Common Function Attributes#

The following attributes are supported on most targets.

access (access-mode, ref-index), access (access-mode, ref-index, size-index)#

The access attribute enables the detection of invalid or unsafe accesses by functions to which they apply or their callers, as well as write-only accesses to objects that are never read from. Such accesses may be diagnosed by warnings such as -Wstringop-overflow, -Wuninitialized, -Wunused, and others.

The access attribute specifies that a function to whose by-reference arguments the attribute applies accesses the referenced object according to access-mode. The access-mode argument is required and must be one of four names: read_only, read_write, write_only, or none. The remaining two are positional arguments.

The required ref-index positional argument denotes a function argument of pointer (or in C++, reference) type that is subject to the access. The same pointer argument can be referenced by at most one distinct access attribute.

The optional size-index positional argument denotes a function argument of integer type that specifies the maximum size of the access. The size is the number of elements of the type referenced by ref-index, or the number of bytes when the pointer type is void*. When no size-index argument is specified, the pointer argument must be either null or point to a space that is suitably aligned and large for at least one object of the referenced type (this implies that a past-the-end pointer is not a valid argument). The actual size of the access may be less but it must not be more.

The read_only access mode specifies that the pointer to which it applies is used to read the referenced object but not write to it. Unless the argument specifying the size of the access denoted by size-index is zero, the referenced object must be initialized. The mode implies a stronger guarantee than the const qualifier which, when cast away from a pointer, does not prevent the pointed-to object from being modified. Examples of the use of the read_only access mode is the argument to the puts function, or the second and third arguments to the memcpy function.

__attribute__ ((access (read_only, 1))) int puts (const char*);
__attribute__ ((access (read_only, 2, 3))) void* memcpy (void*, const void*, size_t);

The read_write access mode applies to arguments of pointer types without the const qualifier. It specifies that the pointer to which it applies is used to both read and write the referenced object. Unless the argument specifying the size of the access denoted by size-index is zero, the object referenced by the pointer must be initialized. An example of the use of the read_write access mode is the first argument to the strcat function.

__attribute__ ((access (read_write, 1), access (read_only, 2))) char* strcat (char*, const char*);

The write_only access mode applies to arguments of pointer types without the const qualifier. It specifies that the pointer to which it applies is used to write to the referenced object but not read from it. The object referenced by the pointer need not be initialized. An example of the use of the write_only access mode is the first argument to the strcpy function, or the first two arguments to the fgets function.

__attribute__ ((access (write_only, 1), access (read_only, 2))) char* strcpy (char*, const char*);
__attribute__ ((access (write_only, 1, 2), access (read_write, 3))) int fgets (char*, int, FILE*);

The access mode none specifies that the pointer to which it applies is not used to access the referenced object at all. Unless the pointer is null the pointed-to object must exist and have at least the size as denoted by the size-index argument. When the optional size-index argument is omitted for an argument of void* type the actual pointer agument is ignored. The referenced object need not be initialized. The mode is intended to be used as a means to help validate the expected object size, for example in functions that call __builtin_object_size. See Object Size Checking Built-in Functions.

Note that the access attribute merely specifies how an object referenced by the pointer argument can be accessed; it does not imply that an access will happen. Also, the access attribute does not imply the attribute nonnull ; it may be appropriate to add both attributes at the declaration of a function that unconditionally manipulates a buffer via a pointer argument. See the nonnull attribute for more information and caveats.

alias ("target")#

The alias attribute causes the declaration to be emitted as an alias for another symbol, which must have been previously declared with the same type, and for variables, also the same size and alignment. Declaring an alias with a different type than the target is undefined and may be diagnosed. As an example, the following declarations:

void __f () { /* Do something. */; }
void f () __attribute__ ((weak, alias ("__f")));

define 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.

This attribute requires assembler and object file support, and may not be available on all targets.

aligned, aligned (alignment)#

The aligned attribute specifies a minimum alignment for the first instruction of the function, measured in bytes. When specified, alignment must be an integer constant power of 2. Specifying no alignment argument implies the ideal alignment for the target. The __alignof__ operator can be used to determine what that is (see Determining the Alignment of Functions, Types or Variables). The attribute has no effect when a definition for the function is not provided in the same translation unit.

