1 | This is Info file gcc.info, produced by Makeinfo version 1.67 from the |
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2 | input file gcc.texi. |
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3 | |
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4 | This file documents the use and the internals of the GNU compiler. |
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5 | |
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6 | Published by the Free Software Foundation 59 Temple Place - Suite 330 |
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7 | Boston, MA 02111-1307 USA |
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8 | |
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9 | Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998 |
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10 | Free Software Foundation, Inc. |
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11 | |
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12 | Permission is granted to make and distribute verbatim copies of this |
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13 | manual provided the copyright notice and this permission notice are |
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14 | preserved on all copies. |
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15 | |
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16 | Permission is granted to copy and distribute modified versions of |
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17 | this manual under the conditions for verbatim copying, provided also |
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18 | that the sections entitled "GNU General Public License," "Funding for |
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19 | Free Software," and "Protect Your Freedom--Fight `Look And Feel'" are |
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20 | included exactly as in the original, and provided that the entire |
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21 | resulting derived work is distributed under the terms of a permission |
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22 | notice identical to this one. |
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23 | |
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24 | Permission is granted to copy and distribute translations of this |
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25 | manual into another language, under the above conditions for modified |
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26 | versions, except that the sections entitled "GNU General Public |
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27 | License," "Funding for Free Software," and "Protect Your Freedom--Fight |
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28 | `Look And Feel'", and this permission notice, may be included in |
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29 | translations approved by the Free Software Foundation instead of in the |
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30 | original English. |
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31 | |
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32 | |
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33 | File: gcc.info, Node: Variable Length, Next: Macro Varargs, Prev: Zero Length, Up: C Extensions |
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34 | |
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35 | Arrays of Variable Length |
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36 | ========================= |
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37 | |
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38 | Variable-length automatic arrays are allowed in GNU C. These arrays |
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39 | are declared like any other automatic arrays, but with a length that is |
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40 | not a constant expression. The storage is allocated at the point of |
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41 | declaration and deallocated when the brace-level is exited. For |
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42 | example: |
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43 | |
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44 | FILE * |
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45 | concat_fopen (char *s1, char *s2, char *mode) |
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46 | { |
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47 | char str[strlen (s1) + strlen (s2) + 1]; |
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48 | strcpy (str, s1); |
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49 | strcat (str, s2); |
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50 | return fopen (str, mode); |
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51 | } |
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52 | |
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53 | Jumping or breaking out of the scope of the array name deallocates |
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54 | the storage. Jumping into the scope is not allowed; you get an error |
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55 | message for it. |
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56 | |
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57 | You can use the function `alloca' to get an effect much like |
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58 | variable-length arrays. The function `alloca' is available in many |
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59 | other C implementations (but not in all). On the other hand, |
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60 | variable-length arrays are more elegant. |
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61 | |
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62 | There are other differences between these two methods. Space |
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63 | allocated with `alloca' exists until the containing *function* returns. |
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64 | The space for a variable-length array is deallocated as soon as the |
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65 | array name's scope ends. (If you use both variable-length arrays and |
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66 | `alloca' in the same function, deallocation of a variable-length array |
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67 | will also deallocate anything more recently allocated with `alloca'.) |
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68 | |
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69 | You can also use variable-length arrays as arguments to functions: |
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70 | |
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71 | struct entry |
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72 | tester (int len, char data[len][len]) |
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73 | { |
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74 | ... |
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75 | } |
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76 | |
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77 | The length of an array is computed once when the storage is allocated |
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78 | and is remembered for the scope of the array in case you access it with |
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79 | `sizeof'. |
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80 | |
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81 | If you want to pass the array first and the length afterward, you can |
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82 | use a forward declaration in the parameter list--another GNU extension. |
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83 | |
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84 | struct entry |
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85 | tester (int len; char data[len][len], int len) |
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86 | { |
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87 | ... |
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88 | } |
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89 | |
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90 | The `int len' before the semicolon is a "parameter forward |
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91 | declaration", and it serves the purpose of making the name `len' known |
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92 | when the declaration of `data' is parsed. |
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93 | |
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94 | You can write any number of such parameter forward declarations in |
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95 | the parameter list. They can be separated by commas or semicolons, but |
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96 | the last one must end with a semicolon, which is followed by the "real" |
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97 | parameter declarations. Each forward declaration must match a "real" |
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98 | declaration in parameter name and data type. |
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99 | |
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100 | |
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101 | File: gcc.info, Node: Macro Varargs, Next: Subscripting, Prev: Variable Length, Up: C Extensions |
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102 | |
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103 | Macros with Variable Numbers of Arguments |
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104 | ========================================= |
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105 | |
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106 | In GNU C, a macro can accept a variable number of arguments, much as |
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107 | a function can. The syntax for defining the macro looks much like that |
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108 | used for a function. Here is an example: |
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109 | |
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110 | #define eprintf(format, args...) \ |
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111 | fprintf (stderr, format , ## args) |
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112 | |
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113 | Here `args' is a "rest argument": it takes in zero or more |
