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<h3 class="section">6.29 Declaring Attributes of Functions</h3>
<p><a name="index-function-attributes-2332"></a><a name="index-declaring-attributes-of-functions-2333"></a><a name="index-functions-that-never-return-2334"></a><a name="index-functions-that-return-more-than-once-2335"></a><a name="index-functions-that-have-no-side-effects-2336"></a><a name="index-functions-in-arbitrary-sections-2337"></a><a name="index-functions-that-behave-like-malloc-2338"></a><a name="index-g_t_0040code_007bvolatile_007d-applied-to-function-2339"></a><a name="index-g_t_0040code_007bconst_007d-applied-to-function-2340"></a><a name="index-functions-with-_0040code_007bprintf_007d_002c-_0040code_007bscanf_007d_002c-_0040code_007bstrftime_007d-or-_0040code_007bstrfmon_007d-style-arguments-2341"></a><a name="index-functions-with-non_002dnull-pointer-arguments-2342"></a><a name="index-functions-that-are-passed-arguments-in-registers-on-the-386-2343"></a><a name="index-functions-that-pop-the-argument-stack-on-the-386-2344"></a><a name="index-functions-that-do-not-pop-the-argument-stack-on-the-386-2345"></a><a name="index-functions-that-have-different-compilation-options-on-the-386-2346"></a><a name="index-functions-that-have-different-optimization-options-2347"></a>
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.
<p>The keyword <code>__attribute__</code> 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:
<code>aligned</code>, <code>alloc_size</code>, <code>noreturn</code>,
<code>returns_twice</code>, <code>noinline</code>, <code>noclone</code>,
<code>always_inline</code>, <code>flatten</code>, <code>pure</code>, <code>const</code>,
<code>nothrow</code>, <code>sentinel</code>, <code>format</code>, <code>format_arg</code>,
<code>no_instrument_function</code>, <code>section</code>, <code>constructor</code>,
<code>destructor</code>, <code>used</code>, <code>unused</code>, <code>deprecated</code>,
<code>weak</code>, <code>malloc</code>, <code>alias</code>, <code>warn_unused_result</code>,
<code>nonnull</code>, <code>gnu_inline</code>, <code>externally_visible</code>,
<code>hot</code>, <code>cold</code>, <code>artificial</code>, <code>error</code> and
<code>warning</code>. Several other attributes are defined for functions on
particular target systems. Other attributes, including <code>section</code>
are supported for variables declarations (see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>)
and for types (see <a href="Type-Attributes.html#Type-Attributes">Type Attributes</a>).
<p>GCC plugins may provide their own attributes.
<p>You may also specify attributes with &lsquo;<samp><span class="samp">__</span></samp>&rsquo; 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 <code>__noreturn__</code> instead of <code>noreturn</code>.
<p>See <a href="Attribute-Syntax.html#Attribute-Syntax">Attribute Syntax</a>, for details of the exact syntax for using
attributes.
<dl>
<!-- Keep this table alphabetized by attribute name. Treat _ as space. -->
<dt><code>alias ("</code><var>target</var><code>")</code><dd><a name="index-g_t_0040code_007balias_007d-attribute-2348"></a>The <code>alias</code> attribute causes the declaration to be emitted as an
alias for another symbol, which must be specified. For instance,
<pre class="smallexample"> void __f () { /* <span class="roman">Do something.</span> */; }
void f () __attribute__ ((weak, alias ("__f")));
</pre>
<p>defines &lsquo;<samp><span class="samp">f</span></samp>&rsquo; to be a weak alias for &lsquo;<samp><span class="samp">__f</span></samp>&rsquo;. In C++, the
mangled name for the target must be used. It is an error if &lsquo;<samp><span class="samp">__f</span></samp>&rsquo;
is not defined in the same translation unit.
<p>Not all target machines support this attribute.
<br><dt><code>aligned (</code><var>alignment</var><code>)</code><dd><a name="index-g_t_0040code_007baligned_007d-attribute-2349"></a>This attribute specifies a minimum alignment for the function,
measured in bytes.
<p>You cannot use this attribute to decrease the alignment of a function,
only to increase it. However, when you explicitly specify a function
alignment this will override the effect of the
<samp><span class="option">-falign-functions</span></samp> (see <a href="Optimize-Options.html#Optimize-Options">Optimize Options</a>) option for this
function.
<p>Note that the effectiveness of <code>aligned</code> attributes may be
limited by inherent limitations in your linker. On many systems, the
linker is only able to arrange for functions to be aligned up to a
certain maximum alignment. (For some linkers, the maximum supported
alignment may be very very small.) See your linker documentation for
further information.
<p>The <code>aligned</code> attribute can also be used for variables and fields
(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>.)
<br><dt><code>alloc_size</code><dd><a name="index-g_t_0040code_007balloc_005fsize_007d-attribute-2350"></a>The <code>alloc_size</code> attribute is used to tell the compiler that the
function return value points to memory, where the size is given by
one or two of the functions parameters. GCC uses this
information to improve the correctness of <code>__builtin_object_size</code>.
<p>The function parameter(s) denoting the allocated size are specified by
one or two integer arguments supplied to the attribute. The allocated size
is either the value of the single function argument specified or the product
of the two function arguments specified. Argument numbering starts at
one.
<p>For instance,
<pre class="smallexample"> void* my_calloc(size_t, size_t) __attribute__((alloc_size(1,2)))
void my_realloc(void*, size_t) __attribute__((alloc_size(2)))
</pre>
<p>declares that my_calloc will return memory of the size given by
the product of parameter 1 and 2 and that my_realloc will return memory
of the size given by parameter 2.
<br><dt><code>always_inline</code><dd><a name="index-g_t_0040code_007balways_005finline_007d-function-attribute-2351"></a>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.
<br><dt><code>gnu_inline</code><dd><a name="index-g_t_0040code_007bgnu_005finline_007d-function-attribute-2352"></a>This attribute should be used with a function which is also declared
with the <code>inline</code> keyword. It directs GCC to treat the function
as if it were defined in gnu90 mode even when compiling in C99 or
gnu99 mode.
<p>If the function is declared <code>extern</code>, then this definition of the
function is used only for inlining. In no case is the function
compiled as a standalone function, not even if you take its address
explicitly. Such an address becomes an external reference, as if you
had only declared the function, and had not defined it. This has
almost the effect of a macro. The way to use this is to put a
function definition in a header file with this attribute, and put
another copy of the function, without <code>extern</code>, 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. Note that the two
definitions of the functions need not be precisely the same, although
if they do not have the same effect your program may behave oddly.
<p>In C, if the function is neither <code>extern</code> nor <code>static</code>, then
the function is compiled as a standalone function, as well as being
inlined where possible.
<p>This is how GCC traditionally handled functions declared
<code>inline</code>. Since ISO C99 specifies a different semantics for
<code>inline</code>, this function attribute is provided as a transition
measure and as a useful feature in its own right. This attribute is
available in GCC 4.1.3 and later. It is available if either of the
preprocessor macros <code>__GNUC_GNU_INLINE__</code> or
<code>__GNUC_STDC_INLINE__</code> are defined. See <a href="Inline.html#Inline">An Inline Function is As Fast As a Macro</a>.
<p>In C++, this attribute does not depend on <code>extern</code> in any way,
but it still requires the <code>inline</code> keyword to enable its special
behavior.
<br><dt><code>artificial</code><dd><a name="index-g_t_0040code_007bartificial_007d-function-attribute-2353"></a>This attribute is useful for small inline wrappers which if possible
should appear during debugging as a unit, depending on the debug
info format it will either mean marking the function as artificial
or using the caller location for all instructions within the inlined
body.
<br><dt><code>bank_switch</code><dd><a name="index-interrupt-handler-functions-2354"></a>When added to an interrupt handler with the M32C port, causes the
prologue and epilogue to use bank switching to preserve the registers
rather than saving them on the stack.
<br><dt><code>flatten</code><dd><a name="index-g_t_0040code_007bflatten_007d-function-attribute-2355"></a>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.
<br><dt><code>error ("</code><var>message</var><code>")</code><dd><a name="index-g_t_0040code_007berror_007d-function-attribute-2356"></a>If this attribute is used on a function declaration and a call to such a function
is not eliminated through dead code elimination or other optimizations, an error
which will include <var>message</var> will be diagnosed. This is useful
for compile time checking, especially together with <code>__builtin_constant_p</code>
and inline functions where checking the inline function arguments is not
possible through <code>extern char [(condition) ? 1 : -1];</code> tricks.
