This is Info file gcc.info, produced by Makeinfo-1.55 from the input file gcc.texi. This file documents the use and the internals of the GNU compiler. Published by the Free Software Foundation 59 Temple Place - Suite 330 Boston, MA 02111-1307 USA Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the sections entitled "GNU General Public License," "Funding for Free Software," and "Protect Your Freedom--Fight `Look And Feel'" are included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the sections entitled "GNU General Public License," "Funding for Free Software," and "Protect Your Freedom--Fight `Look And Feel'", and this permission notice, may be included in translations approved by the Free Software Foundation instead of in the original English.  File: gcc.info, Node: Constructors, Next: Labeled Elements, Prev: Initializers, Up: C Extensions Constructor Expressions ======================= GNU C supports constructor expressions. A constructor looks like a cast containing an initializer. Its value is an object of the type specified in the cast, containing the elements specified in the initializer. Usually, the specified type is a structure. Assume that `struct foo' and `structure' are declared as shown: struct foo {int a; char b[2];} structure; Here is an example of constructing a `struct foo' with a constructor: structure = ((struct foo) {x + y, 'a', 0}); This is equivalent to writing the following: { struct foo temp = {x + y, 'a', 0}; structure = temp; } You can also construct an array. If all the elements of the constructor are (made up of) simple constant expressions, suitable for use in initializers, then the constructor is an lvalue and can be coerced to a pointer to its first element, as shown here: char **foo = (char *[]) { "x", "y", "z" }; Array constructors whose elements are not simple constants are not very useful, because the constructor is not an lvalue. There are only two valid ways to use it: to subscript it, or initialize an array variable with it. The former is probably slower than a `switch' statement, while the latter does the same thing an ordinary C initializer would do. Here is an example of subscripting an array constructor: output = ((int[]) { 2, x, 28 }) [input]; Constructor expressions for scalar types and union types are is also allowed, but then the constructor expression is equivalent to a cast.  File: gcc.info, Node: Labeled Elements, Next: Cast to Union, Prev: Constructors, Up: C Extensions Labeled Elements in Initializers ================================ Standard C requires the elements of an initializer to appear in a fixed order, the same as the order of the elements in the array or structure being initialized. In GNU C you can give the elements in any order, specifying the array indices or structure field names they apply to. This extension is not implemented in GNU C++. To specify an array index, write `[INDEX]' or `[INDEX] =' before the element value. For example, int a[6] = { [4] 29, [2] = 15 }; is equivalent to int a[6] = { 0, 0, 15, 0, 29, 0 }; The index values must be constant expressions, even if the array being initialized is automatic. To initialize a range of elements to the same value, write `[FIRST ... LAST] = VALUE'. For example, int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 }; Note that the length of the array is the highest value specified plus one. In a structure initializer, specify the name of a field to initialize with `FIELDNAME:' before the element value. For example, given the following structure, struct point { int x, y; }; the following initialization struct point p = { y: yvalue, x: xvalue }; is equivalent to struct point p = { xvalue, yvalue }; Another syntax which has the same meaning is `.FIELDNAME ='., as shown here: struct point p = { .y = yvalue, .x = xvalue }; You can also use an element label (with either the colon syntax or the period-equal syntax) when initializing a union, to specify which element of the union should be used. For example, union foo { int i; double d; }; union foo f = { d: 4 }; will convert 4 to a `double' to store it in the union using the second element. By contrast, casting 4 to type `union foo' would store it into the union as the integer `i', since it is an integer. (*Note Cast to Union::.) You can combine this technique of naming elements with ordinary C initialization of successive elements. Each initializer element that does not have a label applies to the next consecutive element of the array or structure. For example, int a[6] = { [1] = v1, v2, [4] = v4 }; is equivalent to int a[6] = { 0, v1, v2, 0, v4, 0 }; Labeling the elements of an array initializer is especially useful when the indices are characters or belong to an `enum' type. For example: int whitespace[256] = { [' '] = 1, ['\t'] = 1, ['\h'] = 1, ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };  File: gcc.info, Node: Case Ranges, Next: Function Attributes, Prev: Cast to Union, Up: C Extensions Case Ranges =========== You can specify a range of consecutive values in a single `case' label, like this: case LOW ... HIGH: This has the same effect as the proper number of individual `case' labels, one for each integer value from LOW to HIGH, inclusive. This feature is especially useful for ranges of ASCII character codes: case 'A' ... 'Z': *Be careful:* Write spaces around the `...', for otherwise it may be parsed wrong when you use it with integer values. For example, write this: case 1 ... 