart the block. |
---|
1373 | |
---|
1374 | =item exec LIST |
---|
1375 | |
---|
1376 | =item exec PROGRAM LIST |
---|
1377 | |
---|
1378 | The C<exec> function executes a system command I<and never returns>-- |
---|
1379 | use C<system> instead of C<exec> if you want it to return. It fails and |
---|
1380 | returns false only if the command does not exist I<and> it is executed |
---|
1381 | directly instead of via your system's command shell (see below). |
---|
1382 | |
---|
1383 | Since it's a common mistake to use C<exec> instead of C<system>, Perl |
---|
1384 | warns you if there is a following statement which isn't C<die>, C<warn>, |
---|
1385 | or C<exit> (if C<-w> is set - but you always do that). If you |
---|
1386 | I<really> want to follow an C<exec> with some other statement, you |
---|
1387 | can use one of these styles to avoid the warning: |
---|
1388 | |
---|
1389 | exec ('foo') or print STDERR "couldn't exec foo: $!"; |
---|
1390 | { exec ('foo') }; print STDERR "couldn't exec foo: $!"; |
---|
1391 | |
---|
1392 | If there is more than one argument in LIST, or if LIST is an array |
---|
1393 | with more than one value, calls execvp(3) with the arguments in LIST. |
---|
1394 | If there is only one scalar argument or an array with one element in it, |
---|
1395 | the argument is checked for shell metacharacters, and if there are any, |
---|
1396 | the entire argument is passed to the system's command shell for parsing |
---|
1397 | (this is C</bin/sh -c> on Unix platforms, but varies on other platforms). |
---|
1398 | If there are no shell metacharacters in the argument, it is split into |
---|
1399 | words and passed directly to C<execvp>, which is more efficient. |
---|
1400 | Examples: |
---|
1401 | |
---|
1402 | exec '/bin/echo', 'Your arguments are: ', @ARGV; |
---|
1403 | exec "sort $outfile | uniq"; |
---|
1404 | |
---|
1405 | If you don't really want to execute the first argument, but want to lie |
---|
1406 | to the program you are executing about its own name, you can specify |
---|
1407 | the program you actually want to run as an "indirect object" (without a |
---|
1408 | comma) in front of the LIST. (This always forces interpretation of the |
---|
1409 | LIST as a multivalued list, even if there is only a single scalar in |
---|
1410 | the list.) Example: |
---|
1411 | |
---|
1412 | $shell = '/bin/csh'; |
---|
1413 | exec $shell '-sh'; # pretend it's a login shell |
---|
1414 | |
---|
1415 | or, more directly, |
---|
1416 | |
---|
1417 | exec {'/bin/csh'} '-sh'; # pretend it's a login shell |
---|
1418 | |
---|
1419 | When the arguments get executed via the system shell, results will |
---|
1420 | be subject to its quirks and capabilities. See L<perlop/"`STRING`"> |
---|
1421 | for details. |
---|
1422 | |
---|
1423 | Using an indirect object with C<exec> or C<system> is also more |
---|
1424 | secure. This usage (which also works fine with system()) forces |
---|
1425 | interpretation of the arguments as a multivalued list, even if the |
---|
1426 | list had just one argument. That way you're safe from the shell |
---|
1427 | expanding wildcards or splitting up words with whitespace in them. |
---|
1428 | |
---|
1429 | @args = ( "echo surprise" ); |
---|
1430 | |
---|
1431 | exec @args; # subject to shell escapes |
---|
1432 | # if @args == 1 |
---|
1433 | exec { $args[0] } @args; # safe even with one-arg list |
---|
1434 | |
---|
1435 | The first version, the one without the indirect object, ran the I<echo> |
---|
1436 | program, passing it C<"surprise"> an argument. The second version |
---|
1437 | didn't--it tried to run a program literally called I<"echo surprise">, |
---|
1438 | didn't find it, and set C<$?> to a non-zero value indicating failure. |
---|
1439 | |
---|
1440 | Beginning with v5.6.0, Perl will attempt to flush all files opened for |
---|
1441 | output before the exec, but this may not be supported on some platforms |
---|
1442 | (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH |
---|
1443 | in English) or call the C<autoflush()> method of C<IO::Handle> on any |
---|
1444 | open handles in order to avoid lost output. |
---|
1445 | |
---|
1446 | Note that C<exec> will not call your C<END> blocks, nor will it call |
---|
1447 | any C<DESTROY> methods in your objects. |
---|
1448 | |
---|
1449 | =item exists EXPR |
---|
1450 | |
---|
1451 | Given an expression that specifies a hash element or array element, |
---|
1452 | returns true if the specified element in the hash or array has ever |
---|
1453 | been initialized, even if the corresponding value is undefined. The |
---|
1454 | element is not autovivified if it doesn't exist. |
---|
1455 | |
---|
1456 | print "Exists\n" if exists $hash{$key}; |
---|
1457 | print "Defined\n" if defined $hash{$key}; |
---|
1458 | print "True\n" if $hash{$key}; |
---|
1459 | |
---|
1460 | print "Exists\n" if exists $array[$index]; |
---|
1461 | print "Defined\n" if defined $array[$index]; |
---|
1462 | print "True\n" if $array[$index]; |
---|
1463 | |
---|
1464 | A hash or array element can be true only if it's defined, and defined if |
---|
1465 | it exists, but the reverse doesn't necessarily hold true. |
---|
1466 | |
---|
1467 | Given an expression that specifies the name of a subroutine, |
---|
1468 | returns true if the specified subroutine has ever been declared, even |
---|
1469 | if it is undefined. Mentioning a subroutine name for exists or defined |
---|
1470 | does not count as declaring it. Note that a subroutine which does not |
---|
1471 | exist may still be callable: its package may have an C<AUTOLOAD> |
---|
1472 | method that makes it spring into existence the first time that it is |
---|
1473 | called -- see L<perlsub>. |
---|
1474 | |
---|
1475 | print "Exists\n" if exists &subroutine; |
---|
1476 | print "Defined\n" if defined &subroutine; |
---|
1477 | |
---|
1478 | Note that the EXPR can be arbitrarily complicated as long as the final |
---|
1479 | operation is a hash or array key lookup or subroutine name: |
---|
1480 | |
---|
1481 | if (exists $ref->{A}->{B}->{$key}) { } |
---|
1482 | if (exists $hash{A}{B}{$key}) { } |
---|
1483 | |
---|
1484 | if (exists $ref->{A}->{B}->[$ix]) { } |
---|
1485 | if (exists $hash{A}{B}[$ix]) { } |
---|
1486 | |
---|
1487 | if (exists &{$ref->{A}{B}{$key}}) { } |
---|
1488 | |
---|
1489 | Although the deepest nested array or hash will not spring into existence |
---|
1490 | just because its existence was tested, any intervening ones will. |
---|
1491 | Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring |
---|
1492 | into existence due to the existence test for the $key element above. |
---|
1493 | This happens anywhere the arrow operator is used, including even: |
---|
1494 | |
---|
1495 | undef $ref; |
---|
1496 | if (exists $ref->{"Some key"}) { } |
---|
1497 | print $ref; # prints HASH(0x80d3d5c) |
---|
1498 | |
---|
1499 | This surprising autovivification in what does not at first--or even |
---|
1500 | second--glance appear to be an lvalue context may be fixed in a future |
---|
1501 | release. |
---|
1502 | |
---|
1503 | See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics |
---|
1504 | on how exists() acts when used on a pseudo-hash. |
---|
1505 | |
---|
1506 | Use of a subroutine call, rather than a subroutine name, as an argument |
---|
1507 | to exists() is an error. |
---|
1508 | |
---|
1509 | exists ⊂ # OK |
---|
1510 | exists &sub(); # Error |
---|
1511 | |
---|
1512 | =item exit EXPR |
---|
1513 | |
---|
1514 | Evaluates EXPR and exits immediately with that value. Example: |
---|
1515 | |
---|
1516 | $ans = <STDIN>; |
---|
1517 | exit 0 if $ans =~ /^[Xx]/; |
---|
1518 | |
---|
1519 | See also C<die>. If EXPR is omitted, exits with C<0> status. The only |
---|
1520 | universally recognized values for EXPR are C<0> for success and C<1> |
---|
1521 | for error; other values are subject to interpretation depending on the |
---|
1522 | environment in which the Perl program is running. For example, exiting |
---|
1523 | 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause |
---|
1524 | the mailer to return the item undelivered, but that's not true everywhere. |
---|
1525 | |
---|
1526 | Don't use C<exit> to abort a subroutine if there's any chance that |
---|
1527 | someone might want to trap whatever error happened. Use C<die> instead, |
---|
1528 | which can be trapped by an C<eval>. |
---|
1529 | |
---|
1530 | The exit() function does not always exit immediately. It calls any |
---|
1531 | defined C<END> routines first, but these C<END> routines may not |
---|
1532 | themselves abort the exit. Likewise any object destructors that need to |
---|
1533 | be called are called before the real exit. If this is a problem, you |
---|
1534 | can call C<POSIX:_exit($status)> to avoid END and destructor processing. |
---|
1535 | See L<perlmod> for details. |
---|
1536 | |
---|
1537 | =item exp EXPR |
---|
1538 | |
---|
1539 | =item exp |
---|
1540 | |
---|
1541 | Returns I<e> (the natural logarithm base) to the power of EXPR. |
---|
1542 | If EXPR is omitted, gives C<exp($_)>. |
---|
1543 | |
---|
1544 | =item fcntl FILEHANDLE,FUNCTION,SCALAR |
---|
1545 | |
---|
1546 | Implements the fcntl(2) function. You'll probably have to say |
---|
1547 | |
---|
1548 | use Fcntl; |
---|
1549 | |
---|
1550 | first to get the correct constant definitions. Argument processing and |
---|
1551 | value return works just like C<ioctl> below. |
---|
1552 | For example: |
---|
1553 | |
---|
1554 | use Fcntl; |
---|
1555 | fcntl($filehandle, F_GETFL, $packed_return_buffer) |
---|
1556 | or die "can't fcntl F_GETFL: $!"; |
---|
1557 | |
---|
1558 | You don't have to check for C<defined> on the return from C<fnctl>. |
---|
1559 | Like C<ioctl>, it maps a C<0> return from the system call into |
---|
1560 | C<"0 but true"> in Perl. This string is true in boolean context and C<0> |
---|
1561 | in numeric context. It is also exempt from the normal B<-w> warnings |
---|
1562 | on improper numeric conversions. |
---|
1563 | |
---|
1564 | Note that C<fcntl> will produce a fatal error if used on a machine that |
---|
1565 | doesn't implement fcntl(2). See the Fcntl module or your fcntl(2) |
---|
1566 | manpage to learn what functions are available on your system. |
---|
1567 | |
---|
1568 | =item fileno FILEHANDLE |
---|
1569 | |
---|
1570 | Returns the file descriptor for a filehandle, or undefined if the |
---|
1571 | filehandle is not open. This is mainly useful for constructing |
---|
1572 | bitmaps for C<select> and low-level POSIX tty-handling operations. |
---|
1573 | If FILEHANDLE is an expression, the value is taken as an indirect |
---|
1574 | filehandle, generally its name. |
---|
1575 | |
---|
1576 | You can use this to find out whether two handles refer to the |
---|
1577 | same underlying descriptor: |
---|
1578 | |
---|
1579 | if (fileno(THIS) == fileno(THAT)) { |
---|
1580 | print "THIS and THAT are dups\n"; |
---|
1581 | } |
---|
1582 | |
---|
1583 | =item flock FILEHANDLE,OPERATION |
---|
1584 | |
---|
1585 | Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true |
---|
1586 | for success, false on failure. Produces a fatal error if used on a |
---|
1587 | machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3). |
---|
1588 | C<flock> is Perl's portable file locking interface, although it locks |
---|
1589 | only entire files, not records. |
---|
1590 | |
---|
1591 | Two potentially non-obvious but traditional C<flock> semantics are |
---|
1592 | that it waits indefinitely until the lock is granted, and that its locks |
---|
1593 | B<merely advisory>. Such discretionary locks are more flexible, but offer |
---|
1594 | fewer guarantees. This means that files locked with C<flock> may be |
---|
1595 | modified by programs that do not also use C<flock>. See L<perlport>, |
---|
1596 | your port's specific documentation, or your system-specific local manpages |
---|
1597 | for details. It's best to assume traditional behavior if you're writing |
---|
1598 | portable programs. (But if you're not, you should as always feel perfectly |
---|
1599 | free to write for your own system's idiosyncrasies (sometimes called |
---|
1600 | "features"). Slavish adherence to portability concerns shouldn't get |
---|
1601 | in the way of your getting your job done.) |
---|
1602 | |
---|
1603 | OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with |
---|
1604 | LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but |
---|
1605 | you can use the symbolic names if you import them from the Fcntl module, |
---|
1606 | either individually, or as a group using the ':flock' tag. LOCK_SH |
---|
1607 | requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN |
---|
1608 | releases a previously requested lock. If LOCK_NB is bitwise-or'ed with |
---|
1609 | LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking |
---|
1610 | waiting for the lock (check the return status to see if you got it). |
---|
1611 | |
---|
1612 | To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE |
---|
1613 | before locking or unlocking it. |
---|
1614 | |
---|
1615 | Note that the emulation built with lockf(3) doesn't provide shared |
---|
1616 | locks, and it requires that FILEHANDLE be open with write intent. These |
---|
1617 | are the semantics that lockf(3) implements. Most if not all systems |
---|
1618 | implement lockf(3) in terms of fcntl(2) locking, though, so the |
---|
1619 | differing semantics shouldn't bite too many people. |
---|
1620 | |
---|
1621 | Note also that some versions of C<flock> cannot lock things over the |
---|
1622 | network; you would need to use the more system-specific C<fcntl> for |
---|
1623 | that. If you like you can force Perl to ignore your system's flock(2) |
---|
1624 | function, and so provide its own fcntl(2)-based emulation, by passing |
---|
1625 | the switch C<-Ud_flock> to the F<Configure> program when you configure |
---|
1626 | perl. |
---|
1627 | |
---|
1628 | Here's a mailbox appender for BSD systems. |
---|
1629 | |
---|
1630 | use Fcntl ':flock'; # import LOCK_* constants |
---|
1631 | |
---|
1632 | sub lock { |
---|
1633 | flock(MBOX,LOCK_EX); |
---|
1634 | # and, in case someone appended |
---|
1635 | # while we were waiting... |
---|
1636 | seek(MBOX, 0, 2); |
---|
1637 | } |
---|
1638 | |
---|
1639 | sub unlock { |
---|
1640 | flock(MBOX,LOCK_UN); |
---|
1641 | } |
---|
1642 | |
---|
1643 | open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}") |
---|
1644 | or die "Can't open mailbox: $!"; |
---|
1645 | |
---|
1646 | lock(); |
---|
1647 | print MBOX $msg,"\n\n"; |
---|
1648 | unlock(); |
---|
1649 | |
---|
1650 | On systems that support a real flock(), locks are inherited across fork() |
---|
1651 | calls, whereas those that must resort to the more capricious fcntl() |
---|
1652 | function lose the locks, making it harder to write servers. |
---|
1653 | |
---|
1654 | See also L<DB_File> for other flock() examples. |
---|
1655 | |
---|
1656 | =item fork |
---|
1657 | |
---|
1658 | Does a fork(2) system call to create a new process running the |
---|
1659 | same program at the same point. It returns the child pid to the |
---|
1660 | parent process, C<0> to the child process, or C<undef> if the fork is |
---|
1661 | unsuccessful. File descriptors (and sometimes locks on those descriptors) |
---|
1662 | are shared, while everything else is copied. On most systems supporting |
---|
1663 | fork(), great care has gone into making it extremely efficient (for |
---|
1664 | example, using copy-on-write technology on data pages), making it the |
---|
1665 | dominant paradigm for multitasking over the last few decades. |
---|
1666 | |
---|
1667 | Beginning with v5.6.0, Perl will attempt to flush all files opened for |
---|
1668 | output before forking the child process, but this may not be supported |
---|
1669 | on some platforms (see L<perlport>). To be safe, you may need to set |
---|
1670 | C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of |
---|
1671 | C<IO::Handle> on any open handles in order to avoid duplicate output. |
---|
1672 | |
---|
1673 | If you C<fork> without ever waiting on your children, you will |
---|
1674 | accumulate zombies. On some systems, you can avoid this by setting |
---|
1675 | C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of |
---|
1676 | forking and reaping moribund children. |
---|
1677 | |
---|
1678 | Note that if your forked child inherits system file descriptors like |
---|
1679 | STDIN and STDOUT that are actually connected by a pipe or socket, even |
---|
1680 | if you exit, then the remote server (such as, say, a CGI script or a |
---|
1681 | backgrounded job launched from a remote shell) won't think you're done. |
---|
1682 | You should reopen those to F</dev/null> if it's any issue. |
---|
1683 | |
---|
1684 | =item format |
---|
1685 | |
---|
1686 | Declare a picture format for use by the C<write> function. For |
---|
1687 | example: |
---|
1688 | |
---|
1689 | format Something = |
---|
1690 | Test: @<<<<<<<< @||||| @>>>>> |
---|
1691 | $str, $%, '$' . int($num) |
---|
1692 | . |
---|
1693 | |
---|
1694 | $str = "widget"; |
---|
1695 | $num = $cost/$quantity; |
---|
1696 | $~ = 'Something'; |
---|
1697 | write; |
---|
1698 | |
---|
1699 | See L<perlform> for many details and examples. |
---|
1700 | |
---|
1701 | =item formline PICTURE,LIST |
---|
1702 | |
---|
1703 | This is an internal function used by C<format>s, though you may call it, |
---|
1704 | too. It formats (see L<perlform>) a list of values according to the |
---|
1705 | contents of PICTURE, placing the output into the format output |
---|
1706 | accumulator, C<$^A> (or C<$ACCUMULATOR> in English). |
---|
1707 | Eventually, when a C<write> is done, the contents of |
---|
1708 | C<$^A> are written to some filehandle, but you could also read C<$^A> |
---|
1709 | yourself and then set C<$^A> back to C<"">. Note that a format typically |
---|
1710 | does one C<formline> per line of form, but the C<formline> function itself |
---|
1711 | doesn't care how many newlines are embedded in the PICTURE. This means |
---|
1712 | that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line. |
---|
1713 | You may therefore need to use multiple formlines to implement a single |
---|
1714 | record format, just like the format compiler. |
---|
1715 | |
---|
1716 | Be careful if you put double quotes around the picture, because an C<@> |
---|
1717 | character may be taken to mean the beginning of an array name. |
---|
1718 | C<formline> always returns true. See L<perlform> for other examples. |
---|
1719 | |
---|
1720 | =item getc FILEHANDLE |
---|
1721 | |
---|
1722 | =item getc |
---|
1723 | |
---|
1724 | Returns the next character from the input file attached to FILEHANDLE, |
---|
1725 | or the undefined value at end of file, or if there was an error. |
---|
1726 | If FILEHANDLE is omitted, reads from STDIN. This is not particularly |
---|
1727 | efficient. However, it cannot be used by itself to fetch single |
---|
1728 | characters without waiting for the user to hit enter. For that, try |
---|
1729 | something more like: |
---|
1730 | |
---|
1731 | if ($BSD_STYLE) { |
---|
1732 | system "stty cbreak </dev/tty >/dev/tty 2>&1"; |
---|
1733 | } |
---|
1734 | else { |
---|
1735 | system "stty", '-icanon', 'eol', "\001"; |
---|
1736 | } |
---|
1737 | |
---|
1738 | $key = getc(STDIN); |
---|
1739 | |
---|
1740 | if ($BSD_STYLE) { |
---|
1741 | system "stty -cbreak </dev/tty >/dev/tty 2>&1"; |
---|
1742 | } |
---|
1743 | else { |
---|
1744 | system "stty", 'icanon', 'eol', '^@'; # ASCII null |
---|
1745 | } |
---|
1746 | print "\n"; |
---|
1747 | |
---|
1748 | Determination of whether $BSD_STYLE should be set |
---|
1749 | is left as an exercise to the reader. |
---|
1750 | |
---|
1751 | The C<POSIX::getattr> function can do this more portably on |
---|
1752 | systems purporting POSIX compliance. See also the C<Term::ReadKey> |
---|
1753 | module from your nearest CPAN site; details on CPAN can be found on |
---|
1754 | L<perlmodlib/CPAN>. |
---|
1755 | |
---|
1756 | =item getlogin |
---|
1757 | |
---|
1758 | Implements the C library function of the same name, which on most |
---|
1759 | systems returns the current login from F</etc/utmp>, if any. If null, |
---|
1760 | use C<getpwuid>. |
---|
1761 | |
---|
1762 | $login = getlogin || getpwuid($<) || "Kilroy"; |
---|
1763 | |
---|
1764 | Do not consider C<getlogin> for authentication: it is not as |
---|
1765 | secure as C<getpwuid>. |
---|
1766 | |
---|
1767 | =item getpeername SOCKET |
---|
1768 | |
---|
1769 | Returns the packed sockaddr address of other end of the SOCKET connection. |
---|
1770 | |
---|
1771 | use Socket; |
---|
1772 | $hersockaddr = getpeername(SOCK); |
---|
1773 | ($port, $iaddr) = sockaddr_in($hersockaddr); |
---|
1774 | $herhostname = gethostbyaddr($iaddr, AF_INET); |
---|
1775 | $herstraddr = inet_ntoa($iaddr); |
---|
1776 | |
---|
1777 | =item getpgrp PID |
---|
1778 | |
---|
1779 | Returns the current process group for the specified PID. Use |
---|
1780 | a PID of C<0> to get the current process group for the |
---|
1781 | current process. Will raise an exception if used on a machine that |
---|
1782 | doesn't implement getpgrp(2). If PID is omitted, returns process |
---|
1783 | group of current process. Note that the POSIX version of C<getpgrp> |
---|
1784 | does not accept a PID argument, so only C<PID==0> is truly portable. |
---|
1785 | |
---|
1786 | =item getppid |
---|
1787 | |
---|
1788 | Returns the process id of the parent process. |
---|
1789 | |
---|
1790 | =item getpriority WHICH,WHO |
---|
1791 | |
---|
1792 | Returns the current priority for a process, a process group, or a user. |
---|
1793 | (See L<getpriority(2)>.) Will raise a fatal exception if used on a |
---|
1794 | machine that doesn't implement getpriority(2). |
---|
1795 | |
---|
1796 | =item getpwnam NAME |
---|
1797 | |
---|
1798 | =item getgrnam NAME |
---|
1799 | |
---|
1800 | =item gethostbyname NAME |
---|
1801 | |
---|
1802 | =item getnetbyname NAME |
---|
1803 | |
---|
1804 | =item getprotobyname NAME |
---|
1805 | |
---|
1806 | =item getpwuid UID |
---|
1807 | |
---|
1808 | =item getgrgid GID |
---|
1809 | |
---|
1810 | =item getservbyname NAME,PROTO |
---|
1811 | |
---|
1812 | =item gethostbyaddr ADDR,ADDRTYPE |
---|
1813 | |
---|
1814 | =item getnetbyaddr ADDR,ADDRTYPE |
---|
1815 | |
---|
1816 | =item getprotobynumber NUMBER |
---|
1817 | |
---|
1818 | =item getservbyport PORT,PROTO |
---|
1819 | |
---|
1820 | =item getpwent |
---|
1821 | |
---|
1822 | =item getgrent |
---|
1823 | |
---|
1824 | =item gethostent |
---|
1825 | |
---|
1826 | =item getnetent |
---|
1827 | |
---|
1828 | =item getprotoent |
---|
1829 | |
---|
1830 | =item getservent |
---|
1831 | |
---|
1832 | =item setpwent |
---|
1833 | |
---|
1834 | =item setgrent |
---|
1835 | |
---|
1836 | =item sethostent STAYOPEN |
---|
1837 | |
---|
1838 | =item setnetent STAYOPEN |
---|
1839 | |
---|
1840 | =item setprotoent STAYOPEN |
---|
1841 | |
---|
1842 | =item setservent STAYOPEN |
---|
1843 | |
---|
1844 | =item endpwent |
---|
1845 | |
---|
1846 | =item endgrent |
---|
1847 | |
---|
1848 | =item endhostent |
---|
1849 | |
---|
1850 | =item endnetent |
---|
1851 | |
---|
1852 | =item endprotoent |
---|
1853 | |
---|
1854 | =item endservent |
---|
1855 | |
---|
1856 | These routines perform the same functions as their counterparts in the |
---|
1857 | system library. In list context, the return values from the |
---|
1858 | various get routines are as follows: |
---|
1859 | |
---|
1860 | ($name,$passwd,$uid,$gid, |
---|
1861 | $quota,$comment,$gcos,$dir,$shell,$expire) = getpw* |
---|
1862 | ($name,$passwd,$gid,$members) = getgr* |
---|
1863 | ($name,$aliases,$addrtype,$length,@addrs) = gethost* |
---|
1864 | ($name,$aliases,$addrtype,$net) = getnet* |
---|
1865 | ($name,$aliases,$proto) = getproto* |
---|
1866 | ($name,$aliases,$port,$proto) = getserv* |
---|
1867 | |
---|
1868 | (If the entry doesn't exist you get a null list.) |
---|
1869 | |
---|
1870 | The exact meaning of the $gcos field varies but it usually contains |
---|
1871 | the real name of the user (as opposed to the login name) and other |
---|
1872 | information pertaining to the user. Beware, however, that in many |
---|
1873 | system users are able to change this information and therefore it |
---|
1874 | cannot be trusted and therefore the $gcos is tainted (see |
---|
1875 | L<perlsec>). The $passwd and $shell, user's encrypted password and |
---|
1876 | login shell, are also tainted, because of the same reason. |
---|
1877 | |
---|
1878 | In scalar context, you get the name, unless the function was a |
---|
1879 | lookup by name, in which case you get the other thing, whatever it is. |
---|
1880 | (If the entry doesn't exist you get the undefined value.) For example: |
---|
1881 | |
---|
1882 | $uid = getpwnam($name); |
---|
1883 | $name = getpwuid($num); |
---|
1884 | $name = getpwent(); |
---|
1885 | $gid = getgrnam($name); |
---|
1886 | $name = getgrgid($num; |
---|
1887 | $name = getgrent(); |
---|
1888 | #etc. |
---|
1889 | |
---|
1890 | In I<getpw*()> the fields $quota, $comment, and $expire are special |
---|
1891 | cases in the sense that in many systems they are unsupported. If the |
---|
1892 | $quota is unsupported, it is an empty scalar. If it is supported, it |
---|
1893 | usually encodes the disk quota. If the $comment field is unsupported, |
---|
1894 | it is an empty scalar. If it is supported it usually encodes some |
---|
1895 | administrative comment about the user. In some systems the $quota |
---|
1896 | field may be $change or $age, fields that have to do with password |
---|
1897 | aging. In some systems the $comment field may be $class. The $expire |
---|
1898 | field, if present, encodes the expiration period of the account or the |
---|
1899 | password. For the availability and the exact meaning of these fields |
---|
1900 | in your system, please consult your getpwnam(3) documentation and your |
---|
1901 | F<pwd.h> file. You can also find out from within Perl what your |
---|
1902 | $quota and $comment fields mean and whether you have the $expire field |
---|
1903 | by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>, |
---|
1904 | C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password |
---|
1905 | files are only supported if your vendor has implemented them in the |
---|
1906 | intuitive fashion that calling the regular C library routines gets the |
---|
1907 | shadow versions if you're running under privilege or if there exists |
---|
1908 | the shadow(3) functions as found in System V ( this includes Solaris |
---|
1909 | and Linux.) Those systems which implement a proprietary shadow password |
---|
1910 | facility are unlikely to be supported. |
---|
1911 | |
---|
1912 | The $members value returned by I<getgr*()> is a space separated list of |
---|
1913 | the login names of the members of the group. |
---|
1914 | |
---|
1915 | For the I<gethost*()> functions, if the C<h_errno> variable is supported in |
---|
1916 | C, it will be returned to you via C<$?> if the function call fails. The |
---|
1917 | C<@addrs> value returned by a successful call is a list of the raw |
---|
1918 | addresses returned by the corresponding system library call. In the |
---|
1919 | Internet domain, each address is four bytes long and you can unpack it |
---|
1920 | by saying something like: |
---|
1921 | |
---|
1922 | ($a,$b,$c,$d) = unpack('C4',$addr[0]); |
---|
1923 | |
---|
1924 | The Socket library makes this slightly easier: |
---|
1925 | |
---|
1926 | use Socket; |
---|
1927 | $iaddr = inet_aton("127.1"); # or whatever address |
---|
1928 | $name = gethostbyaddr($iaddr, AF_INET); |
---|
1929 | |
---|
1930 | # or going the other way |
---|
1931 | $straddr = inet_ntoa($iaddr); |
---|
1932 | |
---|
1933 | If you get tired of remembering which element of the return list |
---|
1934 | contains which return value, by-name interfaces are provided |
---|
1935 | in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>, |
---|
1936 | C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>, |
---|
1937 | and C<User::grent>. These override the normal built-ins, supplying |
---|
1938 | versions that return objects with the appropriate names |
---|
1939 | for each field. For example: |
---|
1940 | |
---|
1941 | use File::stat; |
---|
1942 | use User::pwent; |
---|
1943 | $is_his = (stat($filename)->uid == pwent($whoever)->uid); |
---|
1944 | |
---|