While it is possible to leave the function undefined and thus invoke
a link failure, when using this attribute the problem will be diagnosed
earlier and with exact location of the call even in presence of inline
functions or when not emitting debugging information.
<br><dt><code>warning ("</code><var>message</var><code>")</code><dd><a name="index-g_t_0040code_007bwarning_007d-function-attribute-2357"></a>If this attribute is used on a function declaration and a call to such a function
is not eliminated through dead code elimination or other optimizations, a warning
which will include <var>message</var> will be diagnosed. This is useful
for compile time checking, especially together with <code>__builtin_constant_p</code>
and inline functions. While it is possible to define the function with
a message in <code>.gnu.warning*</code> section, when using this attribute the problem
will be diagnosed earlier and with exact location of the call even in presence
of inline functions or when not emitting debugging information.
<br><dt><code>cdecl</code><dd><a name="index-functions-that-do-pop-the-argument-stack-on-the-386-2358"></a><a name="index-mrtd-2359"></a>On the Intel 386, the <code>cdecl</code> 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 <samp><span class="option">-mrtd</span></samp> switch.
<br><dt><code>const</code><dd><a name="index-g_t_0040code_007bconst_007d-function-attribute-2360"></a>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 <code>pure</code> attribute below, since function is not
allowed to read global memory.
<p><a name="index-pointer-arguments-2361"></a>Note that a function that has pointer arguments and examines the data
pointed to must <em>not</em> be declared <code>const</code>. Likewise, a
function that calls a non-<code>const</code> function usually must not be
<code>const</code>. It does not make sense for a <code>const</code> function to
return <code>void</code>.
<p>The attribute <code>const</code> 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:
<pre class="smallexample"> typedef int intfn ();
extern const intfn square;
</pre>
<p>This approach does not work in GNU C++ from 2.6.0 on, since the language
specifies that the &lsquo;<samp><span class="samp">const</span></samp>&rsquo; must be attached to the return value.
<br><dt><code>constructor</code><dt><code>destructor</code><dt><code>constructor (</code><var>priority</var><code>)</code><dt><code>destructor (</code><var>priority</var><code>)</code><dd><a name="index-g_t_0040code_007bconstructor_007d-function-attribute-2362"></a><a name="index-g_t_0040code_007bdestructor_007d-function-attribute-2363"></a>The <code>constructor</code> attribute causes the function to be called
automatically before execution enters <code>main ()</code>. Similarly, the
<code>destructor</code> attribute causes the function to be called
automatically after <code>main ()</code> has completed or <code>exit ()</code> has
been called. Functions with these attributes are useful for
initializing data that will be used implicitly during the execution of
the program.
<p>You may provide an optional integer priority to control the order in
which constructor and destructor functions are run. A constructor
with a smaller priority number runs before a constructor with a larger
priority number; the opposite relationship holds for destructors. So,
if you have a constructor that allocates a resource and a destructor
that deallocates the same resource, both functions typically have the
same priority. The priorities for constructor and destructor
functions are the same as those specified for namespace-scope C++
objects (see <a href="C_002b_002b-Attributes.html#C_002b_002b-Attributes">C++ Attributes</a>).
<p>These attributes are not currently implemented for Objective-C.
<br><dt><code>deprecated</code><dt><code>deprecated (</code><var>msg</var><code>)</code><dd><a name="index-g_t_0040code_007bdeprecated_007d-attribute_002e-2364"></a>The <code>deprecated</code> 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:
<pre class="smallexample"> int old_fn () __attribute__ ((deprecated));
int old_fn ();
int (*fn_ptr)() = old_fn;
</pre>
<p>results in a warning on line 3 but not line 2. The optional msg
argument, which must be a string, will be printed in the warning if
present.
<p>The <code>deprecated</code> attribute can also be used for variables and
types (see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>, see <a href="Type-Attributes.html#Type-Attributes">Type Attributes</a>.)
<br><dt><code>disinterrupt</code><dd><a name="index-g_t_0040code_007bdisinterrupt_007d-attribute-2365"></a>On MeP targets, this attribute causes the compiler to emit
instructions to disable interrupts for the duration of the given
function.
<br><dt><code>dllexport</code><dd><a name="index-g_t_0040code_007b_005f_005fdeclspec_0028dllexport_0029_007d-2366"></a>On Microsoft Windows targets and Symbian OS targets the
<code>dllexport</code> attribute causes the compiler to provide a global
pointer to a pointer in a DLL, so that it can be referenced with the
<code>dllimport</code> attribute. On Microsoft Windows targets, the pointer
name is formed by combining <code>_imp__</code> and the function or variable
name.
<p>You can use <code>__declspec(dllexport)</code> as a synonym for
<code>__attribute__ ((dllexport))</code> for compatibility with other
compilers.
<p>On systems that support the <code>visibility</code> attribute, this
attribute also implies &ldquo;default&rdquo; visibility. It is an error to
explicitly specify any other visibility.
<p>Currently, the <code>dllexport</code> attribute is ignored for inlined
functions, unless the <samp><span class="option">-fkeep-inline-functions</span></samp> flag has been
used. The attribute is also ignored for undefined symbols.
<p>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.
<p>For Microsoft Windows targets there are alternative methods for
including the symbol in the DLL's export table such as using a
<samp><span class="file">.def</span></samp> file with an <code>EXPORTS</code> section or, with GNU ld, using
the <samp><span class="option">--export-all</span></samp> linker flag.
<br><dt><code>dllimport</code><dd><a name="index-g_t_0040code_007b_005f_005fdeclspec_0028dllimport_0029_007d-2367"></a>On Microsoft Windows and Symbian OS targets, the <code>dllimport</code>
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 <code>extern</code>. On Microsoft Windows
targets, the pointer name is formed by combining <code>_imp__</code> and the
function or variable name.
<p>You can use <code>__declspec(dllimport)</code> as a synonym for
<code>__attribute__ ((dllimport))</code> for compatibility with other
compilers.
<p>On systems that support the <code>visibility</code> attribute, this
attribute also implies &ldquo;default&rdquo; visibility. It is an error to
explicitly specify any other visibility.
<p>Currently, the attribute is ignored for inlined functions. If the
attribute is applied to a symbol <em>definition</em>, an error is reported.
If a symbol previously declared <code>dllimport</code> 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
<code>dllexport</code>.
<p>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.
<p>On the SH Symbian OS target the <code>dllimport</code> attribute also has
another affect&mdash;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 an inline
constructor or destructor and has a key function that is defined in
the current translation unit.
<p>For Microsoft Windows based targets the use of the <code>dllimport</code>
attribute on functions is not necessary, but provides a small
performance benefit by eliminating a thunk in the DLL. The use of the
<code>dllimport</code> attribute on imported variables was required on older
versions of the GNU linker, but can now be avoided by passing the
<samp><span class="option">--enable-auto-import</span></samp> switch to the GNU linker. As with
functions, using the attribute for a variable eliminates a thunk in
the DLL.
<p>One drawback to using this attribute is that a pointer to a
<em>variable</em> marked as <code>dllimport</code> cannot be used as a constant
address. However, a pointer to a <em>function</em> with the
<code>dllimport</code> attribute can be used as a constant initializer; in
this case, the address of a stub function in the import lib is
referenced. On Microsoft Windows targets, the attribute can be disabled
for functions by setting the <samp><span class="option">-mnop-fun-dllimport</span></samp> flag.
<br><dt><code>eightbit_data</code><dd><a name="index-eight-bit-data-on-the-H8_002f300_002c-H8_002f300H_002c-and-H8S-2368"></a>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.
<p>You must use GAS and GLD from GNU binutils version 2.7 or later for
this attribute to work correctly.
<br><dt><code>exception_handler</code><dd><a name="index-exception-handler-functions-on-the-Blackfin-processor-2369"></a>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.
<br><dt><code>externally_visible</code><dd><a name="index-g_t_0040code_007bexternally_005fvisible_007d-attribute_002e-2370"></a>This attribute, attached to a global variable or function, nullifies
the effect of the <samp><span class="option">-fwhole-program</span></samp> command-line option, so the
object remains visible outside the current compilation unit.
<br><dt><code>far</code><dd><a name="index-functions-which-handle-memory-bank-switching-2371"></a>On 68HC11 and 68HC12 the <code>far</code> 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 <samp><span class="option">-mlong-calls</span></samp> option.
<p>On 68HC12 the compiler will use the <code>call</code> and <code>rtc</code> instructions
to call and return from a function.
<p>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 <code>call</code>.
At the end of a function, it will jump to a board-specific routine
instead of using <code>rts</code>. The board-specific return routine simulates
the <code>rtc</code>.