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114 | arguments, as many as the call contains. All of them plus the commas |
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115 | between them form the value of `args', which is substituted into the |
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116 | macro body where `args' is used. Thus, we have this expansion: |
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117 | |
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118 | eprintf ("%s:%d: ", input_file_name, line_number) |
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119 | ==> |
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120 | fprintf (stderr, "%s:%d: " , input_file_name, line_number) |
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121 | |
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122 | Note that the comma after the string constant comes from the definition |
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123 | of `eprintf', whereas the last comma comes from the value of `args'. |
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124 | |
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125 | The reason for using `##' is to handle the case when `args' matches |
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126 | no arguments at all. In this case, `args' has an empty value. In this |
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127 | case, the second comma in the definition becomes an embarrassment: if |
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128 | it got through to the expansion of the macro, we would get something |
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129 | like this: |
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130 | |
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131 | fprintf (stderr, "success!\n" , ) |
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132 | |
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133 | which is invalid C syntax. `##' gets rid of the comma, so we get the |
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134 | following instead: |
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135 | |
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136 | fprintf (stderr, "success!\n") |
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137 | |
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138 | This is a special feature of the GNU C preprocessor: `##' before a |
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139 | rest argument that is empty discards the preceding sequence of |
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140 | non-whitespace characters from the macro definition. (If another macro |
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141 | argument precedes, none of it is discarded.) |
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142 | |
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143 | It might be better to discard the last preprocessor token instead of |
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144 | the last preceding sequence of non-whitespace characters; in fact, we |
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145 | may someday change this feature to do so. We advise you to write the |
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146 | macro definition so that the preceding sequence of non-whitespace |
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147 | characters is just a single token, so that the meaning will not change |
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148 | if we change the definition of this feature. |
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149 | |
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150 | |
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151 | File: gcc.info, Node: Subscripting, Next: Pointer Arith, Prev: Macro Varargs, Up: C Extensions |
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152 | |
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153 | Non-Lvalue Arrays May Have Subscripts |
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154 | ===================================== |
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155 | |
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156 | Subscripting is allowed on arrays that are not lvalues, even though |
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157 | the unary `&' operator is not. For example, this is valid in GNU C |
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158 | though not valid in other C dialects: |
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159 | |
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160 | struct foo {int a[4];}; |
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161 | |
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162 | struct foo f(); |
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163 | |
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164 | bar (int index) |
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165 | { |
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166 | return f().a[index]; |
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167 | } |
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168 | |
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169 | |
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170 | File: gcc.info, Node: Pointer Arith, Next: Initializers, Prev: Subscripting, Up: C Extensions |
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171 | |
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172 | Arithmetic on `void'- and Function-Pointers |
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173 | =========================================== |
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174 | |
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175 | In GNU C, addition and subtraction operations are supported on |
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176 | pointers to `void' and on pointers to functions. This is done by |
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177 | treating the size of a `void' or of a function as 1. |
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178 | |
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179 | A consequence of this is that `sizeof' is also allowed on `void' and |
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180 | on function types, and returns 1. |
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181 | |
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182 | The option `-Wpointer-arith' requests a warning if these extensions |
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183 | are used. |
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184 | |
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185 | |
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186 | File: gcc.info, Node: Initializers, Next: Constructors, Prev: Pointer Arith, Up: C Extensions |
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187 | |
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188 | Non-Constant Initializers |
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189 | ========================= |
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190 | |
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191 | As in standard C++, the elements of an aggregate initializer for an |
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192 | automatic variable are not required to be constant expressions in GNU C. |
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193 | Here is an example of an initializer with run-time varying elements: |
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194 | |
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195 | foo (float f, float g) |
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196 | { |
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197 | float beat_freqs[2] = { f-g, f+g }; |
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198 | ... |
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199 | } |
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200 | |
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201 | |
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202 | File: gcc.info, Node: Constructors, Next: Labeled Elements, Prev: Initializers, Up: C Extensions |
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203 | |
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204 | Constructor Expressions |
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205 | ======================= |
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206 | |
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207 | GNU C supports constructor expressions. A constructor looks like a |
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208 | cast containing an initializer. Its value is an object of the type |
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209 | specified in the cast, containing the elements specified in the |
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210 | initializer. |
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211 | |
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212 | Usually, the specified type is a structure. Assume that `struct |
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213 | foo' and `structure' are declared as shown: |
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214 | |
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215 | struct foo {int a; char b[2];} structure; |
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216 | |
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217 | Here is an example of constructing a `struct foo' with a constructor: |
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218 | |
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219 | structure = ((struct foo) {x + y, 'a', 0}); |
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220 | |
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221 | This is equivalent to writing the following: |
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222 | |
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223 | { |
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224 | struct foo temp = {x + y, 'a', 0}; |
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225 | structure = temp; |
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226 | } |
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227 | |
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228 | You can also construct an array. If all the elements of the |
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229 | constructor are (made up of) simple constant expressions, suitable for |