The attribute cannot be used to decrease the alignment of a function previously declared with a more restrictive alignment; only to increase it. Attempts to do otherwise are diagnosed. Some targets specify a minimum default alignment for functions that is greater than 1. On such targets, specifying a less restrictive alignment is silently ignored. Using the attribute overrides the effect of the -falign-functions (see Options That Control Optimization) option for this function.

Note that the effectiveness of aligned attributes may be limited by inherent limitations in the system linker and/or object file format. On some 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.

The aligned attribute can also be used for variables and fields (see Specifying Attributes of Variables.)

alloc_align (position)#

The alloc_align attribute may be applied to a function that returns a pointer and takes at least one argument of an integer or enumerated type. It indicates that the returned pointer is aligned on a boundary given by the function argument at position. Meaningful alignments are powers of 2 greater than one. GCC uses this information to improve pointer alignment analysis.

The function parameter denoting the allocated alignment is specified by one constant integer argument whose number is the argument of the attribute. Argument numbering starts at one.

For instance,

void* my_memalign (size_t, size_t) __attribute__ ((alloc_align (1)));

declares that my_memalign returns memory with minimum alignment given by parameter 1.

alloc_size (position), alloc_size (position-1, position-2)#

The alloc_size attribute may be applied to a function that returns a pointer and takes at least one argument of an integer or enumerated type. It indicates that the returned pointer points to memory whose size is given by the function argument at position-1, or by the product of the arguments at position-1 and position-2. Meaningful sizes are positive values less than PTRDIFF_MAX. GCC uses this information to improve the results of __builtin_object_size.

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 for ordinary functions, and at two for C++ non-static member functions.

For instance,

void* my_calloc (size_t, size_t) __attribute__ ((alloc_size (1, 2)));
void* my_realloc (void*, size_t) __attribute__ ((alloc_size (2)));

declares that my_calloc returns memory of the size given by the product of parameter 1 and 2 and that my_realloc returns memory of the size given by parameter 2.

always_inline#: Generally, functions are not inlined unless optimization is specified. For functions declared inline, this attribute inlines the function independent of any restrictions that otherwise apply to inlining. Failure to inline such a function is diagnosed as an error. Note that if such a function is called indirectly the compiler may or may not inline it depending on optimization level and a failure to inline an indirect call may or may not be diagnosed.

artificial#: This attribute is useful for small inline wrappers that if possible should appear during debugging as a unit. Depending on the debug info format it either means marking the function as artificial or using the caller location for all instructions within the inlined body.

assume_aligned (alignment), assume_aligned (alignment, offset)#

The assume_aligned attribute may be applied to a function that returns a pointer. It indicates that the returned pointer is aligned on a boundary given by alignment. If the attribute has two arguments, the second argument is misalignment offset. Meaningful values of alignment are powers of 2 greater than one. Meaningful values of offset are greater than zero and less than alignment.

For instance

void* my_alloc1 (size_t) __attribute__((assume_aligned (16)));
void* my_alloc2 (size_t) __attribute__((assume_aligned (32, 8)));

declares that my_alloc1 returns 16-byte aligned pointers and that my_alloc2 returns a pointer whose value modulo 32 is equal to 8.

cold#

The cold attribute on functions is used to inform the compiler that the function is unlikely to be executed. The function is optimized for size rather than speed and on many targets it is placed into a special subsection of the text section so all cold functions appear close together, improving code locality of non-cold parts of program. The paths leading to calls 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 perror, as cold to improve optimization of hot functions that do call marked functions in rare occasions.

When profile feedback is available, via -fprofile-use, cold functions are automatically detected and this attribute is ignored.

const#

Calls to functions whose return value is not affected by changes to the observable state of the program and that have no observable effects on such state other than to return a value may lend themselves to optimizations such as common subexpression elimination. Declaring such functions with the const attribute allows GCC to avoid emitting some calls in repeated invocations of the function with the same argument values.