<p>On MeP targets this causes the compiler to use a calling convention
which assumes the called function is too far away for the built-in
addressing modes.
<br><dt><code>fast_interrupt</code><dd><a name="index-interrupt-handler-functions-2372"></a>Use this attribute on the M32C and RX ports to indicate that the specified
function is a fast interrupt handler. This is just like the
<code>interrupt</code> attribute, except that <code>freit</code> is used to return
instead of <code>reit</code>.
<br><dt><code>fastcall</code><dd><a name="index-functions-that-pop-the-argument-stack-on-the-386-2373"></a>On the Intel 386, the <code>fastcall</code> 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.
<br><dt><code>format (</code><var>archetype</var><code>, </code><var>string-index</var><code>, </code><var>first-to-check</var><code>)</code><dd><a name="index-g_t_0040code_007bformat_007d-function-attribute-2374"></a><a name="index-Wformat-2375"></a>The <code>format</code> attribute specifies that a function takes <code>printf</code>,
<code>scanf</code>, <code>strftime</code> or <code>strfmon</code> style arguments which
should be type-checked against a format string. For example, the
declaration:
<pre class="smallexample"> extern int
my_printf (void *my_object, const char *my_format, ...)
__attribute__ ((format (printf, 2, 3)));
</pre>
<p class="noindent">causes the compiler to check the arguments in calls to <code>my_printf</code>
for consistency with the <code>printf</code> style format string argument
<code>my_format</code>.
<p>The parameter <var>archetype</var> determines how the format string is
interpreted, and should be <code>printf</code>, <code>scanf</code>, <code>strftime</code>,
<code>gnu_printf</code>, <code>gnu_scanf</code>, <code>gnu_strftime</code> or
<code>strfmon</code>. (You can also use <code>__printf__</code>,
<code>__scanf__</code>, <code>__strftime__</code> or <code>__strfmon__</code>.) On
MinGW targets, <code>ms_printf</code>, <code>ms_scanf</code>, and
<code>ms_strftime</code> are also present.
<var>archtype</var> values such as <code>printf</code> refer to the formats accepted
by the system's C run-time library, while <code>gnu_</code> values always refer
to the formats accepted by the GNU C Library. On Microsoft Windows
targets, <code>ms_</code> values refer to the formats accepted by the
<samp><span class="file">msvcrt.dll</span></samp> library.
The parameter <var>string-index</var>
specifies which argument is the format string argument (starting
from 1), while <var>first-to-check</var> 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
<code>vprintf</code>), specify the third parameter as zero. In this case the
compiler only checks the format string for consistency. For
<code>strftime</code> formats, the third parameter is required to be zero.
Since non-static C++ methods have an implicit <code>this</code> argument, the
arguments of such methods should be counted from two, not one, when
giving values for <var>string-index</var> and <var>first-to-check</var>.
<p>In the example above, the format string (<code>my_format</code>) is the second
argument of the function <code>my_print</code>, and the arguments to check
start with the third argument, so the correct parameters for the format
attribute are 2 and 3.
<p><a name="index-ffreestanding-2376"></a><a name="index-fno_002dbuiltin-2377"></a>The <code>format</code> 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
<samp><span class="option">-ffreestanding</span></samp> or <samp><span class="option">-fno-builtin</span></samp> is used) checks formats
for the standard library functions <code>printf</code>, <code>fprintf</code>,
<code>sprintf</code>, <code>scanf</code>, <code>fscanf</code>, <code>sscanf</code>, <code>strftime</code>,
<code>vprintf</code>, <code>vfprintf</code> and <code>vsprintf</code> whenever such
warnings are requested (using <samp><span class="option">-Wformat</span></samp>), so there is no need to
modify the header file <samp><span class="file">stdio.h</span></samp>. In C99 mode, the functions
<code>snprintf</code>, <code>vsnprintf</code>, <code>vscanf</code>, <code>vfscanf</code> and
<code>vsscanf</code> are also checked. Except in strictly conforming C
standard modes, the X/Open function <code>strfmon</code> is also checked as
are <code>printf_unlocked</code> and <code>fprintf_unlocked</code>.
See <a href="C-Dialect-Options.html#C-Dialect-Options">Options Controlling C Dialect</a>.
<p>The target may provide additional types of format checks.
See <a href="Target-Format-Checks.html#Target-Format-Checks">Format Checks Specific to Particular Target Machines</a>.
<br><dt><code>format_arg (</code><var>string-index</var><code>)</code><dd><a name="index-g_t_0040code_007bformat_005farg_007d-function-attribute-2378"></a><a name="index-Wformat_002dnonliteral-2379"></a>The <code>format_arg</code> attribute specifies that a function takes a format
string for a <code>printf</code>, <code>scanf</code>, <code>strftime</code> or
<code>strfmon</code> style function and modifies it (for example, to translate
it into another language), so the result can be passed to a
<code>printf</code>, <code>scanf</code>, <code>strftime</code> or <code>strfmon</code> 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:
<pre class="smallexample"> extern char *
my_dgettext (char *my_domain, const char *my_format)
__attribute__ ((format_arg (2)));
</pre>
<p class="noindent">causes the compiler to check the arguments in calls to a <code>printf</code>,
<code>scanf</code>, <code>strftime</code> or <code>strfmon</code> type function, whose
format string argument is a call to the <code>my_dgettext</code> function, for
consistency with the format string argument <code>my_format</code>. If the
<code>format_arg</code> 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
<samp><span class="option">-Wformat-nonliteral</span></samp> is used, but the calls could not be checked
without the attribute.
<p>The parameter <var>string-index</var> specifies which argument is the format
string argument (starting from one). Since non-static C++ methods have
an implicit <code>this</code> argument, the arguments of such methods should
be counted from two.
<p>The <code>format-arg</code> attribute allows you to identify your own
functions which modify format strings, so that GCC can check the
calls to <code>printf</code>, <code>scanf</code>, <code>strftime</code> or <code>strfmon</code>
type function whose operands are a call to one of your own function.
The compiler always treats <code>gettext</code>, <code>dgettext</code>, and
<code>dcgettext</code> in this manner except when strict ISO C support is
requested by <samp><span class="option">-ansi</span></samp> or an appropriate <samp><span class="option">-std</span></samp> option, or
<samp><span class="option">-ffreestanding</span></samp> or <samp><span class="option">-fno-builtin</span></samp>
is used. See <a href="C-Dialect-Options.html#C-Dialect-Options">Options Controlling C Dialect</a>.
<br><dt><code>function_vector</code><dd><a name="index-calling-functions-through-the-function-vector-on-H8_002f300_002c-M16C_002c-M32C-and-SH2A-processors-2380"></a>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.
<p>In SH2A target, this attribute declares a function to be called using the
TBR relative addressing mode. The argument to this attribute is the entry
number of the same function in a vector table containing all the TBR
relative addressable functions. For the successful jump, register TBR
should contain the start address of this TBR relative vector table.
In the startup routine of the user application, user needs to care of this
TBR register initialization. The TBR relative vector table can have at
max 256 function entries. The jumps to these functions will be generated
using a SH2A specific, non delayed branch instruction JSR/N @(disp8,TBR).
You must use GAS and GLD from GNU binutils version 2.7 or later for
this attribute to work correctly.
<p>Please refer the example of M16C target, to see the use of this
attribute while declaring a function,
<p>In an application, for a function being called once, this attribute will
save at least 8 bytes of code; and if other successive calls are being
made to the same function, it will save 2 bytes of code per each of these
calls.
<p>On M16C/M32C targets, the <code>function_vector</code> attribute declares a
special page subroutine call function. Use of this attribute reduces
the code size by 2 bytes for each call generated to the
subroutine. The argument to the attribute is the vector number entry
from the special page vector table which contains the 16 low-order
bits of the subroutine's entry address. Each vector table has special
page number (18 to 255) which are used in <code>jsrs</code> instruction.
Jump addresses of the routines are generated by adding 0x0F0000 (in
case of M16C targets) or 0xFF0000 (in case of M32C targets), to the 2
byte addresses set in the vector table. Therefore you need to ensure
that all the special page vector routines should get mapped within the
address range 0x0F0000 to 0x0FFFFF (for M16C) and 0xFF0000 to 0xFFFFFF
(for M32C).
<p>In the following example 2 bytes will be saved for each call to
function <code>foo</code>.
<pre class="smallexample"> void foo (void) __attribute__((function_vector(0x18)));
void foo (void)
{
}
void bar (void)
{
foo();
}
</pre>
<p>If functions are defined in one file and are called in another file,
then be sure to write this declaration in both files.