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230 | use in initializers, then the constructor is an lvalue and can be |
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231 | coerced to a pointer to its first element, as shown here: |
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232 | |
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233 | char **foo = (char *[]) { "x", "y", "z" }; |
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234 | |
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235 | Array constructors whose elements are not simple constants are not |
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236 | very useful, because the constructor is not an lvalue. There are only |
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237 | two valid ways to use it: to subscript it, or initialize an array |
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238 | variable with it. The former is probably slower than a `switch' |
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239 | statement, while the latter does the same thing an ordinary C |
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240 | initializer would do. Here is an example of subscripting an array |
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241 | constructor: |
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242 | |
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243 | output = ((int[]) { 2, x, 28 }) [input]; |
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244 | |
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245 | Constructor expressions for scalar types and union types are is also |
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246 | allowed, but then the constructor expression is equivalent to a cast. |
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247 | |
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248 | |
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249 | File: gcc.info, Node: Labeled Elements, Next: Cast to Union, Prev: Constructors, Up: C Extensions |
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250 | |
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251 | Labeled Elements in Initializers |
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252 | ================================ |
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253 | |
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254 | Standard C requires the elements of an initializer to appear in a |
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255 | fixed order, the same as the order of the elements in the array or |
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256 | structure being initialized. |
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257 | |
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258 | In GNU C you can give the elements in any order, specifying the array |
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259 | indices or structure field names they apply to. This extension is not |
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260 | implemented in GNU C++. |
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261 | |
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262 | To specify an array index, write `[INDEX]' or `[INDEX] =' before the |
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263 | element value. For example, |
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264 | |
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265 | int a[6] = { [4] 29, [2] = 15 }; |
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266 | |
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267 | is equivalent to |
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268 | |
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269 | int a[6] = { 0, 0, 15, 0, 29, 0 }; |
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270 | |
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271 | The index values must be constant expressions, even if the array being |
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272 | initialized is automatic. |
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273 | |
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274 | To initialize a range of elements to the same value, write `[FIRST |
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275 | ... LAST] = VALUE'. For example, |
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276 | |
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277 | int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 }; |
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278 | |
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279 | Note that the length of the array is the highest value specified plus |
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280 | one. |
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281 | |
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282 | In a structure initializer, specify the name of a field to initialize |
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283 | with `FIELDNAME:' before the element value. For example, given the |
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284 | following structure, |
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285 | |
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286 | struct point { int x, y; }; |
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287 | |
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288 | the following initialization |
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289 | |
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290 | struct point p = { y: yvalue, x: xvalue }; |
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291 | |
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292 | is equivalent to |
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293 | |
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294 | struct point p = { xvalue, yvalue }; |
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295 | |
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296 | Another syntax which has the same meaning is `.FIELDNAME ='., as |
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297 | shown here: |
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298 | |
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299 | struct point p = { .y = yvalue, .x = xvalue }; |
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300 | |
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301 | You can also use an element label (with either the colon syntax or |
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302 | the period-equal syntax) when initializing a union, to specify which |
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303 | element of the union should be used. For example, |
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304 | |
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305 | union foo { int i; double d; }; |
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306 | |
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307 | union foo f = { d: 4 }; |
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308 | |
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309 | will convert 4 to a `double' to store it in the union using the second |
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310 | element. By contrast, casting 4 to type `union foo' would store it |
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311 | into the union as the integer `i', since it is an integer. (*Note Cast |
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312 | to Union::.) |
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313 | |
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314 | You can combine this technique of naming elements with ordinary C |
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315 | initialization of successive elements. Each initializer element that |
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316 | does not have a label applies to the next consecutive element of the |
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317 | array or structure. For example, |
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318 | |
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319 | int a[6] = { [1] = v1, v2, [4] = v4 }; |
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320 | |
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321 | is equivalent to |
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322 | |
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323 | int a[6] = { 0, v1, v2, 0, v4, 0 }; |
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324 | |
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325 | Labeling the elements of an array initializer is especially useful |
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326 | when the indices are characters or belong to an `enum' type. For |
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327 | example: |
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328 | |
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329 | int whitespace[256] |
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330 | = { [' '] = 1, ['\t'] = 1, ['\h'] = 1, |
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331 | ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 }; |
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332 | |
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333 | |
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334 | File: gcc.info, Node: Case Ranges, Next: Function Attributes, Prev: Cast to Union, Up: C Extensions |
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335 | |
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336 | Case Ranges |
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337 | =========== |
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338 | |
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339 | You can specify a range of consecutive values in a single `case' |
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340 | label, like this: |
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341 | |
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342 | case LOW ... HIGH: |
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343 | |
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344 | This has the same effect as the proper number of individual `case' |
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345 | labels, one for each integer value from LOW to HIGH, inclusive. |
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346 | |
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347 | This feature is especially useful for ranges of ASCII character |
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348 | codes: |