1945 | Even though it looks like they're the same method calls (uid), |
---|
1946 | they aren't, because a C<File::stat> object is different from |
---|
1947 | a C<User::pwent> object. |
---|
1948 | |
---|
1949 | =item getsockname SOCKET |
---|
1950 | |
---|
1951 | Returns the packed sockaddr address of this end of the SOCKET connection, |
---|
1952 | in case you don't know the address because you have several different |
---|
1953 | IPs that the connection might have come in on. |
---|
1954 | |
---|
1955 | use Socket; |
---|
1956 | $mysockaddr = getsockname(SOCK); |
---|
1957 | ($port, $myaddr) = sockaddr_in($mysockaddr); |
---|
1958 | printf "Connect to %s [%s]\n", |
---|
1959 | scalar gethostbyaddr($myaddr, AF_INET), |
---|
1960 | inet_ntoa($myaddr); |
---|
1961 | |
---|
1962 | =item getsockopt SOCKET,LEVEL,OPTNAME |
---|
1963 | |
---|
1964 | Returns the socket option requested, or undef if there is an error. |
---|
1965 | |
---|
1966 | =item glob EXPR |
---|
1967 | |
---|
1968 | =item glob |
---|
1969 | |
---|
1970 | Returns the value of EXPR with filename expansions such as the |
---|
1971 | standard Unix shell F</bin/csh> would do. This is the internal function |
---|
1972 | implementing the C<< <*.c> >> operator, but you can use it directly. |
---|
1973 | If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is |
---|
1974 | discussed in more detail in L<perlop/"I/O Operators">. |
---|
1975 | |
---|
1976 | Beginning with v5.6.0, this operator is implemented using the standard |
---|
1977 | C<File::Glob> extension. See L<File::Glob> for details. |
---|
1978 | |
---|
1979 | =item gmtime EXPR |
---|
1980 | |
---|
1981 | Converts a time as returned by the time function to a 8-element list |
---|
1982 | with the time localized for the standard Greenwich time zone. |
---|
1983 | Typically used as follows: |
---|
1984 | |
---|
1985 | # 0 1 2 3 4 5 6 7 |
---|
1986 | ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) = |
---|
1987 | gmtime(time); |
---|
1988 | |
---|
1989 | All list elements are numeric, and come straight out of the C `struct |
---|
1990 | tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the |
---|
1991 | specified time. $mday is the day of the month, and $mon is the month |
---|
1992 | itself, in the range C<0..11> with 0 indicating January and 11 |
---|
1993 | indicating December. $year is the number of years since 1900. That |
---|
1994 | is, $year is C<123> in year 2023. $wday is the day of the week, with |
---|
1995 | 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of |
---|
1996 | the year, in the range C<0..364> (or C<0..365> in leap years.) |
---|
1997 | |
---|
1998 | Note that the $year element is I<not> simply the last two digits of |
---|
1999 | the year. If you assume it is, then you create non-Y2K-compliant |
---|
2000 | programs--and you wouldn't want to do that, would you? |
---|
2001 | |
---|
2002 | The proper way to get a complete 4-digit year is simply: |
---|
2003 | |
---|
2004 | $year += 1900; |
---|
2005 | |
---|
2006 | And to get the last two digits of the year (e.g., '01' in 2001) do: |
---|
2007 | |
---|
2008 | $year = sprintf("%02d", $year % 100); |
---|
2009 | |
---|
2010 | If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>). |
---|
2011 | |
---|
2012 | In scalar context, C<gmtime()> returns the ctime(3) value: |
---|
2013 | |
---|
2014 | $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994" |
---|
2015 | |
---|
2016 | Also see the C<timegm> function provided by the C<Time::Local> module, |
---|
2017 | and the strftime(3) function available via the POSIX module. |
---|
2018 | |
---|
2019 | This scalar value is B<not> locale dependent (see L<perllocale>), but |
---|
2020 | is instead a Perl builtin. Also see the C<Time::Local> module, and the |
---|
2021 | strftime(3) and mktime(3) functions available via the POSIX module. To |
---|
2022 | get somewhat similar but locale dependent date strings, set up your |
---|
2023 | locale environment variables appropriately (please see L<perllocale>) |
---|
2024 | and try for example: |
---|
2025 | |
---|
2026 | use POSIX qw(strftime); |
---|
2027 | $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime; |
---|
2028 | |
---|
2029 | Note that the C<%a> and C<%b> escapes, which represent the short forms |
---|
2030 | of the day of the week and the month of the year, may not necessarily |
---|
2031 | be three characters wide in all locales. |
---|
2032 | |
---|
2033 | =item goto LABEL |
---|
2034 | |
---|
2035 | =item goto EXPR |
---|
2036 | |
---|
2037 | =item goto &NAME |
---|
2038 | |
---|
2039 | The C<goto-LABEL> form finds the statement labeled with LABEL and resumes |
---|
2040 | execution there. It may not be used to go into any construct that |
---|
2041 | requires initialization, such as a subroutine or a C<foreach> loop. It |
---|
2042 | also can't be used to go into a construct that is optimized away, |
---|
2043 | or to get out of a block or subroutine given to C<sort>. |
---|
2044 | It can be used to go almost anywhere else within the dynamic scope, |
---|
2045 | including out of subroutines, but it's usually better to use some other |
---|
2046 | construct such as C<last> or C<die>. The author of Perl has never felt the |
---|
2047 | need to use this form of C<goto> (in Perl, that is--C is another matter). |
---|
2048 | |
---|
2049 | The C<goto-EXPR> form expects a label name, whose scope will be resolved |
---|
2050 | dynamically. This allows for computed C<goto>s per FORTRAN, but isn't |
---|
2051 | necessarily recommended if you're optimizing for maintainability: |
---|
2052 | |
---|
2053 | goto ("FOO", "BAR", "GLARCH")[$i]; |
---|
2054 | |
---|
2055 | The C<goto-&NAME> form is quite different from the other forms of C<goto>. |
---|
2056 | In fact, it isn't a goto in the normal sense at all, and doesn't have |
---|
2057 | the stigma associated with other gotos. Instead, it |
---|
2058 | substitutes a call to the named subroutine for the currently running |
---|
2059 | subroutine. This is used by C<AUTOLOAD> subroutines that wish to load |
---|
2060 | another subroutine and then pretend that the other subroutine had been |
---|
2061 | called in the first place (except that any modifications to C<@_> |
---|
2062 | in the current subroutine are propagated to the other subroutine.) |
---|
2063 | After the C<goto>, not even C<caller> will be able to tell that this |
---|
2064 | routine was called first. |
---|
2065 | |
---|
2066 | NAME needn't be the name of a subroutine; it can be a scalar variable |
---|
2067 | containing a code reference, or a block which evaluates to a code |
---|
2068 | reference. |
---|
2069 | |
---|
2070 | =item grep BLOCK LIST |
---|
2071 | |
---|
2072 | =item grep EXPR,LIST |
---|
2073 | |
---|
2074 | This is similar in spirit to, but not the same as, grep(1) and its |
---|
2075 | relatives. In particular, it is not limited to using regular expressions. |
---|
2076 | |
---|
2077 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting |
---|
2078 | C<$_> to each element) and returns the list value consisting of those |
---|
2079 | elements for which the expression evaluated to true. In scalar |
---|
2080 | context, returns the number of times the expression was true. |
---|
2081 | |
---|
2082 | @foo = grep(!/^#/, @bar); # weed out comments |
---|
2083 | |
---|
2084 | or equivalently, |
---|
2085 | |
---|
2086 | @foo = grep {!/^#/} @bar; # weed out comments |
---|
2087 | |
---|
2088 | Note that C<$_> is an alias to the list value, so it can be used to |
---|
2089 | modify the elements of the LIST. While this is useful and supported, |
---|
2090 | it can cause bizarre results if the elements of LIST are not variables. |
---|
2091 | Similarly, grep returns aliases into the original list, much as a for |
---|
2092 | loop's index variable aliases the list elements. That is, modifying an |
---|
2093 | element of a list returned by grep (for example, in a C<foreach>, C<map> |
---|
2094 | or another C<grep>) actually modifies the element in the original list. |
---|
2095 | This is usually something to be avoided when writing clear code. |
---|
2096 | |
---|
2097 | See also L</map> for a list composed of the results of the BLOCK or EXPR. |
---|
2098 | |
---|
2099 | =item hex EXPR |
---|
2100 | |
---|
2101 | =item hex |
---|
2102 | |
---|
2103 | Interprets EXPR as a hex string and returns the corresponding value. |
---|
2104 | (To convert strings that might start with either 0, 0x, or 0b, see |
---|
2105 | L</oct>.) If EXPR is omitted, uses C<$_>. |
---|
2106 | |
---|
2107 | print hex '0xAf'; # prints '175' |
---|
2108 | print hex 'aF'; # same |
---|
2109 | |
---|
2110 | Hex strings may only represent integers. Strings that would cause |
---|
2111 | integer overflow trigger a warning. |
---|
2112 | |
---|
2113 | =item import |
---|
2114 | |
---|
2115 | There is no builtin C<import> function. It is just an ordinary |
---|
2116 | method (subroutine) defined (or inherited) by modules that wish to export |
---|
2117 | names to another module. The C<use> function calls the C<import> method |
---|
2118 | for the package used. See also L</use>, L<perlmod>, and L<Exporter>. |
---|
2119 | |
---|
2120 | =item index STR,SUBSTR,POSITION |
---|
2121 | |
---|
2122 | =item index STR,SUBSTR |
---|
2123 | |
---|
2124 | The index function searches for one string within another, but without |
---|
2125 | the wildcard-like behavior of a full regular-expression pattern match. |
---|
2126 | It returns the position of the first occurrence of SUBSTR in STR at |
---|
2127 | or after POSITION. If POSITION is omitted, starts searching from the |
---|
2128 | beginning of the string. The return value is based at C<0> (or whatever |
---|
2129 | you've set the C<$[> variable to--but don't do that). If the substring |
---|
2130 | is not found, returns one less than the base, ordinarily C<-1>. |
---|
2131 | |
---|
2132 | =item int EXPR |
---|
2133 | |
---|
2134 | =item int |
---|
2135 | |
---|
2136 | Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>. |
---|
2137 | You should not use this function for rounding: one because it truncates |
---|
2138 | towards C<0>, and two because machine representations of floating point |
---|
2139 | numbers can sometimes produce counterintuitive results. For example, |
---|
2140 | C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's |
---|
2141 | because it's really more like -268.99999999999994315658 instead. Usually, |
---|
2142 | the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil> |
---|
2143 | functions will serve you better than will int(). |
---|
2144 | |
---|
2145 | =item ioctl FILEHANDLE,FUNCTION,SCALAR |
---|
2146 | |
---|
2147 | Implements the ioctl(2) function. You'll probably first have to say |
---|
2148 | |
---|
2149 | require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph |
---|
2150 | |
---|
2151 | to get the correct function definitions. If F<ioctl.ph> doesn't |
---|
2152 | exist or doesn't have the correct definitions you'll have to roll your |
---|
2153 | own, based on your C header files such as F<< <sys/ioctl.h> >>. |
---|
2154 | (There is a Perl script called B<h2ph> that comes with the Perl kit that |
---|
2155 | may help you in this, but it's nontrivial.) SCALAR will be read and/or |
---|
2156 | written depending on the FUNCTION--a pointer to the string value of SCALAR |
---|
2157 | will be passed as the third argument of the actual C<ioctl> call. (If SCALAR |
---|
2158 | has no string value but does have a numeric value, that value will be |
---|
2159 | passed rather than a pointer to the string value. To guarantee this to be |
---|
2160 | true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack> |
---|
2161 | functions may be needed to manipulate the values of structures used by |
---|
2162 | C<ioctl>. |
---|
2163 | |
---|
2164 | The return value of C<ioctl> (and C<fcntl>) is as follows: |
---|
2165 | |
---|
2166 | if OS returns: then Perl returns: |
---|
2167 | -1 undefined value |
---|
2168 | 0 string "0 but true" |
---|
2169 | anything else that number |
---|
2170 | |
---|
2171 | Thus Perl returns true on success and false on failure, yet you can |
---|
2172 | still easily determine the actual value returned by the operating |
---|
2173 | system: |
---|
2174 | |
---|
2175 | $retval = ioctl(...) || -1; |
---|
2176 | printf "System returned %d\n", $retval; |
---|
2177 | |
---|
2178 | The special string "C<0> but true" is exempt from B<-w> complaints |
---|
2179 | about improper numeric conversions. |
---|
2180 | |
---|
2181 | Here's an example of setting a filehandle named C<REMOTE> to be |
---|
2182 | non-blocking at the system level. You'll have to negotiate C<$|> |
---|
2183 | on your own, though. |
---|
2184 | |
---|
2185 | use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK); |
---|
2186 | |
---|
2187 | $flags = fcntl(REMOTE, F_GETFL, 0) |
---|
2188 | or die "Can't get flags for the socket: $!\n"; |
---|
2189 | |
---|
2190 | $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK) |
---|
2191 | or die "Can't set flags for the socket: $!\n"; |
---|
2192 | |
---|
2193 | =item join EXPR,LIST |
---|
2194 | |
---|
2195 | Joins the separate strings of LIST into a single string with fields |
---|
2196 | separated by the value of EXPR, and returns that new string. Example: |
---|
2197 | |
---|
2198 | $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell); |
---|
2199 | |
---|
2200 | Beware that unlike C<split>, C<join> doesn't take a pattern as its |
---|
2201 | first argument. Compare L</split>. |
---|
2202 | |
---|
2203 | =item keys HASH |
---|
2204 | |
---|
2205 | Returns a list consisting of all the keys of the named hash. (In |
---|
2206 | scalar context, returns the number of keys.) The keys are returned in |
---|
2207 | an apparently random order. The actual random order is subject to |
---|
2208 | change in future versions of perl, but it is guaranteed to be the same |
---|
2209 | order as either the C<values> or C<each> function produces (given |
---|
2210 | that the hash has not been modified). As a side effect, it resets |
---|
2211 | HASH's iterator. |
---|
2212 | |
---|
2213 | Here is yet another way to print your environment: |
---|
2214 | |
---|
2215 | @keys = keys %ENV; |
---|
2216 | @values = values %ENV; |
---|
2217 | while (@keys) { |
---|
2218 | print pop(@keys), '=', pop(@values), "\n"; |
---|
2219 | } |
---|
2220 | |
---|
2221 | or how about sorted by key: |
---|
2222 | |
---|
2223 | foreach $key (sort(keys %ENV)) { |
---|
2224 | print $key, '=', $ENV{$key}, "\n"; |
---|
2225 | } |
---|
2226 | |
---|
2227 | The returned values are copies of the original keys in the hash, so |
---|
2228 | modifying them will not affect the original hash. Compare L</values>. |
---|
2229 | |
---|
2230 | To sort a hash by value, you'll need to use a C<sort> function. |
---|
2231 | Here's a descending numeric sort of a hash by its values: |
---|
2232 | |
---|
2233 | foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) { |
---|
2234 | printf "%4d %s\n", $hash{$key}, $key; |
---|
2235 | } |
---|
2236 | |
---|
2237 | As an lvalue C<keys> allows you to increase the number of hash buckets |
---|
2238 | allocated for the given hash. This can gain you a measure of efficiency if |
---|
2239 | you know the hash is going to get big. (This is similar to pre-extending |
---|
2240 | an array by assigning a larger number to $#array.) If you say |
---|
2241 | |
---|
2242 | keys %hash = 200; |
---|
2243 | |
---|
2244 | then C<%hash> will have at least 200 buckets allocated for it--256 of them, |
---|
2245 | in fact, since it rounds up to the next power of two. These |
---|
2246 | buckets will be retained even if you do C<%hash = ()>, use C<undef |
---|
2247 | %hash> if you want to free the storage while C<%hash> is still in scope. |
---|
2248 | You can't shrink the number of buckets allocated for the hash using |
---|
2249 | C<keys> in this way (but you needn't worry about doing this by accident, |
---|
2250 | as trying has no effect). |
---|
2251 | |
---|
2252 | See also C<each>, C<values> and C<sort>. |
---|
2253 | |
---|
2254 | =item kill SIGNAL, LIST |
---|
2255 | |
---|
2256 | Sends a signal to a list of processes. Returns the number of |
---|
2257 | processes successfully signaled (which is not necessarily the |
---|
2258 | same as the number actually killed). |
---|
2259 | |
---|
2260 | $cnt = kill 1, $child1, $child2; |
---|
2261 | kill 9, @goners; |
---|
2262 | |
---|
2263 | If SIGNAL is zero, no signal is sent to the process. This is a |
---|
2264 | useful way to check that the process is alive and hasn't changed |
---|
2265 | its UID. See L<perlport> for notes on the portability of this |
---|
2266 | construct. |
---|
2267 | |
---|
2268 | Unlike in the shell, if SIGNAL is negative, it kills |
---|
2269 | process groups instead of processes. (On System V, a negative I<PROCESS> |
---|
2270 | number will also kill process groups, but that's not portable.) That |
---|
2271 | means you usually want to use positive not negative signals. You may also |
---|
2272 | use a signal name in quotes. See L<perlipc/"Signals"> for details. |
---|
2273 | |
---|
2274 | =item last LABEL |
---|
2275 | |
---|
2276 | =item last |
---|
2277 | |
---|
2278 | The C<last> command is like the C<break> statement in C (as used in |
---|
2279 | loops); it immediately exits the loop in question. If the LABEL is |
---|
2280 | omitted, the command refers to the innermost enclosing loop. The |
---|
2281 | C<continue> block, if any, is not executed: |
---|
2282 | |
---|
2283 | LINE: while (<STDIN>) { |
---|
2284 | last LINE if /^$/; # exit when done with header |
---|
2285 | #... |
---|
2286 | } |
---|
2287 | |
---|
2288 | C<last> cannot be used to exit a block which returns a value such as |
---|
2289 | C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit |
---|
2290 | a grep() or map() operation. |
---|
2291 | |
---|
2292 | Note that a block by itself is semantically identical to a loop |
---|
2293 | that executes once. Thus C<last> can be used to effect an early |
---|
2294 | exit out of such a block. |
---|
2295 | |
---|
2296 | See also L</continue> for an illustration of how C<last>, C<next>, and |
---|
2297 | C<redo> work. |
---|
2298 | |
---|
2299 | =item lc EXPR |
---|
2300 | |
---|
2301 | =item lc |
---|
2302 | |
---|
2303 | Returns an lowercased version of EXPR. This is the internal function |
---|
2304 | implementing the C<\L> escape in double-quoted strings. |
---|
2305 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale> |
---|
2306 | and L<utf8>. |
---|
2307 | |
---|
2308 | If EXPR is omitted, uses C<$_>. |
---|
2309 | |
---|
2310 | =item lcfirst EXPR |
---|
2311 | |
---|
2312 | =item lcfirst |
---|
2313 | |
---|
2314 | Returns the value of EXPR with the first character lowercased. This is |
---|
2315 | the internal function implementing the C<\l> escape in double-quoted strings. |
---|
2316 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. |
---|
2317 | |
---|
2318 | If EXPR is omitted, uses C<$_>. |
---|
2319 | |
---|
2320 | =item length EXPR |
---|
2321 | |
---|
2322 | =item length |
---|
2323 | |
---|
2324 | Returns the length in characters of the value of EXPR. If EXPR is |
---|
2325 | omitted, returns length of C<$_>. Note that this cannot be used on |
---|
2326 | an entire array or hash to find out how many elements these have. |
---|
2327 | For that, use C<scalar @array> and C<scalar keys %hash> respectively. |
---|
2328 | |
---|
2329 | =item link OLDFILE,NEWFILE |
---|
2330 | |
---|
2331 | Creates a new filename linked to the old filename. Returns true for |
---|
2332 | success, false otherwise. |
---|
2333 | |
---|
2334 | =item listen SOCKET,QUEUESIZE |
---|
2335 | |
---|
2336 | Does the same thing that the listen system call does. Returns true if |
---|
2337 | it succeeded, false otherwise. See the example in |
---|
2338 | L<perlipc/"Sockets: Client/Server Communication">. |
---|
2339 | |
---|
2340 | =item local EXPR |
---|
2341 | |
---|
2342 | You really probably want to be using C<my> instead, because C<local> isn't |
---|
2343 | what most people think of as "local". See |
---|
2344 | L<perlsub/"Private Variables via my()"> for details. |
---|
2345 | |
---|
2346 | A local modifies the listed variables to be local to the enclosing |
---|
2347 | block, file, or eval. If more than one value is listed, the list must |
---|
2348 | be placed in parentheses. See L<perlsub/"Temporary Values via local()"> |
---|
2349 | for details, including issues with tied arrays and hashes. |
---|
2350 | |
---|
2351 | =item localtime EXPR |
---|
2352 | |
---|
2353 | Converts a time as returned by the time function to a 9-element list |
---|
2354 | with the time analyzed for the local time zone. Typically used as |
---|
2355 | follows: |
---|
2356 | |
---|
2357 | # 0 1 2 3 4 5 6 7 8 |
---|
2358 | ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = |
---|
2359 | localtime(time); |
---|
2360 | |
---|
2361 | All list elements are numeric, and come straight out of the C `struct |
---|
2362 | tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the |
---|
2363 | specified time. $mday is the day of the month, and $mon is the month |
---|
2364 | itself, in the range C<0..11> with 0 indicating January and 11 |
---|
2365 | indicating December. $year is the number of years since 1900. That |
---|
2366 | is, $year is C<123> in year 2023. $wday is the day of the week, with |
---|
2367 | 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of |
---|
2368 | the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst |
---|
2369 | is true if the specified time occurs during daylight savings time, |
---|
2370 | false otherwise. |
---|
2371 | |
---|
2372 | Note that the $year element is I<not> simply the last two digits of |
---|
2373 | the year. If you assume it is, then you create non-Y2K-compliant |
---|
2374 | programs--and you wouldn't want to do that, would you? |
---|
2375 | |
---|
2376 | The proper way to get a complete 4-digit year is simply: |
---|
2377 | |
---|
2378 | $year += 1900; |
---|
2379 | |
---|
2380 | And to get the last two digits of the year (e.g., '01' in 2001) do: |
---|
2381 | |
---|
2382 | $year = sprintf("%02d", $year % 100); |
---|
2383 | |
---|
2384 | If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>). |
---|
2385 | |
---|
2386 | In scalar context, C<localtime()> returns the ctime(3) value: |
---|
2387 | |
---|
2388 | $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994" |
---|
2389 | |
---|
2390 | This scalar value is B<not> locale dependent, see L<perllocale>, but |
---|
2391 | instead a Perl builtin. Also see the C<Time::Local> module |
---|
2392 | (to convert the second, minutes, hours, ... back to seconds since the |
---|
2393 | stroke of midnight the 1st of January 1970, the value returned by |
---|
2394 | time()), and the strftime(3) and mktime(3) functions available via the |
---|
2395 | POSIX module. To get somewhat similar but locale dependent date |
---|
2396 | strings, set up your locale environment variables appropriately |
---|
2397 | (please see L<perllocale>) and try for example: |
---|
2398 | |
---|
2399 | use POSIX qw(strftime); |
---|
2400 | $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime; |
---|
2401 | |
---|
2402 | Note that the C<%a> and C<%b>, the short forms of the day of the week |
---|
2403 | and the month of the year, may not necessarily be three characters wide. |
---|
2404 | |
---|
2405 | =item lock |
---|
2406 | |
---|
2407 | lock I<THING> |
---|
2408 | |
---|
2409 | This function places an advisory lock on a variable, subroutine, |
---|
2410 | or referenced object contained in I<THING> until the lock goes out |
---|
2411 | of scope. This is a built-in function only if your version of Perl |
---|
2412 | was built with threading enabled, and if you've said C<use Threads>. |
---|
2413 | Otherwise a user-defined function by this name will be called. See |
---|
2414 | L<Thread>. |
---|
2415 | |
---|
2416 | =item log EXPR |
---|
2417 | |
---|
2418 | =item log |
---|
2419 | |
---|
2420 | Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, |
---|
2421 | returns log of C<$_>. To get the log of another base, use basic algebra: |
---|
2422 | The base-N log of a number is equal to the natural log of that number |
---|
2423 | divided by the natural log of N. For example: |
---|
2424 | |
---|
2425 | sub log10 { |
---|
2426 | my $n = shift; |
---|
2427 | return log($n)/log(10); |
---|
2428 | } |
---|
2429 | |
---|
2430 | See also L</exp> for the inverse operation. |
---|
2431 | |
---|
2432 | =item lstat FILEHANDLE |
---|
2433 | |
---|
2434 | =item lstat EXPR |
---|
2435 | |
---|
2436 | =item lstat |
---|
2437 | |
---|
2438 | Does the same thing as the C<stat> function (including setting the |
---|
2439 | special C<_> filehandle) but stats a symbolic link instead of the file |
---|
2440 | the symbolic link points to. If symbolic links are unimplemented on |
---|
2441 | your system, a normal C<stat> is done. |
---|
2442 | |
---|
2443 | If EXPR is omitted, stats C<$_>. |
---|
2444 | |
---|
2445 | =item m// |
---|
2446 | |
---|
2447 | The match operator. See L<perlop>. |
---|
2448 | |
---|
2449 | =item map BLOCK LIST |
---|
2450 | |
---|
2451 | =item map EXPR,LIST |
---|
2452 | |
---|
2453 | Evaluates the BLOCK or EXPR for each element of LIST (locally setting |
---|
2454 | C<$_> to each element) and returns the list value composed of the |
---|
2455 | results of each such evaluation. In scalar context, returns the |
---|
2456 | total number of elements so generated. Evaluates BLOCK or EXPR in |
---|
2457 | list context, so each element of LIST may produce zero, one, or |
---|
2458 | more elements in the returned value. |
---|
2459 | |
---|
2460 | @chars = map(chr, @nums); |
---|
2461 | |
---|
2462 | translates a list of numbers to the corresponding characters. And |
---|
2463 | |
---|
2464 | %hash = map { getkey($_) => $_ } @array; |
---|
2465 | |
---|
2466 | is just a funny way to write |
---|
2467 | |
---|
2468 | %hash = (); |
---|
2469 | foreach $_ (@array) { |
---|
2470 | $hash{getkey($_)} = $_; |
---|
2471 | } |
---|
2472 | |
---|
2473 | Note that C<$_> is an alias to the list value, so it can be used to |
---|
2474 | modify the elements of the LIST. While this is useful and supported, |
---|
2475 | it can cause bizarre results if the elements of LIST are not variables. |
---|
2476 | Using a regular C<foreach> loop for this purpose would be clearer in |
---|
2477 | most cases. See also L</grep> for an array composed of those items of |
---|
2478 | the original list for which the BLOCK or EXPR evaluates to true. |
---|
2479 | |
---|
2480 | C<{> starts both hash references and blocks, so C<map { ...> could be either |
---|
2481 | the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look |
---|
2482 | ahead for the closing C<}> it has to take a guess at which its dealing with |
---|
2483 | based what it finds just after the C<{>. Usually it gets it right, but if it |
---|
2484 | doesn't it won't realize something is wrong until it gets to the C<}> and |
---|
2485 | encounters the missing (or unexpected) comma. The syntax error will be |
---|
2486 | reported close to the C<}> but you'll need to change something near the C<{> |
---|
2487 | such as using a unary C<+> to give perl some help: |
---|
2488 | |
---|
2489 | %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong |
---|
2490 | %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right |
---|
2491 | %hash = map { ("\L$_", 1) } @array # this also works |
---|
2492 | %hash = map { lc($_), 1 } @array # as does this. |
---|
2493 | %hash = map +( lc($_), 1 ), @array # this is EXPR and works! |
---|
2494 | |
---|
2495 | %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array) |
---|
2496 | |
---|
2497 | or to force an anon hash constructor use C<+{> |
---|
2498 | |
---|
2499 | @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end |
---|
2500 | |
---|
2501 | and you get list of anonymous hashes each with only 1 entry. |
---|
2502 | |
---|
2503 | =item mkdir FILENAME,MASK |
---|
2504 | |
---|
2505 | =item mkdir FILENAME |
---|
2506 | |
---|
2507 | Creates the directory specified by FILENAME, with permissions |
---|
2508 | specified by MASK (as modified by C<umask>). If it succeeds it |
---|
2509 | returns true, otherwise it returns false and sets C<$!> (errno). |
---|
2510 | If omitted, MASK defaults to 0777. |
---|
2511 | |
---|
2512 | In general, it is better to create directories with permissive MASK, |
---|