5: rather than this: case 1...5:  File: gcc.info, Node: Cast to Union, Next: Case Ranges, Prev: Labeled Elements, Up: C Extensions Cast to a Union Type ==================== A cast to union type is similar to other casts, except that the type specified is a union type. You can specify the type either with `union TAG' or with a typedef name. A cast to union is actually a constructor though, not a cast, and hence does not yield an lvalue like normal casts. (*Note Constructors::.) The types that may be cast to the union type are those of the members of the union. Thus, given the following union and variables: union foo { int i; double d; }; int x; double y; both `x' and `y' can be cast to type `union' foo. Using the cast as the right-hand side of an assignment to a variable of union type is equivalent to storing in a member of the union: union foo u; ... u = (union foo) x == u.i = x u = (union foo) y == u.d = y You can also use the union cast as a function argument: void hack (union foo); ... hack ((union foo) x);  File: gcc.info, Node: Function Attributes, Next: Function Prototypes, Prev: Case Ranges, Up: C Extensions Declaring Attributes of Functions ================================= In GNU C, you declare certain things about functions called in your program which help the compiler optimize function calls and check your code more carefully. The keyword `__attribute__' allows you to specify special attributes when making a declaration. This keyword is followed by an attribute specification inside double parentheses. Eight attributes, `noreturn', `const', `format', `section', `constructor', `destructor', `unused' and `weak' are currently defined for functions. Other attributes, including `section' are supported for variables declarations (*note Variable Attributes::.) and for types (*note Type Attributes::.). You may also specify attributes with `__' preceding and following each keyword. This allows you to use them in header files without being concerned about a possible macro of the same name. For example, you may use `__noreturn__' instead of `noreturn'. `noreturn' A few standard library functions, such as `abort' and `exit', cannot return. GNU CC knows this automatically. Some programs define their own functions that never return. You can declare them `noreturn' to tell the compiler this fact. For example, void fatal () __attribute__ ((noreturn)); void fatal (...) { ... /* Print error message. */ ... exit (1); } The `noreturn' keyword tells the compiler to assume that `fatal' cannot return. It can then optimize without regard to what would happen if `fatal' ever did return. This makes slightly better code. More importantly, it helps avoid spurious warnings of uninitialized variables. Do not assume that registers saved by the calling function are restored before calling the `noreturn' function. It does not make sense for a `noreturn' function to have a return type other than `void'. The attribute `noreturn' is not implemented in GNU C versions earlier than 2.5. An alternative way to declare that a function does not return, which works in the current version and in some older versions, is as follows: typedef void voidfn (); volatile voidfn fatal; `const' Many functions do not examine any values except their arguments, and have no effects except the return value. Such a function can be subject to common subexpression elimination and loop optimization just as an arithmetic operator would be. These functions should be declared with the attribute `const'. For example, int square (int) __attribute__ ((const)); says that the hypothetical function `square' is safe to call fewer times than the program says. The attribute `const' is not implemented in GNU C versions earlier than 2.5. An alternative way to declare that a function has no side effects, which works in the current version and in some older versions, is as follows: typedef int intfn (); extern const intfn square; This approach does not work in GNU C++ from 2.6.0 on, since the language specifies that the `const' must be attached to the return value. Note that a function that has pointer arguments and examines the data pointed to must *not* be declared `const'. Likewise, a function that calls a non-`const' function usually must not be `const'. It does not make sense for a `const' function to return `void'. `format (ARCHETYPE, STRING-INDEX, FIRST-TO-CHECK)' The `format' attribute specifies that a function takes `printf' or `scanf' style arguments which should be type-checked against a format string. For example, the declaration: extern int my_printf (void *my_object, const char *my_format, ...) __attribute__ ((format (printf, 2, 3))); causes the compiler to check the arguments in calls to `my_printf' for consistency with the `printf' style format string argument `my_format'. The parameter ARCHETYPE determines how the format string is interpreted, and should be either `printf' or `scanf'. The parameter STRING-INDEX specifies which argument is the format string argument (starting from 1), while FIRST-TO-CHECK is the number of the first argument to check against the format string. For functions where the arguments are not available to be checked (such as `vprintf'), specify the third parameter as zero. In this case the compiler only checks the format string for consistency. In the example above, the format string (`my_format') is the second argument of the function `my_print', and the arguments to check start with the third argument, so the correct parameters for the format attribute are 2 and 3. The `format' attribute allows you to identify your own functions which take format strings as arguments, so that GNU CC can check the calls to these functions for errors. The compiler always checks formats for the ANSI library functions `printf', `fprintf', `sprintf', `scanf', `fscanf', `sscanf', `vprintf', `vfprintf' and `vsprintf' whenever such warnings are requested (using `-Wformat'), so there is no need to modify the header file `stdio.h'. `section ("section-name")' Normally, the compiler places the code it generates in the `text' section. Sometimes, however, you need additional sections, or you