<p>This attribute is ignored for R8C target.
<br><dt><code>interrupt</code><dd><a name="index-interrupt-handler-functions-2381"></a>Use this attribute on the ARM, AVR, CRX, M32C, M32R/D, m68k, MeP, MIPS,
RX 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.
<p>Note, interrupt handlers for the Blackfin, H8/300, H8/300H, H8S, and
SH processors can be specified via the <code>interrupt_handler</code> attribute.
<p>Note, on the AVR, interrupts will be enabled inside the function.
<p>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:
<pre class="smallexample"> void f () __attribute__ ((interrupt ("IRQ")));
</pre>
<p>Permissible values for this parameter are: IRQ, FIQ, SWI, ABORT and UNDEF.
<p>On ARMv7-M the interrupt type is ignored, and the attribute means the function
may be called with a word aligned stack pointer.
<p>On MIPS targets, you can use the following attributes to modify the behavior
of an interrupt handler:
<dl>
<dt><code>use_shadow_register_set</code><dd><a name="index-g_t_0040code_007buse_005fshadow_005fregister_005fset_007d-attribute-2382"></a>Assume that the handler uses a shadow register set, instead of
the main general-purpose registers.
<br><dt><code>keep_interrupts_masked</code><dd><a name="index-g_t_0040code_007bkeep_005finterrupts_005fmasked_007d-attribute-2383"></a>Keep interrupts masked for the whole function. Without this attribute,
GCC tries to reenable interrupts for as much of the function as it can.
<br><dt><code>use_debug_exception_return</code><dd><a name="index-g_t_0040code_007buse_005fdebug_005fexception_005freturn_007d-attribute-2384"></a>Return using the <code>deret</code> instruction. Interrupt handlers that don't
have this attribute return using <code>eret</code> instead.
</dl>
<p>You can use any combination of these attributes, as shown below:
<pre class="smallexample"> void __attribute__ ((interrupt)) v0 ();
void __attribute__ ((interrupt, use_shadow_register_set)) v1 ();
void __attribute__ ((interrupt, keep_interrupts_masked)) v2 ();
void __attribute__ ((interrupt, use_debug_exception_return)) v3 ();
void __attribute__ ((interrupt, use_shadow_register_set,
keep_interrupts_masked)) v4 ();
void __attribute__ ((interrupt, use_shadow_register_set,
use_debug_exception_return)) v5 ();
void __attribute__ ((interrupt, keep_interrupts_masked,
use_debug_exception_return)) v6 ();
void __attribute__ ((interrupt, use_shadow_register_set,
keep_interrupts_masked,
use_debug_exception_return)) v7 ();
</pre>
<br><dt><code>interrupt_handler</code><dd><a name="index-interrupt-handler-functions-on-the-Blackfin_002c-m68k_002c-H8_002f300-and-SH-processors-2385"></a>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.
<br><dt><code>interrupt_thread</code><dd><a name="index-interrupt-thread-functions-on-fido-2386"></a>Use this attribute on fido, a subarchitecture of the m68k, to indicate
that the specified function is an interrupt handler that is designed
to run as a thread. The compiler omits generate prologue/epilogue
sequences and replaces the return instruction with a <code>sleep</code>
instruction. This attribute is available only on fido.
<br><dt><code>isr</code><dd><a name="index-interrupt-service-routines-on-ARM-2387"></a>Use this attribute on ARM to write Interrupt Service Routines. This is an
alias to the <code>interrupt</code> attribute above.
<br><dt><code>kspisusp</code><dd><a name="index-User-stack-pointer-in-interrupts-on-the-Blackfin-2388"></a>When used together with <code>interrupt_handler</code>, <code>exception_handler</code>
or <code>nmi_handler</code>, code will be generated to load the stack pointer
from the USP register in the function prologue.
<br><dt><code>l1_text</code><dd><a name="index-g_t_0040code_007bl1_005ftext_007d-function-attribute-2389"></a>This attribute specifies a function to be placed into L1 Instruction
SRAM. The function will be put into a specific section named <code>.l1.text</code>.
With <samp><span class="option">-mfdpic</span></samp>, function calls with a such function as the callee
or caller will use inlined PLT.
<br><dt><code>l2</code><dd><a name="index-g_t_0040code_007bl2_007d-function-attribute-2390"></a>On the Blackfin, this attribute specifies a function to be placed into L2
SRAM. The function will be put into a specific section named
<code>.l1.text</code>. With <samp><span class="option">-mfdpic</span></samp>, callers of such functions will use
an inlined PLT.
<br><dt><code>long_call/short_call</code><dd><a name="index-indirect-calls-on-ARM-2391"></a>This attribute specifies how a particular function is called on
ARM. Both attributes override the <samp><span class="option">-mlong-calls</span></samp> (see <a href="ARM-Options.html#ARM-Options">ARM Options</a>)
command line switch and <code>#pragma long_calls</code> settings. The
<code>long_call</code> attribute indicates that the function might be far
away from the call site and require a different (more expensive)
calling sequence. The <code>short_call</code> attribute always places
the offset to the function from the call site into the &lsquo;<samp><span class="samp">BL</span></samp>&rsquo;
instruction directly.
<br><dt><code>longcall/shortcall</code><dd><a name="index-functions-called-via-pointer-on-the-RS_002f6000-and-PowerPC-2392"></a>On the Blackfin, RS/6000 and PowerPC, the <code>longcall</code> attribute
indicates that the function might be far away from the call site and
require a different (more expensive) calling sequence. The
<code>shortcall</code> attribute indicates that the function is always close
enough for the shorter calling sequence to be used. These attributes
override both the <samp><span class="option">-mlongcall</span></samp> switch and, on the RS/6000 and
PowerPC, the <code>#pragma longcall</code> setting.
<p>See <a href="RS_002f6000-and-PowerPC-Options.html#RS_002f6000-and-PowerPC-Options">RS/6000 and PowerPC Options</a>, for more information on whether long
calls are necessary.
<br><dt><code>long_call/near/far</code><dd><a name="index-indirect-calls-on-MIPS-2393"></a>These attributes specify how a particular function is called on MIPS.
The attributes override the <samp><span class="option">-mlong-calls</span></samp> (see <a href="MIPS-Options.html#MIPS-Options">MIPS Options</a>)
command-line switch. The <code>long_call</code> and <code>far</code> attributes are
synonyms, and cause the compiler to always call
the function by first loading its address into a register, and then using
the contents of that register. The <code>near</code> attribute has the opposite
effect; it specifies that non-PIC calls should be made using the more
efficient <code>jal</code> instruction.
<br><dt><code>malloc</code><dd><a name="index-g_t_0040code_007bmalloc_007d-attribute-2394"></a>The <code>malloc</code> attribute is used to tell the compiler that a function
may be treated as if any non-<code>NULL</code> pointer it returns cannot
alias any other pointer valid when the function returns.
This will often improve optimization.
Standard functions with this property include <code>malloc</code> and
<code>calloc</code>. <code>realloc</code>-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-<code>NULL</code>
value.
<br><dt><code>mips16/nomips16</code><dd><a name="index-g_t_0040code_007bmips16_007d-attribute-2395"></a><a name="index-g_t_0040code_007bnomips16_007d-attribute-2396"></a>
On MIPS targets, you can use the <code>mips16</code> and <code>nomips16</code>
function attributes to locally select or turn off MIPS16 code generation.
A function with the <code>mips16</code> attribute is emitted as MIPS16 code,
while MIPS16 code generation is disabled for functions with the
<code>nomips16</code> attribute. These attributes override the
<samp><span class="option">-mips16</span></samp> and <samp><span class="option">-mno-mips16</span></samp> options on the command line
(see <a href="MIPS-Options.html#MIPS-Options">MIPS Options</a>).
<p>When compiling files containing mixed MIPS16 and non-MIPS16 code, the
preprocessor symbol <code>__mips16</code> reflects the setting on the command line,
not that within individual functions. Mixed MIPS16 and non-MIPS16 code
may interact badly with some GCC extensions such as <code>__builtin_apply</code>
(see <a href="Constructing-Calls.html#Constructing-Calls">Constructing Calls</a>).
<br><dt><code>micromips/nomicromips</code><dd><a name="index-g_t_0040code_007bmicromips_007d-attribute-2397"></a><a name="index-g_t_0040code_007bnomicromips_007d-attribute-2398"></a>
On MIPS targets, you can use the <code>micromips</code> and <code>nomicromips</code>
function attributes to locally select or turn off microMIPS code generation.