2513 | and let the user modify that with their C<umask>, than it is to supply |
---|
2514 | a restrictive MASK and give the user no way to be more permissive. |
---|
2515 | The exceptions to this rule are when the file or directory should be |
---|
2516 | kept private (mail files, for instance). The perlfunc(1) entry on |
---|
2517 | C<umask> discusses the choice of MASK in more detail. |
---|
2518 | |
---|
2519 | =item msgctl ID,CMD,ARG |
---|
2520 | |
---|
2521 | Calls the System V IPC function msgctl(2). You'll probably have to say |
---|
2522 | |
---|
2523 | use IPC::SysV; |
---|
2524 | |
---|
2525 | first to get the correct constant definitions. If CMD is C<IPC_STAT>, |
---|
2526 | then ARG must be a variable which will hold the returned C<msqid_ds> |
---|
2527 | structure. Returns like C<ioctl>: the undefined value for error, |
---|
2528 | C<"0 but true"> for zero, or the actual return value otherwise. See also |
---|
2529 | L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation. |
---|
2530 | |
---|
2531 | =item msgget KEY,FLAGS |
---|
2532 | |
---|
2533 | Calls the System V IPC function msgget(2). Returns the message queue |
---|
2534 | id, or the undefined value if there is an error. See also |
---|
2535 | L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation. |
---|
2536 | |
---|
2537 | =item msgrcv ID,VAR,SIZE,TYPE,FLAGS |
---|
2538 | |
---|
2539 | Calls the System V IPC function msgrcv to receive a message from |
---|
2540 | message queue ID into variable VAR with a maximum message size of |
---|
2541 | SIZE. Note that when a message is received, the message type as a |
---|
2542 | native long integer will be the first thing in VAR, followed by the |
---|
2543 | actual message. This packing may be opened with C<unpack("l! a*")>. |
---|
2544 | Taints the variable. Returns true if successful, or false if there is |
---|
2545 | an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and |
---|
2546 | C<IPC::SysV::Msg> documentation. |
---|
2547 | |
---|
2548 | =item msgsnd ID,MSG,FLAGS |
---|
2549 | |
---|
2550 | Calls the System V IPC function msgsnd to send the message MSG to the |
---|
2551 | message queue ID. MSG must begin with the native long integer message |
---|
2552 | type, and be followed by the length of the actual message, and finally |
---|
2553 | the message itself. This kind of packing can be achieved with |
---|
2554 | C<pack("l! a*", $type, $message)>. Returns true if successful, |
---|
2555 | or false if there is an error. See also C<IPC::SysV> |
---|
2556 | and C<IPC::SysV::Msg> documentation. |
---|
2557 | |
---|
2558 | =item my EXPR |
---|
2559 | |
---|
2560 | =item my EXPR : ATTRIBUTES |
---|
2561 | |
---|
2562 | A C<my> declares the listed variables to be local (lexically) to the |
---|
2563 | enclosing block, file, or C<eval>. If |
---|
2564 | more than one value is listed, the list must be placed in parentheses. See |
---|
2565 | L<perlsub/"Private Variables via my()"> for details. |
---|
2566 | |
---|
2567 | =item next LABEL |
---|
2568 | |
---|
2569 | =item next |
---|
2570 | |
---|
2571 | The C<next> command is like the C<continue> statement in C; it starts |
---|
2572 | the next iteration of the loop: |
---|
2573 | |
---|
2574 | LINE: while (<STDIN>) { |
---|
2575 | next LINE if /^#/; # discard comments |
---|
2576 | #... |
---|
2577 | } |
---|
2578 | |
---|
2579 | Note that if there were a C<continue> block on the above, it would get |
---|
2580 | executed even on discarded lines. If the LABEL is omitted, the command |
---|
2581 | refers to the innermost enclosing loop. |
---|
2582 | |
---|
2583 | C<next> cannot be used to exit a block which returns a value such as |
---|
2584 | C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit |
---|
2585 | a grep() or map() operation. |
---|
2586 | |
---|
2587 | Note that a block by itself is semantically identical to a loop |
---|
2588 | that executes once. Thus C<next> will exit such a block early. |
---|
2589 | |
---|
2590 | See also L</continue> for an illustration of how C<last>, C<next>, and |
---|
2591 | C<redo> work. |
---|
2592 | |
---|
2593 | =item no Module LIST |
---|
2594 | |
---|
2595 | See the L</use> function, which C<no> is the opposite of. |
---|
2596 | |
---|
2597 | =item oct EXPR |
---|
2598 | |
---|
2599 | =item oct |
---|
2600 | |
---|
2601 | Interprets EXPR as an octal string and returns the corresponding |
---|
2602 | value. (If EXPR happens to start off with C<0x>, interprets it as a |
---|
2603 | hex string. If EXPR starts off with C<0b>, it is interpreted as a |
---|
2604 | binary string.) The following will handle decimal, binary, octal, and |
---|
2605 | hex in the standard Perl or C notation: |
---|
2606 | |
---|
2607 | $val = oct($val) if $val =~ /^0/; |
---|
2608 | |
---|
2609 | If EXPR is omitted, uses C<$_>. To go the other way (produce a number |
---|
2610 | in octal), use sprintf() or printf(): |
---|
2611 | |
---|
2612 | $perms = (stat("filename"))[2] & 07777; |
---|
2613 | $oct_perms = sprintf "%lo", $perms; |
---|
2614 | |
---|
2615 | The oct() function is commonly used when a string such as C<644> needs |
---|
2616 | to be converted into a file mode, for example. (Although perl will |
---|
2617 | automatically convert strings into numbers as needed, this automatic |
---|
2618 | conversion assumes base 10.) |
---|
2619 | |
---|
2620 | =item open FILEHANDLE,MODE,LIST |
---|
2621 | |
---|
2622 | =item open FILEHANDLE,EXPR |
---|
2623 | |
---|
2624 | =item open FILEHANDLE |
---|
2625 | |
---|
2626 | Opens the file whose filename is given by EXPR, and associates it with |
---|
2627 | FILEHANDLE. If FILEHANDLE is an expression, its value is used as the |
---|
2628 | name of the real filehandle wanted. (This is considered a symbolic |
---|
2629 | reference, so C<use strict 'refs'> should I<not> be in effect.) |
---|
2630 | |
---|
2631 | If EXPR is omitted, the scalar |
---|
2632 | variable of the same name as the FILEHANDLE contains the filename. |
---|
2633 | (Note that lexical variables--those declared with C<my>--will not work |
---|
2634 | for this purpose; so if you're using C<my>, specify EXPR in your call |
---|
2635 | to open.) See L<perlopentut> for a kinder, gentler explanation of opening |
---|
2636 | files. |
---|
2637 | |
---|
2638 | If MODE is C<< '<' >> or nothing, the file is opened for input. |
---|
2639 | If MODE is C<< '>' >>, the file is truncated and opened for |
---|
2640 | output, being created if necessary. If MODE is C<<< '>>' >>>, |
---|
2641 | the file is opened for appending, again being created if necessary. |
---|
2642 | You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that |
---|
2643 | you want both read and write access to the file; thus C<< '+<' >> is almost |
---|
2644 | always preferred for read/write updates--the C<< '+>' >> mode would clobber the |
---|
2645 | file first. You can't usually use either read-write mode for updating |
---|
2646 | textfiles, since they have variable length records. See the B<-i> |
---|
2647 | switch in L<perlrun> for a better approach. The file is created with |
---|
2648 | permissions of C<0666> modified by the process' C<umask> value. |
---|
2649 | |
---|
2650 | These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, |
---|
2651 | C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>. |
---|
2652 | |
---|
2653 | In the 2-arguments (and 1-argument) form of the call the mode and |
---|
2654 | filename should be concatenated (in this order), possibly separated by |
---|
2655 | spaces. It is possible to omit the mode if the mode is C<< '<' >>. |
---|
2656 | |
---|
2657 | If the filename begins with C<'|'>, the filename is interpreted as a |
---|
2658 | command to which output is to be piped, and if the filename ends with a |
---|
2659 | C<'|'>, the filename is interpreted as a command which pipes output to |
---|
2660 | us. See L<perlipc/"Using open() for IPC"> |
---|
2661 | for more examples of this. (You are not allowed to C<open> to a command |
---|
2662 | that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, |
---|
2663 | and L<perlipc/"Bidirectional Communication with Another Process"> |
---|
2664 | for alternatives.) |
---|
2665 | |
---|
2666 | If MODE is C<'|-'>, the filename is interpreted as a |
---|
2667 | command to which output is to be piped, and if MODE is |
---|
2668 | C<'-|'>, the filename is interpreted as a command which pipes output to |
---|
2669 | us. In the 2-arguments (and 1-argument) form one should replace dash |
---|
2670 | (C<'-'>) with the command. See L<perlipc/"Using open() for IPC"> |
---|
2671 | for more examples of this. (You are not allowed to C<open> to a command |
---|
2672 | that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>, |
---|
2673 | and L<perlipc/"Bidirectional Communication"> for alternatives.) |
---|
2674 | |
---|
2675 | In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN |
---|
2676 | and opening C<< '>-' >> opens STDOUT. |
---|
2677 | |
---|
2678 | Open returns |
---|
2679 | nonzero upon success, the undefined value otherwise. If the C<open> |
---|
2680 | involved a pipe, the return value happens to be the pid of the |
---|
2681 | subprocess. |
---|
2682 | |
---|
2683 | If you're unfortunate enough to be running Perl on a system that |
---|
2684 | distinguishes between text files and binary files (modern operating |
---|
2685 | systems don't care), then you should check out L</binmode> for tips for |
---|
2686 | dealing with this. The key distinction between systems that need C<binmode> |
---|
2687 | and those that don't is their text file formats. Systems like Unix, MacOS, and |
---|
2688 | Plan9, which delimit lines with a single character, and which encode that |
---|
2689 | character in C as C<"\n">, do not need C<binmode>. The rest need it. |
---|
2690 | |
---|
2691 | When opening a file, it's usually a bad idea to continue normal execution |
---|
2692 | if the request failed, so C<open> is frequently used in connection with |
---|
2693 | C<die>. Even if C<die> won't do what you want (say, in a CGI script, |
---|
2694 | where you want to make a nicely formatted error message (but there are |
---|
2695 | modules that can help with that problem)) you should always check |
---|
2696 | the return value from opening a file. The infrequent exception is when |
---|
2697 | working with an unopened filehandle is actually what you want to do. |
---|
2698 | |
---|
2699 | Examples: |
---|
2700 | |
---|
2701 | $ARTICLE = 100; |
---|
2702 | open ARTICLE or die "Can't find article $ARTICLE: $!\n"; |
---|
2703 | while (<ARTICLE>) {... |
---|
2704 | |
---|
2705 | open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved) |
---|
2706 | # if the open fails, output is discarded |
---|
2707 | |
---|
2708 | open(DBASE, '+<', 'dbase.mine') # open for update |
---|
2709 | or die "Can't open 'dbase.mine' for update: $!"; |
---|
2710 | |
---|
2711 | open(DBASE, '+<dbase.mine') # ditto |
---|
2712 | or die "Can't open 'dbase.mine' for update: $!"; |
---|
2713 | |
---|
2714 | open(ARTICLE, '-|', "caesar <$article") # decrypt article |
---|
2715 | or die "Can't start caesar: $!"; |
---|
2716 | |
---|
2717 | open(ARTICLE, "caesar <$article |") # ditto |
---|
2718 | or die "Can't start caesar: $!"; |
---|
2719 | |
---|
2720 | open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id |
---|
2721 | or die "Can't start sort: $!"; |
---|
2722 | |
---|
2723 | # process argument list of files along with any includes |
---|
2724 | |
---|
2725 | foreach $file (@ARGV) { |
---|
2726 | process($file, 'fh00'); |
---|
2727 | } |
---|
2728 | |
---|
2729 | sub process { |
---|
2730 | my($filename, $input) = @_; |
---|
2731 | $input++; # this is a string increment |
---|
2732 | unless (open($input, $filename)) { |
---|
2733 | print STDERR "Can't open $filename: $!\n"; |
---|
2734 | return; |
---|
2735 | } |
---|
2736 | |
---|
2737 | local $_; |
---|
2738 | while (<$input>) { # note use of indirection |
---|
2739 | if (/^#include "(.*)"/) { |
---|
2740 | process($1, $input); |
---|
2741 | next; |
---|
2742 | } |
---|
2743 | #... # whatever |
---|
2744 | } |
---|
2745 | } |
---|
2746 | |
---|
2747 | You may also, in the Bourne shell tradition, specify an EXPR beginning |
---|
2748 | with C<< '>&' >>, in which case the rest of the string is interpreted as the |
---|
2749 | name of a filehandle (or file descriptor, if numeric) to be |
---|
2750 | duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>, |
---|
2751 | C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The |
---|
2752 | mode you specify should match the mode of the original filehandle. |
---|
2753 | (Duping a filehandle does not take into account any existing contents of |
---|
2754 | stdio buffers.) Duping file handles is not yet supported for 3-argument |
---|
2755 | open(). |
---|
2756 | |
---|
2757 | Here is a script that saves, redirects, and restores STDOUT and |
---|
2758 | STDERR: |
---|
2759 | |
---|
2760 | #!/usr/bin/perl |
---|
2761 | open(OLDOUT, ">&STDOUT"); |
---|
2762 | open(OLDERR, ">&STDERR"); |
---|
2763 | |
---|
2764 | open(STDOUT, '>', "foo.out") || die "Can't redirect stdout"; |
---|
2765 | open(STDERR, ">&STDOUT") || die "Can't dup stdout"; |
---|
2766 | |
---|
2767 | select(STDERR); $| = 1; # make unbuffered |
---|
2768 | select(STDOUT); $| = 1; # make unbuffered |
---|
2769 | |
---|
2770 | print STDOUT "stdout 1\n"; # this works for |
---|
2771 | print STDERR "stderr 1\n"; # subprocesses too |
---|
2772 | |
---|
2773 | close(STDOUT); |
---|
2774 | close(STDERR); |
---|
2775 | |
---|
2776 | open(STDOUT, ">&OLDOUT"); |
---|
2777 | open(STDERR, ">&OLDERR"); |
---|
2778 | |
---|
2779 | print STDOUT "stdout 2\n"; |
---|
2780 | print STDERR "stderr 2\n"; |
---|
2781 | |
---|
2782 | If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an |
---|
2783 | equivalent of C's C<fdopen> of that file descriptor; this is more |
---|
2784 | parsimonious of file descriptors. For example: |
---|
2785 | |
---|
2786 | open(FILEHANDLE, "<&=$fd") |
---|
2787 | |
---|
2788 | Note that this feature depends on the fdopen() C library function. |
---|
2789 | On many UNIX systems, fdopen() is known to fail when file descriptors |
---|
2790 | exceed a certain value, typically 255. If you need more file |
---|
2791 | descriptors than that, consider rebuilding Perl to use the C<sfio> |
---|
2792 | library. |
---|
2793 | |
---|
2794 | If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'> |
---|
2795 | with 2-arguments (or 1-argument) form of open(), then |
---|
2796 | there is an implicit fork done, and the return value of open is the pid |
---|
2797 | of the child within the parent process, and C<0> within the child |
---|
2798 | process. (Use C<defined($pid)> to determine whether the open was successful.) |
---|
2799 | The filehandle behaves normally for the parent, but i/o to that |
---|
2800 | filehandle is piped from/to the STDOUT/STDIN of the child process. |
---|
2801 | In the child process the filehandle isn't opened--i/o happens from/to |
---|
2802 | the new STDOUT or STDIN. Typically this is used like the normal |
---|
2803 | piped open when you want to exercise more control over just how the |
---|
2804 | pipe command gets executed, such as when you are running setuid, and |
---|
2805 | don't want to have to scan shell commands for metacharacters. |
---|
2806 | The following triples are more or less equivalent: |
---|
2807 | |
---|
2808 | open(FOO, "|tr '[a-z]' '[A-Z]'"); |
---|
2809 | open(FOO, '|-', "tr '[a-z]' '[A-Z]'"); |
---|
2810 | open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]'; |
---|
2811 | |
---|
2812 | open(FOO, "cat -n '$file'|"); |
---|
2813 | open(FOO, '-|', "cat -n '$file'"); |
---|
2814 | open(FOO, '-|') || exec 'cat', '-n', $file; |
---|
2815 | |
---|
2816 | See L<perlipc/"Safe Pipe Opens"> for more examples of this. |
---|
2817 | |
---|
2818 | Beginning with v5.6.0, Perl will attempt to flush all files opened for |
---|
2819 | output before any operation that may do a fork, but this may not be |
---|
2820 | supported on some platforms (see L<perlport>). To be safe, you may need |
---|
2821 | to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method |
---|
2822 | of C<IO::Handle> on any open handles. |
---|
2823 | |
---|
2824 | On systems that support a |
---|
2825 | close-on-exec flag on files, the flag will be set for the newly opened |
---|
2826 | file descriptor as determined by the value of $^F. See L<perlvar/$^F>. |
---|
2827 | |
---|
2828 | Closing any piped filehandle causes the parent process to wait for the |
---|
2829 | child to finish, and returns the status value in C<$?>. |
---|
2830 | |
---|
2831 | The filename passed to 2-argument (or 1-argument) form of open() |
---|
2832 | will have leading and trailing |
---|
2833 | whitespace deleted, and the normal redirection characters |
---|
2834 | honored. This property, known as "magic open", |
---|
2835 | can often be used to good effect. A user could specify a filename of |
---|
2836 | F<"rsh cat file |">, or you could change certain filenames as needed: |
---|
2837 | |
---|
2838 | $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/; |
---|
2839 | open(FH, $filename) or die "Can't open $filename: $!"; |
---|
2840 | |
---|
2841 | Use 3-argument form to open a file with arbitrary weird characters in it, |
---|
2842 | |
---|
2843 | open(FOO, '<', $file); |
---|
2844 | |
---|
2845 | otherwise it's necessary to protect any leading and trailing whitespace: |
---|
2846 | |
---|
2847 | $file =~ s#^(\s)#./$1#; |
---|
2848 | open(FOO, "< $file\0"); |
---|
2849 | |
---|
2850 | (this may not work on some bizarre filesystems). One should |
---|
2851 | conscientiously choose between the I<magic> and 3-arguments form |
---|
2852 | of open(): |
---|
2853 | |
---|
2854 | open IN, $ARGV[0]; |
---|
2855 | |
---|
2856 | will allow the user to specify an argument of the form C<"rsh cat file |">, |
---|
2857 | but will not work on a filename which happens to have a trailing space, while |
---|
2858 | |
---|
2859 | open IN, '<', $ARGV[0]; |
---|
2860 | |
---|
2861 | will have exactly the opposite restrictions. |
---|
2862 | |
---|
2863 | If you want a "real" C C<open> (see L<open(2)> on your system), then you |
---|
2864 | should use the C<sysopen> function, which involves no such magic (but |
---|
2865 | may use subtly different filemodes than Perl open(), which is mapped |
---|
2866 | to C fopen()). This is |
---|
2867 | another way to protect your filenames from interpretation. For example: |
---|
2868 | |
---|
2869 | use IO::Handle; |
---|
2870 | sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL) |
---|
2871 | or die "sysopen $path: $!"; |
---|
2872 | $oldfh = select(HANDLE); $| = 1; select($oldfh); |
---|
2873 | print HANDLE "stuff $$\n"; |
---|
2874 | seek(HANDLE, 0, 0); |
---|
2875 | print "File contains: ", <HANDLE>; |
---|
2876 | |
---|
2877 | Using the constructor from the C<IO::Handle> package (or one of its |
---|
2878 | subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous |
---|
2879 | filehandles that have the scope of whatever variables hold references to |
---|
2880 | them, and automatically close whenever and however you leave that scope: |
---|
2881 | |
---|
2882 | use IO::File; |
---|
2883 | #... |
---|
2884 | sub read_myfile_munged { |
---|
2885 | my $ALL = shift; |
---|
2886 | my $handle = new IO::File; |
---|
2887 | open($handle, "myfile") or die "myfile: $!"; |
---|
2888 | $first = <$handle> |
---|
2889 | or return (); # Automatically closed here. |
---|
2890 | mung $first or die "mung failed"; # Or here. |
---|
2891 | return $first, <$handle> if $ALL; # Or here. |
---|
2892 | $first; # Or here. |
---|
2893 | } |
---|
2894 | |
---|
2895 | See L</seek> for some details about mixing reading and writing. |
---|
2896 | |
---|
2897 | =item opendir DIRHANDLE,EXPR |
---|
2898 | |
---|
2899 | Opens a directory named EXPR for processing by C<readdir>, C<telldir>, |
---|
2900 | C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful. |
---|
2901 | DIRHANDLEs have their own namespace separate from FILEHANDLEs. |
---|
2902 | |
---|
2903 | =item ord EXPR |
---|
2904 | |
---|
2905 | =item ord |
---|
2906 | |
---|
2907 | Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If |
---|
2908 | EXPR is omitted, uses C<$_>. For the reverse, see L</chr>. |
---|
2909 | See L<utf8> for more about Unicode. |
---|
2910 | |
---|
2911 | =item our EXPR |
---|
2912 | |
---|
2913 | An C<our> declares the listed variables to be valid globals within |
---|
2914 | the enclosing block, file, or C<eval>. That is, it has the same |
---|
2915 | scoping rules as a "my" declaration, but does not create a local |
---|
2916 | variable. If more than one value is listed, the list must be placed |
---|
2917 | in parentheses. The C<our> declaration has no semantic effect unless |
---|
2918 | "use strict vars" is in effect, in which case it lets you use the |
---|
2919 | declared global variable without qualifying it with a package name. |
---|
2920 | (But only within the lexical scope of the C<our> declaration. In this |
---|
2921 | it differs from "use vars", which is package scoped.) |
---|
2922 | |
---|
2923 | An C<our> declaration declares a global variable that will be visible |
---|
2924 | across its entire lexical scope, even across package boundaries. The |
---|
2925 | package in which the variable is entered is determined at the point |
---|
2926 | of the declaration, not at the point of use. This means the following |
---|
2927 | behavior holds: |
---|
2928 | |
---|
2929 | package Foo; |
---|
2930 | our $bar; # declares $Foo::bar for rest of lexical scope |
---|
2931 | $bar = 20; |
---|
2932 | |
---|
2933 | package Bar; |
---|
2934 | print $bar; # prints 20 |
---|
2935 | |
---|
2936 | Multiple C<our> declarations in the same lexical scope are allowed |
---|
2937 | if they are in different packages. If they happened to be in the same |
---|
2938 | package, Perl will emit warnings if you have asked for them. |
---|
2939 | |
---|
2940 | use warnings; |
---|
2941 | package Foo; |
---|
2942 | our $bar; # declares $Foo::bar for rest of lexical scope |
---|
2943 | $bar = 20; |
---|
2944 | |
---|
2945 | package Bar; |
---|
2946 | our $bar = 30; # declares $Bar::bar for rest of lexical scope |
---|
2947 | print $bar; # prints 30 |
---|
2948 | |
---|
2949 | our $bar; # emits warning |
---|
2950 | |
---|
2951 | =item pack TEMPLATE,LIST |
---|
2952 | |
---|
2953 | Takes a LIST of values and converts it into a string using the rules |
---|
2954 | given by the TEMPLATE. The resulting string is the concatenation of |
---|
2955 | the converted values. Typically, each converted value looks |
---|
2956 | like its machine-level representation. For example, on 32-bit machines |
---|
2957 | a converted integer may be represented by a sequence of 4 bytes. |
---|
2958 | |
---|
2959 | The TEMPLATE is a |
---|
2960 | sequence of characters that give the order and type of values, as |
---|
2961 | follows: |
---|
2962 | |
---|
2963 | a A string with arbitrary binary data, will be null padded. |
---|
2964 | A An ASCII string, will be space padded. |
---|
2965 | Z A null terminated (asciz) string, will be null padded. |
---|
2966 | |
---|
2967 | b A bit string (ascending bit order inside each byte, like vec()). |
---|
2968 | B A bit string (descending bit order inside each byte). |
---|
2969 | h A hex string (low nybble first). |
---|
2970 | H A hex string (high nybble first). |
---|
2971 | |
---|
2972 | c A signed char value. |
---|
2973 | C An unsigned char value. Only does bytes. See U for Unicode. |
---|
2974 | |
---|
2975 | s A signed short value. |
---|
2976 | S An unsigned short value. |
---|
2977 | (This 'short' is _exactly_ 16 bits, which may differ from |
---|
2978 | what a local C compiler calls 'short'. If you want |
---|
2979 | native-length shorts, use the '!' suffix.) |
---|
2980 | |
---|
2981 | i A signed integer value. |
---|
2982 | I An unsigned integer value. |
---|
2983 | (This 'integer' is _at_least_ 32 bits wide. Its exact |
---|
2984 | size depends on what a local C compiler calls 'int', |
---|
2985 | and may even be larger than the 'long' described in |
---|
2986 | the next item.) |
---|
2987 | |
---|
2988 | l A signed long value. |
---|
2989 | L An unsigned long value. |
---|
2990 | (This 'long' is _exactly_ 32 bits, which may differ from |
---|
2991 | what a local C compiler calls 'long'. If you want |
---|
2992 | native-length longs, use the '!' suffix.) |
---|
2993 | |
---|
2994 | n An unsigned short in "network" (big-endian) order. |
---|
2995 | N An unsigned long in "network" (big-endian) order. |
---|
2996 | v An unsigned short in "VAX" (little-endian) order. |
---|
2997 | V An unsigned long in "VAX" (little-endian) order. |
---|
2998 | (These 'shorts' and 'longs' are _exactly_ 16 bits and |
---|
2999 | _exactly_ 32 bits, respectively.) |
---|
3000 | |
---|
3001 | q A signed quad (64-bit) value. |
---|
3002 | Q An unsigned quad value. |
---|
3003 | (Quads are available only if your system supports 64-bit |
---|
3004 | integer values _and_ if Perl has been compiled to support those. |
---|
3005 | Causes a fatal error otherwise.) |
---|
3006 | |
---|
3007 | f A single-precision float in the native format. |
---|
3008 | d A double-precision float in the native format. |
---|
3009 | |
---|
3010 | p A pointer to a null-terminated string. |
---|
3011 | P A pointer to a structure (fixed-length string). |
---|
3012 | |
---|
3013 | u A uuencoded string. |
---|
3014 | U A Unicode character number. Encodes to UTF-8 internally. |
---|
3015 | Works even if C<use utf8> is not in effect. |
---|
3016 | |
---|
3017 | w A BER compressed integer. Its bytes represent an unsigned |
---|
3018 | integer in base 128, most significant digit first, with as |
---|
3019 | few digits as possible. Bit eight (the high bit) is set |
---|
3020 | on each byte except the last. |
---|
3021 | |
---|
3022 | x A null byte. |
---|
3023 | X Back up a byte. |
---|
3024 | @ Null fill to absolute position. |
---|
3025 | |
---|
3026 | The following rules apply: |
---|
3027 | |
---|
3028 | =over 8 |
---|
3029 | |
---|
3030 | =item * |
---|
3031 | |
---|
3032 | Each letter may optionally be followed by a number giving a repeat |
---|
3033 | count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>, |
---|
3034 | C<H>, and C<P> the pack function will gobble up that many values from |
---|
3035 | the LIST. A C<*> for the repeat count means to use however many items are |
---|
3036 | left, except for C<@>, C<x>, C<X>, where it is equivalent |
---|
3037 | to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the |
---|
3038 | same). |
---|
3039 | |
---|
3040 | When used with C<Z>, C<*> results in the addition of a trailing null |
---|
3041 | byte (so the packed result will be one longer than the byte C<length> |
---|
3042 | of the item). |
---|
3043 | |
---|
3044 | The repeat count for C<u> is interpreted as the maximal number of bytes |
---|
3045 | to encode per line of output, with 0 and 1 replaced by 45. |
---|
3046 | |
---|
3047 | =item * |
---|
3048 | |
---|
3049 | The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a |
---|
3050 | string of length count, padding with nulls or spaces as necessary. When |
---|
3051 | unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything |
---|
3052 | after the first null, and C<a> returns data verbatim. When packing, |
---|
3053 | C<a>, and C<Z> are equivalent. |
---|
3054 | |
---|
3055 | If the value-to-pack is too long, it is truncated. If too long and an |
---|
3056 | explicit count is provided, C<Z> packs only C<$count-1> bytes, followed |
---|
3057 | by a null byte. Thus C<Z> always packs a trailing null byte under |
---|
3058 | all circumstances. |
---|
3059 | |
---|
3060 | =item * |
---|
3061 | |
---|
3062 | Likewise, the C<b> and C<B> fields pack a string that many bits long. |
---|
3063 | Each byte of the input field of pack() generates 1 bit of the result. |
---|
3064 | Each result bit is based on the least-significant bit of the corresponding |
---|
3065 | input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and |
---|
3066 | C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">. |
---|
3067 | |
---|
3068 | Starting from the beginning of the input string of pack(), each 8-tuple |
---|
3069 | of bytes is converted to 1 byte of output. With format C<b> |
---|
3070 | the first byte of the 8-tuple determines the least-significant bit of a |
---|
3071 | byte, and with format C<B> it determines the most-significant bit of |
---|
3072 | a byte. |
---|
3073 | |
---|