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349 | |
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350 | case 'A' ... 'Z': |
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351 | |
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352 | *Be careful:* Write spaces around the `...', for otherwise it may be |
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353 | parsed wrong when you use it with integer values. For example, write |
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354 | this: |
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355 | |
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356 | case 1 ... 5: |
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357 | |
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358 | rather than this: |
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359 | |
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360 | case 1...5: |
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361 | |
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362 | |
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363 | File: gcc.info, Node: Cast to Union, Next: Case Ranges, Prev: Labeled Elements, Up: C Extensions |
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364 | |
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365 | Cast to a Union Type |
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366 | ==================== |
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367 | |
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368 | A cast to union type is similar to other casts, except that the type |
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369 | specified is a union type. You can specify the type either with `union |
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370 | TAG' or with a typedef name. A cast to union is actually a constructor |
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371 | though, not a cast, and hence does not yield an lvalue like normal |
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372 | casts. (*Note Constructors::.) |
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373 | |
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374 | The types that may be cast to the union type are those of the members |
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375 | of the union. Thus, given the following union and variables: |
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376 | |
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377 | union foo { int i; double d; }; |
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378 | int x; |
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379 | double y; |
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380 | |
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381 | both `x' and `y' can be cast to type `union' foo. |
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382 | |
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383 | Using the cast as the right-hand side of an assignment to a variable |
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384 | of union type is equivalent to storing in a member of the union: |
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385 | |
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386 | union foo u; |
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387 | ... |
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388 | u = (union foo) x == u.i = x |
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389 | u = (union foo) y == u.d = y |
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390 | |
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391 | You can also use the union cast as a function argument: |
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392 | |
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393 | void hack (union foo); |
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394 | ... |
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395 | hack ((union foo) x); |
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396 | |
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397 | |
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398 | File: gcc.info, Node: Function Attributes, Next: Function Prototypes, Prev: Case Ranges, Up: C Extensions |
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399 | |
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400 | Declaring Attributes of Functions |
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401 | ================================= |
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402 | |
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403 | In GNU C, you declare certain things about functions called in your |
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404 | program which help the compiler optimize function calls and check your |
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405 | code more carefully. |
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406 | |
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407 | The keyword `__attribute__' allows you to specify special attributes |
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408 | when making a declaration. This keyword is followed by an attribute |
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409 | specification inside double parentheses. Eight attributes, `noreturn', |
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410 | `const', `format', `section', `constructor', `destructor', `unused' and |
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411 | `weak' are currently defined for functions. Other attributes, including |
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412 | `section' are supported for variables declarations (*note Variable |
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413 | Attributes::.) and for types (*note Type Attributes::.). |
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414 | |
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415 | You may also specify attributes with `__' preceding and following |
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416 | each keyword. This allows you to use them in header files without |
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417 | being concerned about a possible macro of the same name. For example, |
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418 | you may use `__noreturn__' instead of `noreturn'. |
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419 | |
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420 | `noreturn' |
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421 | A few standard library functions, such as `abort' and `exit', |
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422 | cannot return. GNU CC knows this automatically. Some programs |
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423 | define their own functions that never return. You can declare them |
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424 | `noreturn' to tell the compiler this fact. For example, |
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425 | |
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426 | void fatal () __attribute__ ((noreturn)); |
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427 | |
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428 | void |
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429 | fatal (...) |
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430 | { |
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431 | ... /* Print error message. */ ... |
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432 | exit (1); |
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433 | } |
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434 | |
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435 | The `noreturn' keyword tells the compiler to assume that `fatal' |
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436 | cannot return. It can then optimize without regard to what would |
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437 | happen if `fatal' ever did return. This makes slightly better |
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438 | code. More importantly, it helps avoid spurious warnings of |
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439 | uninitialized variables. |
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440 | |
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441 | Do not assume that registers saved by the calling function are |
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442 | restored before calling the `noreturn' function. |
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443 | |
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444 | It does not make sense for a `noreturn' function to have a return |
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445 | type other than `void'. |
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446 | |
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447 | The attribute `noreturn' is not implemented in GNU C versions |
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448 | earlier than 2.5. An alternative way to declare that a function |
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449 | does not return, which works in the current version and in some |
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450 | older versions, is as follows: |
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451 | |
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452 | typedef void voidfn (); |
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453 | |
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454 | volatile voidfn fatal; |
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455 | |
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456 | `const' |
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457 | Many functions do not examine any values except their arguments, |
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458 | and have no effects except the return value. Such a function can |
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459 | be subject to common subexpression elimination and loop |
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460 | optimization just as an arithmetic operator would be. These |
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461 | functions should be declared with the attribute `const'. For |
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462 | example, |
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463 | |
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464 | int square (int) __attribute__ ((const)); |
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465 | |
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466 | says that the hypothetical function `square' is safe to call fewer |