need certain particular functions to appear in special sections. The `section' attribute specifies that a function lives in a particular section. For example, the declaration: extern void foobar (void) __attribute__ ((section ("bar"))); puts the function `foobar' in the `bar' section. Some file formats do not support arbitrary sections so the `section' attribute is not available on all platforms. If you need to map the entire contents of a module to a particular section, consider using the facilities of the linker instead. `constructor' `destructor' The `constructor' attribute causes the function to be called automatically before execution enters `main ()'. Similarly, the `destructor' attribute causes the function to be called automatically after `main ()' has completed or `exit ()' has been called. Functions with these attributes are useful for initializing data that will be used implicitly during the execution of the program. These attributes are not currently implemented for Objective C. `unused' This attribute, attached to a function, means that the function is meant to be possibly unused. GNU CC will not produce a warning for this function. `weak' The `weak' attribute causes the declaration to be emitted as a weak symbol rather than a global. This is primarily useful in defining library functions which can be overridden in user code, though it can also be used with non-function declarations. Weak symbols are supported for ELF targets, and also for a.out targets when using the GNU assembler and linker. `alias ("target")' The `alias' attribute causes the declaration to be emitted as an alias for another symbol, which must be specified. For instance, void __f () { /* do something */; } void f () __attribute__ ((weak, alias ("__f"))); declares `f' to be a weak alias for `__f'. In C++, the mangled name for the target must be used. `regparm (NUMBER)' On the Intel 386, the `regparm' attribute causes the compiler to pass up to NUMBER integer arguments in registers EAX, EDX, and ECX instead of on the stack. Functions that take a variable number of arguments will continue to be passed all of their arguments on the stack. `stdcall' On the Intel 386, the `stdcall' attribute causes the compiler to assume that the called function will pop off the stack space used to pass arguments, unless it takes a variable number of arguments. `cdecl' On the Intel 386, the `cdecl' attribute causes the compiler to assume that the called function will pop off the stack space used to pass arguments, unless it takes a variable number of arguments. This is useful to override the effects of the `-mrtd' switch. You can specify multiple attributes in a declaration by separating them by commas within the double parentheses or by immediately following an attribute declaration with another attribute declaration. Some people object to the `__attribute__' feature, suggesting that ANSI C's `#pragma' should be used instead. There are two reasons for not doing this. 1. It is impossible to generate `#pragma' commands from a macro. 2. There is no telling what the same `#pragma' might mean in another compiler. These two reasons apply to almost any application that might be proposed for `#pragma'. It is basically a mistake to use `#pragma' for *anything*.  File: gcc.info, Node: Function Prototypes, Next: C++ Comments, Prev: Function Attributes, Up: C Extensions Prototypes and Old-Style Function Definitions ============================================= GNU C extends ANSI C to allow a function prototype to override a later old-style non-prototype definition. Consider the following example: /* Use prototypes unless the compiler is old-fashioned. */ #if __STDC__ #define P(x) x #else #define P(x) () #endif /* Prototype function declaration. */ int isroot P((uid_t)); /* Old-style function definition. */ int isroot (x) /* ??? lossage here ??? */ uid_t x; { return x == 0; } Suppose the type `uid_t' happens to be `short'. ANSI C does not allow this example, because subword arguments in old-style non-prototype definitions are promoted. Therefore in this example the function definition's argument is really an `int', which does not match the prototype argument type of `short'. This restriction of ANSI C makes it hard to write code that is portable to traditional C compilers, because the programmer does not know whether the `uid_t' type is `short', `int', or `long'. Therefore, in cases like these GNU C allows a prototype to override a later old-style definition. More precisely, in GNU C, a function prototype argument type overrides the argument type specified by a later old-style definition if the former type is the same as the latter type before promotion. Thus in GNU C the above example is equivalent to the following: int isroot (uid_t); int isroot (uid_t x) { return x == 0; } GNU C++ does not support old-style function definitions, so this extension is irrelevant.  File: gcc.info, Node: C++ Comments, Next: Dollar Signs, Prev: Function Prototypes, Up: C Extensions C++ Style Comments ================== In GNU C, you may use C++ style comments, which start with `//' and continue until the end of the line. Many other C implementations allow such comments, and they are likely to be in a future C standard. However, C++ style comments are not recognized if you specify `-ansi' or `-traditional', since they are incompatible with traditional constructs like `dividend//*comment*/divisor'.  