3074 | If the length of the input string is not exactly divisible by 8, the |
---|
3075 | remainder is packed as if the input string were padded by null bytes |
---|
3076 | at the end. Similarly, during unpack()ing the "extra" bits are ignored. |
---|
3077 | |
---|
3078 | If the input string of pack() is longer than needed, extra bytes are ignored. |
---|
3079 | A C<*> for the repeat count of pack() means to use all the bytes of |
---|
3080 | the input field. On unpack()ing the bits are converted to a string |
---|
3081 | of C<"0">s and C<"1">s. |
---|
3082 | |
---|
3083 | =item * |
---|
3084 | |
---|
3085 | The C<h> and C<H> fields pack a string that many nybbles (4-bit groups, |
---|
3086 | representable as hexadecimal digits, 0-9a-f) long. |
---|
3087 | |
---|
3088 | Each byte of the input field of pack() generates 4 bits of the result. |
---|
3089 | For non-alphabetical bytes the result is based on the 4 least-significant |
---|
3090 | bits of the input byte, i.e., on C<ord($byte)%16>. In particular, |
---|
3091 | bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes |
---|
3092 | C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result |
---|
3093 | is compatible with the usual hexadecimal digits, so that C<"a"> and |
---|
3094 | C<"A"> both generate the nybble C<0xa==10>. The result for bytes |
---|
3095 | C<"g".."z"> and C<"G".."Z"> is not well-defined. |
---|
3096 | |
---|
3097 | Starting from the beginning of the input string of pack(), each pair |
---|
3098 | of bytes is converted to 1 byte of output. With format C<h> the |
---|
3099 | first byte of the pair determines the least-significant nybble of the |
---|
3100 | output byte, and with format C<H> it determines the most-significant |
---|
3101 | nybble. |
---|
3102 | |
---|
3103 | If the length of the input string is not even, it behaves as if padded |
---|
3104 | by a null byte at the end. Similarly, during unpack()ing the "extra" |
---|
3105 | nybbles are ignored. |
---|
3106 | |
---|
3107 | If the input string of pack() is longer than needed, extra bytes are ignored. |
---|
3108 | A C<*> for the repeat count of pack() means to use all the bytes of |
---|
3109 | the input field. On unpack()ing the bits are converted to a string |
---|
3110 | of hexadecimal digits. |
---|
3111 | |
---|
3112 | =item * |
---|
3113 | |
---|
3114 | The C<p> type packs a pointer to a null-terminated string. You are |
---|
3115 | responsible for ensuring the string is not a temporary value (which can |
---|
3116 | potentially get deallocated before you get around to using the packed result). |
---|
3117 | The C<P> type packs a pointer to a structure of the size indicated by the |
---|
3118 | length. A NULL pointer is created if the corresponding value for C<p> or |
---|
3119 | C<P> is C<undef>, similarly for unpack(). |
---|
3120 | |
---|
3121 | =item * |
---|
3122 | |
---|
3123 | The C</> template character allows packing and unpacking of strings where |
---|
3124 | the packed structure contains a byte count followed by the string itself. |
---|
3125 | You write I<length-item>C</>I<string-item>. |
---|
3126 | |
---|
3127 | The I<length-item> can be any C<pack> template letter, |
---|
3128 | and describes how the length value is packed. |
---|
3129 | The ones likely to be of most use are integer-packing ones like |
---|
3130 | C<n> (for Java strings), C<w> (for ASN.1 or SNMP) |
---|
3131 | and C<N> (for Sun XDR). |
---|
3132 | |
---|
3133 | The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">. |
---|
3134 | For C<unpack> the length of the string is obtained from the I<length-item>, |
---|
3135 | but if you put in the '*' it will be ignored. |
---|
3136 | |
---|
3137 | unpack 'C/a', "\04Gurusamy"; gives 'Guru' |
---|
3138 | unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J') |
---|
3139 | pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world" |
---|
3140 | |
---|
3141 | The I<length-item> is not returned explicitly from C<unpack>. |
---|
3142 | |
---|
3143 | Adding a count to the I<length-item> letter is unlikely to do anything |
---|
3144 | useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a |
---|
3145 | I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters, |
---|
3146 | which Perl does not regard as legal in numeric strings. |
---|
3147 | |
---|
3148 | =item * |
---|
3149 | |
---|
3150 | The integer types C<s>, C<S>, C<l>, and C<L> may be |
---|
3151 | immediately followed by a C<!> suffix to signify native shorts or |
---|
3152 | longs--as you can see from above for example a bare C<l> does mean |
---|
3153 | exactly 32 bits, the native C<long> (as seen by the local C compiler) |
---|
3154 | may be larger. This is an issue mainly in 64-bit platforms. You can |
---|
3155 | see whether using C<!> makes any difference by |
---|
3156 | |
---|
3157 | print length(pack("s")), " ", length(pack("s!")), "\n"; |
---|
3158 | print length(pack("l")), " ", length(pack("l!")), "\n"; |
---|
3159 | |
---|
3160 | C<i!> and C<I!> also work but only because of completeness; |
---|
3161 | they are identical to C<i> and C<I>. |
---|
3162 | |
---|
3163 | The actual sizes (in bytes) of native shorts, ints, longs, and long |
---|
3164 | longs on the platform where Perl was built are also available via |
---|
3165 | L<Config>: |
---|
3166 | |
---|
3167 | use Config; |
---|
3168 | print $Config{shortsize}, "\n"; |
---|
3169 | print $Config{intsize}, "\n"; |
---|
3170 | print $Config{longsize}, "\n"; |
---|
3171 | print $Config{longlongsize}, "\n"; |
---|
3172 | |
---|
3173 | (The C<$Config{longlongsize}> will be undefine if your system does |
---|
3174 | not support long longs.) |
---|
3175 | |
---|
3176 | =item * |
---|
3177 | |
---|
3178 | The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L> |
---|
3179 | are inherently non-portable between processors and operating systems |
---|
3180 | because they obey the native byteorder and endianness. For example a |
---|
3181 | 4-byte integer 0x12345678 (305419896 decimal) be ordered natively |
---|
3182 | (arranged in and handled by the CPU registers) into bytes as |
---|
3183 | |
---|
3184 | 0x12 0x34 0x56 0x78 # big-endian |
---|
3185 | 0x78 0x56 0x34 0x12 # little-endian |
---|
3186 | |
---|
3187 | Basically, the Intel and VAX CPUs are little-endian, while everybody |
---|
3188 | else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and |
---|
3189 | Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq |
---|
3190 | used/uses them in little-endian mode; SGI/Cray uses them in big-endian mode. |
---|
3191 | |
---|
3192 | The names `big-endian' and `little-endian' are comic references to |
---|
3193 | the classic "Gulliver's Travels" (via the paper "On Holy Wars and a |
---|
3194 | Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and |
---|
3195 | the egg-eating habits of the Lilliputians. |
---|
3196 | |
---|
3197 | Some systems may have even weirder byte orders such as |
---|
3198 | |
---|
3199 | 0x56 0x78 0x12 0x34 |
---|
3200 | 0x34 0x12 0x78 0x56 |
---|
3201 | |
---|
3202 | You can see your system's preference with |
---|
3203 | |
---|
3204 | print join(" ", map { sprintf "%#02x", $_ } |
---|
3205 | unpack("C*",pack("L",0x12345678))), "\n"; |
---|
3206 | |
---|
3207 | The byteorder on the platform where Perl was built is also available |
---|
3208 | via L<Config>: |
---|
3209 | |
---|
3210 | use Config; |
---|
3211 | print $Config{byteorder}, "\n"; |
---|
3212 | |
---|
3213 | Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'> |
---|
3214 | and C<'87654321'> are big-endian. |
---|
3215 | |
---|
3216 | If you want portable packed integers use the formats C<n>, C<N>, |
---|
3217 | C<v>, and C<V>, their byte endianness and size is known. |
---|
3218 | See also L<perlport>. |
---|
3219 | |
---|
3220 | =item * |
---|
3221 | |
---|
3222 | Real numbers (floats and doubles) are in the native machine format only; |
---|
3223 | due to the multiplicity of floating formats around, and the lack of a |
---|
3224 | standard "network" representation, no facility for interchange has been |
---|
3225 | made. This means that packed floating point data written on one machine |
---|
3226 | may not be readable on another - even if both use IEEE floating point |
---|
3227 | arithmetic (as the endian-ness of the memory representation is not part |
---|
3228 | of the IEEE spec). See also L<perlport>. |
---|
3229 | |
---|
3230 | Note that Perl uses doubles internally for all numeric calculation, and |
---|
3231 | converting from double into float and thence back to double again will |
---|
3232 | lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general |
---|
3233 | equal $foo). |
---|
3234 | |
---|
3235 | =item * |
---|
3236 | |
---|
3237 | If the pattern begins with a C<U>, the resulting string will be treated |
---|
3238 | as Unicode-encoded. You can force UTF8 encoding on in a string with an |
---|
3239 | initial C<U0>, and the bytes that follow will be interpreted as Unicode |
---|
3240 | characters. If you don't want this to happen, you can begin your pattern |
---|
3241 | with C<C0> (or anything else) to force Perl not to UTF8 encode your |
---|
3242 | string, and then follow this with a C<U*> somewhere in your pattern. |
---|
3243 | |
---|
3244 | =item * |
---|
3245 | |
---|
3246 | You must yourself do any alignment or padding by inserting for example |
---|
3247 | enough C<'x'>es while packing. There is no way to pack() and unpack() |
---|
3248 | could know where the bytes are going to or coming from. Therefore |
---|
3249 | C<pack> (and C<unpack>) handle their output and input as flat |
---|
3250 | sequences of bytes. |
---|
3251 | |
---|
3252 | =item * |
---|
3253 | |
---|
3254 | A comment in a TEMPLATE starts with C<#> and goes to the end of line. |
---|
3255 | |
---|
3256 | =item * |
---|
3257 | |
---|
3258 | If TEMPLATE requires more arguments to pack() than actually given, pack() |
---|
3259 | assumes additional C<""> arguments. If TEMPLATE requires less arguments |
---|
3260 | to pack() than actually given, extra arguments are ignored. |
---|
3261 | |
---|
3262 | =back |
---|
3263 | |
---|
3264 | Examples: |
---|
3265 | |
---|
3266 | $foo = pack("CCCC",65,66,67,68); |
---|
3267 | # foo eq "ABCD" |
---|
3268 | $foo = pack("C4",65,66,67,68); |
---|
3269 | # same thing |
---|
3270 | $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9); |
---|
3271 | # same thing with Unicode circled letters |
---|
3272 | |
---|
3273 | $foo = pack("ccxxcc",65,66,67,68); |
---|
3274 | # foo eq "AB\0\0CD" |
---|
3275 | |
---|
3276 | # note: the above examples featuring "C" and "c" are true |
---|
3277 | # only on ASCII and ASCII-derived systems such as ISO Latin 1 |
---|
3278 | # and UTF-8. In EBCDIC the first example would be |
---|
3279 | # $foo = pack("CCCC",193,194,195,196); |
---|
3280 | |
---|
3281 | $foo = pack("s2",1,2); |
---|
3282 | # "\1\0\2\0" on little-endian |
---|
3283 | # "\0\1\0\2" on big-endian |
---|
3284 | |
---|
3285 | $foo = pack("a4","abcd","x","y","z"); |
---|
3286 | # "abcd" |
---|
3287 | |
---|
3288 | $foo = pack("aaaa","abcd","x","y","z"); |
---|
3289 | # "axyz" |
---|
3290 | |
---|
3291 | $foo = pack("a14","abcdefg"); |
---|
3292 | # "abcdefg\0\0\0\0\0\0\0" |
---|
3293 | |
---|
3294 | $foo = pack("i9pl", gmtime); |
---|
3295 | # a real struct tm (on my system anyway) |
---|
3296 | |
---|
3297 | $utmp_template = "Z8 Z8 Z16 L"; |
---|
3298 | $utmp = pack($utmp_template, @utmp1); |
---|
3299 | # a struct utmp (BSDish) |
---|
3300 | |
---|
3301 | @utmp2 = unpack($utmp_template, $utmp); |
---|
3302 | # "@utmp1" eq "@utmp2" |
---|
3303 | |
---|
3304 | sub bintodec { |
---|
3305 | unpack("N", pack("B32", substr("0" x 32 . shift, -32))); |
---|
3306 | } |
---|
3307 | |
---|
3308 | $foo = pack('sx2l', 12, 34); |
---|
3309 | # short 12, two zero bytes padding, long 34 |
---|
3310 | $bar = pack('s@4l', 12, 34); |
---|
3311 | # short 12, zero fill to position 4, long 34 |
---|
3312 | # $foo eq $bar |
---|
3313 | |
---|
3314 | The same template may generally also be used in unpack(). |
---|
3315 | |
---|
3316 | =item package NAMESPACE |
---|
3317 | |
---|
3318 | =item package |
---|
3319 | |
---|
3320 | Declares the compilation unit as being in the given namespace. The scope |
---|
3321 | of the package declaration is from the declaration itself through the end |
---|
3322 | of the enclosing block, file, or eval (the same as the C<my> operator). |
---|
3323 | All further unqualified dynamic identifiers will be in this namespace. |
---|
3324 | A package statement affects only dynamic variables--including those |
---|
3325 | you've used C<local> on--but I<not> lexical variables, which are created |
---|
3326 | with C<my>. Typically it would be the first declaration in a file to |
---|
3327 | be included by the C<require> or C<use> operator. You can switch into a |
---|
3328 | package in more than one place; it merely influences which symbol table |
---|
3329 | is used by the compiler for the rest of that block. You can refer to |
---|
3330 | variables and filehandles in other packages by prefixing the identifier |
---|
3331 | with the package name and a double colon: C<$Package::Variable>. |
---|
3332 | If the package name is null, the C<main> package as assumed. That is, |
---|
3333 | C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>, |
---|
3334 | still seen in older code). |
---|
3335 | |
---|
3336 | If NAMESPACE is omitted, then there is no current package, and all |
---|
3337 | identifiers must be fully qualified or lexicals. This is stricter |
---|
3338 | than C<use strict>, since it also extends to function names. |
---|
3339 | |
---|
3340 | See L<perlmod/"Packages"> for more information about packages, modules, |
---|
3341 | and classes. See L<perlsub> for other scoping issues. |
---|
3342 | |
---|
3343 | =item pipe READHANDLE,WRITEHANDLE |
---|
3344 | |
---|
3345 | Opens a pair of connected pipes like the corresponding system call. |
---|
3346 | Note that if you set up a loop of piped processes, deadlock can occur |
---|
3347 | unless you are very careful. In addition, note that Perl's pipes use |
---|
3348 | stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE |
---|
3349 | after each command, depending on the application. |
---|
3350 | |
---|
3351 | See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication"> |
---|
3352 | for examples of such things. |
---|
3353 | |
---|
3354 | On systems that support a close-on-exec flag on files, the flag will be set |
---|
3355 | for the newly opened file descriptors as determined by the value of $^F. |
---|
3356 | See L<perlvar/$^F>. |
---|
3357 | |
---|
3358 | =item pop ARRAY |
---|
3359 | |
---|
3360 | =item pop |
---|
3361 | |
---|
3362 | Pops and returns the last value of the array, shortening the array by |
---|
3363 | one element. Has an effect similar to |
---|
3364 | |
---|
3365 | $ARRAY[$#ARRAY--] |
---|
3366 | |
---|
3367 | If there are no elements in the array, returns the undefined value |
---|
3368 | (although this may happen at other times as well). If ARRAY is |
---|
3369 | omitted, pops the C<@ARGV> array in the main program, and the C<@_> |
---|
3370 | array in subroutines, just like C<shift>. |
---|
3371 | |
---|
3372 | =item pos SCALAR |
---|
3373 | |
---|
3374 | =item pos |
---|
3375 | |
---|
3376 | Returns the offset of where the last C<m//g> search left off for the variable |
---|
3377 | in question (C<$_> is used when the variable is not specified). May be |
---|
3378 | modified to change that offset. Such modification will also influence |
---|
3379 | the C<\G> zero-width assertion in regular expressions. See L<perlre> and |
---|
3380 | L<perlop>. |
---|
3381 | |
---|
3382 | =item print FILEHANDLE LIST |
---|
3383 | |
---|
3384 | =item print LIST |
---|
3385 | |
---|
3386 | =item print |
---|
3387 | |
---|
3388 | Prints a string or a list of strings. Returns true if successful. |
---|
3389 | FILEHANDLE may be a scalar variable name, in which case the variable |
---|
3390 | contains the name of or a reference to the filehandle, thus introducing |
---|
3391 | one level of indirection. (NOTE: If FILEHANDLE is a variable and |
---|
3392 | the next token is a term, it may be misinterpreted as an operator |
---|
3393 | unless you interpose a C<+> or put parentheses around the arguments.) |
---|
3394 | If FILEHANDLE is omitted, prints by default to standard output (or |
---|
3395 | to the last selected output channel--see L</select>). If LIST is |
---|
3396 | also omitted, prints C<$_> to the currently selected output channel. |
---|
3397 | To set the default output channel to something other than STDOUT |
---|
3398 | use the select operation. The current value of C<$,> (if any) is |
---|
3399 | printed between each LIST item. The current value of C<$\> (if |
---|
3400 | any) is printed after the entire LIST has been printed. Because |
---|
3401 | print takes a LIST, anything in the LIST is evaluated in list |
---|
3402 | context, and any subroutine that you call will have one or more of |
---|
3403 | its expressions evaluated in list context. Also be careful not to |
---|
3404 | follow the print keyword with a left parenthesis unless you want |
---|
3405 | the corresponding right parenthesis to terminate the arguments to |
---|
3406 | the print--interpose a C<+> or put parentheses around all the |
---|
3407 | arguments. |
---|
3408 | |
---|
3409 | Note that if you're storing FILEHANDLES in an array or other expression, |
---|
3410 | you will have to use a block returning its value instead: |
---|
3411 | |
---|
3412 | print { $files[$i] } "stuff\n"; |
---|
3413 | print { $OK ? STDOUT : STDERR } "stuff\n"; |
---|
3414 | |
---|
3415 | =item printf FILEHANDLE FORMAT, LIST |
---|
3416 | |
---|
3417 | =item printf FORMAT, LIST |
---|
3418 | |
---|
3419 | Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\> |
---|
3420 | (the output record separator) is not appended. The first argument |
---|
3421 | of the list will be interpreted as the C<printf> format. If C<use locale> is |
---|
3422 | in effect, the character used for the decimal point in formatted real numbers |
---|
3423 | is affected by the LC_NUMERIC locale. See L<perllocale>. |
---|
3424 | |
---|
3425 | Don't fall into the trap of using a C<printf> when a simple |
---|
3426 | C<print> would do. The C<print> is more efficient and less |
---|
3427 | error prone. |
---|
3428 | |
---|
3429 | =item prototype FUNCTION |
---|
3430 | |
---|
3431 | Returns the prototype of a function as a string (or C<undef> if the |
---|
3432 | function has no prototype). FUNCTION is a reference to, or the name of, |
---|
3433 | the function whose prototype you want to retrieve. |
---|
3434 | |
---|
3435 | If FUNCTION is a string starting with C<CORE::>, the rest is taken as a |
---|
3436 | name for Perl builtin. If the builtin is not I<overridable> (such as |
---|
3437 | C<qw//>) or its arguments cannot be expressed by a prototype (such as |
---|
3438 | C<system>) returns C<undef> because the builtin does not really behave |
---|
3439 | like a Perl function. Otherwise, the string describing the equivalent |
---|
3440 | prototype is returned. |
---|
3441 | |
---|
3442 | =item push ARRAY,LIST |
---|
3443 | |
---|
3444 | Treats ARRAY as a stack, and pushes the values of LIST |
---|
3445 | onto the end of ARRAY. The length of ARRAY increases by the length of |
---|
3446 | LIST. Has the same effect as |
---|
3447 | |
---|
3448 | for $value (LIST) { |
---|
3449 | $ARRAY[++$#ARRAY] = $value; |
---|
3450 | } |
---|
3451 | |
---|
3452 | but is more efficient. Returns the new number of elements in the array. |
---|
3453 | |
---|
3454 | =item q/STRING/ |
---|
3455 | |
---|
3456 | =item qq/STRING/ |
---|
3457 | |
---|
3458 | =item qr/STRING/ |
---|
3459 | |
---|
3460 | =item qx/STRING/ |
---|
3461 | |
---|
3462 | =item qw/STRING/ |
---|
3463 | |
---|
3464 | Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">. |
---|
3465 | |
---|
3466 | =item quotemeta EXPR |
---|
3467 | |
---|
3468 | =item quotemeta |
---|
3469 | |
---|
3470 | Returns the value of EXPR with all non-"word" |
---|
3471 | characters backslashed. (That is, all characters not matching |
---|
3472 | C</[A-Za-z_0-9]/> will be preceded by a backslash in the |
---|
3473 | returned string, regardless of any locale settings.) |
---|
3474 | This is the internal function implementing |
---|
3475 | the C<\Q> escape in double-quoted strings. |
---|
3476 | |
---|
3477 | If EXPR is omitted, uses C<$_>. |
---|
3478 | |
---|
3479 | =item rand EXPR |
---|
3480 | |
---|
3481 | =item rand |
---|
3482 | |
---|
3483 | Returns a random fractional number greater than or equal to C<0> and less |
---|
3484 | than the value of EXPR. (EXPR should be positive.) If EXPR is |
---|
3485 | omitted, the value C<1> is used. Automatically calls C<srand> unless |
---|
3486 | C<srand> has already been called. See also C<srand>. |
---|
3487 | |
---|
3488 | (Note: If your rand function consistently returns numbers that are too |
---|
3489 | large or too small, then your version of Perl was probably compiled |
---|
3490 | with the wrong number of RANDBITS.) |
---|
3491 | |
---|
3492 | =item read FILEHANDLE,SCALAR,LENGTH,OFFSET |
---|
3493 | |
---|
3494 | =item read FILEHANDLE,SCALAR,LENGTH |
---|
3495 | |
---|
3496 | Attempts to read LENGTH bytes of data into variable SCALAR from the |
---|
3497 | specified FILEHANDLE. Returns the number of bytes actually read, C<0> |
---|
3498 | at end of file, or undef if there was an error. SCALAR will be grown |
---|
3499 | or shrunk to the length actually read. If SCALAR needs growing, the |
---|
3500 | new bytes will be zero bytes. An OFFSET may be specified to place |
---|
3501 | the read data into some other place in SCALAR than the beginning. |
---|
3502 | The call is actually implemented in terms of stdio's fread(3) call. |
---|
3503 | To get a true read(2) system call, see C<sysread>. |
---|
3504 | |
---|
3505 | =item readdir DIRHANDLE |
---|
3506 | |
---|
3507 | Returns the next directory entry for a directory opened by C<opendir>. |
---|
3508 | If used in list context, returns all the rest of the entries in the |
---|
3509 | directory. If there are no more entries, returns an undefined value in |
---|
3510 | scalar context or a null list in list context. |
---|
3511 | |
---|
3512 | If you're planning to filetest the return values out of a C<readdir>, you'd |
---|
3513 | better prepend the directory in question. Otherwise, because we didn't |
---|
3514 | C<chdir> there, it would have been testing the wrong file. |
---|
3515 | |
---|
3516 | opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!"; |
---|
3517 | @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR); |
---|
3518 | closedir DIR; |
---|
3519 | |
---|
3520 | =item readline EXPR |
---|
3521 | |
---|
3522 | Reads from the filehandle whose typeglob is contained in EXPR. In scalar |
---|
3523 | context, each call reads and returns the next line, until end-of-file is |
---|
3524 | reached, whereupon the subsequent call returns undef. In list context, |
---|
3525 | reads until end-of-file is reached and returns a list of lines. Note that |
---|
3526 | the notion of "line" used here is however you may have defined it |
---|
3527 | with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">. |
---|
3528 | |
---|
3529 | When C<$/> is set to C<undef>, when readline() is in scalar |
---|
3530 | context (i.e. file slurp mode), and when an empty file is read, it |
---|
3531 | returns C<''> the first time, followed by C<undef> subsequently. |
---|
3532 | |
---|
3533 | This is the internal function implementing the C<< <EXPR> >> |
---|
3534 | operator, but you can use it directly. The C<< <EXPR> >> |
---|
3535 | operator is discussed in more detail in L<perlop/"I/O Operators">. |
---|
3536 | |
---|
3537 | $line = <STDIN>; |
---|
3538 | $line = readline(*STDIN); # same thing |
---|
3539 | |
---|
3540 | =item readlink EXPR |
---|
3541 | |
---|
3542 | =item readlink |
---|
3543 | |
---|
3544 | Returns the value of a symbolic link, if symbolic links are |
---|
3545 | implemented. If not, gives a fatal error. If there is some system |
---|
3546 | error, returns the undefined value and sets C<$!> (errno). If EXPR is |
---|
3547 | omitted, uses C<$_>. |
---|
3548 | |
---|
3549 | =item readpipe EXPR |
---|
3550 | |
---|
3551 | EXPR is executed as a system command. |
---|
3552 | The collected standard output of the command is returned. |
---|
3553 | In scalar context, it comes back as a single (potentially |
---|
3554 | multi-line) string. In list context, returns a list of lines |
---|
3555 | (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>). |
---|
3556 | This is the internal function implementing the C<qx/EXPR/> |
---|
3557 | operator, but you can use it directly. The C<qx/EXPR/> |
---|
3558 | operator is discussed in more detail in L<perlop/"I/O Operators">. |
---|
3559 | |
---|
3560 | =item recv SOCKET,SCALAR,LENGTH,FLAGS |
---|
3561 | |
---|
3562 | Receives a message on a socket. Attempts to receive LENGTH bytes of |
---|
3563 | data into variable SCALAR from the specified SOCKET filehandle. SCALAR |
---|
3564 | will be grown or shrunk to the length actually read. Takes the same |
---|
3565 | flags as the system call of the same name. Returns the address of the |
---|
3566 | sender if SOCKET's protocol supports this; returns an empty string |
---|
3567 | otherwise. If there's an error, returns the undefined value. This call |
---|
3568 | is actually implemented in terms of recvfrom(2) system call. See |
---|
3569 | L<perlipc/"UDP: Message Passing"> for examples. |
---|
3570 | |
---|
3571 | =item redo LABEL |
---|
3572 | |
---|
3573 | =item redo |
---|
3574 | |
---|
3575 | The C<redo> command restarts the loop block without evaluating the |
---|
3576 | conditional again. The C<continue> block, if any, is not executed. If |
---|
3577 | the LABEL is omitted, the command refers to the innermost enclosing |
---|
3578 | loop. This command is normally used by programs that want to lie to |
---|
3579 | themselves about what was just input: |
---|
3580 | |
---|
3581 | # a simpleminded Pascal comment stripper |
---|
3582 | # (warning: assumes no { or } in strings) |
---|
3583 | LINE: while (<STDIN>) { |
---|
3584 | while (s|({.*}.*){.*}|$1 |) {} |
---|
3585 | s|{.*}| |; |
---|
3586 | if (s|{.*| |) { |
---|
3587 | $front = $_; |
---|
3588 | while (<STDIN>) { |
---|
3589 | if (/}/) { # end of comment? |
---|
3590 | s|^|$front\{|; |
---|
3591 | redo LINE; |
---|
3592 | } |
---|
3593 | } |
---|
3594 | } |
---|
3595 | print; |
---|
3596 | } |
---|
3597 | |
---|
3598 | C<redo> cannot be used to retry a block which returns a value such as |
---|
3599 | C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit |
---|
3600 | a grep() or map() operation. |
---|
3601 | |
---|
3602 | Note that a block by itself is semantically identical to a loop |
---|
3603 | that executes once. Thus C<redo> inside such a block will effectively |
---|
3604 | turn it into a looping construct. |
---|
3605 | |
---|
3606 | See also L</continue> for an illustration of how C<last>, C<next>, and |
---|
3607 | C<redo> work. |
---|
3608 | |
---|
3609 | =item ref EXPR |
---|
3610 | |
---|
3611 | =item ref |
---|
3612 | |
---|
3613 | Returns a true value if EXPR is a reference, false otherwise. If EXPR |
---|
3614 | is not specified, C<$_> will be used. The value returned depends on the |
---|
3615 | type of thing the reference is a reference to. |
---|
3616 | Builtin types include: |
---|
3617 | |
---|
3618 | SCALAR |
---|
3619 | ARRAY |
---|
3620 | HASH |
---|
3621 | CODE |
---|
3622 | REF |
---|
3623 | GLOB |
---|
3624 | LVALUE |
---|
3625 | |
---|
3626 | If the referenced object has been blessed into a package, then that package |
---|
3627 | name is returned instead. You can think of C<ref> as a C<typeof> operator. |
---|
3628 | |
---|
3629 | if (ref($r) eq "HASH") { |
---|
3630 | print "r is a reference to a hash.\n"; |
---|
3631 | } |
---|
3632 | unless (ref($r)) { |
---|
3633 | print "r is not a reference at all.\n"; |
---|
3634 | } |
---|
3635 | if (UNIVERSAL::isa($r, "HASH")) { # for subclassing |
---|
3636 | print "r is a reference to something that isa hash.\n"; |
---|
3637 | } |
---|
3638 | |
---|
3639 | See also L<perlref>. |
---|
3640 | |
---|
3641 | =item rename OLDNAME,NEWNAME |
---|
3642 | |
---|
3643 | Changes the name of a file; an existing file NEWNAME will be |
---|
3644 | clobbered. Returns true for success, false otherwise. |
---|
3645 | |
---|
3646 | Behavior of this function varies wildly depending on your system |
---|