A function with the <code>micromips</code> attribute is emitted as microMIPS code,
while microMIPS code generation is disabled for functions with the
<code>nomicromips</code> attribute. These attributes override the
<samp><span class="option">-mmicromips</span></samp> and <samp><span class="option">-mno-micromips</span></samp> options on the command line
(see <a href="MIPS-Options.html#MIPS-Options">MIPS Options</a>).
<p>When compiling files containing mixed microMIPS and non-microMIPS code, the
preprocessor symbol <code>__mips_micromips</code> reflects the setting on the
command line,
not that within individual functions. Mixed microMIPS and non-microMIPS code
may interact badly with some GCC extensions such as <code>__builtin_apply</code>
(see <a href="Constructing-Calls.html#Constructing-Calls">Constructing Calls</a>).
<br><dt><code>model (</code><var>model-name</var><code>)</code><dd><a name="index-function-addressability-on-the-M32R_002fD-2399"></a><a name="index-variable-addressability-on-the-IA_002d64-2400"></a>
On the M32R/D, use this attribute to set the addressability of an
object, and of the code generated for a function. The identifier
<var>model-name</var> is one of <code>small</code>, <code>medium</code>, or
<code>large</code>, representing each of the code models.
<p>Small model objects live in the lower 16MB of memory (so that their
addresses can be loaded with the <code>ld24</code> instruction), and are
callable with the <code>bl</code> instruction.
<p>Medium model objects may live anywhere in the 32-bit address space (the
compiler will generate <code>seth/add3</code> instructions to load their addresses),
and are callable with the <code>bl</code> instruction.
<p>Large model objects may live anywhere in the 32-bit address space (the
compiler will generate <code>seth/add3</code> instructions to load their addresses),
and may not be reachable with the <code>bl</code> instruction (the compiler will
generate the much slower <code>seth/add3/jl</code> instruction sequence).
<p>On IA-64, use this attribute to set the addressability of an object.
At present, the only supported identifier for <var>model-name</var> is
<code>small</code>, indicating addressability via &ldquo;small&rdquo; (22-bit)
addresses (so that their addresses can be loaded with the <code>addl</code>
instruction). Caveat: such addressing is by definition not position
independent and hence this attribute must not be used for objects
defined by shared libraries.
<br><dt><code>ms_abi/sysv_abi</code><dd><a name="index-g_t_0040code_007bms_005fabi_007d-attribute-2401"></a><a name="index-g_t_0040code_007bsysv_005fabi_007d-attribute-2402"></a>
On 64-bit x86_64-*-* targets, you can use an ABI attribute to indicate
which calling convention should be used for a function. The <code>ms_abi</code>
attribute tells the compiler to use the Microsoft ABI, while the
<code>sysv_abi</code> attribute tells the compiler to use the ABI used on
GNU/Linux and other systems. The default is to use the Microsoft ABI
when targeting Windows. On all other systems, the default is the AMD ABI.
<p>Note, the <code>ms_abi</code> attribute for Windows targets currently requires
the <samp><span class="option">-maccumulate-outgoing-args</span></samp> option.
<br><dt><code>ms_hook_prologue</code><dd><a name="index-g_t_0040code_007bms_005fhook_005fprologue_007d-attribute-2403"></a>
On 32 bit i[34567]86-*-* targets, you can use this function attribute to make
gcc generate the "hot-patching" function prologue used in Win32 API
functions in Microsoft Windows XP Service Pack 2 and newer. This requires
support for the swap suffix in the assembler. (GNU Binutils 2.19.51 or later)
<br><dt><code>naked</code><dd><a name="index-function-without-a-prologue_002fepilogue-code-2404"></a>Use this attribute on the ARM, AVR, IP2K, RX and SPU 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. The
only statements that can be safely included in naked functions are
<code>asm</code> statements that do not have operands. All other statements,
including declarations of local variables, <code>if</code> statements, and so
forth, should be avoided. Naked functions should be used to implement the
body of an assembly function, while allowing the compiler to construct
the requisite function declaration for the assembler.
<br><dt><code>near</code><dd><a name="index-functions-which-do-not-handle-memory-bank-switching-on-68HC11_002f68HC12-2405"></a>On 68HC11 and 68HC12 the <code>near</code> attribute causes the compiler to
use the normal calling convention based on <code>jsr</code> and <code>rts</code>.
This attribute can be used to cancel the effect of the <samp><span class="option">-mlong-calls</span></samp>
option.
<p>On MeP targets this attribute causes the compiler to assume the called
function is close enough to use the normal calling convention,
overriding the <code>-mtf</code> command line option.
<br><dt><code>nesting</code><dd><a name="index-Allow-nesting-in-an-interrupt-handler-on-the-Blackfin-processor_002e-2406"></a>Use this attribute together with <code>interrupt_handler</code>,
<code>exception_handler</code> or <code>nmi_handler</code> to indicate that the function
entry code should enable nested interrupts or exceptions.
<br><dt><code>nmi_handler</code><dd><a name="index-NMI-handler-functions-on-the-Blackfin-processor-2407"></a>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.
<br><dt><code>no_instrument_function</code><dd><a name="index-g_t_0040code_007bno_005finstrument_005ffunction_007d-function-attribute-2408"></a><a name="index-finstrument_002dfunctions-2409"></a>If <samp><span class="option">-finstrument-functions</span></samp> 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.
<br><dt><code>noinline</code><dd><a name="index-g_t_0040code_007bnoinline_007d-function-attribute-2410"></a>This function attribute prevents a function from being considered for
inlining.
<!-- Don't enumerate the optimizations by name here; we try to be -->
<!-- future-compatible with this mechanism. -->
If the function does not have side-effects, there are optimizations
other than inlining that causes function calls to be optimized away,
although the function call is live. To keep such calls from being
optimized away, put
<pre class="smallexample"> asm ("");
</pre>
<p>(see <a href="Extended-Asm.html#Extended-Asm">Extended Asm</a>) in the called function, to serve as a special
side-effect.
<br><dt><code>noclone</code><dd><a name="index-g_t_0040code_007bnoclone_007d-function-attribute-2411"></a>This function attribute prevents a function from being considered for
cloning - a mechanism which produces specialized copies of functions
and which is (currently) performed by interprocedural constant
propagation.
<br><dt><code>nonnull (</code><var>arg-index</var><code>, ...)</code><dd><a name="index-g_t_0040code_007bnonnull_007d-function-attribute-2412"></a>The <code>nonnull</code> attribute specifies that some function parameters should
be non-null pointers. For instance, the declaration:
<pre class="smallexample"> extern void *
my_memcpy (void *dest, const void *src, size_t len)
__attribute__((nonnull (1, 2)));
</pre>
<p class="noindent">causes the compiler to check that, in calls to <code>my_memcpy</code>,
arguments <var>dest</var> and <var>src</var> are non-null. If the compiler
determines that a null pointer is passed in an argument slot marked
as non-null, and the <samp><span class="option">-Wnonnull</span></samp> 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.
<p>If no argument index list is given to the <code>nonnull</code> attribute,
all pointer arguments are marked as non-null. To illustrate, the
following declaration is equivalent to the previous example:
<pre class="smallexample"> extern void *
my_memcpy (void *dest, const void *src, size_t len)
__attribute__((nonnull));
</pre>
<br><dt><code>noreturn</code><dd><a name="index-g_t_0040code_007bnoreturn_007d-function-attribute-2413"></a>A few standard library functions, such as <code>abort</code> and <code>exit</code>,
cannot return. GCC knows this automatically. Some programs define
their own functions that never return. You can declare them
<code>noreturn</code> to tell the compiler this fact. For example,
<pre class="smallexample"> void fatal () __attribute__ ((noreturn));
void
fatal (/* <span class="roman">...</span> */)
{
/* <span class="roman">...</span> */ /* <span class="roman">Print error message.</span> */ /* <span class="roman">...</span> */
exit (1);
}
</pre>
<p>The <code>noreturn</code> keyword tells the compiler to assume that
<code>fatal</code> cannot return. It can then optimize without regard to what
would happen if <code>fatal</code> ever did return. This makes slightly
better code. More importantly, it helps avoid spurious warnings of
uninitialized variables.
<p>The <code>noreturn</code> keyword does not affect the exceptional path when that
applies: a <code>noreturn</code>-marked function may still return to the caller
by throwing an exception or calling <code>longjmp</code>.
<p>Do not assume that registers saved by the calling function are
restored before calling the <code>noreturn</code> function.
<p>It does not make sense for a <code>noreturn</code> function to have a return
type other than <code>void</code>.
<p>The attribute <code>noreturn</code> 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:
<pre class="smallexample"> typedef void voidfn ();
volatile voidfn fatal;
</pre>
<p>This approach does not work in GNU C++.