File: gcc.info, Node: Dollar Signs, Next: Character Escapes, Prev: C++ Comments, Up: C Extensions Dollar Signs in Identifier Names ================================ In GNU C, you may use dollar signs in identifier names. This is because many traditional C implementations allow such identifiers. On some machines, dollar signs are allowed in identifiers if you specify `-traditional'. On a few systems they are allowed by default, even if you do not use `-traditional'. But they are never allowed if you specify `-ansi'. There are certain ANSI C programs (obscure, to be sure) that would compile incorrectly if dollar signs were permitted in identifiers. For example: #define foo(a) #a #define lose(b) foo (b) #define test$ lose (test)  File: gcc.info, Node: Character Escapes, Next: Variable Attributes, Prev: Dollar Signs, Up: C Extensions The Character ESC in Constants ============================== You can use the sequence `\e' in a string or character constant to stand for the ASCII character ESC.  File: gcc.info, Node: Alignment, Next: Inline, Prev: Type Attributes, Up: C Extensions Inquiring on Alignment of Types or Variables ============================================ The keyword `__alignof__' allows you to inquire about how an object is aligned, or the minimum alignment usually required by a type. Its syntax is just like `sizeof'. For example, if the target machine requires a `double' value to be aligned on an 8-byte boundary, then `__alignof__ (double)' is 8. This is true on many RISC machines. On more traditional machine designs, `__alignof__ (double)' is 4 or even 2. Some machines never actually require alignment; they allow reference to any data type even at an odd addresses. For these machines, `__alignof__' reports the *recommended* alignment of a type. When the operand of `__alignof__' is an lvalue rather than a type, the value is the largest alignment that the lvalue is known to have. It may have this alignment as a result of its data type, or because it is part of a structure and inherits alignment from that structure. For example, after this declaration: struct foo { int x; char y; } foo1; the value of `__alignof__ (foo1.y)' is probably 2 or 4, the same as `__alignof__ (int)', even though the data type of `foo1.y' does not itself demand any alignment. A related feature which lets you specify the alignment of an object is `__attribute__ ((aligned (ALIGNMENT)))'; see the following section.  File: gcc.info, Node: Variable Attributes, Next: Type Attributes, Prev: Character Escapes, Up: C Extensions Specifying Attributes of Variables ================================== The keyword `__attribute__' allows you to specify special attributes of variables or structure fields. This keyword is followed by an attribute specification inside double parentheses. Eight attributes are currently defined for variables: `aligned', `mode', `nocommon', `packed', `section', `transparent_union', `unused', and `weak'. Other attributes are available for functions (*note Function Attributes::.) and for types (*note Type Attributes::.). You may also specify attributes with `__' preceding and following each keyword. This allows you to use them in header files without being concerned about a possible macro of the same name. For example, you may use `__aligned__' instead of `aligned'. `aligned (ALIGNMENT)' This attribute specifies a minimum alignment for the variable or structure field, measured in bytes. For example, the declaration: int x __attribute__ ((aligned (16))) = 0; causes the compiler to allocate the global variable `x' on a 16-byte boundary. On a 68040, this could be used in conjunction with an `asm' expression to access the `move16' instruction which requires 16-byte aligned operands. You can also specify the alignment of structure fields. For example, to create a double-word aligned `int' pair, you could write: struct foo { int x[2] __attribute__ ((aligned (8))); }; This is an alternative to creating a union with a `double' member that forces the union to be double-word aligned. It is not possible to specify the alignment of functions; the alignment of functions is determined by the machine's requirements and cannot be changed. You cannot specify alignment for a typedef name because such a name is just an alias, not a distinct type. As in the preceding examples, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given variable or structure field. Alternatively, you can leave out the alignment factor and just ask the compiler to align a variable or field to the maximum useful alignment for the target machine you are compiling for. For example, you could write: short array[3] __attribute__ ((aligned)); Whenever you leave out the alignment factor in an `aligned' attribute specification, the compiler automatically sets the alignment for the declared variable or field to the largest alignment which is ever used for any data type on the target machine you are compiling for. Doing this can often make copy operations more efficient, because the compiler can use whatever instructions copy the biggest chunks of memory when performing copies to or from the variables or fields that you have aligned this way. The `aligned' attribute can only increase the alignment; but you can decrease it by specifying `packed' as well. See below. Note that the effectiveness of `aligned' attributes may be limited by inherent limitations in your linker. On many systems, the linker is only able to arrange for variables to be aligned up to a certain maximum alignment. (For some linkers, the maximum supported alignment may be very very small.) If your linker is only able to align variables up to a maximum of 8 byte alignment, then specifying `aligned(16)' in an `__attribute__' will still only provide you with 8 byte alignment. See your linker documentation for further information. `mode (MODE)' This attribute specifies the data type for the declaration--whichever type corresponds to the mode MODE. This in effect lets you request an integer or floating point type according to its width. You may also specify a mode of `byte' or `__byte__' to indicate the mode corresponding to a one-byte integer, `word' or `__word__' for