3647 | implementation. For example, it will usually not work across file system |
---|
3648 | boundaries, even though the system I<mv> command sometimes compensates |
---|
3649 | for this. Other restrictions include whether it works on directories, |
---|
3650 | open files, or pre-existing files. Check L<perlport> and either the |
---|
3651 | rename(2) manpage or equivalent system documentation for details. |
---|
3652 | |
---|
3653 | =item require VERSION |
---|
3654 | |
---|
3655 | =item require EXPR |
---|
3656 | |
---|
3657 | =item require |
---|
3658 | |
---|
3659 | Demands some semantics specified by EXPR, or by C<$_> if EXPR is not |
---|
3660 | supplied. |
---|
3661 | |
---|
3662 | If a VERSION is specified as a literal of the form v5.6.1, |
---|
3663 | demands that the current version of Perl (C<$^V> or $PERL_VERSION) be |
---|
3664 | at least as recent as that version, at run time. (For compatibility |
---|
3665 | with older versions of Perl, a numeric argument will also be interpreted |
---|
3666 | as VERSION.) Compare with L</use>, which can do a similar check at |
---|
3667 | compile time. |
---|
3668 | |
---|
3669 | require v5.6.1; # run time version check |
---|
3670 | require 5.6.1; # ditto |
---|
3671 | require 5.005_03; # float version allowed for compatibility |
---|
3672 | |
---|
3673 | Otherwise, demands that a library file be included if it hasn't already |
---|
3674 | been included. The file is included via the do-FILE mechanism, which is |
---|
3675 | essentially just a variety of C<eval>. Has semantics similar to the following |
---|
3676 | subroutine: |
---|
3677 | |
---|
3678 | sub require { |
---|
3679 | my($filename) = @_; |
---|
3680 | return 1 if $INC{$filename}; |
---|
3681 | my($realfilename,$result); |
---|
3682 | ITER: { |
---|
3683 | foreach $prefix (@INC) { |
---|
3684 | $realfilename = "$prefix/$filename"; |
---|
3685 | if (-f $realfilename) { |
---|
3686 | $INC{$filename} = $realfilename; |
---|
3687 | $result = do $realfilename; |
---|
3688 | last ITER; |
---|
3689 | } |
---|
3690 | } |
---|
3691 | die "Can't find $filename in \@INC"; |
---|
3692 | } |
---|
3693 | delete $INC{$filename} if $@ || !$result; |
---|
3694 | die $@ if $@; |
---|
3695 | die "$filename did not return true value" unless $result; |
---|
3696 | return $result; |
---|
3697 | } |
---|
3698 | |
---|
3699 | Note that the file will not be included twice under the same specified |
---|
3700 | name. The file must return true as the last statement to indicate |
---|
3701 | successful execution of any initialization code, so it's customary to |
---|
3702 | end such a file with C<1;> unless you're sure it'll return true |
---|
3703 | otherwise. But it's better just to put the C<1;>, in case you add more |
---|
3704 | statements. |
---|
3705 | |
---|
3706 | If EXPR is a bareword, the require assumes a "F<.pm>" extension and |
---|
3707 | replaces "F<::>" with "F</>" in the filename for you, |
---|
3708 | to make it easy to load standard modules. This form of loading of |
---|
3709 | modules does not risk altering your namespace. |
---|
3710 | |
---|
3711 | In other words, if you try this: |
---|
3712 | |
---|
3713 | require Foo::Bar; # a splendid bareword |
---|
3714 | |
---|
3715 | The require function will actually look for the "F<Foo/Bar.pm>" file in the |
---|
3716 | directories specified in the C<@INC> array. |
---|
3717 | |
---|
3718 | But if you try this: |
---|
3719 | |
---|
3720 | $class = 'Foo::Bar'; |
---|
3721 | require $class; # $class is not a bareword |
---|
3722 | #or |
---|
3723 | require "Foo::Bar"; # not a bareword because of the "" |
---|
3724 | |
---|
3725 | The require function will look for the "F<Foo::Bar>" file in the @INC array and |
---|
3726 | will complain about not finding "F<Foo::Bar>" there. In this case you can do: |
---|
3727 | |
---|
3728 | eval "require $class"; |
---|
3729 | |
---|
3730 | For a yet-more-powerful import facility, see L</use> and L<perlmod>. |
---|
3731 | |
---|
3732 | =item reset EXPR |
---|
3733 | |
---|
3734 | =item reset |
---|
3735 | |
---|
3736 | Generally used in a C<continue> block at the end of a loop to clear |
---|
3737 | variables and reset C<??> searches so that they work again. The |
---|
3738 | expression is interpreted as a list of single characters (hyphens |
---|
3739 | allowed for ranges). All variables and arrays beginning with one of |
---|
3740 | those letters are reset to their pristine state. If the expression is |
---|
3741 | omitted, one-match searches (C<?pattern?>) are reset to match again. Resets |
---|
3742 | only variables or searches in the current package. Always returns |
---|
3743 | 1. Examples: |
---|
3744 | |
---|
3745 | reset 'X'; # reset all X variables |
---|
3746 | reset 'a-z'; # reset lower case variables |
---|
3747 | reset; # just reset ?one-time? searches |
---|
3748 | |
---|
3749 | Resetting C<"A-Z"> is not recommended because you'll wipe out your |
---|
3750 | C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package |
---|
3751 | variables--lexical variables are unaffected, but they clean themselves |
---|
3752 | up on scope exit anyway, so you'll probably want to use them instead. |
---|
3753 | See L</my>. |
---|
3754 | |
---|
3755 | =item return EXPR |
---|
3756 | |
---|
3757 | =item return |
---|
3758 | |
---|
3759 | Returns from a subroutine, C<eval>, or C<do FILE> with the value |
---|
3760 | given in EXPR. Evaluation of EXPR may be in list, scalar, or void |
---|
3761 | context, depending on how the return value will be used, and the context |
---|
3762 | may vary from one execution to the next (see C<wantarray>). If no EXPR |
---|
3763 | is given, returns an empty list in list context, the undefined value in |
---|
3764 | scalar context, and (of course) nothing at all in a void context. |
---|
3765 | |
---|
3766 | (Note that in the absence of a explicit C<return>, a subroutine, eval, |
---|
3767 | or do FILE will automatically return the value of the last expression |
---|
3768 | evaluated.) |
---|
3769 | |
---|
3770 | =item reverse LIST |
---|
3771 | |
---|
3772 | In list context, returns a list value consisting of the elements |
---|
3773 | of LIST in the opposite order. In scalar context, concatenates the |
---|
3774 | elements of LIST and returns a string value with all characters |
---|
3775 | in the opposite order. |
---|
3776 | |
---|
3777 | print reverse <>; # line tac, last line first |
---|
3778 | |
---|
3779 | undef $/; # for efficiency of <> |
---|
3780 | print scalar reverse <>; # character tac, last line tsrif |
---|
3781 | |
---|
3782 | This operator is also handy for inverting a hash, although there are some |
---|
3783 | caveats. If a value is duplicated in the original hash, only one of those |
---|
3784 | can be represented as a key in the inverted hash. Also, this has to |
---|
3785 | unwind one hash and build a whole new one, which may take some time |
---|
3786 | on a large hash, such as from a DBM file. |
---|
3787 | |
---|
3788 | %by_name = reverse %by_address; # Invert the hash |
---|
3789 | |
---|
3790 | =item rewinddir DIRHANDLE |
---|
3791 | |
---|
3792 | Sets the current position to the beginning of the directory for the |
---|
3793 | C<readdir> routine on DIRHANDLE. |
---|
3794 | |
---|
3795 | =item rindex STR,SUBSTR,POSITION |
---|
3796 | |
---|
3797 | =item rindex STR,SUBSTR |
---|
3798 | |
---|
3799 | Works just like index() except that it returns the position of the LAST |
---|
3800 | occurrence of SUBSTR in STR. If POSITION is specified, returns the |
---|
3801 | last occurrence at or before that position. |
---|
3802 | |
---|
3803 | =item rmdir FILENAME |
---|
3804 | |
---|
3805 | =item rmdir |
---|
3806 | |
---|
3807 | Deletes the directory specified by FILENAME if that directory is empty. If it |
---|
3808 | succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If |
---|
3809 | FILENAME is omitted, uses C<$_>. |
---|
3810 | |
---|
3811 | =item s/// |
---|
3812 | |
---|
3813 | The substitution operator. See L<perlop>. |
---|
3814 | |
---|
3815 | =item scalar EXPR |
---|
3816 | |
---|
3817 | Forces EXPR to be interpreted in scalar context and returns the value |
---|
3818 | of EXPR. |
---|
3819 | |
---|
3820 | @counts = ( scalar @a, scalar @b, scalar @c ); |
---|
3821 | |
---|
3822 | There is no equivalent operator to force an expression to |
---|
3823 | be interpolated in list context because in practice, this is never |
---|
3824 | needed. If you really wanted to do so, however, you could use |
---|
3825 | the construction C<@{[ (some expression) ]}>, but usually a simple |
---|
3826 | C<(some expression)> suffices. |
---|
3827 | |
---|
3828 | Because C<scalar> is unary operator, if you accidentally use for EXPR a |
---|
3829 | parenthesized list, this behaves as a scalar comma expression, evaluating |
---|
3830 | all but the last element in void context and returning the final element |
---|
3831 | evaluated in scalar context. This is seldom what you want. |
---|
3832 | |
---|
3833 | The following single statement: |
---|
3834 | |
---|
3835 | print uc(scalar(&foo,$bar)),$baz; |
---|
3836 | |
---|
3837 | is the moral equivalent of these two: |
---|
3838 | |
---|
3839 | &foo; |
---|
3840 | print(uc($bar),$baz); |
---|
3841 | |
---|
3842 | See L<perlop> for more details on unary operators and the comma operator. |
---|
3843 | |
---|
3844 | =item seek FILEHANDLE,POSITION,WHENCE |
---|
3845 | |
---|
3846 | Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>. |
---|
3847 | FILEHANDLE may be an expression whose value gives the name of the |
---|
3848 | filehandle. The values for WHENCE are C<0> to set the new position to |
---|
3849 | POSITION, C<1> to set it to the current position plus POSITION, and |
---|
3850 | C<2> to set it to EOF plus POSITION (typically negative). For WHENCE |
---|
3851 | you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> |
---|
3852 | (start of the file, current position, end of the file) from the Fcntl |
---|
3853 | module. Returns C<1> upon success, C<0> otherwise. |
---|
3854 | |
---|
3855 | If you want to position file for C<sysread> or C<syswrite>, don't use |
---|
3856 | C<seek>--buffering makes its effect on the file's system position |
---|
3857 | unpredictable and non-portable. Use C<sysseek> instead. |
---|
3858 | |
---|
3859 | Due to the rules and rigors of ANSI C, on some systems you have to do a |
---|
3860 | seek whenever you switch between reading and writing. Amongst other |
---|
3861 | things, this may have the effect of calling stdio's clearerr(3). |
---|
3862 | A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position: |
---|
3863 | |
---|
3864 | seek(TEST,0,1); |
---|
3865 | |
---|
3866 | This is also useful for applications emulating C<tail -f>. Once you hit |
---|
3867 | EOF on your read, and then sleep for a while, you might have to stick in a |
---|
3868 | seek() to reset things. The C<seek> doesn't change the current position, |
---|
3869 | but it I<does> clear the end-of-file condition on the handle, so that the |
---|
3870 | next C<< <FILE> >> makes Perl try again to read something. We hope. |
---|
3871 | |
---|
3872 | If that doesn't work (some stdios are particularly cantankerous), then |
---|
3873 | you may need something more like this: |
---|
3874 | |
---|
3875 | for (;;) { |
---|
3876 | for ($curpos = tell(FILE); $_ = <FILE>; |
---|
3877 | $curpos = tell(FILE)) { |
---|
3878 | # search for some stuff and put it into files |
---|
3879 | } |
---|
3880 | sleep($for_a_while); |
---|
3881 | seek(FILE, $curpos, 0); |
---|
3882 | } |
---|
3883 | |
---|
3884 | =item seekdir DIRHANDLE,POS |
---|
3885 | |
---|
3886 | Sets the current position for the C<readdir> routine on DIRHANDLE. POS |
---|
3887 | must be a value returned by C<telldir>. Has the same caveats about |
---|
3888 | possible directory compaction as the corresponding system library |
---|
3889 | routine. |
---|
3890 | |
---|
3891 | =item select FILEHANDLE |
---|
3892 | |
---|
3893 | =item select |
---|
3894 | |
---|
3895 | Returns the currently selected filehandle. Sets the current default |
---|
3896 | filehandle for output, if FILEHANDLE is supplied. This has two |
---|
3897 | effects: first, a C<write> or a C<print> without a filehandle will |
---|
3898 | default to this FILEHANDLE. Second, references to variables related to |
---|
3899 | output will refer to this output channel. For example, if you have to |
---|
3900 | set the top of form format for more than one output channel, you might |
---|
3901 | do the following: |
---|
3902 | |
---|
3903 | select(REPORT1); |
---|
3904 | $^ = 'report1_top'; |
---|
3905 | select(REPORT2); |
---|
3906 | $^ = 'report2_top'; |
---|
3907 | |
---|
3908 | FILEHANDLE may be an expression whose value gives the name of the |
---|
3909 | actual filehandle. Thus: |
---|
3910 | |
---|
3911 | $oldfh = select(STDERR); $| = 1; select($oldfh); |
---|
3912 | |
---|
3913 | Some programmers may prefer to think of filehandles as objects with |
---|
3914 | methods, preferring to write the last example as: |
---|
3915 | |
---|
3916 | use IO::Handle; |
---|
3917 | STDERR->autoflush(1); |
---|
3918 | |
---|
3919 | =item select RBITS,WBITS,EBITS,TIMEOUT |
---|
3920 | |
---|
3921 | This calls the select(2) system call with the bit masks specified, which |
---|
3922 | can be constructed using C<fileno> and C<vec>, along these lines: |
---|
3923 | |
---|
3924 | $rin = $win = $ein = ''; |
---|
3925 | vec($rin,fileno(STDIN),1) = 1; |
---|
3926 | vec($win,fileno(STDOUT),1) = 1; |
---|
3927 | $ein = $rin | $win; |
---|
3928 | |
---|
3929 | If you want to select on many filehandles you might wish to write a |
---|
3930 | subroutine: |
---|
3931 | |
---|
3932 | sub fhbits { |
---|
3933 | my(@fhlist) = split(' ',$_[0]); |
---|
3934 | my($bits); |
---|
3935 | for (@fhlist) { |
---|
3936 | vec($bits,fileno($_),1) = 1; |
---|
3937 | } |
---|
3938 | $bits; |
---|
3939 | } |
---|
3940 | $rin = fhbits('STDIN TTY SOCK'); |
---|
3941 | |
---|
3942 | The usual idiom is: |
---|
3943 | |
---|
3944 | ($nfound,$timeleft) = |
---|
3945 | select($rout=$rin, $wout=$win, $eout=$ein, $timeout); |
---|
3946 | |
---|
3947 | or to block until something becomes ready just do this |
---|
3948 | |
---|
3949 | $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef); |
---|
3950 | |
---|
3951 | Most systems do not bother to return anything useful in $timeleft, so |
---|
3952 | calling select() in scalar context just returns $nfound. |
---|
3953 | |
---|
3954 | Any of the bit masks can also be undef. The timeout, if specified, is |
---|
3955 | in seconds, which may be fractional. Note: not all implementations are |
---|
3956 | capable of returning the$timeleft. If not, they always return |
---|
3957 | $timeleft equal to the supplied $timeout. |
---|
3958 | |
---|
3959 | You can effect a sleep of 250 milliseconds this way: |
---|
3960 | |
---|
3961 | select(undef, undef, undef, 0.25); |
---|
3962 | |
---|
3963 | B<WARNING>: One should not attempt to mix buffered I/O (like C<read> |
---|
3964 | or <FH>) with C<select>, except as permitted by POSIX, and even |
---|
3965 | then only on POSIX systems. You have to use C<sysread> instead. |
---|
3966 | |
---|
3967 | =item semctl ID,SEMNUM,CMD,ARG |
---|
3968 | |
---|
3969 | Calls the System V IPC function C<semctl>. You'll probably have to say |
---|
3970 | |
---|
3971 | use IPC::SysV; |
---|
3972 | |
---|
3973 | first to get the correct constant definitions. If CMD is IPC_STAT or |
---|
3974 | GETALL, then ARG must be a variable which will hold the returned |
---|
3975 | semid_ds structure or semaphore value array. Returns like C<ioctl>: |
---|
3976 | the undefined value for error, "C<0 but true>" for zero, or the actual |
---|
3977 | return value otherwise. The ARG must consist of a vector of native |
---|
3978 | short integers, which may be created with C<pack("s!",(0)x$nsem)>. |
---|
3979 | See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore> |
---|
3980 | documentation. |
---|
3981 | |
---|
3982 | =item semget KEY,NSEMS,FLAGS |
---|
3983 | |
---|
3984 | Calls the System V IPC function semget. Returns the semaphore id, or |
---|
3985 | the undefined value if there is an error. See also |
---|
3986 | L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore> |
---|
3987 | documentation. |
---|
3988 | |
---|
3989 | =item semop KEY,OPSTRING |
---|
3990 | |
---|
3991 | Calls the System V IPC function semop to perform semaphore operations |
---|
3992 | such as signaling and waiting. OPSTRING must be a packed array of |
---|
3993 | semop structures. Each semop structure can be generated with |
---|
3994 | C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore |
---|
3995 | operations is implied by the length of OPSTRING. Returns true if |
---|
3996 | successful, or false if there is an error. As an example, the |
---|
3997 | following code waits on semaphore $semnum of semaphore id $semid: |
---|
3998 | |
---|
3999 | $semop = pack("sss", $semnum, -1, 0); |
---|
4000 | die "Semaphore trouble: $!\n" unless semop($semid, $semop); |
---|
4001 | |
---|
4002 | To signal the semaphore, replace C<-1> with C<1>. See also |
---|
4003 | L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore> |
---|
4004 | documentation. |
---|
4005 | |
---|
4006 | =item send SOCKET,MSG,FLAGS,TO |
---|
4007 | |
---|
4008 | =item send SOCKET,MSG,FLAGS |
---|
4009 | |
---|
4010 | Sends a message on a socket. Takes the same flags as the system call |
---|
4011 | of the same name. On unconnected sockets you must specify a |
---|
4012 | destination to send TO, in which case it does a C C<sendto>. Returns |
---|
4013 | the number of characters sent, or the undefined value if there is an |
---|
4014 | error. The C system call sendmsg(2) is currently unimplemented. |
---|
4015 | See L<perlipc/"UDP: Message Passing"> for examples. |
---|
4016 | |
---|
4017 | =item setpgrp PID,PGRP |
---|
4018 | |
---|
4019 | Sets the current process group for the specified PID, C<0> for the current |
---|
4020 | process. Will produce a fatal error if used on a machine that doesn't |
---|
4021 | implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted, |
---|
4022 | it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not |
---|
4023 | accept any arguments, so only C<setpgrp(0,0)> is portable. See also |
---|
4024 | C<POSIX::setsid()>. |
---|
4025 | |
---|
4026 | =item setpriority WHICH,WHO,PRIORITY |
---|
4027 | |
---|
4028 | Sets the current priority for a process, a process group, or a user. |
---|
4029 | (See setpriority(2).) Will produce a fatal error if used on a machine |
---|
4030 | that doesn't implement setpriority(2). |
---|
4031 | |
---|
4032 | =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL |
---|
4033 | |
---|
4034 | Sets the socket option requested. Returns undefined if there is an |
---|
4035 | error. OPTVAL may be specified as C<undef> if you don't want to pass an |
---|
4036 | argument. |
---|
4037 | |
---|
4038 | =item shift ARRAY |
---|
4039 | |
---|
4040 | =item shift |
---|
4041 | |
---|
4042 | Shifts the first value of the array off and returns it, shortening the |
---|
4043 | array by 1 and moving everything down. If there are no elements in the |
---|
4044 | array, returns the undefined value. If ARRAY is omitted, shifts the |
---|
4045 | C<@_> array within the lexical scope of subroutines and formats, and the |
---|
4046 | C<@ARGV> array at file scopes or within the lexical scopes established by |
---|
4047 | the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}> |
---|
4048 | constructs. |
---|
4049 | |
---|
4050 | See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the |
---|
4051 | same thing to the left end of an array that C<pop> and C<push> do to the |
---|
4052 | right end. |
---|
4053 | |
---|
4054 | =item shmctl ID,CMD,ARG |
---|
4055 | |
---|
4056 | Calls the System V IPC function shmctl. You'll probably have to say |
---|
4057 | |
---|
4058 | use IPC::SysV; |
---|
4059 | |
---|
4060 | first to get the correct constant definitions. If CMD is C<IPC_STAT>, |
---|
4061 | then ARG must be a variable which will hold the returned C<shmid_ds> |
---|
4062 | structure. Returns like ioctl: the undefined value for error, "C<0> but |
---|
4063 | true" for zero, or the actual return value otherwise. |
---|
4064 | See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. |
---|
4065 | |
---|
4066 | =item shmget KEY,SIZE,FLAGS |
---|
4067 | |
---|
4068 | Calls the System V IPC function shmget. Returns the shared memory |
---|
4069 | segment id, or the undefined value if there is an error. |
---|
4070 | See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation. |
---|
4071 | |
---|
4072 | =item shmread ID,VAR,POS,SIZE |
---|
4073 | |
---|
4074 | =item shmwrite ID,STRING,POS,SIZE |
---|
4075 | |
---|
4076 | Reads or writes the System V shared memory segment ID starting at |
---|
4077 | position POS for size SIZE by attaching to it, copying in/out, and |
---|
4078 | detaching from it. When reading, VAR must be a variable that will |
---|
4079 | hold the data read. When writing, if STRING is too long, only SIZE |
---|
4080 | bytes are used; if STRING is too short, nulls are written to fill out |
---|
4081 | SIZE bytes. Return true if successful, or false if there is an error. |
---|
4082 | shmread() taints the variable. See also L<perlipc/"SysV IPC">, |
---|
4083 | C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN. |
---|
4084 | |
---|
4085 | =item shutdown SOCKET,HOW |
---|
4086 | |
---|
4087 | Shuts down a socket connection in the manner indicated by HOW, which |
---|
4088 | has the same interpretation as in the system call of the same name. |
---|
4089 | |
---|
4090 | shutdown(SOCKET, 0); # I/we have stopped reading data |
---|
4091 | shutdown(SOCKET, 1); # I/we have stopped writing data |
---|
4092 | shutdown(SOCKET, 2); # I/we have stopped using this socket |
---|
4093 | |
---|
4094 | This is useful with sockets when you want to tell the other |
---|
4095 | side you're done writing but not done reading, or vice versa. |
---|
4096 | It's also a more insistent form of close because it also |
---|
4097 | disables the file descriptor in any forked copies in other |
---|
4098 | processes. |
---|
4099 | |
---|
4100 | =item sin EXPR |
---|
4101 | |
---|
4102 | =item sin |
---|
4103 | |
---|
4104 | Returns the sine of EXPR (expressed in radians). If EXPR is omitted, |
---|
4105 | returns sine of C<$_>. |
---|
4106 | |
---|
4107 | For the inverse sine operation, you may use the C<Math::Trig::asin> |
---|
4108 | function, or use this relation: |
---|
4109 | |
---|
4110 | sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) } |
---|
4111 | |
---|
4112 | =item sleep EXPR |
---|
4113 | |
---|
4114 | =item sleep |
---|
4115 | |
---|
4116 | Causes the script to sleep for EXPR seconds, or forever if no EXPR. |
---|
4117 | May be interrupted if the process receives a signal such as C<SIGALRM>. |
---|
4118 | Returns the number of seconds actually slept. You probably cannot |
---|
4119 | mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented |
---|
4120 | using C<alarm>. |
---|
4121 | |
---|
4122 | On some older systems, it may sleep up to a full second less than what |
---|
4123 | you requested, depending on how it counts seconds. Most modern systems |
---|
4124 | always sleep the full amount. They may appear to sleep longer than that, |
---|
4125 | however, because your process might not be scheduled right away in a |
---|
4126 | busy multitasking system. |
---|
4127 | |
---|
4128 | For delays of finer granularity than one second, you may use Perl's |
---|
4129 | C<syscall> interface to access setitimer(2) if your system supports |
---|
4130 | it, or else see L</select> above. The Time::HiRes module from CPAN |
---|
4131 | may also help. |
---|
4132 | |
---|
4133 | See also the POSIX module's C<pause> function. |
---|
4134 | |
---|
4135 | =item socket SOCKET,DOMAIN,TYPE,PROTOCOL |
---|
4136 | |
---|
4137 | Opens a socket of the specified kind and attaches it to filehandle |
---|
4138 | SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for |
---|
4139 | the system call of the same name. You should C<use Socket> first |
---|
4140 | to get the proper definitions imported. See the examples in |
---|
4141 | L<perlipc/"Sockets: Client/Server Communication">. |
---|
4142 | |
---|
4143 | On systems that support a close-on-exec flag on files, the flag will |
---|
4144 | be set for the newly opened file descriptor, as determined by the |
---|
4145 | value of $^F. See L<perlvar/$^F>. |
---|
4146 | |
---|
4147 | =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL |
---|
4148 | |
---|
4149 | Creates an unnamed pair of sockets in the specified domain, of the |
---|
4150 | specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as |
---|
4151 | for the system call of the same name. If unimplemented, yields a fatal |
---|
4152 | error. Returns true if successful. |
---|
4153 | |
---|
4154 | On systems that support a close-on-exec flag on files, the flag will |
---|
4155 | be set for the newly opened file descriptors, as determined by the value |
---|
4156 | of $^F. See L<perlvar/$^F>. |
---|
4157 | |
---|
4158 | Some systems defined C<pipe> in terms of C<socketpair>, in which a call |
---|
4159 | to C<pipe(Rdr, Wtr)> is essentially: |
---|
4160 | |
---|
4161 | use Socket; |
---|
4162 | socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC); |
---|
4163 | shutdown(Rdr, 1); # no more writing for reader |
---|
4164 | shutdown(Wtr, 0); # no more reading for writer |
---|
4165 | |
---|
4166 | See L<perlipc> for an example of socketpair use. |
---|
4167 | |
---|
4168 | =item sort SUBNAME LIST |
---|
4169 | |
---|
4170 | =item sort BLOCK LIST |
---|
4171 | |
---|
4172 | =item sort LIST |
---|
4173 | |
---|
4174 | Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK |
---|
4175 | is omitted, C<sort>s in standard string comparison order. If SUBNAME is |
---|
4176 | specified, it gives the name of a subroutine that returns an integer |
---|
4177 | less than, equal to, or greater than C<0>, depending on how the elements |
---|
4178 | of the list are to be ordered. (The C<< <=> >> and C<cmp> |
---|
4179 | operators are extremely useful in such routines.) SUBNAME may be a |
---|
4180 | scalar variable name (unsubscripted), in which case the value provides |
---|
4181 | the name of (or a reference to) the actual subroutine to use. In place |
---|
4182 | of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort |
---|
4183 | subroutine. |
---|
4184 | |
---|
4185 | If the subroutine's prototype is C<($$)>, the elements to be compared |
---|
4186 | are passed by reference in C<@_>, as for a normal subroutine. This is |
---|
4187 | slower than unprototyped subroutines, where the elements to be |
---|
4188 | compared are passed into the subroutine |
---|
4189 | as the package global variables $a and $b (see example below). Note that |
---|
4190 | in the latter case, it is usually counter-productive to declare $a and |
---|
4191 | $b as lexicals. |
---|
4192 | |
---|
4193 | In either case, the subroutine may not be recursive. The values to be |
---|
4194 | compared are always passed by reference, so don't modify them. |
---|
4195 | |
---|
4196 | You also cannot exit out of the sort block or subroutine using any of the |
---|
4197 | loop control operators described in L<perlsyn> or with C<goto>. |
---|
4198 | |
---|
4199 | When C<use locale> is in effect, C<sort LIST> sorts LIST according to the |
---|
4200 | current collation locale. See L<perllocale>. |
---|
4201 | |
---|
4202 | Examples: |
---|
4203 | |
---|
4204 | # sort lexically |
---|
4205 | @articles = sort @files; |
---|
4206 | |
---|
4207 | # same thing, but with explicit sort routine |
---|
4208 | @articles = sort {$a cmp $b} @files; |
---|
4209 | |
---|
4210 | # now case-insensitively |
---|
4211 | @articles = sort {uc($a) cmp uc($b)} @files; |
---|
4212 | |
---|
4213 | # same thing in reversed order |
---|
4214 | @articles = sort {$b cmp $a} @files; |
---|
4215 | |
---|
4216 | # sort numerically ascending |
---|
4217 | @articles = sort {$a <=> $b} @files; |
---|
4218 | |
---|
4219 | # sort numerically descending |
---|
4220 | @articles = sort {$b <=> $a} @files; |
---|
4221 | |
---|
4222 | # this sorts the %age hash by value instead of key |
---|
4223 | # using an in-line function |
---|
4224 | @eldest = sort { $age{$b} <=> $age{$a} } keys %age; |
---|
4225 | |
---|
4226 | # sort using explicit subroutine name |
---|
4227 | sub byage { |
---|
4228 | $age{$a} <=> $age{$b}; # presuming numeric |
