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(gcc-300.info)Function Attributes
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. Fourteen attributes, `noreturn', `pure', `const', `format', `format_arg', `no_instrument_function', `section', `constructor', `destructor', `unused', `weak', `malloc', `alias' and `no_check_memory_usage' are currently defined for functions. Several other attributes are defined for functions on particular target systems. 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'. Note: Attribute Syntax, for details of the exact syntax for using attributes. `noreturn' A few standard library functions, such as `abort' and `exit', cannot return. GCC 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 GCC 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; `pure' Many functions have no effects except the return value and their return value depends only on the parameters and/or global variables. 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 `pure'. For example, int square (int) __attribute__ ((pure)); says that the hypothetical function `square' is safe to call fewer times than the program says. Some of common examples of pure functions are `strlen' or `memcmp'. Interesting non-pure functions are functions with infinite loops or those depending on volatile memory or other system resource, that may change between two consecutive calls (such as `feof' in a multithreading environment). The attribute `pure' is not implemented in GCC versions earlier than 2.96. `const' Many functions do not examine any values except their arguments, and have no effects except the return value. Basically this is just slightly more strict class than the `pure' attribute above, since function is not allowed to read global memory. 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'. The attribute `const' is not implemented in GCC 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. `format (ARCHETYPE, STRING-INDEX, FIRST-TO-CHECK)' The `format' attribute specifies that a function takes `printf', `scanf', `strftime' or `strfmon' 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 `printf', `scanf', `strftime' or `strfmon'. (You can also use `__printf__', `__scanf__', `__strftime__' or `__strfmon__'.) 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. For `strftime' formats, the third parameter is required to be zero. 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 GCC can check the calls to these functions for errors. The compiler always (unless `-ffreestanding' is used) checks formats for the standard library functions `printf', `fprintf', `sprintf', `scanf', `fscanf', `sscanf', `strftime', `vprintf', `vfprintf' and `vsprintf' whenever such warnings are requested (using `-Wformat'), so there is no need to modify the header file `stdio.h'. In C99 mode, the functions `snprintf', `vsnprintf', `vscanf', `vfscanf' and `vsscanf' are also checked. Except in strictly conforming C standard modes, the X/Open function `strfmon' is also checked. Note: Options Controlling C Dialect. `format_arg (STRING-INDEX)' The `format_arg' attribute specifies that a function takes a format string for a `printf', `scanf', `strftime' or `strfmon' style function and modifies it (for example, to translate it into another language), so the result can be passed to a `printf', `scanf', `strftime' or `strfmon' style function (with the remaining arguments to the format function the same as they would have been for the unmodified string). For example, the declaration: extern char * my_dgettext (char *my_domain, const char *my_format) __attribute__ ((format_arg (2))); causes the compiler to check the arguments in calls to a `printf', `scanf', `strftime' or `strfmon' type function, whose format string argument is a call to the `my_dgettext' function, for consistency with the format string argument `my_format'. If the `format_arg' attribute had not been specified, all the compiler could tell in such calls to format functions would be that the format string argument is not constant; this would generate a warning when `-Wformat-nonliteral' is used, but the calls could not be checked without the attribute. The parameter STRING-INDEX specifies which argument is the format string argument (starting from 1). The `format-arg' attribute allows you to identify your own functions which modify format strings, so that GCC can check the calls to `printf', `scanf', `strftime' or `strfmon' type function whose operands are a call to one of your own function. The compiler always treats `gettext', `dgettext', and `dcgettext' in this manner except when strict ISO C support is requested by `-ansi' or an appropriate `-std' option, or `-ffreestanding' is used. Note: Options Controlling C Dialect. `no_instrument_function' If `-finstrument-functions' is given, profiling function calls will be generated at entry and exit of most user-compiled functions. Functions with this attribute will not be so instrumented. `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. GCC will not produce a warning for this function. GNU C++ does not currently support this attribute as definitions without parameters are valid in C++. `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. `malloc' The `malloc' attribute is used to tell the compiler that a function may be treated as if it were the malloc function. The compiler assumes that calls to malloc result in a pointers that cannot alias anything. This will often improve optimization. `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. Not all target machines support this attribute. `no_check_memory_usage' The `no_check_memory_usage' attribute causes GCC to omit checks of memory references when it generates code for that function. Normally if you specify `-fcheck-memory-usage' (see Note: Code Gen Options), GCC generates calls to support routines before most memory accesses to permit support code to record usage and detect uses of uninitialized or unallocated storage. Since GCC cannot handle `asm' statements properly they are not allowed in such functions. If you declare a function with this attribute, GCC will not generate memory checking code for that function, permitting the use of `asm' statements without having to compile that function with different options. This also allows you to write support routines of your own if you wish, without getting infinite recursion if they get compiled with `-fcheck-memory-usage'. `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. The PowerPC compiler for Windows NT currently ignores the `stdcall' attribute. `cdecl' On the Intel 386, the `cdecl' attribute causes the compiler to assume that the calling function will pop off the stack space used