the mode of a one-word integer, and `pointer' or `__pointer__' for the mode used to represent pointers. `nocommon' This attribute specifies requests GNU CC not to place a variable "common" but instead to allocate space for it directly. If you specify the `-fno-common' flag, GNU CC will do this for all variables. Specifying the `nocommon' attribute for a variable provides an initialization of zeros. A variable may only be initialized in one source file. `packed' The `packed' attribute specifies that a variable or structure field should have the smallest possible alignment--one byte for a variable, and one bit for a field, unless you specify a larger value with the `aligned' attribute. Here is a structure in which the field `x' is packed, so that it immediately follows `a': struct foo { char a; int x[2] __attribute__ ((packed)); }; `section ("section-name")' Normally, the compiler places the objects it generates in sections like `data' and `bss'. Sometimes, however, you need additional sections, or you need certain particular variables to appear in special sections, for example to map to special hardware. The `section' attribute specifies that a variable (or function) lives in a particular section. For example, this small program uses several specific section names: struct duart a __attribute__ ((section ("DUART_A"))) = { 0 }; struct duart b __attribute__ ((section ("DUART_B"))) = { 0 }; char stack[10000] __attribute__ ((section ("STACK"))) = { 0 }; int init_data_copy __attribute__ ((section ("INITDATACOPY"))) = 0; main() { /* Initialize stack pointer */ init_sp (stack + sizeof (stack)); /* Initialize initialized data */ memcpy (&init_data_copy, &data, &edata - &data); /* Turn on the serial ports */ init_duart (&a); init_duart (&b); } Use the `section' attribute with an *initialized* definition of a *global* variable, as shown in the example. GNU CC issues a warning and otherwise ignores the `section' attribute in uninitialized variable declarations. You may only use the `section' attribute with a fully initialized global definition because of the way linkers work. The linker requires each object be defined once, with the exception that uninitialized variables tentatively go in the `common' (or `bss') section and can be multiply "defined". You can force a variable to be initialized with the `-fno-common' flag or the `nocommon' attribute. Some file formats do not support arbitrary sections so the `section' attribute is not available on all platforms. If you need to map the entire contents of a module to a particular section, consider using the facilities of the linker instead. `transparent_union' This attribute, attached to a function argument variable which is a union, means to pass the argument in the same way that the first union member would be passed. You can also use this attribute on a `typedef' for a union data type; then it applies to all function arguments with that type. `unused' This attribute, attached to a variable, means that the variable is meant to be possibly unused. GNU CC will not produce a warning for this variable. `weak' The `weak' attribute is described in *Note Function Attributes::. To specify multiple attributes, separate them by commas within the double parentheses: for example, `__attribute__ ((aligned (16), packed))'.  File: gcc.info, Node: Type Attributes, Next: Alignment, Prev: Variable Attributes, Up: C Extensions Specifying Attributes of Types ============================== The keyword `__attribute__' allows you to specify special attributes of `struct' and `union' types when you define such types. This keyword is followed by an attribute specification inside double parentheses. Three attributes are currently defined for types: `aligned', `packed', and `transparent_union'. Other attributes are defined for functions (*note Function Attributes::.) and for variables (*note Variable Attributes::.). You may also specify any one of these attributes with `__' preceding and following its keyword. This allows you to use these attributes in header files without being concerned about a possible macro of the same name. For example, you may use `__aligned__' instead of `aligned'. You may specify the `aligned' and `transparent_union' attributes either in a `typedef' declaration or just past the closing curly brace of a complete enum, struct or union type *definition* and the `packed' attribute only past the closing brace of a definition. `aligned (ALIGNMENT)' This attribute specifies a minimum alignment (in bytes) for variables of the specified type. For example, the declarations: struct S { short f[3]; } __attribute__ ((aligned (8)); typedef int more_aligned_int __attribute__ ((aligned (8)); force the compiler to insure (as fas as it can) that each variable whose type is `struct S' or `more_aligned_int' will be allocated and aligned *at least* on a 8-byte boundary. On a Sparc, having all variables of type `struct S' aligned to 8-byte boundaries allows the compiler to use the `ldd' and `std' (doubleword load and store) instructions when copying one variable of type `struct S' to another, thus improving run-time efficiency. Note that the alignment of any given `struct' or `union' type is required by the ANSI C standard to be at least a perfect multiple of the lowest common multiple of the alignments of all of the members of the `struct' or `union' in question. This means that you *can* effectively adjust the alignment of a `struct' or `union' type by attaching an `aligned' attribute to any one of the members of such a type, but the notation illustrated in the example above is a more obvious, intuitive, and readable way to request the compiler to adjust the alignment of an entire `struct' or `union' type. As in the preceding example, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given `struct' or `union' type. Alternatively, you can leave out the alignment factor and just ask the compiler to align a type to the maximum useful alignment for the target machine you are compiling for. For example, you could write: struct S { short f[3]; } __attribute__ ((aligned)); Whenever you leave out the alignment factor in an `aligned' attribute specification, the compiler automatically sets the alignment for the type to the largest alignment which is ever used for any data type on the target machine you are compiling for. Doing this can often make copy operations more efficient, because the compiler can use whatever instructions copy the biggest chunks of memory when performing copies to or from the variables which have types that you have aligned this way. In the example above, if the size of each `short' is 2 bytes, then the size of the entire `struct S' type is 6 bytes. The smallest power of two which is greater than or equal to that is 8, so the compiler sets the alignment for the entire `struct S' type to 8 bytes. Note that although you can ask the compiler to select a time-efficient alignment for a given type and then declare only individual stand-alone objects of that type, the compiler's ability to select a time-efficient alignment is primarily useful only when you plan to create arrays of variables having the relevant (efficiently aligned) type. If you declare or use arrays of variables of an efficiently-aligned type, then it is likely that your program will also be doing pointer arithmetic (or subscripting, which amounts to the same thing) on pointers to the relevant type, and the code that the compiler generates for these pointer arithmetic operations will often be more efficient for efficiently-aligned types than for other types. The `aligned' attribute can only increase the alignment; but you can decrease it by specifying `packed' as well. See below. Note that the effectiveness of `aligned' attributes may be limited by inherent limitations in your linker. On many systems, the linker is only able to arrange for variables to be aligned up to a certain maximum alignment. (For some linkers, the maximum supported alignment may be very very small.) If your linker is only able to align variables up to a maximum of 8 byte alignment, then specifying `aligned(16)' in an `__attribute__' will still only provide you with 8 byte alignment. See your linker documentation for further information. `packed' This attribute, attached to an `enum', `struct', or `union' type definition, specified that the minimum required memory be used to represent the type. Specifying this attribute for `struct' and `union' types is equivalent to specifying the `packed' attribute on each of the structure or union members. Specifying the `-fshort-enums' flag on the line is equivalent to specifying the `packed' attribute on all `enum' definitions. You may only specify this attribute after a closing curly brace on an `enum' definition, not in a `typedef' declaration. `transparent_union' This attribute, attached to a `union' type definition, indicates that any variable having that union type should, if passed to a function, be passed in the same way that the first union member would be passed. For example: union foo { char a; int x[2]; } __attribute__ ((transparent_union)); To specify multiple attributes, separate them by commas within the double parentheses: for example, `__attribute__ ((aligned (16), packed))'.  File: gcc.info, Node: Inline, Next: Extended Asm, Prev: Alignment, Up: C Extensions An Inline Function is As Fast As a Macro ======================================== By declaring a function `inline', you can direct GNU CC to integrate that function's code into the code for its callers. This makes execution faster by eliminating the function-call overhead; in addition, if any of the actual argument values are constant, their known values may permit simplifications at compile time so that not all of the inline function's code needs to be included. The effect on code size is less predictable; object code may be larger or smaller with function inlining, depending on the particular case. Inlining of functions is an optimization and it really "works" only in optimizing compilation. If you don't use `-O', no function is really inline. To declare a function inline, use the `inline' keyword in its declaration, like this: inline int inc (int *a) { (*a)++; } (If you are writing a header file to be included in ANSI C programs, write `__inline__' instead of `inline'. *Note Alternate Keywords::.) You can also make all "simple enough" functions inline with the option `-finline-functions'. Note that certain usages in a function definition can make it unsuitable for inline substitution. Note that in C and Objective C, unlike C++, the `inline' keyword does not affect the linkage of the function. GNU CC automatically inlines member functions defined within the class body of C++ programs even if they are not explicitly declared `inline'. (You can override this with `-fno-default-inline'; *note Options Controlling C++ Dialect: C++ Dialect Options..) When a function is both inline and `static', if all calls to the function are integrated into the caller, and the function's address is never used, then the function's own assembler code is never referenced. In this case, GNU CC does not actually output assembler code for the function, unless you specify the option `-fkeep-inline-functions'. Some calls cannot be integrated for various reasons (in particular, calls that precede the function's definition cannot be integrated, and neither can recursive calls within the definition). If there is a nonintegrated call, then the function is compiled to assembler code as usual. The function must also be compiled as usual if the program refers