---|
4229 | } |
---|
4230 | @sortedclass = sort byage @class; |
---|
4231 | |
---|
4232 | sub backwards { $b cmp $a } |
---|
4233 | @harry = qw(dog cat x Cain Abel); |
---|
4234 | @george = qw(gone chased yz Punished Axed); |
---|
4235 | print sort @harry; |
---|
4236 | # prints AbelCaincatdogx |
---|
4237 | print sort backwards @harry; |
---|
4238 | # prints xdogcatCainAbel |
---|
4239 | print sort @george, 'to', @harry; |
---|
4240 | # prints AbelAxedCainPunishedcatchaseddoggonetoxyz |
---|
4241 | |
---|
4242 | # inefficiently sort by descending numeric compare using |
---|
4243 | # the first integer after the first = sign, or the |
---|
4244 | # whole record case-insensitively otherwise |
---|
4245 | |
---|
4246 | @new = sort { |
---|
4247 | ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0] |
---|
4248 | || |
---|
4249 | uc($a) cmp uc($b) |
---|
4250 | } @old; |
---|
4251 | |
---|
4252 | # same thing, but much more efficiently; |
---|
4253 | # we'll build auxiliary indices instead |
---|
4254 | # for speed |
---|
4255 | @nums = @caps = (); |
---|
4256 | for (@old) { |
---|
4257 | push @nums, /=(\d+)/; |
---|
4258 | push @caps, uc($_); |
---|
4259 | } |
---|
4260 | |
---|
4261 | @new = @old[ sort { |
---|
4262 | $nums[$b] <=> $nums[$a] |
---|
4263 | || |
---|
4264 | $caps[$a] cmp $caps[$b] |
---|
4265 | } 0..$#old |
---|
4266 | ]; |
---|
4267 | |
---|
4268 | # same thing, but without any temps |
---|
4269 | @new = map { $_->[0] } |
---|
4270 | sort { $b->[1] <=> $a->[1] |
---|
4271 | || |
---|
4272 | $a->[2] cmp $b->[2] |
---|
4273 | } map { [$_, /=(\d+)/, uc($_)] } @old; |
---|
4274 | |
---|
4275 | # using a prototype allows you to use any comparison subroutine |
---|
4276 | # as a sort subroutine (including other package's subroutines) |
---|
4277 | package other; |
---|
4278 | sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here |
---|
4279 | |
---|
4280 | package main; |
---|
4281 | @new = sort other::backwards @old; |
---|
4282 | |
---|
4283 | If you're using strict, you I<must not> declare $a |
---|
4284 | and $b as lexicals. They are package globals. That means |
---|
4285 | if you're in the C<main> package and type |
---|
4286 | |
---|
4287 | @articles = sort {$b <=> $a} @files; |
---|
4288 | |
---|
4289 | then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>), |
---|
4290 | but if you're in the C<FooPack> package, it's the same as typing |
---|
4291 | |
---|
4292 | @articles = sort {$FooPack::b <=> $FooPack::a} @files; |
---|
4293 | |
---|
4294 | The comparison function is required to behave. If it returns |
---|
4295 | inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and |
---|
4296 | sometimes saying the opposite, for example) the results are not |
---|
4297 | well-defined. |
---|
4298 | |
---|
4299 | =item splice ARRAY,OFFSET,LENGTH,LIST |
---|
4300 | |
---|
4301 | =item splice ARRAY,OFFSET,LENGTH |
---|
4302 | |
---|
4303 | =item splice ARRAY,OFFSET |
---|
4304 | |
---|
4305 | =item splice ARRAY |
---|
4306 | |
---|
4307 | Removes the elements designated by OFFSET and LENGTH from an array, and |
---|
4308 | replaces them with the elements of LIST, if any. In list context, |
---|
4309 | returns the elements removed from the array. In scalar context, |
---|
4310 | returns the last element removed, or C<undef> if no elements are |
---|
4311 | removed. The array grows or shrinks as necessary. |
---|
4312 | If OFFSET is negative then it starts that far from the end of the array. |
---|
4313 | If LENGTH is omitted, removes everything from OFFSET onward. |
---|
4314 | If LENGTH is negative, leaves that many elements off the end of the array. |
---|
4315 | If both OFFSET and LENGTH are omitted, removes everything. |
---|
4316 | |
---|
4317 | The following equivalences hold (assuming C<$[ == 0>): |
---|
4318 | |
---|
4319 | push(@a,$x,$y) splice(@a,@a,0,$x,$y) |
---|
4320 | pop(@a) splice(@a,-1) |
---|
4321 | shift(@a) splice(@a,0,1) |
---|
4322 | unshift(@a,$x,$y) splice(@a,0,0,$x,$y) |
---|
4323 | $a[$x] = $y splice(@a,$x,1,$y) |
---|
4324 | |
---|
4325 | Example, assuming array lengths are passed before arrays: |
---|
4326 | |
---|
4327 | sub aeq { # compare two list values |
---|
4328 | my(@a) = splice(@_,0,shift); |
---|
4329 | my(@b) = splice(@_,0,shift); |
---|
4330 | return 0 unless @a == @b; # same len? |
---|
4331 | while (@a) { |
---|
4332 | return 0 if pop(@a) ne pop(@b); |
---|
4333 | } |
---|
4334 | return 1; |
---|
4335 | } |
---|
4336 | if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... } |
---|
4337 | |
---|
4338 | =item split /PATTERN/,EXPR,LIMIT |
---|
4339 | |
---|
4340 | =item split /PATTERN/,EXPR |
---|
4341 | |
---|
4342 | =item split /PATTERN/ |
---|
4343 | |
---|
4344 | =item split |
---|
4345 | |
---|
4346 | Splits a string into a list of strings and returns that list. By default, |
---|
4347 | empty leading fields are preserved, and empty trailing ones are deleted. |
---|
4348 | |
---|
4349 | In scalar context, returns the number of fields found and splits into |
---|
4350 | the C<@_> array. Use of split in scalar context is deprecated, however, |
---|
4351 | because it clobbers your subroutine arguments. |
---|
4352 | |
---|
4353 | If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted, |
---|
4354 | splits on whitespace (after skipping any leading whitespace). Anything |
---|
4355 | matching PATTERN is taken to be a delimiter separating the fields. (Note |
---|
4356 | that the delimiter may be longer than one character.) |
---|
4357 | |
---|
4358 | If LIMIT is specified and positive, splits into no more than that |
---|
4359 | many fields (though it may split into fewer). If LIMIT is unspecified |
---|
4360 | or zero, trailing null fields are stripped (which potential users |
---|
4361 | of C<pop> would do well to remember). If LIMIT is negative, it is |
---|
4362 | treated as if an arbitrarily large LIMIT had been specified. |
---|
4363 | |
---|
4364 | A pattern matching the null string (not to be confused with |
---|
4365 | a null pattern C<//>, which is just one member of the set of patterns |
---|
4366 | matching a null string) will split the value of EXPR into separate |
---|
4367 | characters at each point it matches that way. For example: |
---|
4368 | |
---|
4369 | print join(':', split(/ */, 'hi there')); |
---|
4370 | |
---|
4371 | produces the output 'h:i:t:h:e:r:e'. |
---|
4372 | |
---|
4373 | Empty leading (or trailing) fields are produced when there positive width |
---|
4374 | matches at the beginning (or end) of the string; a zero-width match at the |
---|
4375 | beginning (or end) of the string does not produce an empty field. For |
---|
4376 | example: |
---|
4377 | |
---|
4378 | print join(':', split(/(?=\w)/, 'hi there!')); |
---|
4379 | |
---|
4380 | produces the output 'h:i :t:h:e:r:e!'. |
---|
4381 | |
---|
4382 | The LIMIT parameter can be used to split a line partially |
---|
4383 | |
---|
4384 | ($login, $passwd, $remainder) = split(/:/, $_, 3); |
---|
4385 | |
---|
4386 | When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT |
---|
4387 | one larger than the number of variables in the list, to avoid |
---|
4388 | unnecessary work. For the list above LIMIT would have been 4 by |
---|
4389 | default. In time critical applications it behooves you not to split |
---|
4390 | into more fields than you really need. |
---|
4391 | |
---|
4392 | If the PATTERN contains parentheses, additional list elements are |
---|
4393 | created from each matching substring in the delimiter. |
---|
4394 | |
---|
4395 | split(/([,-])/, "1-10,20", 3); |
---|
4396 | |
---|
4397 | produces the list value |
---|
4398 | |
---|
4399 | (1, '-', 10, ',', 20) |
---|
4400 | |
---|
4401 | If you had the entire header of a normal Unix email message in $header, |
---|
4402 | you could split it up into fields and their values this way: |
---|
4403 | |
---|
4404 | $header =~ s/\n\s+/ /g; # fix continuation lines |
---|
4405 | %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header); |
---|
4406 | |
---|
4407 | The pattern C</PATTERN/> may be replaced with an expression to specify |
---|
4408 | patterns that vary at runtime. (To do runtime compilation only once, |
---|
4409 | use C</$variable/o>.) |
---|
4410 | |
---|
4411 | As a special case, specifying a PATTERN of space (C<' '>) will split on |
---|
4412 | white space just as C<split> with no arguments does. Thus, C<split(' ')> can |
---|
4413 | be used to emulate B<awk>'s default behavior, whereas C<split(/ /)> |
---|
4414 | will give you as many null initial fields as there are leading spaces. |
---|
4415 | A C<split> on C</\s+/> is like a C<split(' ')> except that any leading |
---|
4416 | whitespace produces a null first field. A C<split> with no arguments |
---|
4417 | really does a C<split(' ', $_)> internally. |
---|
4418 | |
---|
4419 | A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't |
---|
4420 | much use otherwise. |
---|
4421 | |
---|
4422 | Example: |
---|
4423 | |
---|
4424 | open(PASSWD, '/etc/passwd'); |
---|
4425 | while (<PASSWD>) { |
---|
4426 | chomp; |
---|
4427 | ($login, $passwd, $uid, $gid, |
---|
4428 | $gcos, $home, $shell) = split(/:/); |
---|
4429 | #... |
---|
4430 | } |
---|
4431 | |
---|
4432 | |
---|
4433 | =item sprintf FORMAT, LIST |
---|
4434 | |
---|
4435 | Returns a string formatted by the usual C<printf> conventions of the C |
---|
4436 | library function C<sprintf>. See below for more details |
---|
4437 | and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of |
---|
4438 | the general principles. |
---|
4439 | |
---|
4440 | For example: |
---|
4441 | |
---|
4442 | # Format number with up to 8 leading zeroes |
---|
4443 | $result = sprintf("%08d", $number); |
---|
4444 | |
---|
4445 | # Round number to 3 digits after decimal point |
---|
4446 | $rounded = sprintf("%.3f", $number); |
---|
4447 | |
---|
4448 | Perl does its own C<sprintf> formatting--it emulates the C |
---|
4449 | function C<sprintf>, but it doesn't use it (except for floating-point |
---|
4450 | numbers, and even then only the standard modifiers are allowed). As a |
---|
4451 | result, any non-standard extensions in your local C<sprintf> are not |
---|
4452 | available from Perl. |
---|
4453 | |
---|
4454 | Unlike C<printf>, C<sprintf> does not do what you probably mean when you |
---|
4455 | pass it an array as your first argument. The array is given scalar context, |
---|
4456 | and instead of using the 0th element of the array as the format, Perl will |
---|
4457 | use the count of elements in the array as the format, which is almost never |
---|
4458 | useful. |
---|
4459 | |
---|
4460 | Perl's C<sprintf> permits the following universally-known conversions: |
---|
4461 | |
---|
4462 | %% a percent sign |
---|
4463 | %c a character with the given number |
---|
4464 | %s a string |
---|
4465 | %d a signed integer, in decimal |
---|
4466 | %u an unsigned integer, in decimal |
---|
4467 | %o an unsigned integer, in octal |
---|
4468 | %x an unsigned integer, in hexadecimal |
---|
4469 | %e a floating-point number, in scientific notation |
---|
4470 | %f a floating-point number, in fixed decimal notation |
---|
4471 | %g a floating-point number, in %e or %f notation |
---|
4472 | |
---|
4473 | In addition, Perl permits the following widely-supported conversions: |
---|
4474 | |
---|
4475 | %X like %x, but using upper-case letters |
---|
4476 | %E like %e, but using an upper-case "E" |
---|
4477 | %G like %g, but with an upper-case "E" (if applicable) |
---|
4478 | %b an unsigned integer, in binary |
---|
4479 | %p a pointer (outputs the Perl value's address in hexadecimal) |
---|
4480 | %n special: *stores* the number of characters output so far |
---|
4481 | into the next variable in the parameter list |
---|
4482 | |
---|
4483 | Finally, for backward (and we do mean "backward") compatibility, Perl |
---|
4484 | permits these unnecessary but widely-supported conversions: |
---|
4485 | |
---|
4486 | %i a synonym for %d |
---|
4487 | %D a synonym for %ld |
---|
4488 | %U a synonym for %lu |
---|
4489 | %O a synonym for %lo |
---|
4490 | %F a synonym for %f |
---|
4491 | |
---|
4492 | Note that the number of exponent digits in the scientific notation by |
---|
4493 | C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the |
---|
4494 | exponent less than 100 is system-dependent: it may be three or less |
---|
4495 | (zero-padded as necessary). In other words, 1.23 times ten to the |
---|
4496 | 99th may be either "1.23e99" or "1.23e099". |
---|
4497 | |
---|
4498 | Perl permits the following universally-known flags between the C<%> |
---|
4499 | and the conversion letter: |
---|
4500 | |
---|
4501 | space prefix positive number with a space |
---|
4502 | + prefix positive number with a plus sign |
---|
4503 | - left-justify within the field |
---|
4504 | 0 use zeros, not spaces, to right-justify |
---|
4505 | # prefix non-zero octal with "0", non-zero hex with "0x" |
---|
4506 | number minimum field width |
---|
4507 | .number "precision": digits after decimal point for |
---|
4508 | floating-point, max length for string, minimum length |
---|
4509 | for integer |
---|
4510 | l interpret integer as C type "long" or "unsigned long" |
---|
4511 | h interpret integer as C type "short" or "unsigned short" |
---|
4512 | If no flags, interpret integer as C type "int" or "unsigned" |
---|
4513 | |
---|
4514 | There are also two Perl-specific flags: |
---|
4515 | |
---|
4516 | V interpret integer as Perl's standard integer type |
---|
4517 | v interpret string as a vector of integers, output as |
---|
4518 | numbers separated either by dots, or by an arbitrary |
---|
4519 | string received from the argument list when the flag |
---|
4520 | is preceded by C<*> |
---|
4521 | |
---|
4522 | Where a number would appear in the flags, an asterisk (C<*>) may be |
---|
4523 | used instead, in which case Perl uses the next item in the parameter |
---|
4524 | list as the given number (that is, as the field width or precision). |
---|
4525 | If a field width obtained through C<*> is negative, it has the same |
---|
4526 | effect as the C<-> flag: left-justification. |
---|
4527 | |
---|
4528 | The C<v> flag is useful for displaying ordinal values of characters |
---|
4529 | in arbitrary strings: |
---|
4530 | |
---|
4531 | printf "version is v%vd\n", $^V; # Perl's version |
---|
4532 | printf "address is %*vX\n", ":", $addr; # IPv6 address |
---|
4533 | printf "bits are %*vb\n", " ", $bits; # random bitstring |
---|
4534 | |
---|
4535 | If C<use locale> is in effect, the character used for the decimal |
---|
4536 | point in formatted real numbers is affected by the LC_NUMERIC locale. |
---|
4537 | See L<perllocale>. |
---|
4538 | |
---|
4539 | If Perl understands "quads" (64-bit integers) (this requires |
---|
4540 | either that the platform natively support quads or that Perl |
---|
4541 | be specifically compiled to support quads), the characters |
---|
4542 | |
---|
4543 | d u o x X b i D U O |
---|
4544 | |
---|
4545 | print quads, and they may optionally be preceded by |
---|
4546 | |
---|
4547 | ll L q |
---|
4548 | |
---|
4549 | For example |
---|
4550 | |
---|
4551 | %lld %16LX %qo |
---|
4552 | |
---|
4553 | You can find out whether your Perl supports quads via L<Config>: |
---|
4554 | |
---|
4555 | use Config; |
---|
4556 | ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) && |
---|
4557 | print "quads\n"; |
---|
4558 | |
---|
4559 | If Perl understands "long doubles" (this requires that the platform |
---|
4560 | support long doubles), the flags |
---|
4561 | |
---|
4562 | e f g E F G |
---|
4563 | |
---|
4564 | may optionally be preceded by |
---|
4565 | |
---|
4566 | ll L |
---|
4567 | |
---|
4568 | For example |
---|
4569 | |
---|
4570 | %llf %Lg |
---|
4571 | |
---|
4572 | You can find out whether your Perl supports long doubles via L<Config>: |
---|
4573 | |
---|
4574 | use Config; |
---|
4575 | $Config{d_longdbl} eq 'define' && print "long doubles\n"; |
---|
4576 | |
---|
4577 | =item sqrt EXPR |
---|
4578 | |
---|
4579 | =item sqrt |
---|
4580 | |
---|
4581 | Return the square root of EXPR. If EXPR is omitted, returns square |
---|
4582 | root of C<$_>. Only works on non-negative operands, unless you've |
---|
4583 | loaded the standard Math::Complex module. |
---|
4584 | |
---|
4585 | use Math::Complex; |
---|
4586 | print sqrt(-2); # prints 1.4142135623731i |
---|
4587 | |
---|
4588 | =item srand EXPR |
---|
4589 | |
---|
4590 | =item srand |
---|
4591 | |
---|
4592 | Sets the random number seed for the C<rand> operator. If EXPR is |
---|
4593 | omitted, uses a semi-random value supplied by the kernel (if it supports |
---|
4594 | the F</dev/urandom> device) or based on the current time and process |
---|
4595 | ID, among other things. In versions of Perl prior to 5.004 the default |
---|
4596 | seed was just the current C<time>. This isn't a particularly good seed, |
---|
4597 | so many old programs supply their own seed value (often C<time ^ $$> or |
---|
4598 | C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more. |
---|
4599 | |
---|
4600 | In fact, it's usually not necessary to call C<srand> at all, because if |
---|
4601 | it is not called explicitly, it is called implicitly at the first use of |
---|
4602 | the C<rand> operator. However, this was not the case in version of Perl |
---|
4603 | before 5.004, so if your script will run under older Perl versions, it |
---|
4604 | should call C<srand>. |
---|
4605 | |
---|
4606 | Note that you need something much more random than the default seed for |
---|
4607 | cryptographic purposes. Checksumming the compressed output of one or more |
---|
4608 | rapidly changing operating system status programs is the usual method. For |
---|
4609 | example: |
---|
4610 | |
---|
4611 | srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`); |
---|
4612 | |
---|
4613 | If you're particularly concerned with this, see the C<Math::TrulyRandom> |
---|
4614 | module in CPAN. |
---|
4615 | |
---|
4616 | Do I<not> call C<srand> multiple times in your program unless you know |
---|
4617 | exactly what you're doing and why you're doing it. The point of the |
---|
4618 | function is to "seed" the C<rand> function so that C<rand> can produce |
---|
4619 | a different sequence each time you run your program. Just do it once at the |
---|
4620 | top of your program, or you I<won't> get random numbers out of C<rand>! |
---|
4621 | |
---|
4622 | Frequently called programs (like CGI scripts) that simply use |
---|
4623 | |
---|
4624 | time ^ $$ |
---|
4625 | |
---|
4626 | for a seed can fall prey to the mathematical property that |
---|
4627 | |
---|
4628 | a^b == (a+1)^(b+1) |
---|
4629 | |
---|
4630 | one-third of the time. So don't do that. |
---|
4631 | |
---|
4632 | =item stat FILEHANDLE |
---|
4633 | |
---|
4634 | =item stat EXPR |
---|
4635 | |
---|
4636 | =item stat |
---|
4637 | |
---|
4638 | Returns a 13-element list giving the status info for a file, either |
---|
4639 | the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, |
---|
4640 | it stats C<$_>. Returns a null list if the stat fails. Typically used |
---|
4641 | as follows: |
---|
4642 | |
---|
4643 | ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size, |
---|
4644 | $atime,$mtime,$ctime,$blksize,$blocks) |
---|
4645 | = stat($filename); |
---|
4646 | |
---|
4647 | Not all fields are supported on all filesystem types. Here are the |
---|
4648 | meaning of the fields: |
---|
4649 | |
---|
4650 | 0 dev device number of filesystem |
---|
4651 | 1 ino inode number |
---|
4652 | 2 mode file mode (type and permissions) |
---|
4653 | 3 nlink number of (hard) links to the file |
---|
4654 | 4 uid numeric user ID of file's owner |
---|
4655 | 5 gid numeric group ID of file's owner |
---|
4656 | 6 rdev the device identifier (special files only) |
---|
4657 | 7 size total size of file, in bytes |
---|
4658 | 8 atime last access time in seconds since the epoch |
---|
4659 | 9 mtime last modify time in seconds since the epoch |
---|
4660 | 10 ctime inode change time (NOT creation time!) in seconds since the epoch |
---|
4661 | 11 blksize preferred block size for file system I/O |
---|
4662 | 12 blocks actual number of blocks allocated |
---|
4663 | |
---|
4664 | (The epoch was at 00:00 January 1, 1970 GMT.) |
---|
4665 | |
---|
4666 | If stat is passed the special filehandle consisting of an underline, no |
---|
4667 | stat is done, but the current contents of the stat structure from the |
---|
4668 | last stat or filetest are returned. Example: |
---|
4669 | |
---|
4670 | if (-x $file && (($d) = stat(_)) && $d < 0) { |
---|
4671 | print "$file is executable NFS file\n"; |
---|
4672 | } |
---|
4673 | |
---|
4674 | (This works on machines only for which the device number is negative |
---|
4675 | under NFS.) |
---|
4676 | |
---|
4677 | Because the mode contains both the file type and its permissions, you |
---|
4678 | should mask off the file type portion and (s)printf using a C<"%o"> |
---|
4679 | if you want to see the real permissions. |
---|
4680 | |
---|
4681 | $mode = (stat($filename))[2]; |
---|
4682 | printf "Permissions are %04o\n", $mode & 07777; |
---|
4683 | |
---|
4684 | In scalar context, C<stat> returns a boolean value indicating success |
---|
4685 | or failure, and, if successful, sets the information associated with |
---|
4686 | the special filehandle C<_>. |
---|
4687 | |
---|
4688 | The File::stat module provides a convenient, by-name access mechanism: |
---|
4689 | |
---|
4690 | use File::stat; |
---|
4691 | $sb = stat($filename); |
---|
4692 | printf "File is %s, size is %s, perm %04o, mtime %s\n", |
---|
4693 | $filename, $sb->size, $sb->mode & 07777, |
---|
4694 | scalar localtime $sb->mtime; |
---|
4695 | |
---|
4696 | You can import symbolic mode constants (C<S_IF*>) and functions |
---|
4697 | (C<S_IS*>) from the Fcntl module: |
---|
4698 | |
---|
4699 | use Fcntl ':mode'; |
---|
4700 | |
---|
4701 | $mode = (stat($filename))[2]; |
---|
4702 | |
---|
4703 | $user_rwx = ($mode & S_IRWXU) >> 6; |
---|
4704 | $group_read = ($mode & S_IRGRP) >> 3; |
---|
4705 | $other_execute = $mode & S_IXOTH; |
---|
4706 | |
---|
4707 | printf "Permissions are %04o\n", S_ISMODE($mode), "\n"; |
---|
4708 | |
---|
4709 | $is_setuid = $mode & S_ISUID; |
---|
4710 | $is_setgid = S_ISDIR($mode); |
---|
4711 | |
---|
4712 | You could write the last two using the C<-u> and C<-d> operators. |
---|
4713 | The commonly available S_IF* constants are |
---|
4714 | |
---|
4715 | # Permissions: read, write, execute, for user, group, others. |
---|
4716 | |
---|
4717 | S_IRWXU S_IRUSR S_IWUSR S_IXUSR |
---|
4718 | S_IRWXG S_IRGRP S_IWGRP S_IXGRP |
---|
4719 | S_IRWXO S_IROTH S_IWOTH S_IXOTH |
---|
4720 | |
---|
4721 | # Setuid/Setgid/Stickiness. |
---|
4722 | |
---|
4723 | S_ISUID S_ISGID S_ISVTX S_ISTXT |
---|
4724 | |
---|
4725 | # File types. Not necessarily all are available on your system. |
---|
4726 | |
---|
4727 | S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT |
---|
4728 | |
---|
4729 | # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR. |
---|
4730 | |
---|
4731 | S_IREAD S_IWRITE S_IEXEC |
---|
4732 | |
---|
4733 | and the S_IF* functions are |
---|
4734 | |
---|
4735 | S_IFMODE($mode) the part of $mode containing the permission bits |
---|
4736 | and the setuid/setgid/sticky bits |
---|
4737 | |
---|
4738 | S_IFMT($mode) the part of $mode containing the file type |
---|
4739 | which can be bit-anded with e.g. S_IFREG |
---|
4740 | or with the following functions |
---|
4741 | |
---|
4742 | # The operators -f, -d, -l, -b, -c, -p, and -s. |
---|
4743 | |
---|
4744 | S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode) |
---|
4745 | S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode) |
---|
4746 | |
---|
4747 | # No direct -X operator counterpart, but for the first one |
---|
4748 | # the -g operator is often equivalent. The ENFMT stands for |
---|
4749 | # record flocking enforcement, a platform-dependent feature. |
---|
4750 | |
---|
4751 | S_ISENFMT($mode) S_ISWHT($mode) |
---|
4752 | |
---|
4753 | See your native chmod(2) and stat(2) documentation for more details |
---|
4754 | about the S_* constants. |
---|
4755 | |
---|
4756 | =item study SCALAR |
---|
4757 | |
---|
4758 | =item study |
---|
4759 | |
---|
4760 | Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of |
---|
4761 | doing many pattern matches on the string before it is next modified. |
---|
4762 | This may or may not save time, depending on the nature and number of |
---|
4763 | patterns you are searching on, and on the distribution of character |
---|
4764 | frequencies in the string to be searched--you probably want to compare |
---|
4765 | run times with and without it to see which runs faster. Those loops |
---|
4766 | which scan for many short constant strings (including the constant |
---|
4767 | parts of more complex patterns) will benefit most. You may have only |
---|
4768 | one C<study> active at a time--if you study a different scalar the first |
---|
4769 | is "unstudied". (The way C<study> works is this: a linked list of every |
---|
4770 | character in the string to be searched is made, so we know, for |
---|
4771 | example, where all the C<'k'> characters are. From each search string, |
---|
4772 | the rarest character is selected, based on some static frequency tables |
---|
4773 | constructed from some C programs and English text. Only those places |
---|
4774 | that contain this "rarest" character are examined.) |
---|
4775 | |
---|
4776 | For example, here is a loop that inserts index producing entries |
---|
4777 | before any line containing a certain pattern: |
---|
4778 | |
---|
4779 | while (<>) { |
---|
4780 | study; |
---|
4781 | print ".IX foo\n" if /\bfoo\b/; |
---|
4782 | print ".IX bar\n" if /\bbar\b/; |
---|
4783 | print ".IX blurfl\n" if /\bblurfl\b/; |
---|
4784 | # ... |
---|
4785 | print; |
---|
4786 | } |
---|
4787 | |
---|
4788 | In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f> |
---|
4789 | will be looked at, because C<f> is rarer than C<o>. In general, this is |
---|
4790 | a big win except in pathological cases. The only question is whether |
---|
4791 | it saves you more time than it took to build the linked list in the |
---|
4792 | first place. |
---|
4793 | |
---|
4794 | Note that if you have to look for strings that you don't know till |
---|
4795 | runtime, you can build an entire loop as a string and C<eval> that to |
---|
4796 | avoid recompiling all your patterns all the time. Together with |
---|
4797 | undefining C<$/> to input entire files as one record, this can be very |
---|
4798 | fast, often faster than specialized programs like fgrep(1). The following |
---|
4799 | scans a list of files (C<@files>) for a list of words (C<@words>), and prints |
---|
4800 | out the names of those files that contain a match: |
---|
4801 | |
---|
4802 | $search = 'while (<>) { study;'; |
---|
4803 | foreach $word (@words) { |
---|
4804 | $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n"; |
---|
4805 | } |
---|
4806 | $search .= "}"; |
---|
4807 | @ARGV = @files; |
---|
4808 | undef $/; |
---|
4809 | eval $search; # this screams |
---|
4810 | $/ = "\n"; # put back to normal input delimiter |
---|
4811 | foreach $file (sort keys(%seen)) { |
---|
4812 | print $file, "\n"; |
---|
4813 | } |
---|
4814 | |
---|
4815 | =item sub BLOCK |
---|
4816 | |
---|
4817 | =item sub NAME |
---|
4818 | |
---|
4819 | =item sub NAME BLOCK |
---|
4820 | |
---|
4821 | This is subroutine definition, not a real function I<per se>. With just a |
---|
4822 | NAME (and possibly prototypes or attributes), it's just a forward declaration. |
---|
4823 | Without a NAME, it's an anonymous function declaration, and does actually |
---|
4824 | return a value: the CODE ref of the closure you just created. See L<perlsub> |
---|
4825 | and L<perlref> for details. |
---|
4826 | |
---|
4827 | =item substr EXPR,OFFSET,LENGTH,REPLACEMENT |
---|
4828 | |
---|
4829 | =item substr EXPR,OFFSET,LENGTH |
---|
4830 | |
---|
4831 | =item substr EXPR,OFFSET |
---|
4832 | |
---|
4833 | Extracts a substring out of EXPR and returns it. First character is at |
---|
4834 | offset C<0>, or whatever you've set C<$[> to (but don't do that). |
---|
4835 | If OFFSET is negative (or more precisely, less than C<$[>), starts |
---|
4836 | that far from the end of the string. If LENGTH is omitted, returns |
---|
4837 | everything to the end of the string. If LENGTH is negative, leaves that |
---|