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467 | times than the program says. |
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468 | |
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469 | The attribute `const' is not implemented in GNU C versions earlier |
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470 | than 2.5. An alternative way to declare that a function has no |
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471 | side effects, which works in the current version and in some older |
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472 | versions, is as follows: |
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473 | |
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474 | typedef int intfn (); |
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475 | |
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476 | extern const intfn square; |
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477 | |
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478 | This approach does not work in GNU C++ from 2.6.0 on, since the |
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479 | language specifies that the `const' must be attached to the return |
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480 | value. |
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481 | |
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482 | Note that a function that has pointer arguments and examines the |
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483 | data pointed to must *not* be declared `const'. Likewise, a |
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484 | function that calls a non-`const' function usually must not be |
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485 | `const'. It does not make sense for a `const' function to return |
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486 | `void'. |
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487 | |
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488 | `format (ARCHETYPE, STRING-INDEX, FIRST-TO-CHECK)' |
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489 | The `format' attribute specifies that a function takes `printf' or |
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490 | `scanf' style arguments which should be type-checked against a |
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491 | format string. For example, the declaration: |
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492 | |
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493 | extern int |
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494 | my_printf (void *my_object, const char *my_format, ...) |
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495 | __attribute__ ((format (printf, 2, 3))); |
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496 | |
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497 | causes the compiler to check the arguments in calls to `my_printf' |
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498 | for consistency with the `printf' style format string argument |
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499 | `my_format'. |
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500 | |
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501 | The parameter ARCHETYPE determines how the format string is |
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502 | interpreted, and should be either `printf' or `scanf'. The |
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503 | parameter STRING-INDEX specifies which argument is the format |
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504 | string argument (starting from 1), while FIRST-TO-CHECK is the |
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505 | number of the first argument to check against the format string. |
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506 | For functions where the arguments are not available to be checked |
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507 | (such as `vprintf'), specify the third parameter as zero. In this |
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508 | case the compiler only checks the format string for consistency. |
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509 | |
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510 | In the example above, the format string (`my_format') is the second |
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511 | argument of the function `my_print', and the arguments to check |
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512 | start with the third argument, so the correct parameters for the |
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513 | format attribute are 2 and 3. |
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514 | |
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515 | The `format' attribute allows you to identify your own functions |
---|
516 | which take format strings as arguments, so that GNU CC can check |
---|
517 | the calls to these functions for errors. The compiler always |
---|
518 | checks formats for the ANSI library functions `printf', `fprintf', |
---|
519 | `sprintf', `scanf', `fscanf', `sscanf', `vprintf', `vfprintf' and |
---|
520 | `vsprintf' whenever such warnings are requested (using |
---|
521 | `-Wformat'), so there is no need to modify the header file |
---|
522 | `stdio.h'. |
---|
523 | |
---|
524 | `format_arg (STRING-INDEX)' |
---|
525 | The `format_arg' attribute specifies that a function takes |
---|
526 | `printf' or `scanf' style arguments, modifies it (for example, to |
---|
527 | translate it into another language), and passes it to a `printf' |
---|
528 | or `scanf' style function. For example, the declaration: |
---|
529 | |
---|
530 | extern char * |
---|
531 | my_dgettext (char *my_domain, const char *my_format) |
---|
532 | __attribute__ ((format_arg (2))); |
---|
533 | |
---|
534 | causes the compiler to check the arguments in calls to |
---|
535 | `my_dgettext' whose result is passed to a `printf' or `scanf' type |
---|
536 | function for consistency with the `printf' style format string |
---|
537 | argument `my_format'. |
---|
538 | |
---|
539 | The parameter STRING-INDEX specifies which argument is the format |
---|
540 | string argument (starting from 1). |
---|
541 | |
---|
542 | The `format-arg' attribute allows you to identify your own |
---|
543 | functions which modify format strings, so that GNU CC can check the |
---|
544 | calls to `printf' and `scanf' function whose operands are a call |
---|
545 | to one of your own function. The compiler always treats |
---|
546 | `gettext', `dgettext', and `dcgettext' in this manner. |
---|
547 | |
---|
548 | `section ("section-name")' |
---|
549 | Normally, the compiler places the code it generates in the `text' |
---|
550 | section. Sometimes, however, you need additional sections, or you |
---|
551 | need certain particular functions to appear in special sections. |
---|
552 | The `section' attribute specifies that a function lives in a |
---|
553 | particular section. For example, the declaration: |
---|
554 | |
---|
555 | extern void foobar (void) __attribute__ ((section ("bar"))); |
---|
556 | |
---|
557 | puts the function `foobar' in the `bar' section. |
---|
558 | |
---|
559 | Some file formats do not support arbitrary sections so the |
---|
560 | `section' attribute is not available on all platforms. If you |
---|
561 | need to map the entire contents of a module to a particular |
---|
562 | section, consider using the facilities of the linker instead. |
---|
563 | |
---|
564 | `constructor' |
---|
565 | `destructor' |
---|
566 | The `constructor' attribute causes the function to be called |
---|
567 | automatically before execution enters `main ()'. Similarly, the |
---|
568 | `destructor' attribute causes the function to be called |
---|
569 | automatically after `main ()' has completed or `exit ()' has been |
---|
570 | called. Functions with these attributes are useful for |
---|
571 | initializing data that will be used implicitly during the |
---|
572 | execution of the program. |
---|
573 | |
---|
574 | These attributes are not currently implemented for Objective C. |
---|
575 | |
---|
576 | `unused' |
---|
577 | This attribute, attached to a function, means that the function is |
---|
578 | meant to be possibly unused. GNU CC will not produce a warning |
---|
579 | for this function. GNU C++ does not currently support this |
---|
580 | attribute as definitions without parameters are valid in C++. |
---|
581 | |
---|
582 | `weak' |
---|
583 | The `weak' attribute causes the declaration to be emitted as a weak |
---|
584 | symbol rather than a global. This is primarily useful in defining |
---|
585 | library functions which can be overridden in user code, though it |
---|
586 | can also be used with non-function declarations. Weak symbols are |
---|
587 | supported for ELF targets, and also for a.out targets when using |
---|
588 | the GNU assembler and linker. |
---|
589 | |
---|
590 | `alias ("target")' |
---|
591 | The `alias' attribute causes the declaration to be emitted as an |
---|
592 | alias for another symbol, which must be specified. For instance, |
---|
593 | |
---|
594 | void __f () { /* do something */; } |
---|
595 | void f () __attribute__ ((weak, alias ("__f"))); |
---|
596 | |
---|
597 | declares `f' to be a weak alias for `__f'. In C++, the mangled |
---|
598 | name for the target must be used. |
---|
599 | |
---|
600 | Not all target machines support this attribute. |
---|
601 | |
---|
602 | `regparm (NUMBER)' |
---|
603 | On the Intel 386, the `regparm' attribute causes the compiler to |
---|
604 | pass up to NUMBER integer arguments in registers EAX, EDX, and ECX |
---|
605 | instead of on the stack. Functions that take a variable number of |
---|
606 | arguments will continue to be passed all of their arguments on the |
---|
607 | stack. |
---|
608 | |
---|
609 | `stdcall' |
---|
610 | On the Intel 386, the `stdcall' attribute causes the compiler to |
---|
611 | assume that the called function will pop off the stack space used |
---|
612 | to pass arguments, unless it takes a variable number of arguments. |
---|
613 | |
---|
614 | The PowerPC compiler for Windows NT currently ignores the `stdcall' |
---|
615 | attribute. |
---|
616 | |
---|
617 | `cdecl' |
---|
618 | On the Intel 386, the `cdecl' attribute causes the compiler to |
---|
619 | assume that the calling function will pop off the stack space used |
---|
620 | to pass arguments. This is useful to override the effects of the |
---|
621 | `-mrtd' switch. |
---|
622 | |
---|
623 | The PowerPC compiler for Windows NT currently ignores the `cdecl' |
---|
624 | attribute. |
---|
625 | |
---|
626 | `longcall' |
---|
