### 5.24 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`, `returns_twice`, `noinline`, `always_inline`, `flatten`, `pure`, `const`, `nothrow`, `sentinel`, `format`, `format_arg`, `no_instrument_function`, `section`, `constructor`, `destructor`, `used`, `unused`, `deprecated`, `weak`, `malloc`, `alias`, `warn_unused_result`, `nonnull` and `externally_visible`. 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.

`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. It is an error if ``__f` ' is not defined in the same translation unit.

  Not all target machines support this attribute.

`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.

`flatten`
  Generally, inlining into a function is limited. For a function marked with this attribute, every call inside this function will be inlined, if possible. Whether the function itself is considered for inlining depends on its size and the current inlining parameters. The `flatten` attribute only works reliably in unit-at-a-time mode.
`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.
`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 below, 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.

`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.

`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.)

`dllexport`
  On Microsoft Windows targets and Symbian OS 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. On Microsoft Windows targets, the pointer name is formed by combining `_imp__` and the function or variable name.

  You can use `__declspec(dllexport)` as a synonym for `__attribute__ ((dllexport))` for compatibility with other compilers.

  On systems that support the `visibility` attribute, this attribute also implies “default” visibility, unless a `visibility` attribute is explicitly specified. You should avoid the use of `dllexport` with “hidden” or “internal” visibility; in the future GCC may issue an error for those cases.

  Currently, the `dllexport` attribute is ignored for inlined functions, unless the `-fkeep-inline-functions` flag has been used. 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.

  For Microsoft Windows targets there are alternative methods for including the symbol in the DLL's export table such as using a `.def` file with an `EXPORTS` section or, with GNU ld, using the `--export-all` linker flag.

`dllimport`
  On Microsoft Windows and Symbian OS 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 DLL exporting the symbol. The attribute implies `extern` storage. On Microsoft Windows targets, the pointer name is formed by combining `_imp__` and the function or variable name.

  You can use `__declspec(dllimport)` as a synonym for `__attribute__ ((dllimport))` for compatibility with other compilers.

  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 the SH Symbian OS target the `dllimport` attribute also has another affect—it can cause the vtable and run-time type information for a class to be exported. This happens when the class has a dllimport'ed constructor or a non-inline, non-pure virtual function and, for either of those two conditions, the class also has a inline constructor or destructor and has a key function that is defined in the current translation unit.

  For Microsoft Windows based targets 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 the GNU linker, but can now be avoided by passing the `--enable-auto-import` switch to the GNU linker. 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. On Microsoft Windows targets, the attribute can be disabled for functions by setting the `-mnop-fun-dllimport` flag.

`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.

`exception_handler`
  Use this attribute on the Blackfin to indicate that the specified function is an exception handler. The compiler will generate function entry and exit sequences suitable for use in an exception handler when this attribute is present.
`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`.

`fastcall`
  On the Intel 386, the `fastcall` attribute causes the compiler to pass the first argument (if of integral type) in the register ECX and the second argument (if of integral type) in the register EDX. Subsequent and other typed 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.
`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` or `-fno-builtin` 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.

  The target may provide additional types of format checks. See Format Checks Specific to Particular Target Machines.

`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` or `-fno-builtin` is used. See Options Controlling C Dialect.

`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, CRX, M32C, M32R/D, MS1, 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 Blackfin, 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 Blackfin, 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.
`kspisusp`
  When used together with `interrupt_handler`, `exception_handler` or `nmi_handler`, code will be generated to load the stack pointer from the USP register in the function prologue.
`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.
`longcall/shortcall`
  On the Blackfin, 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, on the RS/6000 and PowerPC, the `#pragma longcall` setting.

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

`long_call`
  This attribute specifies how a particular function is called on MIPS. The attribute overrides the `-mlong-calls` (see MIPS Options) command line switch. This 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.
`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.
`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.

`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.
`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.
`nesting`
  Use this attribute together with `interrupt_handler`, `exception_handler` or `nmi_handler` to indicate that the function entry code should enable nested interrupts or exceptions.
`nmi_handler`
  Use this attribute on the Blackfin to indicate that the specified function is an NMI handler. The compiler will generate function entry and exit sequences suitable for use in an NMI handler when this attribute is present.
`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.
`noinline`
  This function attribute prevents a function from being considered for inlining.
`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));

`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 or calling `longjmp`.

  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;

  This approach does not work in GNU C++.

`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 3.3.
`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.

`regparm (` number `)`
  On the Intel 386, the `regparm` attribute causes the compiler to pass arguments number one to number if they are of integral type 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.)

`sseregparm`
  On the Intel 386 with SSE support, the `sseregparm` attribute causes the compiler to pass up to 8 floating point arguments in SSE registers instead of on the stack. Functions that take a variable number of arguments will continue to pass all of their floating point arguments on the stack.
`returns_twice`
  The `returns_twice` attribute tells the compiler that a function may return more than one time. The compiler will ensure that all registers are dead before calling such a function and will emit a warning about the variables that may be clobbered after the second return from the function. Examples of such functions are `setjmp` and `vfork`. The `longjmp`-like counterpart of such function, if any, might need to be marked with the `noreturn` attribute.
`saveall`
  Use this attribute on the Blackfin, 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.
`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.

`sentinel`
  This function attribute ensures that a parameter in a function call is an explicit `NULL`. The attribute is only valid on variadic functions. By default, the sentinel is located at position zero, the last parameter of the function call. If an optional integer position argument P is supplied to the attribute, the sentinel must be located at position P counting backwards from the end of the argument list.

                __attribute__ ((sentinel))
                is equivalent to
                __attribute__ ((sentinel(0)))

  The attribute is automatically set with a position of 0 for the built-in functions `execl` and `execlp`. The built-in function `execle` has the attribute set with a position of 1.

  A valid `NULL` in this context is defined as zero with any pointer type. If your system defines the `NULL` macro with an integer type then you need to add an explicit cast. GCC replaces `stddef.h` with a copy that redefines NULL appropriately.

  The warnings for missing or incorrect sentinels are enabled with `-Wformat` .

`short_call`
  See long_call/short_call.
`shortcall`
  See longcall/shortcall.
`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.
`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")));

`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.
`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.
`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.
`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.
`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.
  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.
  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.

  Not all ELF targets support this attribute.

`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.
`weakref`
`weakref ("` target `")`
  The `weakref` attribute marks a declaration as a weak reference. Without arguments, it should be accompanied by an `alias` attribute naming the target symbol. Optionally, the target may be given as an argument to `weakref` itself. In either case, `weakref` implicitly marks the declaration as `weak`. Without a target, given as an argument to `weakref` or to `alias`, `weakref` is equivalent to `weak`.

                extern int x() __attribute__ ((weakref ("y")));
                /* is equivalent to... */
                extern int x() __attribute__ ((weak, weakref, alias ("y")));
                /* and to... */
                extern int x() __attribute__ ((weakref));
                extern int x() __attribute__ ((alias ("y")));

  A weak reference is an alias that does not by itself require a definition to be given for the target symbol. If the target symbol is only referenced through weak references, then the becomes a `weak` undefined symbol. If it is directly referenced, however, then such strong references prevail, and a definition will be required for the symbol, not necessarily in the same translation unit.

  The effect is equivalent to moving all references to the alias to a separate translation unit, renaming the alias to the aliased symbol, declaring it as weak, compiling the two separate translation units and performing a reloadable link on them.

`externally_visible`
  This attribute, attached to a global variable or function nullify effect of `-fwhole-program` command line option, so the object remain visible outside the current compilation unit

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.