4838 | many characters off the end of the string. |
---|
4839 | |
---|
4840 | You can use the substr() function as an lvalue, in which case EXPR |
---|
4841 | must itself be an lvalue. If you assign something shorter than LENGTH, |
---|
4842 | the string will shrink, and if you assign something longer than LENGTH, |
---|
4843 | the string will grow to accommodate it. To keep the string the same |
---|
4844 | length you may need to pad or chop your value using C<sprintf>. |
---|
4845 | |
---|
4846 | If OFFSET and LENGTH specify a substring that is partly outside the |
---|
4847 | string, only the part within the string is returned. If the substring |
---|
4848 | is beyond either end of the string, substr() returns the undefined |
---|
4849 | value and produces a warning. When used as an lvalue, specifying a |
---|
4850 | substring that is entirely outside the string is a fatal error. |
---|
4851 | Here's an example showing the behavior for boundary cases: |
---|
4852 | |
---|
4853 | my $name = 'fred'; |
---|
4854 | substr($name, 4) = 'dy'; # $name is now 'freddy' |
---|
4855 | my $null = substr $name, 6, 2; # returns '' (no warning) |
---|
4856 | my $oops = substr $name, 7; # returns undef, with warning |
---|
4857 | substr($name, 7) = 'gap'; # fatal error |
---|
4858 | |
---|
4859 | An alternative to using substr() as an lvalue is to specify the |
---|
4860 | replacement string as the 4th argument. This allows you to replace |
---|
4861 | parts of the EXPR and return what was there before in one operation, |
---|
4862 | just as you can with splice(). |
---|
4863 | |
---|
4864 | =item symlink OLDFILE,NEWFILE |
---|
4865 | |
---|
4866 | Creates a new filename symbolically linked to the old filename. |
---|
4867 | Returns C<1> for success, C<0> otherwise. On systems that don't support |
---|
4868 | symbolic links, produces a fatal error at run time. To check for that, |
---|
4869 | use eval: |
---|
4870 | |
---|
4871 | $symlink_exists = eval { symlink("",""); 1 }; |
---|
4872 | |
---|
4873 | =item syscall LIST |
---|
4874 | |
---|
4875 | Calls the system call specified as the first element of the list, |
---|
4876 | passing the remaining elements as arguments to the system call. If |
---|
4877 | unimplemented, produces a fatal error. The arguments are interpreted |
---|
4878 | as follows: if a given argument is numeric, the argument is passed as |
---|
4879 | an int. If not, the pointer to the string value is passed. You are |
---|
4880 | responsible to make sure a string is pre-extended long enough to |
---|
4881 | receive any result that might be written into a string. You can't use a |
---|
4882 | string literal (or other read-only string) as an argument to C<syscall> |
---|
4883 | because Perl has to assume that any string pointer might be written |
---|
4884 | through. If your |
---|
4885 | integer arguments are not literals and have never been interpreted in a |
---|
4886 | numeric context, you may need to add C<0> to them to force them to look |
---|
4887 | like numbers. This emulates the C<syswrite> function (or vice versa): |
---|
4888 | |
---|
4889 | require 'syscall.ph'; # may need to run h2ph |
---|
4890 | $s = "hi there\n"; |
---|
4891 | syscall(&SYS_write, fileno(STDOUT), $s, length $s); |
---|
4892 | |
---|
4893 | Note that Perl supports passing of up to only 14 arguments to your system call, |
---|
4894 | which in practice should usually suffice. |
---|
4895 | |
---|
4896 | Syscall returns whatever value returned by the system call it calls. |
---|
4897 | If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno). |
---|
4898 | Note that some system calls can legitimately return C<-1>. The proper |
---|
4899 | way to handle such calls is to assign C<$!=0;> before the call and |
---|
4900 | check the value of C<$!> if syscall returns C<-1>. |
---|
4901 | |
---|
4902 | There's a problem with C<syscall(&SYS_pipe)>: it returns the file |
---|
4903 | number of the read end of the pipe it creates. There is no way |
---|
4904 | to retrieve the file number of the other end. You can avoid this |
---|
4905 | problem by using C<pipe> instead. |
---|
4906 | |
---|
4907 | =item sysopen FILEHANDLE,FILENAME,MODE |
---|
4908 | |
---|
4909 | =item sysopen FILEHANDLE,FILENAME,MODE,PERMS |
---|
4910 | |
---|
4911 | Opens the file whose filename is given by FILENAME, and associates it |
---|
4912 | with FILEHANDLE. If FILEHANDLE is an expression, its value is used as |
---|
4913 | the name of the real filehandle wanted. This function calls the |
---|
4914 | underlying operating system's C<open> function with the parameters |
---|
4915 | FILENAME, MODE, PERMS. |
---|
4916 | |
---|
4917 | The possible values and flag bits of the MODE parameter are |
---|
4918 | system-dependent; they are available via the standard module C<Fcntl>. |
---|
4919 | See the documentation of your operating system's C<open> to see which |
---|
4920 | values and flag bits are available. You may combine several flags |
---|
4921 | using the C<|>-operator. |
---|
4922 | |
---|
4923 | Some of the most common values are C<O_RDONLY> for opening the file in |
---|
4924 | read-only mode, C<O_WRONLY> for opening the file in write-only mode, |
---|
4925 | and C<O_RDWR> for opening the file in read-write mode, and. |
---|
4926 | |
---|
4927 | For historical reasons, some values work on almost every system |
---|
4928 | supported by perl: zero means read-only, one means write-only, and two |
---|
4929 | means read/write. We know that these values do I<not> work under |
---|
4930 | OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to |
---|
4931 | use them in new code. |
---|
4932 | |
---|
4933 | If the file named by FILENAME does not exist and the C<open> call creates |
---|
4934 | it (typically because MODE includes the C<O_CREAT> flag), then the value of |
---|
4935 | PERMS specifies the permissions of the newly created file. If you omit |
---|
4936 | the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>. |
---|
4937 | These permission values need to be in octal, and are modified by your |
---|
4938 | process's current C<umask>. |
---|
4939 | |
---|
4940 | In many systems the C<O_EXCL> flag is available for opening files in |
---|
4941 | exclusive mode. This is B<not> locking: exclusiveness means here that |
---|
4942 | if the file already exists, sysopen() fails. The C<O_EXCL> wins |
---|
4943 | C<O_TRUNC>. |
---|
4944 | |
---|
4945 | Sometimes you may want to truncate an already-existing file: C<O_TRUNC>. |
---|
4946 | |
---|
4947 | You should seldom if ever use C<0644> as argument to C<sysopen>, because |
---|
4948 | that takes away the user's option to have a more permissive umask. |
---|
4949 | Better to omit it. See the perlfunc(1) entry on C<umask> for more |
---|
4950 | on this. |
---|
4951 | |
---|
4952 | Note that C<sysopen> depends on the fdopen() C library function. |
---|
4953 | On many UNIX systems, fdopen() is known to fail when file descriptors |
---|
4954 | exceed a certain value, typically 255. If you need more file |
---|
4955 | descriptors than that, consider rebuilding Perl to use the C<sfio> |
---|
4956 | library, or perhaps using the POSIX::open() function. |
---|
4957 | |
---|
4958 | See L<perlopentut> for a kinder, gentler explanation of opening files. |
---|
4959 | |
---|
4960 | =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET |
---|
4961 | |
---|
4962 | =item sysread FILEHANDLE,SCALAR,LENGTH |
---|
4963 | |
---|
4964 | Attempts to read LENGTH bytes of data into variable SCALAR from the |
---|
4965 | specified FILEHANDLE, using the system call read(2). It bypasses stdio, |
---|
4966 | so mixing this with other kinds of reads, C<print>, C<write>, |
---|
4967 | C<seek>, C<tell>, or C<eof> can cause confusion because stdio |
---|
4968 | usually buffers data. Returns the number of bytes actually read, C<0> |
---|
4969 | at end of file, or undef if there was an error. SCALAR will be grown or |
---|
4970 | shrunk so that the last byte actually read is the last byte of the |
---|
4971 | scalar after the read. |
---|
4972 | |
---|
4973 | An OFFSET may be specified to place the read data at some place in the |
---|
4974 | string other than the beginning. A negative OFFSET specifies |
---|
4975 | placement at that many bytes counting backwards from the end of the |
---|
4976 | string. A positive OFFSET greater than the length of SCALAR results |
---|
4977 | in the string being padded to the required size with C<"\0"> bytes before |
---|
4978 | the result of the read is appended. |
---|
4979 | |
---|
4980 | There is no syseof() function, which is ok, since eof() doesn't work |
---|
4981 | very well on device files (like ttys) anyway. Use sysread() and check |
---|
4982 | for a return value for 0 to decide whether you're done. |
---|
4983 | |
---|
4984 | =item sysseek FILEHANDLE,POSITION,WHENCE |
---|
4985 | |
---|
4986 | Sets FILEHANDLE's system position using the system call lseek(2). It |
---|
4987 | bypasses stdio, so mixing this with reads (other than C<sysread>), |
---|
4988 | C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion. |
---|
4989 | FILEHANDLE may be an expression whose value gives the name of the |
---|
4990 | filehandle. The values for WHENCE are C<0> to set the new position to |
---|
4991 | POSITION, C<1> to set the it to the current position plus POSITION, |
---|
4992 | and C<2> to set it to EOF plus POSITION (typically negative). For |
---|
4993 | WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and |
---|
4994 | C<SEEK_END> (start of the file, current position, end of the file) |
---|
4995 | from the Fcntl module. |
---|
4996 | |
---|
4997 | Returns the new position, or the undefined value on failure. A position |
---|
4998 | of zero is returned as the string C<"0 but true">; thus C<sysseek> returns |
---|
4999 | true on success and false on failure, yet you can still easily determine |
---|
5000 | the new position. |
---|
5001 | |
---|
5002 | =item system LIST |
---|
5003 | |
---|
5004 | =item system PROGRAM LIST |
---|
5005 | |
---|
5006 | Does exactly the same thing as C<exec LIST>, except that a fork is |
---|
5007 | done first, and the parent process waits for the child process to |
---|
5008 | complete. Note that argument processing varies depending on the |
---|
5009 | number of arguments. If there is more than one argument in LIST, |
---|
5010 | or if LIST is an array with more than one value, starts the program |
---|
5011 | given by the first element of the list with arguments given by the |
---|
5012 | rest of the list. If there is only one scalar argument, the argument |
---|
5013 | is checked for shell metacharacters, and if there are any, the |
---|
5014 | entire argument is passed to the system's command shell for parsing |
---|
5015 | (this is C</bin/sh -c> on Unix platforms, but varies on other |
---|
5016 | platforms). If there are no shell metacharacters in the argument, |
---|
5017 | it is split into words and passed directly to C<execvp>, which is |
---|
5018 | more efficient. |
---|
5019 | |
---|
5020 | Beginning with v5.6.0, Perl will attempt to flush all files opened for |
---|
5021 | output before any operation that may do a fork, but this may not be |
---|
5022 | supported on some platforms (see L<perlport>). To be safe, you may need |
---|
5023 | to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method |
---|
5024 | of C<IO::Handle> on any open handles. |
---|
5025 | |
---|
5026 | The return value is the exit status of the program as |
---|
5027 | returned by the C<wait> call. To get the actual exit value divide by |
---|
5028 | 256. See also L</exec>. This is I<not> what you want to use to capture |
---|
5029 | the output from a command, for that you should use merely backticks or |
---|
5030 | C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1 |
---|
5031 | indicates a failure to start the program (inspect $! for the reason). |
---|
5032 | |
---|
5033 | Like C<exec>, C<system> allows you to lie to a program about its name if |
---|
5034 | you use the C<system PROGRAM LIST> syntax. Again, see L</exec>. |
---|
5035 | |
---|
5036 | Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the |
---|
5037 | program they're running doesn't actually interrupt your program. |
---|
5038 | |
---|
5039 | @args = ("command", "arg1", "arg2"); |
---|
5040 | system(@args) == 0 |
---|
5041 | or die "system @args failed: $?" |
---|
5042 | |
---|
5043 | You can check all the failure possibilities by inspecting |
---|
5044 | C<$?> like this: |
---|
5045 | |
---|
5046 | $exit_value = $? >> 8; |
---|
5047 | $signal_num = $? & 127; |
---|
5048 | $dumped_core = $? & 128; |
---|
5049 | |
---|
5050 | When the arguments get executed via the system shell, results |
---|
5051 | and return codes will be subject to its quirks and capabilities. |
---|
5052 | See L<perlop/"`STRING`"> and L</exec> for details. |
---|
5053 | |
---|
5054 | =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET |
---|
5055 | |
---|
5056 | =item syswrite FILEHANDLE,SCALAR,LENGTH |
---|
5057 | |
---|
5058 | =item syswrite FILEHANDLE,SCALAR |
---|
5059 | |
---|
5060 | Attempts to write LENGTH bytes of data from variable SCALAR to the |
---|
5061 | specified FILEHANDLE, using the system call write(2). If LENGTH |
---|
5062 | is not specified, writes whole SCALAR. It bypasses stdio, so mixing |
---|
5063 | this with reads (other than C<sysread())>, C<print>, C<write>, |
---|
5064 | C<seek>, C<tell>, or C<eof> may cause confusion because stdio |
---|
5065 | usually buffers data. Returns the number of bytes actually written, |
---|
5066 | or C<undef> if there was an error. If the LENGTH is greater than |
---|
5067 | the available data in the SCALAR after the OFFSET, only as much |
---|
5068 | data as is available will be written. |
---|
5069 | |
---|
5070 | An OFFSET may be specified to write the data from some part of the |
---|
5071 | string other than the beginning. A negative OFFSET specifies writing |
---|
5072 | that many bytes counting backwards from the end of the string. In the |
---|
5073 | case the SCALAR is empty you can use OFFSET but only zero offset. |
---|
5074 | |
---|
5075 | =item tell FILEHANDLE |
---|
5076 | |
---|
5077 | =item tell |
---|
5078 | |
---|
5079 | Returns the current position for FILEHANDLE, or -1 on error. FILEHANDLE |
---|
5080 | may be an expression whose value gives the name of the actual filehandle. |
---|
5081 | If FILEHANDLE is omitted, assumes the file last read. |
---|
5082 | |
---|
5083 | The return value of tell() for the standard streams like the STDIN |
---|
5084 | depends on the operating system: it may return -1 or something else. |
---|
5085 | tell() on pipes, fifos, and sockets usually returns -1. |
---|
5086 | |
---|
5087 | There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that. |
---|
5088 | |
---|
5089 | =item telldir DIRHANDLE |
---|
5090 | |
---|
5091 | Returns the current position of the C<readdir> routines on DIRHANDLE. |
---|
5092 | Value may be given to C<seekdir> to access a particular location in a |
---|
5093 | directory. Has the same caveats about possible directory compaction as |
---|
5094 | the corresponding system library routine. |
---|
5095 | |
---|
5096 | =item tie VARIABLE,CLASSNAME,LIST |
---|
5097 | |
---|
5098 | This function binds a variable to a package class that will provide the |
---|
5099 | implementation for the variable. VARIABLE is the name of the variable |
---|
5100 | to be enchanted. CLASSNAME is the name of a class implementing objects |
---|
5101 | of correct type. Any additional arguments are passed to the C<new> |
---|
5102 | method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>, |
---|
5103 | or C<TIEHASH>). Typically these are arguments such as might be passed |
---|
5104 | to the C<dbm_open()> function of C. The object returned by the C<new> |
---|
5105 | method is also returned by the C<tie> function, which would be useful |
---|
5106 | if you want to access other methods in CLASSNAME. |
---|
5107 | |
---|
5108 | Note that functions such as C<keys> and C<values> may return huge lists |
---|
5109 | when used on large objects, like DBM files. You may prefer to use the |
---|
5110 | C<each> function to iterate over such. Example: |
---|
5111 | |
---|
5112 | # print out history file offsets |
---|
5113 | use NDBM_File; |
---|
5114 | tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0); |
---|
5115 | while (($key,$val) = each %HIST) { |
---|
5116 | print $key, ' = ', unpack('L',$val), "\n"; |
---|
5117 | } |
---|
5118 | untie(%HIST); |
---|
5119 | |
---|
5120 | A class implementing a hash should have the following methods: |
---|
5121 | |
---|
5122 | TIEHASH classname, LIST |
---|
5123 | FETCH this, key |
---|
5124 | STORE this, key, value |
---|
5125 | DELETE this, key |
---|
5126 | CLEAR this |
---|
5127 | EXISTS this, key |
---|
5128 | FIRSTKEY this |
---|
5129 | NEXTKEY this, lastkey |
---|
5130 | DESTROY this |
---|
5131 | UNTIE this |
---|
5132 | |
---|
5133 | A class implementing an ordinary array should have the following methods: |
---|
5134 | |
---|
5135 | TIEARRAY classname, LIST |
---|
5136 | FETCH this, key |
---|
5137 | STORE this, key, value |
---|
5138 | FETCHSIZE this |
---|
5139 | STORESIZE this, count |
---|
5140 | CLEAR this |
---|
5141 | PUSH this, LIST |
---|
5142 | POP this |
---|
5143 | SHIFT this |
---|
5144 | UNSHIFT this, LIST |
---|
5145 | SPLICE this, offset, length, LIST |
---|
5146 | EXTEND this, count |
---|
5147 | DESTROY this |
---|
5148 | UNTIE this |
---|
5149 | |
---|
5150 | A class implementing a file handle should have the following methods: |
---|
5151 | |
---|
5152 | TIEHANDLE classname, LIST |
---|
5153 | READ this, scalar, length, offset |
---|
5154 | READLINE this |
---|
5155 | GETC this |
---|
5156 | WRITE this, scalar, length, offset |
---|
5157 | PRINT this, LIST |
---|
5158 | PRINTF this, format, LIST |
---|
5159 | BINMODE this |
---|
5160 | EOF this |
---|
5161 | FILENO this |
---|
5162 | SEEK this, position, whence |
---|
5163 | TELL this |
---|
5164 | OPEN this, mode, LIST |
---|
5165 | CLOSE this |
---|
5166 | DESTROY this |
---|
5167 | UNTIE this |
---|
5168 | |
---|
5169 | A class implementing a scalar should have the following methods: |
---|
5170 | |
---|
5171 | TIESCALAR classname, LIST |
---|
5172 | FETCH this, |
---|
5173 | STORE this, value |
---|
5174 | DESTROY this |
---|
5175 | UNTIE this |
---|
5176 | |
---|
5177 | Not all methods indicated above need be implemented. See L<perltie>, |
---|
5178 | L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>. |
---|
5179 | |
---|
5180 | Unlike C<dbmopen>, the C<tie> function will not use or require a module |
---|
5181 | for you--you need to do that explicitly yourself. See L<DB_File> |
---|
5182 | or the F<Config> module for interesting C<tie> implementations. |
---|
5183 | |
---|
5184 | For further details see L<perltie>, L<"tied VARIABLE">. |
---|
5185 | |
---|
5186 | =item tied VARIABLE |
---|
5187 | |
---|
5188 | Returns a reference to the object underlying VARIABLE (the same value |
---|
5189 | that was originally returned by the C<tie> call that bound the variable |
---|
5190 | to a package.) Returns the undefined value if VARIABLE isn't tied to a |
---|
5191 | package. |
---|
5192 | |
---|
5193 | =item time |
---|
5194 | |
---|
5195 | Returns the number of non-leap seconds since whatever time the system |
---|
5196 | considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS, |
---|
5197 | and 00:00:00 UTC, January 1, 1970 for most other systems). |
---|
5198 | Suitable for feeding to C<gmtime> and C<localtime>. |
---|
5199 | |
---|
5200 | For measuring time in better granularity than one second, |
---|
5201 | you may use either the Time::HiRes module from CPAN, or |
---|
5202 | if you have gettimeofday(2), you may be able to use the |
---|
5203 | C<syscall> interface of Perl, see L<perlfaq8> for details. |
---|
5204 | |
---|
5205 | =item times |
---|
5206 | |
---|
5207 | Returns a four-element list giving the user and system times, in |
---|
5208 | seconds, for this process and the children of this process. |
---|
5209 | |
---|
5210 | ($user,$system,$cuser,$csystem) = times; |
---|
5211 | |
---|
5212 | =item tr/// |
---|
5213 | |
---|
5214 | The transliteration operator. Same as C<y///>. See L<perlop>. |
---|
5215 | |
---|
5216 | =item truncate FILEHANDLE,LENGTH |
---|
5217 | |
---|
5218 | =item truncate EXPR,LENGTH |
---|
5219 | |
---|
5220 | Truncates the file opened on FILEHANDLE, or named by EXPR, to the |
---|
5221 | specified length. Produces a fatal error if truncate isn't implemented |
---|
5222 | on your system. Returns true if successful, the undefined value |
---|
5223 | otherwise. |
---|
5224 | |
---|
5225 | =item uc EXPR |
---|
5226 | |
---|
5227 | =item uc |
---|
5228 | |
---|
5229 | Returns an uppercased version of EXPR. This is the internal function |
---|
5230 | implementing the C<\U> escape in double-quoted strings. |
---|
5231 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>. |
---|
5232 | Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It |
---|
5233 | does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.) |
---|
5234 | |
---|
5235 | If EXPR is omitted, uses C<$_>. |
---|
5236 | |
---|
5237 | =item ucfirst EXPR |
---|
5238 | |
---|
5239 | =item ucfirst |
---|
5240 | |
---|
5241 | Returns the value of EXPR with the first character |
---|
5242 | in uppercase (titlecase in Unicode). This is |
---|
5243 | the internal function implementing the C<\u> escape in double-quoted strings. |
---|
5244 | Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale> |
---|
5245 | and L<utf8>. |
---|
5246 | |
---|
5247 | If EXPR is omitted, uses C<$_>. |
---|
5248 | |
---|
5249 | =item umask EXPR |
---|
5250 | |
---|
5251 | =item umask |
---|
5252 | |
---|
5253 | Sets the umask for the process to EXPR and returns the previous value. |
---|
5254 | If EXPR is omitted, merely returns the current umask. |
---|
5255 | |
---|
5256 | The Unix permission C<rwxr-x---> is represented as three sets of three |
---|
5257 | bits, or three octal digits: C<0750> (the leading 0 indicates octal |
---|
5258 | and isn't one of the digits). The C<umask> value is such a number |
---|
5259 | representing disabled permissions bits. The permission (or "mode") |
---|
5260 | values you pass C<mkdir> or C<sysopen> are modified by your umask, so |
---|
5261 | even if you tell C<sysopen> to create a file with permissions C<0777>, |
---|
5262 | if your umask is C<0022> then the file will actually be created with |
---|
5263 | permissions C<0755>. If your C<umask> were C<0027> (group can't |
---|
5264 | write; others can't read, write, or execute), then passing |
---|
5265 | C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~ |
---|
5266 | 027> is C<0640>). |
---|
5267 | |
---|
5268 | Here's some advice: supply a creation mode of C<0666> for regular |
---|
5269 | files (in C<sysopen>) and one of C<0777> for directories (in |
---|
5270 | C<mkdir>) and executable files. This gives users the freedom of |
---|
5271 | choice: if they want protected files, they might choose process umasks |
---|
5272 | of C<022>, C<027>, or even the particularly antisocial mask of C<077>. |
---|
5273 | Programs should rarely if ever make policy decisions better left to |
---|
5274 | the user. The exception to this is when writing files that should be |
---|
5275 | kept private: mail files, web browser cookies, I<.rhosts> files, and |
---|
5276 | so on. |
---|
5277 | |
---|
5278 | If umask(2) is not implemented on your system and you are trying to |
---|
5279 | restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a |
---|
5280 | fatal error at run time. If umask(2) is not implemented and you are |
---|
5281 | not trying to restrict access for yourself, returns C<undef>. |
---|
5282 | |
---|
5283 | Remember that a umask is a number, usually given in octal; it is I<not> a |
---|
5284 | string of octal digits. See also L</oct>, if all you have is a string. |
---|
5285 | |
---|
5286 | =item undef EXPR |
---|
5287 | |
---|
5288 | =item undef |
---|
5289 | |
---|
5290 | Undefines the value of EXPR, which must be an lvalue. Use only on a |
---|
5291 | scalar value, an array (using C<@>), a hash (using C<%>), a subroutine |
---|
5292 | (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}> |
---|
5293 | will probably not do what you expect on most predefined variables or |
---|
5294 | DBM list values, so don't do that; see L<delete>.) Always returns the |
---|
5295 | undefined value. You can omit the EXPR, in which case nothing is |
---|
5296 | undefined, but you still get an undefined value that you could, for |
---|
5297 | instance, return from a subroutine, assign to a variable or pass as a |
---|
5298 | parameter. Examples: |
---|
5299 | |
---|
5300 | undef $foo; |
---|
5301 | undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'}; |
---|
5302 | undef @ary; |
---|
5303 | undef %hash; |
---|
5304 | undef &mysub; |
---|
5305 | undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc. |
---|
5306 | return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it; |
---|
5307 | select undef, undef, undef, 0.25; |
---|
5308 | ($a, $b, undef, $c) = &foo; # Ignore third value returned |
---|
5309 | |
---|
5310 | Note that this is a unary operator, not a list operator. |
---|
5311 | |
---|
5312 | =item unlink LIST |
---|
5313 | |
---|
5314 | =item unlink |
---|
5315 | |
---|
5316 | Deletes a list of files. Returns the number of files successfully |
---|
5317 | deleted. |
---|
5318 | |
---|
5319 | $cnt = unlink 'a', 'b', 'c'; |
---|
5320 | unlink @goners; |
---|
5321 | unlink <*.bak>; |
---|
5322 | |
---|
5323 | Note: C<unlink> will not delete directories unless you are superuser and |
---|
5324 | the B<-U> flag is supplied to Perl. Even if these conditions are |
---|
5325 | met, be warned that unlinking a directory can inflict damage on your |
---|
5326 | filesystem. Use C<rmdir> instead. |
---|
5327 | |
---|
5328 | If LIST is omitted, uses C<$_>. |
---|
5329 | |
---|
5330 | =item unpack TEMPLATE,EXPR |
---|
5331 | |
---|
5332 | C<unpack> does the reverse of C<pack>: it takes a string |
---|
5333 | and expands it out into a list of values. |
---|
5334 | (In scalar context, it returns merely the first value produced.) |
---|
5335 | |
---|
5336 | The string is broken into chunks described by the TEMPLATE. Each chunk |
---|
5337 | is converted separately to a value. Typically, either the string is a result |
---|
5338 | of C<pack>, or the bytes of the string represent a C structure of some |
---|
5339 | kind. |
---|
5340 | |
---|
5341 | The TEMPLATE has the same format as in the C<pack> function. |
---|
5342 | Here's a subroutine that does substring: |
---|
5343 | |
---|
5344 | sub substr { |
---|
5345 | my($what,$where,$howmuch) = @_; |
---|
5346 | unpack("x$where a$howmuch", $what); |
---|
5347 | } |
---|
5348 | |
---|
5349 | and then there's |
---|
5350 | |
---|
5351 | sub ordinal { unpack("c",$_[0]); } # same as ord() |
---|
5352 | |
---|
5353 | In addition to fields allowed in pack(), you may prefix a field with |
---|
5354 | a %<number> to indicate that |
---|
5355 | you want a <number>-bit checksum of the items instead of the items |
---|
5356 | themselves. Default is a 16-bit checksum. Checksum is calculated by |
---|
5357 | summing numeric values of expanded values (for string fields the sum of |
---|
5358 | C<ord($char)> is taken, for bit fields the sum of zeroes and ones). |
---|
5359 | |
---|
5360 | For example, the following |
---|
5361 | computes the same number as the System V sum program: |
---|
5362 | |
---|
5363 | $checksum = do { |
---|
5364 | local $/; # slurp! |
---|
5365 | unpack("%32C*",<>) % 65535; |
---|
5366 | }; |
---|
5367 | |
---|
5368 | The following efficiently counts the number of set bits in a bit vector: |
---|
5369 | |
---|
5370 | $setbits = unpack("%32b*", $selectmask); |
---|
5371 | |
---|
5372 | The C<p> and C<P> formats should be used with care. Since Perl |
---|
5373 | has no way of checking whether the value passed to C<unpack()> |
---|
5374 | corresponds to a valid memory location, passing a pointer value that's |
---|
5375 | not known to be valid is likely to have disastrous consequences. |
---|
5376 | |
---|
5377 | If the repeat count of a field is larger than what the remainder of |
---|
5378 | the input string allows, repeat count is decreased. If the input string |
---|
5379 | is longer than one described by the TEMPLATE, the rest is ignored. |
---|
5380 | |
---|
5381 | See L</pack> for more examples and notes. |