<br><dt><code>nothrow</code><dd><a name="index-g_t_0040code_007bnothrow_007d-function-attribute-2414"></a>The <code>nothrow</code> 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 <code>qsort</code> and <code>bsearch</code> that
take function pointer arguments. The <code>nothrow</code> attribute is not
implemented in GCC versions earlier than 3.3.
<br><dt><code>optimize</code><dd><a name="index-g_t_0040code_007boptimize_007d-function-attribute-2415"></a>The <code>optimize</code> attribute is used to specify that a function is to
be compiled with different optimization options than specified on the
command line. Arguments can either be numbers or strings. Numbers
are assumed to be an optimization level. Strings that begin with
<code>O</code> are assumed to be an optimization option, while other options
are assumed to be used with a <code>-f</code> prefix. You can also use the
&lsquo;<samp><span class="samp">#pragma GCC optimize</span></samp>&rsquo; pragma to set the optimization options
that affect more than one function.
See <a href="Function-Specific-Option-Pragmas.html#Function-Specific-Option-Pragmas">Function Specific Option Pragmas</a>, for details about the
&lsquo;<samp><span class="samp">#pragma GCC optimize</span></samp>&rsquo; pragma.
<p>This can be used for instance to have frequently executed functions
compiled with more aggressive optimization options that produce faster
and larger code, while other functions can be called with less
aggressive options.
<br><dt><code>pcs</code><dd><a name="index-g_t_0040code_007bpcs_007d-function-attribute-2416"></a>
The <code>pcs</code> attribute can be used to control the calling convention
used for a function on ARM. The attribute takes an argument that specifies
the calling convention to use.
<p>When compiling using the AAPCS ABI (or a variant of that) then valid
values for the argument are <code>"aapcs"</code> and <code>"aapcs-vfp"</code>. In
order to use a variant other than <code>"aapcs"</code> then the compiler must
be permitted to use the appropriate co-processor registers (i.e., the
VFP registers must be available in order to use <code>"aapcs-vfp"</code>).
For example,
<pre class="smallexample"> /* Argument passed in r0, and result returned in r0+r1. */
double f2d (float) __attribute__((pcs("aapcs")));
</pre>
<p>Variadic functions always use the <code>"aapcs"</code> calling convention and
the compiler will reject attempts to specify an alternative.
<br><dt><code>pure</code><dd><a name="index-g_t_0040code_007bpure_007d-function-attribute-2417"></a>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 <code>pure</code>. For example,
<pre class="smallexample"> int square (int) __attribute__ ((pure));
</pre>
<p class="noindent">says that the hypothetical function <code>square</code> is safe to call
fewer times than the program says.
<p>Some of common examples of pure functions are <code>strlen</code> or <code>memcmp</code>.
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 <code>feof</code> in a multithreading environment).
<p>The attribute <code>pure</code> is not implemented in GCC versions earlier
than 2.96.
<br><dt><code>hot</code><dd><a name="index-g_t_0040code_007bhot_007d-function-attribute-2418"></a>The <code>hot</code> attribute is used to inform the compiler that a function is a
hot spot of the compiled program. The function is optimized more aggressively
and on many target it is placed into special subsection of the text section so
all hot functions appears close together improving locality.
<p>When profile feedback is available, via <samp><span class="option">-fprofile-use</span></samp>, hot functions
are automatically detected and this attribute is ignored.
<p>The <code>hot</code> attribute is not implemented in GCC versions earlier
than 4.3.
<br><dt><code>cold</code><dd><a name="index-g_t_0040code_007bcold_007d-function-attribute-2419"></a>The <code>cold</code> attribute is used to inform the compiler that a function is
unlikely executed. The function is optimized for size rather than speed and on
many targets it is placed into special subsection of the text section so all
cold functions appears close together improving code locality of non-cold parts
of program. The paths leading to call of cold functions within code are marked
as unlikely by the branch prediction mechanism. It is thus useful to mark
functions used to handle unlikely conditions, such as <code>perror</code>, as cold to
improve optimization of hot functions that do call marked functions in rare
occasions.
<p>When profile feedback is available, via <samp><span class="option">-fprofile-use</span></samp>, hot functions
are automatically detected and this attribute is ignored.
<p>The <code>cold</code> attribute is not implemented in GCC versions earlier than 4.3.
<br><dt><code>regparm (</code><var>number</var><code>)</code><dd><a name="index-g_t_0040code_007bregparm_007d-attribute-2420"></a><a name="index-functions-that-are-passed-arguments-in-registers-on-the-386-2421"></a>On the Intel 386, the <code>regparm</code> attribute causes the compiler to
pass arguments number one to <var>number</var> 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.
<p>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 EAX, EDX and ECX. (Lazy binding can be
disabled with the linker or the loader if desired, to avoid the
problem.)
<br><dt><code>sseregparm</code><dd><a name="index-g_t_0040code_007bsseregparm_007d-attribute-2422"></a>On the Intel 386 with SSE support, the <code>sseregparm</code> attribute
causes the compiler to pass up to 3 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.
<br><dt><code>force_align_arg_pointer</code><dd><a name="index-g_t_0040code_007bforce_005falign_005farg_005fpointer_007d-attribute-2423"></a>On the Intel x86, the <code>force_align_arg_pointer</code> attribute may be
applied to individual function definitions, generating an alternate
prologue and epilogue that realigns the runtime stack if necessary.
This supports mixing legacy codes that run with a 4-byte aligned stack
with modern codes that keep a 16-byte stack for SSE compatibility.
<br><dt><code>resbank</code><dd><a name="index-g_t_0040code_007bresbank_007d-attribute-2424"></a>On the SH2A target, this attribute enables the high-speed register
saving and restoration using a register bank for <code>interrupt_handler</code>
routines. Saving to the bank is performed automatically after the CPU
accepts an interrupt that uses a register bank.
<p>The nineteen 32-bit registers comprising general register R0 to R14,
control register GBR, and system registers MACH, MACL, and PR and the
vector table address offset are saved into a register bank. Register
banks are stacked in first-in last-out (FILO) sequence. Restoration
from the bank is executed by issuing a RESBANK instruction.
<br><dt><code>returns_twice</code><dd><a name="index-g_t_0040code_007breturns_005ftwice_007d-attribute-2425"></a>The <code>returns_twice</code> 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 <code>setjmp</code> and <code>vfork</code>.
The <code>longjmp</code>-like counterpart of such function, if any, might need
to be marked with the <code>noreturn</code> attribute.
<br><dt><code>saveall</code><dd><a name="index-save-all-registers-on-the-Blackfin_002c-H8_002f300_002c-H8_002f300H_002c-and-H8S-2426"></a>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.
<br><dt><code>section ("</code><var>section-name</var><code>")</code><dd><a name="index-g_t_0040code_007bsection_007d-function-attribute-2427"></a>Normally, the compiler places the code it generates in the <code>text</code> section.
Sometimes, however, you need additional sections, or you need certain
particular functions to appear in special sections. The <code>section</code>
attribute specifies that a function lives in a particular section.
For example, the declaration:
<pre class="smallexample"> extern void foobar (void) __attribute__ ((section ("bar")));
</pre>
<p class="noindent">puts the function <code>foobar</code> in the <code>bar</code> section.
<p>Some file formats do not support arbitrary sections so the <code>section</code>
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.
<br><dt><code>sentinel</code><dd><a name="index-g_t_0040code_007bsentinel_007d-function-attribute-2428"></a>This function attribute ensures that a parameter in a function call is
an explicit <code>NULL</code>. 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.
<pre class="smallexample"> __attribute__ ((sentinel))
is equivalent to
__attribute__ ((sentinel(0)))
</pre>
<p>The attribute is automatically set with a position of 0 for the built-in
functions <code>execl</code> and <code>execlp</code>. The built-in function
<code>execle</code> has the attribute set with a position of 1.
<p>A valid <code>NULL</code> in this context is defined as zero with any pointer
type. If your system defines the <code>NULL</code> macro with an integer type
then you need to add an explicit cast. GCC replaces <code>stddef.h</code>
with a copy that redefines NULL appropriately.
<p>The warnings for missing or incorrect sentinels are enabled with
<samp><span class="option">-Wformat</span></samp>.
<br><dt><code>short_call</code><dd>See long_call/short_call.
<br><dt><code>shortcall</code><dd>See longcall/shortcall.
<br><dt><code>signal</code><dd><a name="index-signal-handler-functions-on-the-AVR-processors-2429"></a>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.
