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### 5.25 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. The following attributes are currently defined for functions on all targets: `noreturn`, `noinline`, `always_inline`, `pure`, `const`, `nothrow`, `format`, `format_arg`, `no_instrument_function`, `section`, `constructor`, `destructor`, `used`, `unused`, `deprecated`, `weak`, `malloc`, `alias`, `warn_unused_result` and `nonnull`. Several other attributes are defined for functions on particular target systems. Other attributes, including `section` are supported for variables declarations (see Variable Attributes) and for types (see 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`.

See 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.

  The `noreturn` keyword does not affect the exceptional path when that applies: a `noreturn`-marked function may still return to the caller by throwing an exception.

  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;

`noinline`
  This function attribute prevents a function from being considered for inlining.

`always_inline`
  Generally, functions are not inlined unless optimization is specified. For functions declared inline, this attribute inlines the function even if no optimization level was specified.

`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.

`nothrow`
  The `nothrow` attribute is used to inform the compiler that a function cannot throw an exception. For example, most functions in the standard C library can be guaranteed not to throw an exception with the notable exceptions of `qsort` and `bsearch` that take function pointer arguments. The `nothrow` attribute is not implemented in GCC versions earlier than 3.2.
`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. Since non-static C++ methods have an implicit `this` argument, the arguments of such methods should be counted from two, not one, when giving values for string-index and first-to-check.

  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 as are `printf_unlocked` and `fprintf_unlocked`. See 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 one). Since non-static C++ methods have an implicit `this` argument, the arguments of such methods should be counted from two.

  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. See Options Controlling C Dialect.

`nonnull (`arg-index`, ...)`
  The `nonnull` attribute specifies that some function parameters should be non-null pointers. For instance, the declaration:

                extern void *
                my_memcpy (void *dest, const void *src, size_t len)
                      __attribute__((nonnull (1, 2)));

  causes the compiler to check that, in calls to `my_memcpy`, arguments dest and src are non-null. If the compiler determines that a null pointer is passed in an argument slot marked as non-null, and the -Wnonnull option is enabled, a warning is issued. The compiler may also choose to make optimizations based on the knowledge that certain function arguments will not be null.

  If no argument index list is given to the `nonnull` attribute, all pointer arguments are marked as non-null. To illustrate, the following declaration is equivalent to the previous example:

                extern void *
                my_memcpy (void *dest, const void *src, size_t len)
                      __attribute__((nonnull));

`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.

`used`
  This attribute, attached to a function, means that code must be emitted for the function even if it appears that the function is not referenced. This is useful, for example, when the function is referenced only in inline assembly.

`deprecated`
  The `deprecated` attribute results in a warning if the function is used anywhere in the source file. This is useful when identifying functions that are expected to be removed in a future version of a program. The warning also includes the location of the declaration of the deprecated function, to enable users to easily find further information about why the function is deprecated, or what they should do instead. Note that the warnings only occurs for uses:

                int old_fn () __attribute__ ((deprecated));
                int old_fn ();
                int (*fn_ptr)() = old_fn;

  results in a warning on line 3 but not line 2.

  The `deprecated` attribute can also be used for variables and types (see Variable Attributes, see Type Attributes.)

`warn_unused_result`
  The `warn_unused_result` attribute causes a warning to be emitted if a caller of the function with this attribute does not use its return value. This is useful for functions where not checking the result is either a security problem or always a bug, such as `realloc`.

                int fn () __attribute__ ((warn_unused_result));
                int foo ()
                {
                  if (fn () < 0) return -1;
                  fn ();
                  return 0;
                }

  results in warning on line 5.

`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 any non-`NULL` pointer it returns cannot alias any other pointer valid when the function returns. This will often improve optimization. Standard functions with this property include `malloc` and `calloc`. `realloc`-like functions have this property as long as the old pointer is never referred to (including comparing it to the new pointer) after the function returns a non-`NULL` value.
`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.

`visibility ("`visibility_type`")`
  The `visibility` attribute on ELF targets causes the declaration to be emitted with default, hidden, protected or internal visibility.

                void __attribute__ ((visibility ("protected")))
                f () { /* Do something. */; }
                int i __attribute__ ((visibility ("hidden")));

  See the ELF gABI for complete details, but the short story is:

  default
    Default visibility is the normal case for ELF. This value is available for the visibility attribute to override other options that may change the assumed visibility of symbols.
  hidden
    Hidden visibility indicates that the symbol will not be placed into the dynamic symbol table, so no other module (executable or shared library) can reference it directly.
  protected
    Protected visibility indicates that the symbol will be placed in the dynamic symbol table, but that references within the defining module will bind to the local symbol. That is, the symbol cannot be overridden by another module.
  internal
    Internal visibility is like hidden visibility, but with additional processor specific semantics. Unless otherwise specified by the psABI, GCC defines internal visibility to mean that the function is never called from another module. Note that hidden symbols, while they cannot be referenced directly by other modules, can be referenced indirectly via function pointers. By indicating that a symbol cannot be called from outside the module, GCC may for instance omit the load of a PIC register since it is known that the calling function loaded the correct value.

  Not all ELF targets support this attribute.

`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.

  Beware that on some ELF systems this attribute is unsuitable for global functions in shared libraries with lazy binding (which is the default). Lazy binding will send the first call via resolving code in the loader, which might assume EAX, EDX and ECX can be clobbered, as per the standard calling conventions. Solaris 8 is affected by this. GNU systems with GLIBC 2.1 or higher, and FreeBSD, are believed to be safe since the loaders there save all registers. (Lazy binding can be disabled with the linker or the loader if desired, to avoid the problem.)