to its address, because that can't be inlined. When an inline function is not `static', then the compiler must assume that there may be calls from other source files; since a global symbol can be defined only once in any program, the function must not be defined in the other source files, so the calls therein cannot be integrated. Therefore, a non-`static' inline function is always compiled on its own in the usual fashion. If you specify both `inline' and `extern' in the function definition, then the definition is used only for inlining. In no case is the function compiled on its own, not even if you refer to its address explicitly. Such an address becomes an external reference, as if you had only declared the function, and had not defined it. This combination of `inline' and `extern' has almost the effect of a macro. The way to use it is to put a function definition in a header file with these keywords, and put another copy of the definition (lacking `inline' and `extern') in a library file. The definition in the header file will cause most calls to the function to be inlined. If any uses of the function remain, they will refer to the single copy in the library. GNU C does not inline any functions when not optimizing. It is not clear whether it is better to inline or not, in this case, but we found that a correct implementation when not optimizing was difficult. So we did the easy thing, and turned it off.  File: gcc.info, Node: Extended Asm, Next: Asm Labels, Prev: Inline, Up: C Extensions Assembler Instructions with C Expression Operands ================================================= In an assembler instruction using `asm', you can now specify the operands of the instruction using C expressions. This means no more guessing which registers or memory locations will contain the data you want to use. You must specify an assembler instruction template much like what appears in a machine description, plus an operand constraint string for each operand. For example, here is how to use the 68881's `fsinx' instruction: asm ("fsinx %1,%0" : "=f" (result) : "f" (angle)); Here `angle' is the C expression for the input operand while `result' is that of the output operand. Each has `"f"' as its operand constraint, saying that a floating point register is required. The `=' in `=f' indicates that the operand is an output; all output operands' constraints must use `='. The constraints use the same language used in the machine description (*note Constraints::.). Each operand is described by an operand-constraint string followed by the C expression in parentheses. A colon separates the assembler template from the first output operand, and another separates the last output operand from the first input, if any. Commas separate output operands and separate inputs. The total number of operands is limited to ten or to the maximum number of operands in any instruction pattern in the machine description, whichever is greater. If there are no output operands, and there are input operands, then there must be two consecutive colons surrounding the place where the output operands would go. Output operand expressions must be lvalues; the compiler can check this. The input operands need not be lvalues. The compiler cannot check whether the operands have data types that are reasonable for the instruction being executed. It does not parse the assembler instruction template and does not know what it means, or whether it is valid assembler input. The extended `asm' feature is most often used for machine instructions that the compiler itself does not know exist. If the output expression cannot be directly addressed (for example, it is a bit field), your constraint must allow a register. In that case, GNU CC will use the register as the output of the `asm', and then store that register into the output. The output operands must be write-only; GNU CC will assume that the values in these operands before the instruction are dead and need not be generated. Extended asm does not support input-output or read-write operands. For this reason, the constraint character `+', which indicates such an operand, may not be used. When the assembler instruction has a read-write operand, or an operand in which only some of the bits are to be changed, you must logically split its function into two separate operands, one input operand and one write-only output operand. The connection between them is expressed by constraints which say they need to be in the same location when the instruction executes. You can use the same C expression for both operands, or different expressions. For example, here we write the (fictitious) `combine' instruction with `bar' as its read-only source operand and `foo' as its read-write destination: asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar)); The constraint `"0"' for operand 1 says that it must occupy the same location as operand 0. A digit in constraint is allowed only in an input operand, and it must refer to an output operand. Only a digit in the constraint can guarantee that one operand will be in the same place as another. The mere fact that `foo' is the value of both operands is not enough to guarantee that they will be in the same place in the generated assembler code. The following would not work: asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar)); Various optimizations or reloading could cause operands 0 and 1 to be in different registers; GNU CC knows no reason not to do so. For example, the compiler might find a copy of the value of `foo' in one register and use it for operand 1, but generate the output operand 0 in a different register (copying it afterward to `foo''s own address). Of course, since the register for operand 1 is not even mentioned in the assembler code, the result will not