<br><dt><code>sp_switch</code><dd>Use this attribute on the SH to indicate an <code>interrupt_handler</code>
function should switch to an alternate stack. It expects a string
argument that names a global variable holding the address of the
alternate stack.
<pre class="smallexample"> void *alt_stack;
void f () __attribute__ ((interrupt_handler,
sp_switch ("alt_stack")));
</pre>
<br><dt><code>stdcall</code><dd><a name="index-functions-that-pop-the-argument-stack-on-the-386-2430"></a>On the Intel 386, the <code>stdcall</code> 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.
<br><dt><code>syscall_linkage</code><dd><a name="index-g_t_0040code_007bsyscall_005flinkage_007d-attribute-2431"></a>This attribute is used to modify the IA64 calling convention by marking
all input registers as live at all function exits. This makes it possible
to restart a system call after an interrupt without having to save/restore
the input registers. This also prevents kernel data from leaking into
application code.
<br><dt><code>target</code><dd><a name="index-g_t_0040code_007btarget_007d-function-attribute-2432"></a>The <code>target</code> attribute is used to specify that a function is to
be compiled with different target options than specified on the
command line. This can be used for instance to have functions
compiled with a different ISA (instruction set architecture) than the
default. You can also use the &lsquo;<samp><span class="samp">#pragma GCC target</span></samp>&rsquo; pragma to set
more than one function to be compiled with specific target options.
See <a href="Function-Specific-Option-Pragmas.html#Function-Specific-Option-Pragmas">Function Specific Option Pragmas</a>, for details about the
&lsquo;<samp><span class="samp">#pragma GCC target</span></samp>&rsquo; pragma.
<p>For instance on a 386, you could compile one function with
<code>target("sse4.1,arch=core2")</code> and another with
<code>target("sse4a,arch=amdfam10")</code> that would be equivalent to
compiling the first function with <samp><span class="option">-msse4.1</span></samp> and
<samp><span class="option">-march=core2</span></samp> options, and the second function with
<samp><span class="option">-msse4a</span></samp> and <samp><span class="option">-march=amdfam10</span></samp> options. It is up to the
user to make sure that a function is only invoked on a machine that
supports the particular ISA it was compiled for (for example by using
<code>cpuid</code> on 386 to determine what feature bits and architecture
family are used).
<pre class="smallexample"> int core2_func (void) __attribute__ ((__target__ ("arch=core2")));
int sse3_func (void) __attribute__ ((__target__ ("sse3")));
</pre>
<p>On the 386, the following options are allowed:
<dl>
<dt>&lsquo;<samp><span class="samp">abm</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-abm</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022abm_0022_0029_007d-attribute-2433"></a>Enable/disable the generation of the advanced bit instructions.
<br><dt>&lsquo;<samp><span class="samp">aes</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-aes</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022aes_0022_0029_007d-attribute-2434"></a>Enable/disable the generation of the AES instructions.
<br><dt>&lsquo;<samp><span class="samp">mmx</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-mmx</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022mmx_0022_0029_007d-attribute-2435"></a>Enable/disable the generation of the MMX instructions.
<br><dt>&lsquo;<samp><span class="samp">pclmul</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-pclmul</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022pclmul_0022_0029_007d-attribute-2436"></a>Enable/disable the generation of the PCLMUL instructions.
<br><dt>&lsquo;<samp><span class="samp">popcnt</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-popcnt</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022popcnt_0022_0029_007d-attribute-2437"></a>Enable/disable the generation of the POPCNT instruction.
<br><dt>&lsquo;<samp><span class="samp">sse</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-sse</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022sse_0022_0029_007d-attribute-2438"></a>Enable/disable the generation of the SSE instructions.
<br><dt>&lsquo;<samp><span class="samp">sse2</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-sse2</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022sse2_0022_0029_007d-attribute-2439"></a>Enable/disable the generation of the SSE2 instructions.
<br><dt>&lsquo;<samp><span class="samp">sse3</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-sse3</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022sse3_0022_0029_007d-attribute-2440"></a>Enable/disable the generation of the SSE3 instructions.
<br><dt>&lsquo;<samp><span class="samp">sse4</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-sse4</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022sse4_0022_0029_007d-attribute-2441"></a>Enable/disable the generation of the SSE4 instructions (both SSE4.1
and SSE4.2).
<br><dt>&lsquo;<samp><span class="samp">sse4.1</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-sse4.1</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022sse4_002e1_0022_0029_007d-attribute-2442"></a>Enable/disable the generation of the sse4.1 instructions.
<br><dt>&lsquo;<samp><span class="samp">sse4.2</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-sse4.2</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022sse4_002e2_0022_0029_007d-attribute-2443"></a>Enable/disable the generation of the sse4.2 instructions.
<br><dt>&lsquo;<samp><span class="samp">sse4a</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-sse4a</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022sse4a_0022_0029_007d-attribute-2444"></a>Enable/disable the generation of the SSE4A instructions.
<br><dt>&lsquo;<samp><span class="samp">fma4</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-fma4</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022fma4_0022_0029_007d-attribute-2445"></a>Enable/disable the generation of the FMA4 instructions.
<br><dt>&lsquo;<samp><span class="samp">xop</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-xop</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022xop_0022_0029_007d-attribute-2446"></a>Enable/disable the generation of the XOP instructions.
<br><dt>&lsquo;<samp><span class="samp">lwp</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-lwp</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022lwp_0022_0029_007d-attribute-2447"></a>Enable/disable the generation of the LWP instructions.
<br><dt>&lsquo;<samp><span class="samp">ssse3</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-ssse3</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022ssse3_0022_0029_007d-attribute-2448"></a>Enable/disable the generation of the SSSE3 instructions.
<br><dt>&lsquo;<samp><span class="samp">cld</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-cld</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022cld_0022_0029_007d-attribute-2449"></a>Enable/disable the generation of the CLD before string moves.
<br><dt>&lsquo;<samp><span class="samp">fancy-math-387</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-fancy-math-387</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022fancy_002dmath_002d387_0022_0029_007d-attribute-2450"></a>Enable/disable the generation of the <code>sin</code>, <code>cos</code>, and
<code>sqrt</code> instructions on the 387 floating point unit.
<br><dt>&lsquo;<samp><span class="samp">fused-madd</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-fused-madd</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022fused_002dmadd_0022_0029_007d-attribute-2451"></a>Enable/disable the generation of the fused multiply/add instructions.
<br><dt>&lsquo;<samp><span class="samp">ieee-fp</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-ieee-fp</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022ieee_002dfp_0022_0029_007d-attribute-2452"></a>Enable/disable the generation of floating point that depends on IEEE arithmetic.
<br><dt>&lsquo;<samp><span class="samp">inline-all-stringops</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-inline-all-stringops</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022inline_002dall_002dstringops_0022_0029_007d-attribute-2453"></a>Enable/disable inlining of string operations.
<br><dt>&lsquo;<samp><span class="samp">inline-stringops-dynamically</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-inline-stringops-dynamically</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022inline_002dstringops_002ddynamically_0022_0029_007d-attribute-2454"></a>Enable/disable the generation of the inline code to do small string
operations and calling the library routines for large operations.
<br><dt>&lsquo;<samp><span class="samp">align-stringops</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-align-stringops</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022align_002dstringops_0022_0029_007d-attribute-2455"></a>Do/do not align destination of inlined string operations.
<br><dt>&lsquo;<samp><span class="samp">recip</span></samp>&rsquo;<dt>&lsquo;<samp><span class="samp">no-recip</span></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022recip_0022_0029_007d-attribute-2456"></a>Enable/disable the generation of RCPSS, RCPPS, RSQRTSS and RSQRTPS
instructions followed an additional Newton-Raphson step instead of
doing a floating point division.
<br><dt>&lsquo;<samp><span class="samp">arch=</span><var>ARCH</var></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022arch_003d_0040var_007bARCH_007d_0022_0029_007d-attribute-2457"></a>Specify the architecture to generate code for in compiling the function.
<br><dt>&lsquo;<samp><span class="samp">tune=</span><var>TUNE</var></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022tune_003d_0040var_007bTUNE_007d_0022_0029_007d-attribute-2458"></a>Specify the architecture to tune for in compiling the function.
<br><dt>&lsquo;<samp><span class="samp">fpmath=</span><var>FPMATH</var></samp>&rsquo;<dd><a name="index-g_t_0040code_007btarget_0028_0022fpmath_003d_0040var_007bFPMATH_007d_0022_0029_007d-attribute-2459"></a>Specify which floating point unit to use. The
<code>target("fpmath=sse,387")</code> option must be specified as
<code>target("fpmath=sse+387")</code> because the comma would separate
different options.