627 | On the RS/6000 and PowerPC, the `longcall' attribute causes the |
---|
628 | compiler to always call the function via a pointer, so that |
---|
629 | functions which reside further than 64 megabytes (67,108,864 |
---|
630 | bytes) from the current location can be called. |
---|
631 | |
---|
632 | `dllimport' |
---|
633 | On the PowerPC running Windows NT, the `dllimport' attribute causes |
---|
634 | the compiler to call the function via a global pointer to the |
---|
635 | function pointer that is set up by the Windows NT dll library. |
---|
636 | The pointer name is formed by combining `__imp_' and the function |
---|
637 | name. |
---|
638 | |
---|
639 | `dllexport' |
---|
640 | On the PowerPC running Windows NT, the `dllexport' attribute causes |
---|
641 | the compiler to provide a global pointer to the function pointer, |
---|
642 | so that it can be called with the `dllimport' attribute. The |
---|
643 | pointer name is formed by combining `__imp_' and the function name. |
---|
644 | |
---|
645 | `exception (EXCEPT-FUNC [, EXCEPT-ARG])' |
---|
646 | On the PowerPC running Windows NT, the `exception' attribute causes |
---|
647 | the compiler to modify the structured exception table entry it |
---|
648 | emits for the declared function. The string or identifier |
---|
649 | EXCEPT-FUNC is placed in the third entry of the structured |
---|
650 | exception table. It represents a function, which is called by the |
---|
651 | exception handling mechanism if an exception occurs. If it was |
---|
652 | specified, the string or identifier EXCEPT-ARG is placed in the |
---|
653 | fourth entry of the structured exception table. |
---|
654 | |
---|
655 | `function_vector' |
---|
656 | Use this option on the H8/300 and H8/300H to indicate that the |
---|
657 | specified function should be called through the function vector. |
---|
658 | Calling a function through the function vector will reduce code |
---|
659 | size, however; the function vector has a limited size (maximum 128 |
---|
660 | entries on the H8/300 and 64 entries on the H8/300H) and shares |
---|
661 | space with the interrupt vector. |
---|
662 | |
---|
663 | You must use GAS and GLD from GNU binutils version 2.7 or later for |
---|
664 | this option to work correctly. |
---|
665 | |
---|
666 | `interrupt_handler' |
---|
667 | Use this option on the H8/300 and H8/300H to indicate that the |
---|
668 | specified function is an interrupt handler. The compiler will |
---|
669 | generate function entry and exit sequences suitable for use in an |
---|
670 | interrupt handler when this attribute is present. |
---|
671 | |
---|
672 | `eightbit_data' |
---|
673 | Use this option on the H8/300 and H8/300H to indicate that the |
---|
674 | specified variable should be placed into the eight bit data |
---|
675 | section. The compiler will generate more efficient code for |
---|
676 | certain operations on data in the eight bit data area. Note the |
---|
677 | eight bit data area is limited to 256 bytes of data. |
---|
678 | |
---|
679 | You must use GAS and GLD from GNU binutils version 2.7 or later for |
---|
680 | this option to work correctly. |
---|
681 | |
---|
682 | `tiny_data' |
---|
683 | Use this option on the H8/300H to indicate that the specified |
---|
684 | variable should be placed into the tiny data section. The |
---|
685 | compiler will generate more efficient code for loads and stores on |
---|
686 | data in the tiny data section. Note the tiny data area is limited |
---|
687 | to slightly under 32kbytes of data. |
---|
688 | |
---|
689 | `interrupt' |
---|
690 | Use this option on the M32R/D to indicate that the specified |
---|
691 | function is an interrupt handler. The compiler will generate |
---|
692 | function entry and exit sequences suitable for use in an interrupt |
---|
693 | handler when this attribute is present. |
---|
694 | |
---|
695 | `model (MODEL-NAME)' |
---|
696 | Use this attribute on the M32R/D to set the addressability of an |
---|
697 | object, and the code generated for a function. The identifier |
---|
698 | MODEL-NAME is one of `small', `medium', or `large', representing |
---|
699 | each of the code models. |
---|
700 | |
---|
701 | Small model objects live in the lower 16MB of memory (so that their |
---|
702 | addresses can be loaded with the `ld24' instruction), and are |
---|
703 | callable with the `bl' instruction. |
---|
704 | |
---|
705 | Medium model objects may live anywhere in the 32 bit address space |
---|
706 | (the compiler will generate `seth/add3' instructions to load their |
---|
707 | addresses), and are callable with the `bl' instruction. |
---|
708 | |
---|
709 | Large model objects may live anywhere in the 32 bit address space |
---|
710 | (the compiler will generate `seth/add3' instructions to load their |
---|
711 | addresses), and may not be reachable with the `bl' instruction |
---|
712 | (the compiler will generate the much slower `seth/add3/jl' |
---|
713 | instruction sequence). |
---|
714 | |
---|
715 | You can specify multiple attributes in a declaration by separating |
---|
716 | them by commas within the double parentheses or by immediately |
---|
717 | following an attribute declaration with another attribute declaration. |
---|
718 | |
---|
719 | Some people object to the `__attribute__' feature, suggesting that |
---|
720 | ANSI C's `#pragma' should be used instead. There are two reasons for |
---|
721 | not doing this. |
---|
722 | |
---|
723 | 1. It is impossible to generate `#pragma' commands from a macro. |
---|
724 | |
---|
725 | 2. There is no telling what the same `#pragma' might mean in another |
---|
726 | compiler. |
---|
727 | |
---|
728 | These two reasons apply to almost any application that might be |
---|
729 | proposed for `#pragma'. It is basically a mistake to use `#pragma' for |
---|
730 | *anything*. |
---|
731 | |
---|
732 | |
---|
733 | File: gcc.info, Node: Function Prototypes, Next: C++ Comments, Prev: Function Attributes, Up: C Extensions |
---|
734 | |
---|
735 | Prototypes and Old-Style Function Definitions |
---|
736 | ============================================= |
---|
737 | |
---|
738 | GNU C extends ANSI C to allow a function prototype to override a |
---|
739 | later old-style non-prototype definition. Consider the following |
---|
740 | example: |
---|
741 | |
---|
742 | /* Use prototypes unless the compiler is old-fashioned. */ |
---|
743 | #ifdef __STDC__ |
---|
744 | #define P(x) x |
---|
745 | #else |
---|
746 | #define P(x) () |
---|
747 | #endif |
---|
748 | |
---|
749 | /* Prototype function declaration. */ |
---|
750 | int isroot P((uid_t)); |
---|
751 | |
---|
752 | /* Old-style function definition. */ |
---|
753 | int |
---|
754 | isroot (x) /* ??? lossage here ??? */ |
---|
755 | uid_t x; |
---|
756 | { |
---|
757 | return x == 0; |
---|
758 | } |
---|
759 | |
---|
760 | Suppose the type `uid_t' happens to be `short'. ANSI C does not |
---|
761 | allow this example, because subword arguments in old-style |
---|
762 | non-prototype definitions are promoted. Therefore in this example the |
---|
763 | function definition's argument is really an `int', which does not match |
---|
764 | the prototype argument type of `short'. |
---|
765 | |
---|
766 | This restriction of ANSI C makes it hard to write code that is |
---|
767 | portable to traditional C compilers, because the programmer does not |
---|
768 | know whether the `uid_t' type is `short', `int', or `long'. Therefore, |
---|
769 | in cases like these GNU C allows a prototype to override a later |
---|
770 | old-style definition. More precisely, in GNU C, a function prototype |
---|
771 | argument type overrides the argument type specified by a later |
---|
772 | old-style definition if the former type is the same as the latter type |
---|
773 | before promotion. Thus in GNU C the above example is equivalent to the |
---|
774 | following: |
---|
775 | |
---|
776 | int isroot (uid_t); |
---|
777 | |
---|
778 | int |
---|
779 | isroot (uid_t x) |
---|
780 | { |
---|
781 | return x == 0; |
---|
782 | } |
---|
783 | |
---|
784 | GNU C++ does not support old-style function definitions, so this |
---|
785 | extension is irrelevant. |
---|
786 | |
---|
787 | |
---|
788 | File: gcc.info, Node: C++ Comments, Next: Dollar Signs, Prev: Function Prototypes, Up: C Extensions |
---|
789 | |
---|
790 | C++ Style Comments |
---|
791 | ================== |
---|
792 | |
---|
793 | In GNU C, you may use C++ style comments, which start with `//' and |
---|
794 | continue until the end of the line. Many other C implementations allow |
---|
795 | such comments, and they are likely to be in a future C standard. |
---|
796 | However, C++ style comments are not recognized if you specify `-ansi' |
---|
797 | or `-traditional', since they are incompatible with traditional |
---|
798 | constructs like `dividend//*comment*/divisor'. |
---|
799 | |
---|
800 | |
---|
801 | File: gcc.info, Node: Dollar Signs, Next: Character Escapes, Prev: C++ Comments, Up: C Extensions |
---|
802 | |
---|
803 | Dollar Signs in Identifier Names |
---|
804 | ================================ |
---|
805 | |
---|
806 | In GNU C, you may normally use dollar signs in identifier names. |
---|
807 | This is because many traditional C implementations allow such |
---|
808 | identifiers. However, dollar signs in identifiers are not supported on |
---|
809 | a few target machines, typically because the target assembler does not |
---|
810 | allow them. |
---|
811 | |
---|
812 | |
---|
813 | File: gcc.info, Node: Character Escapes, Next: Variable Attributes, Prev: Dollar Signs, Up: C Extensions |
---|
814 | |
---|
815 | The Character <ESC> in Constants |
---|
816 | ================================ |
---|
817 | |
---|
818 | You can use the sequence `\e' in a string or character constant to |
---|
819 | stand for the ASCII character <ESC>. |
---|
820 | |
---|
821 | |
---|
822 | File: gcc.info, Node: Alignment, Next: Inline, Prev: Type Attributes, Up: C Extensions |
---|
823 | |
---|
824 | Inquiring on Alignment of Types or Variables |
---|
825 | ============================================ |
---|
826 | |
---|
827 | The keyword `__alignof__' allows you to inquire about how an object |
---|
828 | is aligned, or the minimum alignment usually required by a type. Its |
---|
829 | syntax is just like `sizeof'. |
---|
830 | |
---|
831 | For example, if the target machine requires a `double' value to be |
---|
832 | aligned on an 8-byte boundary, then `__alignof__ (double)' is 8. This |
---|
833 | is true on many RISC machines. On more traditional machine designs, |
---|
834 | `__alignof__ (double)' is 4 or even 2. |
---|
835 | |
---|
836 | Some machines never actually require alignment; they allow reference |
---|
837 | to any data type even at an odd addresses. For these machines, |
---|
838 | `__alignof__' reports the *recommended* alignment of a type. |
---|
839 | |