work, but GNU CC can't tell that. Some instructions clobber specific hard registers. To describe this, write a third colon after the input operands, followed by the names of the clobbered hard registers (given as strings). Here is a realistic example for the Vax: asm volatile ("movc3 %0,%1,%2" : /* no outputs */ : "g" (from), "g" (to), "g" (count) : "r0", "r1", "r2", "r3", "r4", "r5"); If you refer to a particular hardware register from the assembler code, then you will probably have to list the register after the third colon to tell the compiler that the register's value is modified. In many assemblers, the register names begin with `%'; to produce one `%' in the assembler code, you must write `%%' in the input. If your assembler instruction can alter the condition code register, add `cc' to the list of clobbered registers. GNU CC on some machines represents the condition codes as a specific hardware register; `cc' serves to name this register. On other machines, the condition code is handled differently, and specifying `cc' has no effect. But it is valid no matter what the machine. If your assembler instruction modifies memory in an unpredictable fashion, add `memory' to the list of clobbered registers. This will cause GNU CC to not keep memory values cached in registers across the assembler instruction. You can put multiple assembler instructions together in a single `asm' template, separated either with newlines (written as `\n') or with semicolons if the assembler allows such semicolons. The GNU assembler allows semicolons and all Unix assemblers seem to do so. The input operands are guaranteed not to use any of the clobbered registers, and neither will the output operands' addresses, so you can read and write the clobbered registers as many times as you like. Here is an example of multiple instructions in a template; it assumes that the subroutine `_foo' accepts arguments in registers 9 and 10: asm ("movl %0,r9;movl %1,r10;call _foo" : /* no outputs */ : "g" (from), "g" (to) : "r9", "r10"); Unless an output operand has the `&' constraint modifier, GNU CC may allocate it in the same register as an unrelated input operand, on the assumption that the inputs are consumed before the outputs are produced. This assumption may be false if the assembler code actually consists of more than one instruction. In such a case, use `&' for each output operand that may not overlap an input. *Note Modifiers::. If you want to test the condition code produced by an assembler instruction, you must include a branch and a label in the `asm' construct, as follows: asm ("clr %0;frob %1;beq 0f;mov #1,%0;0:" : "g" (result) : "g" (input)); This assumes your assembler supports local labels, as the GNU assembler and most Unix assemblers do. Speaking of labels, jumps from one `asm' to another are not supported. The compiler's optimizers do not know about these jumps, and therefore they cannot take account of them when deciding how to optimize. Usually the most convenient way to use these `asm' instructions is to encapsulate them in macros that look like functions. For example, #define sin(x) \ ({ double __value, __arg = (x); \ asm ("fsinx %1,%0": "=f" (__value): "f" (__arg)); \ __value; }) Here the variable `__arg' is used to make sure that the instruction operates on a proper `double' value, and to accept only those arguments `x' which can convert automatically to a `double'. Another way to make sure the instruction operates on the correct data type is to use a cast in the `asm'. This is different from using a variable `__arg' in that it converts more different types. For example, if the desired type were `int', casting the argument to `int' would accept a pointer with no complaint, while assigning the argument to an `int' variable named `__arg' would warn about using a pointer unless the caller explicitly casts it. If an `asm' has output operands, GNU CC assumes for optimization purposes that the instruction has no side effects except to change the output operands. This does not mean that instructions with a side effect cannot be used, but you must be careful, because the compiler may eliminate them if the output operands aren't used, or move them out of loops, or replace two with one if they constitute a common subexpression. Also, if your instruction does have a side effect on a variable that otherwise appears not to change, the old value of the variable may be reused later if it happens to be found in a register. You can prevent an `asm' instruction from being deleted, moved significantly, or combined, by writing the keyword `volatile' after the `asm'. For example: #define set_priority(x) \ asm volatile ("set_priority %0": /* no outputs */ : "g" (x)) An instruction without output operands will not be deleted or moved significantly, regardless, unless it is unreachable. Note that even a volatile `asm' instruction can be moved in ways that appear insignificant to the compiler, such as across jump instructions. You can't expect a sequence of volatile `asm' instructions to remain perfectly consecutive. If you want consecutive output, use a single `asm'. It is a natural idea to look for a way to give access to the condition code left by the assembler instruction. However, when we attempted to implement this, we found no way to make it work reliably. The problem is that output operands might need reloading, which would result in additional following "store" instructions. On most machines, these instructions would alter the condition code before there was time to test it. This problem doesn't arise for ordinary "test" and "compare" instructions because they don't have any output operands. If you are writing a header file that should be includable in ANSI C programs, write `__asm__' instead of `asm'. *Note Alternate Keywords::.