to pass arguments. This is useful to override the effects of the `-mrtd' switch. The PowerPC compiler for Windows NT currently ignores the `cdecl' attribute. `longcall' On the RS/6000 and PowerPC, the `longcall' attribute causes the compiler to always call the function via a pointer, so that functions which reside further than 64 megabytes (67,108,864 bytes) from the current location can be called. `long_call/short_call' This attribute allows to specify how to call a particular function on ARM. Both attributes override the `-mlong-calls' (Note: ARM Options) command line switch and `#pragma long_calls' settings. The `long_call' attribute causes the compiler to always call the function by first loading its address into a register and then using the contents of that register. The `short_call' attribute always places the offset to the function from the call site into the `BL' instruction directly. `dllimport' On the PowerPC running Windows NT, the `dllimport' attribute causes the compiler to call the function via a global pointer to the function pointer that is set up by the Windows NT dll library. The pointer name is formed by combining `__imp_' and the function name. `dllexport' On the PowerPC running Windows NT, the `dllexport' attribute causes the compiler to provide a global pointer to the function pointer, so that it can be called with the `dllimport' attribute. The pointer name is formed by combining `__imp_' and the function name. `exception (EXCEPT-FUNC [, EXCEPT-ARG])' On the PowerPC running Windows NT, the `exception' attribute causes the compiler to modify the structured exception table entry it emits for the declared function. The string or identifier EXCEPT-FUNC is placed in the third entry of the structured exception table. It represents a function, which is called by the exception handling mechanism if an exception occurs. If it was specified, the string or identifier EXCEPT-ARG is placed in the fourth entry of the structured exception table. `function_vector' Use this option on the H8/300 and H8/300H to indicate that the specified function should be called through the function vector. Calling a function through the function vector will reduce code size, however; the function vector has a limited size (maximum 128 entries on the H8/300 and 64 entries on the H8/300H) and shares space with the interrupt vector. You must use GAS and GLD from GNU binutils version 2.7 or later for this option to work correctly. `interrupt' Use this option on the ARM, AVR and M32R/D ports to indicate that the specified function is an interrupt handler. The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present. Note, interrupt handlers for the H8/300, H8/300H and SH processors can be specified via the `interrupt_handler' attribute. Note, on the AVR interrupts will be enabled inside the function. Note, for the ARM you can specify the kind of interrupt to be handled by adding an optional parameter to the interrupt attribute like this: void f () __attribute__ ((interrupt ("IRQ"))); Permissible values for this parameter are: IRQ, FIQ, SWI, ABORT and UNDEF. `interrupt_handler' Use this option on the H8/300, H8/300H and SH to indicate that the specified function is an interrupt handler. The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present. `sp_switch' Use this option on the SH to indicate an `interrupt_handler' function should switch to an alternate stack. It expects a string argument that names a global variable holding the address of the alternate stack. void *alt_stack; void f () __attribute__ ((interrupt_handler, sp_switch ("alt_stack"))); `trap_exit' Use this option on the SH for an `interrupt_handle' to return using `trapa' instead of `rte'. This attribute expects an integer argument specifying the trap number to be used. `eightbit_data' Use this option on the H8/300 and H8/300H to indicate that the specified variable should be placed into the eight bit data section. The compiler will generate more efficient code for certain operations on data in the eight bit data area. Note the eight bit data area is limited to 256 bytes of data. You must use GAS and GLD from GNU binutils version 2.7 or later for this option to work correctly. `tiny_data' Use this option on the H8/300H to indicate that the specified variable should be placed into the tiny data section. The compiler will generate more efficient code for loads and stores on data in the tiny data section. Note the tiny data area is limited to slightly under 32kbytes of data. `signal' Use this option on the AVR to indicate that the specified function is an signal handler. The compiler will generate function entry and exit sequences suitable for use in an signal handler when this attribute is present. Interrupts will be disabled inside function. `naked' Use this option on the ARM or AVR ports to indicate that the specified function do not need prologue/epilogue sequences generated by the compiler. It is up to the programmer to provide these sequences. `model (MODEL-NAME)' Use this attribute on the M32R/D to set the addressability of an object, and the code generated for a function. The identifier MODEL-NAME is one of `small', `medium', or `large', representing each of the code models. Small model objects live in the lower 16MB of memory (so that their addresses can be loaded with the `ld24' instruction), and are callable with the `bl' instruction. Medium model objects may live anywhere in the 32-bit address space (the compiler will generate `seth/add3' instructions to load their addresses), and are callable with the `bl' instruction. Large model objects may live anywhere in the 32-bit address space (the compiler will generate `seth/add3' instructions to load their addresses), and may not be reachable with the `bl' instruction (the compiler will generate the much slower `seth/add3/jl' instruction sequence). 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 ISO C's `#pragma' should be used instead. At the time `__attribute__' was designed, there were 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 applied to almost any application that might have been proposed for `#pragma'. It was basically a mistake to use `#pragma' for _anything_. The ISO C99 standard includes `_Pragma', which now allows pragmas to be generated from macros. In addition, a `#pragma GCC' namespace is now in use for GCC-specific pragmas. However, it has been found convenient to use `__attribute__' to achieve a natural attachment of attributes to their corresponding declarations, whereas `#pragma GCC' is of use for constructs that do not naturally form part of the grammar. Note: Miscellaneous Preprocessing Directives.
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