---|
840 | When the operand of `__alignof__' is an lvalue rather than a type, |
---|
841 | the value is the largest alignment that the lvalue is known to have. |
---|
842 | It may have this alignment as a result of its data type, or because it |
---|
843 | is part of a structure and inherits alignment from that structure. For |
---|
844 | example, after this declaration: |
---|
845 | |
---|
846 | struct foo { int x; char y; } foo1; |
---|
847 | |
---|
848 | the value of `__alignof__ (foo1.y)' is probably 2 or 4, the same as |
---|
849 | `__alignof__ (int)', even though the data type of `foo1.y' does not |
---|
850 | itself demand any alignment. |
---|
851 | |
---|
852 | A related feature which lets you specify the alignment of an object |
---|
853 | is `__attribute__ ((aligned (ALIGNMENT)))'; see the following section. |
---|
854 | |
---|
855 | |
---|
856 | File: gcc.info, Node: Variable Attributes, Next: Type Attributes, Prev: Character Escapes, Up: C Extensions |
---|
857 | |
---|
858 | Specifying Attributes of Variables |
---|
859 | ================================== |
---|
860 | |
---|
861 | The keyword `__attribute__' allows you to specify special attributes |
---|
862 | of variables or structure fields. This keyword is followed by an |
---|
863 | attribute specification inside double parentheses. Eight attributes |
---|
864 | are currently defined for variables: `aligned', `mode', `nocommon', |
---|
865 | `packed', `section', `transparent_union', `unused', and `weak'. Other |
---|
866 | attributes are available for functions (*note Function Attributes::.) |
---|
867 | and for types (*note Type Attributes::.). |
---|
868 | |
---|
869 | You may also specify attributes with `__' preceding and following |
---|
870 | each keyword. This allows you to use them in header files without |
---|
871 | being concerned about a possible macro of the same name. For example, |
---|
872 | you may use `__aligned__' instead of `aligned'. |
---|
873 | |
---|
874 | `aligned (ALIGNMENT)' |
---|
875 | This attribute specifies a minimum alignment for the variable or |
---|
876 | structure field, measured in bytes. For example, the declaration: |
---|
877 | |
---|
878 | int x __attribute__ ((aligned (16))) = 0; |
---|
879 | |
---|
880 | causes the compiler to allocate the global variable `x' on a |
---|
881 | 16-byte boundary. On a 68040, this could be used in conjunction |
---|
882 | with an `asm' expression to access the `move16' instruction which |
---|
883 | requires 16-byte aligned operands. |
---|
884 | |
---|
885 | You can also specify the alignment of structure fields. For |
---|
886 | example, to create a double-word aligned `int' pair, you could |
---|
887 | write: |
---|
888 | |
---|
889 | struct foo { int x[2] __attribute__ ((aligned (8))); }; |
---|
890 | |
---|
891 | This is an alternative to creating a union with a `double' member |
---|
892 | that forces the union to be double-word aligned. |
---|
893 | |
---|
894 | It is not possible to specify the alignment of functions; the |
---|
895 | alignment of functions is determined by the machine's requirements |
---|
896 | and cannot be changed. You cannot specify alignment for a typedef |
---|
897 | name because such a name is just an alias, not a distinct type. |
---|
898 | |
---|
899 | As in the preceding examples, you can explicitly specify the |
---|
900 | alignment (in bytes) that you wish the compiler to use for a given |
---|
901 | variable or structure field. Alternatively, you can leave out the |
---|
902 | alignment factor and just ask the compiler to align a variable or |
---|
903 | field to the maximum useful alignment for the target machine you |
---|
904 | are compiling for. For example, you could write: |
---|
905 | |
---|
906 | short array[3] __attribute__ ((aligned)); |
---|
907 | |
---|
908 | Whenever you leave out the alignment factor in an `aligned' |
---|
909 | attribute specification, the compiler automatically sets the |
---|
910 | alignment for the declared variable or field to the largest |
---|
911 | alignment which is ever used for any data type on the target |
---|
912 | machine you are compiling for. Doing this can often make copy |
---|
913 | operations more efficient, because the compiler can use whatever |
---|
914 | instructions copy the biggest chunks of memory when performing |
---|
915 | copies to or from the variables or fields that you have aligned |
---|
916 | this way. |
---|
917 | |
---|
918 | The `aligned' attribute can only increase the alignment; but you |
---|
919 | can decrease it by specifying `packed' as well. See below. |
---|
920 | |
---|
921 | Note that the effectiveness of `aligned' attributes may be limited |
---|
922 | by inherent limitations in your linker. On many systems, the |
---|
923 | linker is only able to arrange for variables to be aligned up to a |
---|
924 | certain maximum alignment. (For some linkers, the maximum |
---|
925 | supported alignment may be very very small.) If your linker is |
---|
926 | only able to align variables up to a maximum of 8 byte alignment, |
---|
927 | then specifying `aligned(16)' in an `__attribute__' will still |
---|
928 | only provide you with 8 byte alignment. See your linker |
---|
929 | documentation for further information. |
---|
930 | |
---|
931 | `mode (MODE)' |
---|
932 | This attribute specifies the data type for the |
---|
933 | declaration--whichever type corresponds to the mode MODE. This in |
---|
934 | effect lets you request an integer or floating point type |
---|
935 | according to its width. |
---|
936 | |
---|
937 | You may also specify a mode of `byte' or `__byte__' to indicate |
---|
938 | the mode corresponding to a one-byte integer, `word' or `__word__' |
---|
939 | for the mode of a one-word integer, and `pointer' or `__pointer__' |
---|
940 | for the mode used to represent pointers. |
---|
941 | |
---|
942 | `nocommon' |
---|
943 | This attribute specifies requests GNU CC not to place a variable |
---|
944 | "common" but instead to allocate space for it directly. If you |
---|
945 | specify the `-fno-common' flag, GNU CC will do this for all |
---|
946 | variables. |
---|
947 | |
---|
948 | Specifying the `nocommon' attribute for a variable provides an |
---|
949 | initialization of zeros. A variable may only be initialized in one |
---|
950 | source file. |
---|
951 | |
---|
952 | `packed' |
---|
953 | The `packed' attribute specifies that a variable or structure field |
---|
954 | should have the smallest possible alignment--one byte for a |
---|
955 | variable, and one bit for a field, unless you specify a larger |
---|
956 | value with the `aligned' attribute. |
---|
957 | |
---|
958 | Here is a structure in which the field `x' is packed, so that it |
---|
959 | immediately follows `a': |
---|
960 | |
---|
961 | struct foo |
---|
962 | { |
---|
963 | char a; |
---|
964 | int x[2] __attribute__ ((packed)); |
---|
965 | }; |
---|
966 | |
---|
967 | `section ("section-name")' |
---|
968 | Normally, the compiler places the objects it generates in sections |
---|
969 | like `data' and `bss'. Sometimes, however, you need additional |
---|
970 | sections, or you need certain particular variables to appear in |
---|
971 | special sections, for example to map to special hardware. The |
---|
972 | `section' attribute specifies that a variable (or function) lives |
---|
973 | in a particular section. For example, this small program uses |
---|
974 | several specific section names: |
---|
975 | |
---|
976 | struct duart a __attribute__ ((section ("DUART_A"))) = { 0 }; |
---|
977 | struct duart b __attribute__ ((section ("DUART_B"))) = { 0 }; |
---|
978 | char stack[10000] __attribute__ ((section ("STACK"))) = { 0 }; |
---|
979 | int init_data __attribute__ ((section ("INITDATA"))) = 0; |
---|
980 | |
---|
981 | main() |
---|
982 | { |
---|
983 | /* Initialize stack pointer */ |
---|
984 | init_sp (stack + sizeof (stack)); |
---|
985 | |
---|
986 | /* Initialize initialized data */ |
---|
987 | memcpy (&init_data, &data, &edata - &data); |
---|
988 | |
---|
989 | /* Turn on the serial ports */ |
---|
990 | init_duart (&a); |
---|
991 | init_duart (&b); |
---|
992 | } |
---|
993 | |
---|
994 | Use the `section' attribute with an *initialized* definition of a |
---|
995 | *global* variable, as shown in the example. GNU CC issues a |
---|
996 | warning and otherwise ignores the `section' attribute in |
---|
997 | uninitialized variable declarations. |
---|
998 | |
---|
999 | You may only use the `section' attribute with a fully initialized |
---|
1000 | global definition because of the way linkers work. The linker |
---|
1001 | requires each object be defined once, with the exception that |
---|
1002 | uninitialized variables tentatively go in the `common' (or `bss') |
---|
1003 | section and can be multiply "defined". You can force a variable |
---|
1004 | to be initialized with the `-fno-common' flag or the `nocommon' |
---|
1005 | attribute. |
---|
1006 | |
---|
1007 | Some file formats do not support arbitrary sections so the |
---|
1008 | `section' attribute is not available on all platforms. If you |
---|
1009 | need to map the entire contents of a module to a particular |
---|
1010 | section, consider using the facilities of the linker instead. |
---|
1011 | |
---|
1012 | `transparent_union' |
---|
1013 | This attribute, attached to a function parameter which is a union, |
---|
1014 | means that the corresponding argument may have the type of any |
---|
1015 | union member, but the argument is passed as if its type were that |
---|
1016 | of the first union member. For more details see *Note Type |
---|
1017 | Attributes::. You can also use this attribute on a `typedef' for |
---|
1018 | a union data type; then it applies to all function parameters with |
---|
1019 | that type. |
---|
1020 | |
---|
1021 | `unused' |
---|
1022 | This attribute, attached to a variable, means that the variable is |
---|
1023 | meant to be possibly unused. GNU CC will not produce a warning |
---|
1024 | for this variable. |
---|
1025 | |
---|
1026 | `weak' |
---|
1027 | The `weak' attribute is described in *Note Function Attributes::. |
---|
1028 | |
---|
1029 | `model (MODEL-NAME)' |
---|
1030 | Use this attribute on the M32R/D to set the addressability of an |
---|
1031 | object. The identifier MODEL-NAME is one of `small', `medium', or |
---|
1032 | `large', representing each of the code models. |
---|
1033 | |
---|
1034 | Small model objects live in the lower 16MB of memory (so that their |
---|
1035 | addresses can be loaded with the `ld24' instruction). |
---|
1036 | |
---|
1037 | Medium and large model objects may live anywhere in the 32 bit |
---|
1038 | address space (the compiler will generate `seth/add3' instructions |
---|
1039 | to load their addresses). |
---|
1040 | |
---|
1041 | To specify multiple attributes, separate them by commas within the |
---|