</dl>
<p>On the 386, you can use either multiple strings to specify multiple
options, or you can separate the option with a comma (<code>,</code>).
<p>On the 386, the inliner will not inline a function that has different
target options than the caller, unless the callee has a subset of the
target options of the caller. For example a function declared with
<code>target("sse3")</code> can inline a function with
<code>target("sse2")</code>, since <code>-msse3</code> implies <code>-msse2</code>.
<p>The <code>target</code> attribute is not implemented in GCC versions earlier
than 4.4, and at present only the 386 uses it.
<br><dt><code>tiny_data</code><dd><a name="index-tiny-data-section-on-the-H8_002f300H-and-H8S-2460"></a>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.
<br><dt><code>trap_exit</code><dd>Use this attribute on the SH for an <code>interrupt_handler</code> to return using
<code>trapa</code> instead of <code>rte</code>. This attribute expects an integer
argument specifying the trap number to be used.
<br><dt><code>unused</code><dd><a name="index-g_t_0040code_007bunused_007d-attribute_002e-2461"></a>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.
<br><dt><code>used</code><dd><a name="index-g_t_0040code_007bused_007d-attribute_002e-2462"></a>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.
<br><dt><code>version_id</code><dd><a name="index-g_t_0040code_007bversion_005fid_007d-attribute-2463"></a>This IA64 HP-UX attribute, attached to a global variable or function, renames a
symbol to contain a version string, thus allowing for function level
versioning. HP-UX system header files may use version level functioning
for some system calls.
<pre class="smallexample"> extern int foo () __attribute__((version_id ("20040821")));
</pre>
<p>Calls to <var>foo</var> will be mapped to calls to <var>foo{20040821}</var>.
<br><dt><code>visibility ("</code><var>visibility_type</var><code>")</code><dd><a name="index-g_t_0040code_007bvisibility_007d-attribute-2464"></a>This attribute affects the linkage of the declaration to which it is attached.
There are four supported <var>visibility_type</var> values: default,
hidden, protected or internal visibility.
<pre class="smallexample"> void __attribute__ ((visibility ("protected")))
f () { /* <span class="roman">Do something.</span> */; }
int i __attribute__ ((visibility ("hidden")));
</pre>
<p>The possible values of <var>visibility_type</var> correspond to the
visibility settings in the ELF gABI.
<dl>
<!-- keep this list of visibilities in alphabetical order. -->
<dt><dfn>default</dfn><dd>Default visibility is the normal case for the object file format.
This value is available for the visibility attribute to override other
options that may change the assumed visibility of entities.
<p>On ELF, default visibility means that the declaration is visible to other
modules and, in shared libraries, means that the declared entity may be
overridden.
<p>On Darwin, default visibility means that the declaration is visible to
other modules.
<p>Default visibility corresponds to &ldquo;external linkage&rdquo; in the language.
<br><dt><dfn>hidden</dfn><dd>Hidden visibility indicates that the entity declared will have a new
form of linkage, which we'll call &ldquo;hidden linkage&rdquo;. Two
declarations of an object with hidden linkage refer to the same object
if they are in the same shared object.
<br><dt><dfn>internal</dfn><dd>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 a function is
<em>never</em> called from another module. Compare this with hidden
functions which, while they cannot be referenced directly by other
modules, can be referenced indirectly via function pointers. By
indicating that a function 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.
<br><dt><dfn>protected</dfn><dd>Protected visibility is like default visibility except that it
indicates that references within the defining module will bind to the
definition in that module. That is, the declared entity cannot be
overridden by another module.
</dl>
<p>All visibilities are supported on many, but not all, ELF targets
(supported when the assembler supports the &lsquo;<samp><span class="samp">.visibility</span></samp>&rsquo;
pseudo-op). Default visibility is supported everywhere. Hidden
visibility is supported on Darwin targets.
<p>The visibility attribute should be applied only to declarations which
would otherwise have external linkage. The attribute should be applied
consistently, so that the same entity should not be declared with
different settings of the attribute.
<p>In C++, the visibility attribute applies to types as well as functions
and objects, because in C++ types have linkage. A class must not have
greater visibility than its non-static data member types and bases,
and class members default to the visibility of their class. Also, a
declaration without explicit visibility is limited to the visibility
of its type.
<p>In C++, you can mark member functions and static member variables of a
class with the visibility attribute. This is useful if you know a
particular method or static member variable should only be used from
one shared object; then you can mark it hidden while the rest of the
class has default visibility. Care must be taken to avoid breaking
the One Definition Rule; for example, it is usually not useful to mark
an inline method as hidden without marking the whole class as hidden.
<p>A C++ namespace declaration can also have the visibility attribute.
This attribute applies only to the particular namespace body, not to
other definitions of the same namespace; it is equivalent to using
&lsquo;<samp><span class="samp">#pragma GCC visibility</span></samp>&rsquo; before and after the namespace
definition (see <a href="Visibility-Pragmas.html#Visibility-Pragmas">Visibility Pragmas</a>).
<p>In C++, if a template argument has limited visibility, this
restriction is implicitly propagated to the template instantiation.
Otherwise, template instantiations and specializations default to the
visibility of their template.
<p>If both the template and enclosing class have explicit visibility, the
visibility from the template is used.
<br><dt><code>vliw</code><dd><a name="index-g_t_0040code_007bvliw_007d-attribute-2465"></a>On MeP, the <code>vliw</code> attribute tells the compiler to emit
instructions in VLIW mode instead of core mode. Note that this
attribute is not allowed unless a VLIW coprocessor has been configured
and enabled through command line options.
<br><dt><code>warn_unused_result</code><dd><a name="index-g_t_0040code_007bwarn_005funused_005fresult_007d-attribute-2466"></a>The <code>warn_unused_result</code> 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
<code>realloc</code>.
<pre class="smallexample"> int fn () __attribute__ ((warn_unused_result));
int foo ()
{
if (fn () &lt; 0) return -1;
fn ();
return 0;
}
</pre>
<p>results in warning on line 5.
<br><dt><code>weak</code><dd><a name="index-g_t_0040code_007bweak_007d-attribute-2467"></a>The <code>weak</code> 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.
<br><dt><code>weakref</code><dt><code>weakref ("</code><var>target</var><code>")</code><dd><a name="index-g_t_0040code_007bweakref_007d-attribute-2468"></a>The <code>weakref</code> attribute marks a declaration as a weak reference.
Without arguments, it should be accompanied by an <code>alias</code> attribute
naming the target symbol. Optionally, the <var>target</var> may be given as
an argument to <code>weakref</code> itself. In either case, <code>weakref</code>
implicitly marks the declaration as <code>weak</code>. Without a
<var>target</var>, given as an argument to <code>weakref</code> or to <code>alias</code>,
<code>weakref</code> is equivalent to <code>weak</code>.
<pre class="smallexample"> static int x() __attribute__ ((weakref ("y")));
/* is equivalent to... */
static int x() __attribute__ ((weak, weakref, alias ("y")));
/* and to... */
static int x() __attribute__ ((weakref));
static int x() __attribute__ ((alias ("y")));
</pre>
<p>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 <code>weak</code>
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.
<p>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.
<p>At present, a declaration to which <code>weakref</code> is attached can
only be <code>static</code>.
</dl>
<p>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.
<p><a name="index-g_t_0040code_007b_0023pragma_007d_002c-reason-for-not-using-2469"></a><a name="index-pragma_002c-reason-for-not-using-2470"></a>Some people object to the <code>__attribute__</code> feature, suggesting that
ISO C's <code>#pragma</code> should be used instead. At the time
<code>__attribute__</code> was designed, there were two reasons for not doing
this.
<ol type=1 start=1>
<li>It is impossible to generate <code>#pragma</code> commands from a macro.
<li>There is no telling what the same <code>#pragma</code> might mean in another
compiler.
</ol>
<p>These two reasons applied to almost any application that might have been
proposed for <code>#pragma</code>. It was basically a mistake to use
<code>#pragma</code> for <em>anything</em>.
<p>The ISO C99 standard includes <code>_Pragma</code>, which now allows pragmas
to be generated from macros. In addition, a <code>#pragma GCC</code>
namespace is now in use for GCC-specific pragmas. However, it has been
found convenient to use <code>__attribute__</code> to achieve a natural
attachment of attributes to their corresponding declarations, whereas
<code>#pragma GCC</code> is of use for constructs that do not naturally form
part of the grammar. See <a href="../cpp/Other-Directives.html#Other-Directives">Miscellaneous Preprocessing Directives</a>.
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