---|
5382 | |
---|
5383 | =item untie VARIABLE |
---|
5384 | |
---|
5385 | Breaks the binding between a variable and a package. (See C<tie>.) |
---|
5386 | |
---|
5387 | =item unshift ARRAY,LIST |
---|
5388 | |
---|
5389 | Does the opposite of a C<shift>. Or the opposite of a C<push>, |
---|
5390 | depending on how you look at it. Prepends list to the front of the |
---|
5391 | array, and returns the new number of elements in the array. |
---|
5392 | |
---|
5393 | unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/; |
---|
5394 | |
---|
5395 | Note the LIST is prepended whole, not one element at a time, so the |
---|
5396 | prepended elements stay in the same order. Use C<reverse> to do the |
---|
5397 | reverse. |
---|
5398 | |
---|
5399 | =item use Module VERSION LIST |
---|
5400 | |
---|
5401 | =item use Module VERSION |
---|
5402 | |
---|
5403 | =item use Module LIST |
---|
5404 | |
---|
5405 | =item use Module |
---|
5406 | |
---|
5407 | =item use VERSION |
---|
5408 | |
---|
5409 | Imports some semantics into the current package from the named module, |
---|
5410 | generally by aliasing certain subroutine or variable names into your |
---|
5411 | package. It is exactly equivalent to |
---|
5412 | |
---|
5413 | BEGIN { require Module; import Module LIST; } |
---|
5414 | |
---|
5415 | except that Module I<must> be a bareword. |
---|
5416 | |
---|
5417 | VERSION, which can be specified as a literal of the form v5.6.1, demands |
---|
5418 | that the current version of Perl (C<$^V> or $PERL_VERSION) be at least |
---|
5419 | as recent as that version. (For compatibility with older versions of Perl, |
---|
5420 | a numeric literal will also be interpreted as VERSION.) If the version |
---|
5421 | of the running Perl interpreter is less than VERSION, then an error |
---|
5422 | message is printed and Perl exits immediately without attempting to |
---|
5423 | parse the rest of the file. Compare with L</require>, which can do a |
---|
5424 | similar check at run time. |
---|
5425 | |
---|
5426 | use v5.6.1; # compile time version check |
---|
5427 | use 5.6.1; # ditto |
---|
5428 | use 5.005_03; # float version allowed for compatibility |
---|
5429 | |
---|
5430 | This is often useful if you need to check the current Perl version before |
---|
5431 | C<use>ing library modules that have changed in incompatible ways from |
---|
5432 | older versions of Perl. (We try not to do this more than we have to.) |
---|
5433 | |
---|
5434 | The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The |
---|
5435 | C<require> makes sure the module is loaded into memory if it hasn't been |
---|
5436 | yet. The C<import> is not a builtin--it's just an ordinary static method |
---|
5437 | call into the C<Module> package to tell the module to import the list of |
---|
5438 | features back into the current package. The module can implement its |
---|
5439 | C<import> method any way it likes, though most modules just choose to |
---|
5440 | derive their C<import> method via inheritance from the C<Exporter> class that |
---|
5441 | is defined in the C<Exporter> module. See L<Exporter>. If no C<import> |
---|
5442 | method can be found then the call is skipped. |
---|
5443 | |
---|
5444 | If you do not want to call the package's C<import> method (for instance, |
---|
5445 | to stop your namespace from being altered), explicitly supply the empty list: |
---|
5446 | |
---|
5447 | use Module (); |
---|
5448 | |
---|
5449 | That is exactly equivalent to |
---|
5450 | |
---|
5451 | BEGIN { require Module } |
---|
5452 | |
---|
5453 | If the VERSION argument is present between Module and LIST, then the |
---|
5454 | C<use> will call the VERSION method in class Module with the given |
---|
5455 | version as an argument. The default VERSION method, inherited from |
---|
5456 | the UNIVERSAL class, croaks if the given version is larger than the |
---|
5457 | value of the variable C<$Module::VERSION>. |
---|
5458 | |
---|
5459 | Again, there is a distinction between omitting LIST (C<import> called |
---|
5460 | with no arguments) and an explicit empty LIST C<()> (C<import> not |
---|
5461 | called). Note that there is no comma after VERSION! |
---|
5462 | |
---|
5463 | Because this is a wide-open interface, pragmas (compiler directives) |
---|
5464 | are also implemented this way. Currently implemented pragmas are: |
---|
5465 | |
---|
5466 | use constant; |
---|
5467 | use diagnostics; |
---|
5468 | use integer; |
---|
5469 | use sigtrap qw(SEGV BUS); |
---|
5470 | use strict qw(subs vars refs); |
---|
5471 | use subs qw(afunc blurfl); |
---|
5472 | use warnings qw(all); |
---|
5473 | |
---|
5474 | Some of these pseudo-modules import semantics into the current |
---|
5475 | block scope (like C<strict> or C<integer>, unlike ordinary modules, |
---|
5476 | which import symbols into the current package (which are effective |
---|
5477 | through the end of the file). |
---|
5478 | |
---|
5479 | There's a corresponding C<no> command that unimports meanings imported |
---|
5480 | by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>. |
---|
5481 | |
---|
5482 | no integer; |
---|
5483 | no strict 'refs'; |
---|
5484 | no warnings; |
---|
5485 | |
---|
5486 | If no C<unimport> method can be found the call fails with a fatal error. |
---|
5487 | |
---|
5488 | See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun> |
---|
5489 | for the C<-M> and C<-m> command-line options to perl that give C<use> |
---|
5490 | functionality from the command-line. |
---|
5491 | |
---|
5492 | =item utime LIST |
---|
5493 | |
---|
5494 | Changes the access and modification times on each file of a list of |
---|
5495 | files. The first two elements of the list must be the NUMERICAL access |
---|
5496 | and modification times, in that order. Returns the number of files |
---|
5497 | successfully changed. The inode change time of each file is set |
---|
5498 | to the current time. This code has the same effect as the C<touch> |
---|
5499 | command if the files already exist: |
---|
5500 | |
---|
5501 | #!/usr/bin/perl |
---|
5502 | $now = time; |
---|
5503 | utime $now, $now, @ARGV; |
---|
5504 | |
---|
5505 | =item values HASH |
---|
5506 | |
---|
5507 | Returns a list consisting of all the values of the named hash. (In a |
---|
5508 | scalar context, returns the number of values.) The values are |
---|
5509 | returned in an apparently random order. The actual random order is |
---|
5510 | subject to change in future versions of perl, but it is guaranteed to |
---|
5511 | be the same order as either the C<keys> or C<each> function would |
---|
5512 | produce on the same (unmodified) hash. |
---|
5513 | |
---|
5514 | Note that the values are not copied, which means modifying them will |
---|
5515 | modify the contents of the hash: |
---|
5516 | |
---|
5517 | for (values %hash) { s/foo/bar/g } # modifies %hash values |
---|
5518 | for (@hash{keys %hash}) { s/foo/bar/g } # same |
---|
5519 | |
---|
5520 | As a side effect, calling values() resets the HASH's internal iterator. |
---|
5521 | See also C<keys>, C<each>, and C<sort>. |
---|
5522 | |
---|
5523 | =item vec EXPR,OFFSET,BITS |
---|
5524 | |
---|
5525 | Treats the string in EXPR as a bit vector made up of elements of |
---|
5526 | width BITS, and returns the value of the element specified by OFFSET |
---|
5527 | as an unsigned integer. BITS therefore specifies the number of bits |
---|
5528 | that are reserved for each element in the bit vector. This must |
---|
5529 | be a power of two from 1 to 32 (or 64, if your platform supports |
---|
5530 | that). |
---|
5531 | |
---|
5532 | If BITS is 8, "elements" coincide with bytes of the input string. |
---|
5533 | |
---|
5534 | If BITS is 16 or more, bytes of the input string are grouped into chunks |
---|
5535 | of size BITS/8, and each group is converted to a number as with |
---|
5536 | pack()/unpack() with big-endian formats C<n>/C<N> (and analogously |
---|
5537 | for BITS==64). See L<"pack"> for details. |
---|
5538 | |
---|
5539 | If bits is 4 or less, the string is broken into bytes, then the bits |
---|
5540 | of each byte are broken into 8/BITS groups. Bits of a byte are |
---|
5541 | numbered in a little-endian-ish way, as in C<0x01>, C<0x02>, |
---|
5542 | C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example, |
---|
5543 | breaking the single input byte C<chr(0x36)> into two groups gives a list |
---|
5544 | C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>. |
---|
5545 | |
---|
5546 | C<vec> may also be assigned to, in which case parentheses are needed |
---|
5547 | to give the expression the correct precedence as in |
---|
5548 | |
---|
5549 | vec($image, $max_x * $x + $y, 8) = 3; |
---|
5550 | |
---|
5551 | If the selected element is outside the string, the value 0 is returned. |
---|
5552 | If an element off the end of the string is written to, Perl will first |
---|
5553 | extend the string with sufficiently many zero bytes. It is an error |
---|
5554 | to try to write off the beginning of the string (i.e. negative OFFSET). |
---|
5555 | |
---|
5556 | The string should not contain any character with the value > 255 (which |
---|
5557 | can only happen if you're using UTF8 encoding). If it does, it will be |
---|
5558 | treated as something which is not UTF8 encoded. When the C<vec> was |
---|
5559 | assigned to, other parts of your program will also no longer consider the |
---|
5560 | string to be UTF8 encoded. In other words, if you do have such characters |
---|
5561 | in your string, vec() will operate on the actual byte string, and not the |
---|
5562 | conceptual character string. |
---|
5563 | |
---|
5564 | Strings created with C<vec> can also be manipulated with the logical |
---|
5565 | operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit |
---|
5566 | vector operation is desired when both operands are strings. |
---|
5567 | See L<perlop/"Bitwise String Operators">. |
---|
5568 | |
---|
5569 | The following code will build up an ASCII string saying C<'PerlPerlPerl'>. |
---|
5570 | The comments show the string after each step. Note that this code works |
---|
5571 | in the same way on big-endian or little-endian machines. |
---|
5572 | |
---|
5573 | my $foo = ''; |
---|
5574 | vec($foo, 0, 32) = 0x5065726C; # 'Perl' |
---|
5575 | |
---|
5576 | # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits |
---|
5577 | print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P') |
---|
5578 | |
---|
5579 | vec($foo, 2, 16) = 0x5065; # 'PerlPe' |
---|
5580 | vec($foo, 3, 16) = 0x726C; # 'PerlPerl' |
---|
5581 | vec($foo, 8, 8) = 0x50; # 'PerlPerlP' |
---|
5582 | vec($foo, 9, 8) = 0x65; # 'PerlPerlPe' |
---|
5583 | vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02" |
---|
5584 | vec($foo, 21, 4) = 7; # 'PerlPerlPer' |
---|
5585 | # 'r' is "\x72" |
---|
5586 | vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c" |
---|
5587 | vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c" |
---|
5588 | vec($foo, 94, 1) = 1; # 'PerlPerlPerl' |
---|
5589 | # 'l' is "\x6c" |
---|
5590 | |
---|
5591 | To transform a bit vector into a string or list of 0's and 1's, use these: |
---|
5592 | |
---|
5593 | $bits = unpack("b*", $vector); |
---|
5594 | @bits = split(//, unpack("b*", $vector)); |
---|
5595 | |
---|
5596 | If you know the exact length in bits, it can be used in place of the C<*>. |
---|
5597 | |
---|
5598 | Here is an example to illustrate how the bits actually fall in place: |
---|
5599 | |
---|
5600 | #!/usr/bin/perl -wl |
---|
5601 | |
---|
5602 | print <<'EOT'; |
---|
5603 | 0 1 2 3 |
---|
5604 | unpack("V",$_) 01234567890123456789012345678901 |
---|
5605 | ------------------------------------------------------------------ |
---|
5606 | EOT |
---|
5607 | |
---|
5608 | for $w (0..3) { |
---|
5609 | $width = 2**$w; |
---|
5610 | for ($shift=0; $shift < $width; ++$shift) { |
---|
5611 | for ($off=0; $off < 32/$width; ++$off) { |
---|
5612 | $str = pack("B*", "0"x32); |
---|
5613 | $bits = (1<<$shift); |
---|
5614 | vec($str, $off, $width) = $bits; |
---|
5615 | $res = unpack("b*",$str); |
---|
5616 | $val = unpack("V", $str); |
---|
5617 | write; |
---|
5618 | } |
---|
5619 | } |
---|
5620 | } |
---|
5621 | |
---|
5622 | format STDOUT = |
---|
5623 | vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> |
---|
5624 | $off, $width, $bits, $val, $res |
---|
5625 | . |
---|
5626 | __END__ |
---|
5627 | |
---|
5628 | Regardless of the machine architecture on which it is run, the above |
---|
5629 | example should print the following table: |
---|
5630 | |
---|
5631 | 0 1 2 3 |
---|
5632 | unpack("V",$_) 01234567890123456789012345678901 |
---|
5633 | ------------------------------------------------------------------ |
---|
5634 | vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000 |
---|
5635 | vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000 |
---|
5636 | vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000 |
---|
5637 | vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000 |
---|
5638 | vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000 |
---|
5639 | vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000 |
---|
5640 | vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000 |
---|
5641 | vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000 |
---|
5642 | vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000 |
---|
5643 | vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000 |
---|
5644 | vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000 |
---|
5645 | vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000 |
---|
5646 | vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000 |
---|
5647 | vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000 |
---|
5648 | vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000 |
---|
5649 | vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000 |
---|
5650 | vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000 |
---|
5651 | vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000 |
---|
5652 | vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000 |
---|
5653 | vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000 |
---|
5654 | vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000 |
---|
5655 | vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000 |
---|
5656 | vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000 |
---|
5657 | vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000 |
---|
5658 | vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000 |
---|
5659 | vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000 |
---|
5660 | vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000 |
---|
5661 | vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000 |
---|
5662 | vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000 |
---|
5663 | vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100 |
---|
5664 | vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010 |
---|
5665 | vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001 |
---|
5666 | vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000 |
---|
5667 | vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000 |
---|
5668 | vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000 |
---|
5669 | vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000 |
---|
5670 | vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000 |
---|
5671 | vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000 |
---|
5672 | vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000 |
---|
5673 | vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000 |
---|
5674 | vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000 |
---|
5675 | vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000 |
---|
5676 | vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000 |
---|
5677 | vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000 |
---|
5678 | vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000 |
---|
5679 | vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000 |
---|
5680 | vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000 |
---|
5681 | vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010 |
---|
5682 | vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000 |
---|
5683 | vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000 |
---|
5684 | vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000 |
---|
5685 | vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000 |
---|
5686 | vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000 |
---|
5687 | vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000 |
---|
5688 | vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000 |
---|
5689 | vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000 |
---|
5690 | vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000 |
---|
5691 | vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000 |
---|
5692 | vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000 |
---|
5693 | vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000 |
---|
5694 | vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000 |
---|
5695 | vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000 |
---|
5696 | vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100 |
---|
5697 | vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001 |
---|
5698 | vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000 |
---|
5699 | vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000 |
---|
5700 | vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000 |
---|
5701 | vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000 |
---|
5702 | vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000 |
---|
5703 | vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000 |
---|
5704 | vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000 |
---|
5705 | vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000 |
---|
5706 | vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000 |
---|
5707 | vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000 |
---|
5708 | vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000 |
---|
5709 | vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000 |
---|
5710 | vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000 |
---|
5711 | vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000 |
---|
5712 | vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000 |
---|
5713 | vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100 |
---|
5714 | vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000 |
---|
5715 | vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000 |
---|
5716 | vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000 |
---|
5717 | vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000 |
---|
5718 | vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000 |
---|
5719 | vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000 |
---|
5720 | vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000 |
---|
5721 | vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010 |
---|
5722 | vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000 |
---|
5723 | vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000 |
---|
5724 | vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000 |
---|
5725 | vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000 |
---|
5726 | vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000 |
---|
5727 | vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000 |
---|
5728 | vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000 |
---|
5729 | vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001 |
---|
5730 | vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000 |
---|
5731 | vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000 |
---|
5732 | vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000 |
---|
5733 | vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000 |
---|
5734 | vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000 |
---|
5735 | vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000 |
---|
5736 | vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000 |
---|
5737 | vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000 |
---|
5738 | vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000 |
---|
5739 | vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000 |
---|
5740 | vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000 |
---|
5741 | vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000 |
---|
5742 | vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000 |
---|
5743 | vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000 |
---|
5744 | vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000 |
---|
5745 | vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000 |
---|
5746 | vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000 |
---|
5747 | vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000 |
---|
5748 | vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000 |
---|
5749 | vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000 |
---|
5750 | vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000 |
---|
5751 | vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000 |
---|
5752 | vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000 |
---|
5753 | vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100 |
---|
5754 | vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000 |
---|
5755 | vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000 |
---|
5756 | vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000 |
---|
5757 | vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010 |
---|
5758 | vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000 |
---|
5759 | vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000 |
---|
5760 | vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000 |
---|
5761 | vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001 |
---|
5762 | |
---|
5763 | =item wait |
---|
5764 | |
---|
5765 | Behaves like the wait(2) system call on your system: it waits for a child |
---|
5766 | process to terminate and returns the pid of the deceased process, or |
---|
5767 | C<-1> if there are no child processes. The status is returned in C<$?>. |
---|
5768 | Note that a return value of C<-1> could mean that child processes are |
---|
5769 | being automatically reaped, as described in L<perlipc>. |
---|
5770 | |
---|
5771 | =item waitpid PID,FLAGS |
---|
5772 | |
---|
5773 | Waits for a particular child process to terminate and returns the pid of |
---|
5774 | the deceased process, or C<-1> if there is no such child process. On some |
---|
5775 | systems, a value of 0 indicates that there are processes still running. |
---|
5776 | The status is returned in C<$?>. If you say |
---|
5777 | |
---|
5778 | use POSIX ":sys_wait_h"; |
---|
5779 | #... |
---|
5780 | do { |
---|
5781 | $kid = waitpid(-1,&WNOHANG); |
---|
5782 | } until $kid == -1; |
---|
5783 | |
---|
5784 | then you can do a non-blocking wait for all pending zombie processes. |
---|
5785 | Non-blocking wait is available on machines supporting either the |
---|
5786 | waitpid(2) or wait4(2) system calls. However, waiting for a particular |
---|
5787 | pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the |
---|
5788 | system call by remembering the status values of processes that have |
---|
5789 | exited but have not been harvested by the Perl script yet.) |
---|
5790 | |
---|
5791 | Note that on some systems, a return value of C<-1> could mean that child |
---|
5792 | processes are being automatically reaped. See L<perlipc> for details, |
---|
5793 | and for other examples. |
---|
5794 | |
---|
5795 | =item wantarray |
---|
5796 | |
---|
5797 | Returns true if the context of the currently executing subroutine is |
---|
5798 | looking for a list value. Returns false if the context is looking |
---|
5799 | for a scalar. Returns the undefined value if the context is looking |
---|
5800 | for no value (void context). |
---|
5801 | |
---|
5802 | return unless defined wantarray; # don't bother doing more |
---|
5803 | my @a = complex_calculation(); |
---|
5804 | return wantarray ? @a : "@a"; |
---|
5805 | |
---|
5806 | This function should have been named wantlist() instead. |
---|
5807 | |
---|
5808 | =item warn LIST |
---|
5809 | |
---|
5810 | Produces a message on STDERR just like C<die>, but doesn't exit or throw |
---|
5811 | an exception. |
---|
5812 | |
---|
5813 | If LIST is empty and C<$@> already contains a value (typically from a |
---|
5814 | previous eval) that value is used after appending C<"\t...caught"> |
---|
5815 | to C<$@>. This is useful for staying almost, but not entirely similar to |
---|
5816 | C<die>. |
---|
5817 | |
---|
5818 | If C<$@> is empty then the string C<"Warning: Something's wrong"> is used. |
---|
5819 | |
---|
5820 | No message is printed if there is a C<$SIG{__WARN__}> handler |
---|
5821 | installed. It is the handler's responsibility to deal with the message |
---|
5822 | as it sees fit (like, for instance, converting it into a C<die>). Most |
---|
5823 | handlers must therefore make arrangements to actually display the |
---|
5824 | warnings that they are not prepared to deal with, by calling C<warn> |
---|
5825 | again in the handler. Note that this is quite safe and will not |
---|
5826 | produce an endless loop, since C<__WARN__> hooks are not called from |
---|
5827 | inside one. |
---|
5828 | |
---|
5829 | You will find this behavior is slightly different from that of |
---|
5830 | C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can |
---|
5831 | instead call C<die> again to change it). |
---|
5832 | |
---|
5833 | Using a C<__WARN__> handler provides a powerful way to silence all |
---|
5834 | warnings (even the so-called mandatory ones). An example: |
---|
5835 | |
---|
5836 | # wipe out *all* compile-time warnings |
---|
5837 | BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } } |
---|
5838 | my $foo = 10; |
---|
5839 | my $foo = 20; # no warning about duplicate my $foo, |
---|
5840 | # but hey, you asked for it! |
---|
5841 | # no compile-time or run-time warnings before here |
---|
5842 | $DOWARN = 1; |
---|
5843 | |
---|
5844 | # run-time warnings enabled after here |
---|
5845 | warn "\$foo is alive and $foo!"; # does show up |
---|
5846 | |
---|
5847 | See L<perlvar> for details on setting C<%SIG> entries, and for more |
---|
5848 | examples. See the Carp module for other kinds of warnings using its |
---|
5849 | carp() and cluck() functions. |
---|
5850 | |
---|
5851 | =item write FILEHANDLE |
---|
5852 | |
---|
5853 | =item write EXPR |
---|
5854 | |
---|
5855 | =item write |
---|
5856 | |
---|
5857 | Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, |
---|
5858 | using the format associated with that file. By default the format for |
---|
5859 | a file is the one having the same name as the filehandle, but the |
---|
5860 | format for the current output channel (see the C<select> function) may be set |
---|
5861 | explicitly by assigning the name of the format to the C<$~> variable. |
---|
5862 | |
---|
5863 | Top of form processing is handled automatically: if there is |
---|
5864 | insufficient room on the current page for the formatted record, the |
---|
5865 | page is advanced by writing a form feed, a special top-of-page format |
---|
5866 | is used to format the new page header, and then the record is written. |
---|
5867 | By default the top-of-page format is the name of the filehandle with |
---|
5868 | "_TOP" appended, but it may be dynamically set to the format of your |
---|
5869 | choice by assigning the name to the C<$^> variable while the filehandle is |
---|
5870 | selected. The number of lines remaining on the current page is in |
---|
5871 | variable C<$->, which can be set to C<0> to force a new page. |
---|
5872 | |
---|
5873 | If FILEHANDLE is unspecified, output goes to the current default output |
---|
5874 | channel, which starts out as STDOUT but may be changed by the |
---|
5875 | C<select> operator. If the FILEHANDLE is an EXPR, then the expression |
---|
5876 | is evaluated and the resulting string is used to look up the name of |
---|
5877 | the FILEHANDLE at run time. For more on formats, see L<perlform>. |
---|
5878 | |
---|
5879 | Note that write is I<not> the opposite of C<read>. Unfortunately. |
---|
5880 | |
---|
5881 | =item y/// |
---|
5882 | |
---|
5883 | The transliteration operator. Same as C<tr///>. See L<perlop>. |
---|
5884 | |
---|
5885 | =back |
---|