`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.
`fastcall`
  On the Intel 386, the `fastcall` attribute causes the compiler to pass the first two arguments in the registers ECX and EDX. Subsequent arguments are passed on the stack. The called function will pop the arguments off the stack. If the number of arguments is variable all arguments are pushed on the stack.
`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.
`longcall/shortcall`
  On the RS/6000 and PowerPC, the `longcall` attribute causes the compiler to always call this function via a pointer, just as it would if the -mlongcall option had been specified. The `shortcall` attribute causes the compiler not to do this. These attributes override both the -mlongcall switch and the `#pragma longcall` setting.

  See RS/6000 and PowerPC Options, for more information on whether long calls are necessary.

`long_call/short_call`
  This attribute specifies how a particular function is called on ARM. Both attributes override the -mlong-calls (see 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.
`function_vector`
  Use this attribute on the H8/300, H8/300H, and H8S 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 H8S) and shares space with the interrupt vector.

  You must use GAS and GLD from GNU binutils version 2.7 or later for this attribute to work correctly.

`interrupt`
  Use this attribute on the ARM, AVR, C4x, M32R/D and Xstormy16 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 m68k, H8/300, H8/300H, H8S, 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 attribute on the m68k, H8/300, H8/300H, H8S, 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 attribute 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 attribute on the SH for an `interrupt_handler` to return using `trapa` instead of `rte`. This attribute expects an integer argument specifying the trap number to be used.
`eightbit_data`
  Use this attribute on the H8/300, H8/300H, and H8S 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 attribute to work correctly.

`tiny_data`
  Use this attribute on the H8/300H and H8S 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.
`saveall`
  Use this attribute on the H8/300, H8/300H, and H8S to indicate that all registers except the stack pointer should be saved in the prologue regardless of whether they are used or not.
`signal`
  Use this attribute on the AVR to indicate that the specified function is a signal handler. The compiler will generate function entry and exit sequences suitable for use in a signal handler when this attribute is present. Interrupts will be disabled inside the function.
`naked`
  Use this attribute on the ARM, AVR, C4x and IP2K ports to indicate that the specified function does not need prologue/epilogue sequences generated by the compiler. It is up to the programmer to provide these sequences.
`model (`model-name`)`
  On the M32R/D, use this attribute to set the addressability of an object, and of 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).

  On IA-64, use this attribute to set the addressability of an object. At present, the only supported identifier for model-name is `small`, indicating addressability via “small” (22-bit) addresses (so that their addresses can be loaded with the `addl` instruction). Caveat: such addressing is by definition not position independent and hence this attribute must not be used for objects defined by shared libraries.

`far`
  On 68HC11 and 68HC12 the `far` attribute causes the compiler to use a calling convention that takes care of switching memory banks when entering and leaving a function. This calling convention is also the default when using the -mlong-calls option.

  On 68HC12 the compiler will use the `call` and `rtc` instructions to call and return from a function.

  On 68HC11 the compiler will generate a sequence of instructions to invoke a board-specific routine to switch the memory bank and call the real function. The board-specific routine simulates a `call`. At the end of a function, it will jump to a board-specific routine instead of using `rts`. The board-specific return routine simulates the `rtc`.

`near`
  On 68HC11 and 68HC12 the `near` attribute causes the compiler to use the normal calling convention based on `jsr` and `rts`. This attribute can be used to cancel the effect of the -mlong-calls option.
`dllimport`
  On Microsoft Windows targets, the `dllimport` attribute causes the compiler to reference a function or variable via a global pointer to a pointer that is set up by the Microsoft Windows dll library. The pointer name is formed by combining `_imp__` and the function or variable name. The attribute implies `extern` storage.

  Currently, the attribute is ignored for inlined functions. If the attribute is applied to a symbol definition, an error is reported. If a symbol previously declared `dllimport` is later defined, the attribute is ignored in subsequent references, and a warning is emitted. The attribute is also overridden by a subsequent declaration as `dllexport`.

  When applied to C++ classes, the attribute marks non-inlined member functions and static data members as imports. However, the attribute is ignored for virtual methods to allow creation of vtables using thunks.

  On cygwin, mingw and arm-pe targets, `__declspec(dllimport)` is recognized as a synonym for `__attribute__ ((dllimport))` for compatibility with other Microsoft Windows compilers.

  The use of the `dllimport` attribute on functions is not necessary, but provides a small performance benefit by eliminating a thunk in the dll. The use of the `dllimport` attribute on imported variables was required on older versions of GNU ld, but can now be avoided by passing the --enable-auto-import switch to ld. As with functions, using the attribute for a variable eliminates a thunk in the dll.

  One drawback to using this attribute is that a pointer to a function or variable marked as dllimport cannot be used as a constant address. The attribute can be disabled for functions by setting the -mnop-fun-dllimport flag.

`dllexport`
  On Microsoft Windows targets the `dllexport` attribute causes the compiler to provide a global pointer to a pointer in a dll, so that it can be referenced with the `dllimport` attribute. The pointer name is formed by combining `_imp__` and the function or variable name.

  Currently, the `dllexport`attribute is ignored for inlined functions, but export can be forced by using the -fkeep-inline-functions flag. The attribute is also ignored for undefined symbols.

  When applied to C++ classes. the attribute marks defined non-inlined member functions and static data members as exports. Static consts initialized in-class are not marked unless they are also defined out-of-class.

  On cygwin, mingw and arm-pe targets, `__declspec(dllexport)` is recognized as a synonym for `__attribute__ ((dllexport))` for compatibility with other Microsoft Windows compilers.

  Alternative methods for including the symbol in the dll's export table are to use a .def file with an `EXPORTS` section or, with GNU ld, using the --export-all linker flag.

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. See Miscellaneous Preprocessing Directives (The GNU C Preprocessor).