1042 | double parentheses: for example, `__attribute__ ((aligned (16), |
---|
1043 | packed))'. |
---|
1044 | |
---|
1045 | |
---|
1046 | File: gcc.info, Node: Type Attributes, Next: Alignment, Prev: Variable Attributes, Up: C Extensions |
---|
1047 | |
---|
1048 | Specifying Attributes of Types |
---|
1049 | ============================== |
---|
1050 | |
---|
1051 | The keyword `__attribute__' allows you to specify special attributes |
---|
1052 | of `struct' and `union' types when you define such types. This keyword |
---|
1053 | is followed by an attribute specification inside double parentheses. |
---|
1054 | Three attributes are currently defined for types: `aligned', `packed', |
---|
1055 | and `transparent_union'. Other attributes are defined for functions |
---|
1056 | (*note Function Attributes::.) and for variables (*note Variable |
---|
1057 | Attributes::.). |
---|
1058 | |
---|
1059 | You may also specify any one of these attributes with `__' preceding |
---|
1060 | and following its keyword. This allows you to use these attributes in |
---|
1061 | header files without being concerned about a possible macro of the same |
---|
1062 | name. For example, you may use `__aligned__' instead of `aligned'. |
---|
1063 | |
---|
1064 | You may specify the `aligned' and `transparent_union' attributes |
---|
1065 | either in a `typedef' declaration or just past the closing curly brace |
---|
1066 | of a complete enum, struct or union type *definition* and the `packed' |
---|
1067 | attribute only past the closing brace of a definition. |
---|
1068 | |
---|
1069 | `aligned (ALIGNMENT)' |
---|
1070 | This attribute specifies a minimum alignment (in bytes) for |
---|
1071 | variables of the specified type. For example, the declarations: |
---|
1072 | |
---|
1073 | struct S { short f[3]; } __attribute__ ((aligned (8))); |
---|
1074 | typedef int more_aligned_int __attribute__ ((aligned (8))); |
---|
1075 | |
---|
1076 | force the compiler to insure (as far as it can) that each variable |
---|
1077 | whose type is `struct S' or `more_aligned_int' will be allocated |
---|
1078 | and aligned *at least* on a 8-byte boundary. On a Sparc, having |
---|
1079 | all variables of type `struct S' aligned to 8-byte boundaries |
---|
1080 | allows the compiler to use the `ldd' and `std' (doubleword load and |
---|
1081 | store) instructions when copying one variable of type `struct S' to |
---|
1082 | another, thus improving run-time efficiency. |
---|
1083 | |
---|
1084 | Note that the alignment of any given `struct' or `union' type is |
---|
1085 | required by the ANSI C standard to be at least a perfect multiple |
---|
1086 | of the lowest common multiple of the alignments of all of the |
---|
1087 | members of the `struct' or `union' in question. This means that |
---|
1088 | you *can* effectively adjust the alignment of a `struct' or `union' |
---|
1089 | type by attaching an `aligned' attribute to any one of the members |
---|
1090 | of such a type, but the notation illustrated in the example above |
---|
1091 | is a more obvious, intuitive, and readable way to request the |
---|
1092 | compiler to adjust the alignment of an entire `struct' or `union' |
---|
1093 | type. |
---|
1094 | |
---|
1095 | As in the preceding example, you can explicitly specify the |
---|
1096 | alignment (in bytes) that you wish the compiler to use for a given |
---|
1097 | `struct' or `union' type. Alternatively, you can leave out the |
---|
1098 | alignment factor and just ask the compiler to align a type to the |
---|
1099 | maximum useful alignment for the target machine you are compiling |
---|
1100 | for. For example, you could write: |
---|
1101 | |
---|
1102 | struct S { short f[3]; } __attribute__ ((aligned)); |
---|
1103 | |
---|
1104 | Whenever you leave out the alignment factor in an `aligned' |
---|
1105 | attribute specification, the compiler automatically sets the |
---|
1106 | alignment for the type to the largest alignment which is ever used |
---|
1107 | for any data type on the target machine you are compiling for. |
---|
1108 | Doing this can often make copy operations more efficient, because |
---|
1109 | the compiler can use whatever instructions copy the biggest chunks |
---|
1110 | of memory when performing copies to or from the variables which |
---|
1111 | have types that you have aligned this way. |
---|
1112 | |
---|
1113 | In the example above, if the size of each `short' is 2 bytes, then |
---|
1114 | the size of the entire `struct S' type is 6 bytes. The smallest |
---|
1115 | power of two which is greater than or equal to that is 8, so the |
---|
1116 | compiler sets the alignment for the entire `struct S' type to 8 |
---|
1117 | bytes. |
---|
1118 | |
---|
1119 | Note that although you can ask the compiler to select a |
---|
1120 | time-efficient alignment for a given type and then declare only |
---|
1121 | individual stand-alone objects of that type, the compiler's |
---|
1122 | ability to select a time-efficient alignment is primarily useful |
---|
1123 | only when you plan to create arrays of variables having the |
---|
1124 | relevant (efficiently aligned) type. If you declare or use arrays |
---|
1125 | of variables of an efficiently-aligned type, then it is likely |
---|
1126 | that your program will also be doing pointer arithmetic (or |
---|
1127 | subscripting, which amounts to the same thing) on pointers to the |
---|
1128 | relevant type, and the code that the compiler generates for these |
---|
1129 | pointer arithmetic operations will often be more efficient for |
---|
1130 | efficiently-aligned types than for other types. |
---|
1131 | |
---|
1132 | The `aligned' attribute can only increase the alignment; but you |
---|
1133 | can decrease it by specifying `packed' as well. See below. |
---|
1134 | |
---|
1135 | Note that the effectiveness of `aligned' attributes may be limited |
---|
1136 | by inherent limitations in your linker. On many systems, the |
---|
1137 | linker is only able to arrange for variables to be aligned up to a |
---|
1138 | certain maximum alignment. (For some linkers, the maximum |
---|
1139 | supported alignment may be very very small.) If your linker is |
---|
1140 | only able to align variables up to a maximum of 8 byte alignment, |
---|
1141 | then specifying `aligned(16)' in an `__attribute__' will still |
---|
1142 | only provide you with 8 byte alignment. See your linker |
---|
1143 | documentation for further information. |
---|
1144 | |
---|
1145 | `packed' |
---|
1146 | This attribute, attached to an `enum', `struct', or `union' type |
---|
1147 | definition, specified that the minimum required memory be used to |
---|
1148 | represent the type. |
---|
1149 | |
---|
1150 | Specifying this attribute for `struct' and `union' types is |
---|
1151 | equivalent to specifying the `packed' attribute on each of the |
---|
1152 | structure or union members. Specifying the `-fshort-enums' flag |
---|
1153 | on the line is equivalent to specifying the `packed' attribute on |
---|
1154 | all `enum' definitions. |
---|
1155 | |
---|
1156 | You may only specify this attribute after a closing curly brace on |
---|
1157 | an `enum' definition, not in a `typedef' declaration, unless that |
---|
1158 | declaration also contains the definition of the `enum'. |
---|
1159 | |
---|
1160 | `transparent_union' |
---|
1161 | This attribute, attached to a `union' type definition, indicates |
---|
1162 | that any function parameter having that union type causes calls to |
---|
1163 | that function to be treated in a special way. |
---|
1164 | |
---|
1165 | First, the argument corresponding to a transparent union type can |
---|
1166 | be of any type in the union; no cast is required. Also, if the |
---|
1167 | union contains a pointer type, the corresponding argument can be a |
---|
1168 | null pointer constant or a void pointer expression; and if the |
---|
1169 | union contains a void pointer type, the corresponding argument can |
---|
1170 | be any pointer expression. If the union member type is a pointer, |
---|
1171 | qualifiers like `const' on the referenced type must be respected, |
---|
1172 | just as with normal pointer conversions. |
---|
1173 | |
---|
1174 | Second, the argument is passed to the function using the calling |
---|
1175 | conventions of first member of the transparent union, not the |
---|
1176 | calling conventions of the union itself. All members of the union |
---|
1177 | must have the same machine representation; this is necessary for |
---|
1178 | this argument passing to work properly. |
---|
1179 | |
---|
1180 | Transparent unions are designed for library functions that have |
---|
1181 | multiple interfaces for compatibility reasons. For example, |
---|
1182 | suppose the `wait' function must accept either a value of type |
---|
1183 | `int *' to comply with Posix, or a value of type `union wait *' to |
---|
1184 | comply with the 4.1BSD interface. If `wait''s parameter were |
---|
1185 | `void *', `wait' would accept both kinds of arguments, but it |
---|
1186 | would also accept any other pointer type and this would make |
---|
1187 | argument type checking less useful. Instead, `<sys/wait.h>' might |
---|
1188 | define the interface as follows: |
---|
1189 | |
---|
1190 | typedef union |
---|
1191 | { |
---|
1192 | int *__ip; |
---|
1193 | union wait *__up; |
---|
1194 | } wait_status_ptr_t __attribute__ ((__transparent_union__)); |
---|
1195 | |
---|
1196 | pid_t wait (wait_status_ptr_t); |
---|
1197 | |
---|
1198 | This interface allows either `int *' or `union wait *' arguments |
---|
1199 | to be passed, using the `int *' calling convention. The program |
---|
1200 | can call `wait' with arguments of either type: |
---|
1201 | |
---|
1202 | int w1 () { int w; return wait (&w); } |
---|
1203 | int w2 () { union wait w; return wait (&w); } |
---|
1204 | |
---|
1205 | With this interface, `wait''s implementation might look like this: |
---|
1206 | |
---|
1207 | pid_t wait (wait_status_ptr_t p) |
---|
1208 | { |
---|
1209 | return waitpid (-1, p.__ip, 0); |
---|
1210 | } |
---|
1211 | |
---|
1212 | `unused' |
---|
1213 | When attached to a type (including a `union' or a `struct'), this |
---|
1214 | attribute means that variables of that type are meant to appear |
---|
1215 | possibly unused. GNU CC will not produce a warning for any |
---|
1216 | variables of that type, even if the variable appears to do |
---|
1217 | nothing. This is often the case with lock or thread classes, |
---|
1218 | which are usually defined and then not referenced, but contain |
---|
1219 | constructors and destructors that have nontrivial bookkeeping |
---|
1220 | functions. |
---|
1221 | |
---|
1222 | To specify multiple attributes, separate them by commas within the |
---|
1223 | double parentheses: for example, `__attribute__ ((aligned (16), |
---|
1224 | packed))'. |
---|
1225 | |
---|