For example,

int square (int) __attribute__ ((const));

tells GCC that subsequent calls to function square with the same argument value can be replaced by the result of the first call regardless of the statements in between.

The const attribute prohibits a function from reading objects that affect its return value between successive invocations. However, functions declared with the attribute can safely read objects that do not change their return value, such as non-volatile constants.

The const attribute imposes greater restrictions on a function’s definition than the similar pure attribute. Declaring the same function with both the const and the pure attribute is diagnosed. Because a const function cannot have any observable side effects it does not make sense for it to return void. Declaring such a function is diagnosed.

Note that a function that has pointer arguments and examines the data pointed to must not be declared const if the pointed-to data might change between successive invocations of the function. In general, since a function cannot distinguish data that might change from data that cannot, const functions should never take pointer or, in C++, reference arguments. Likewise, a function that calls a non-const function usually must not be const itself.

constructor, destructor, constructor (priority), destructor (priority)#

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 () completes or exit () is called. Functions with these attributes are useful for initializing data that is used implicitly during the execution of the program.

On some targets the attributes also accept an integer argument to specify a 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. Note that priorities 0-100 are reserved. 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 C++-Specific Variable, Function, and Type Attributes). However, at present, the order in which constructors for C++ objects with static storage duration and functions decorated with attribute constructor are invoked is unspecified. In mixed declarations, attribute init_priority can be used to impose a specific ordering.

Using the argument forms of the constructor and destructor attributes on targets where the feature is not supported is rejected with an error.

copy, copy (function)#

The copy attribute applies the set of attributes with which function has been declared to the declaration of the function to which the attribute is applied. The attribute is designed for libraries that define aliases or function resolvers that are expected to specify the same set of attributes as their targets. The copy attribute can be used with functions, variables, or types. However, the kind of symbol to which the attribute is applied (either function or variable) must match the kind of symbol to which the argument refers. The copy attribute copies only syntactic and semantic attributes but not attributes that affect a symbol’s linkage or visibility such as alias, visibility, or weak. The deprecated and target_clones attribute are also not copied. See Common Type Attributes. See Common Variable Attributes.

For example, the StrongAlias macro below makes use of the alias and copy attributes to define an alias named alloc for function allocate declared with attributes alloc_size, malloc, and nothrow. Thanks to the __typeof__ operator the alias has the same type as the target function. As a result of the copy attribute the alias also shares the same attributes as the target.

#define StrongAlias(TargetFunc, AliasDecl)  \
  extern __typeof__ (TargetFunc) AliasDecl  \
    __attribute__ ((alias (#TargetFunc), copy (TargetFunc)));

extern __attribute__ ((alloc_size (1), malloc, nothrow))
  void* allocate (size_t);
StrongAlias (allocate, alloc);

deprecated, deprecated (msg)#

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 optional msg argument, which must be a string, is printed in the warning if present.

The deprecated attribute can also be used for variables and types (see Specifying Attributes of Variables, see Specifying Attributes of Types.)

The message attached to the attribute is affected by the setting of the -fmessage-length option.

unavailable, unavailable (msg)#

The unavailable attribute results in an error if the function is used anywhere in the source file. This is useful when identifying functions that have been removed from a particular variation of an interface. Other than emitting an error rather than a warning, the unavailable attribute behaves in the same manner as deprecated.

The unavailable attribute can also be used for variables and types (see Specifying Attributes of Variables, see Specifying Attributes of Types.)

error ("message"), warning ("message")#

If the error or warning 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 or warning (respectively) that includes message is diagnosed. This is useful for compile-time checking, especially together with __builtin_constant_p and inline functions where checking the inline function arguments is not possible through extern char [(condition) ? 1 : -1]; tricks.

While it is possible to leave the function undefined and thus invoke a link failure (to define the function with a message in .gnu.warning* section), when using these attributes the problem is diagnosed earlier and with exact location of the call even in presence of inline functions or when not emitting debugging information.

externally_visible#

This attribute, attached to a global variable or function, nullifies the effect of the -fwhole-program command-line option, so the object remains visible outside the current compilation unit.

If -fwhole-program is used together with -flto and gold is used as the linker plugin, externally_visible attributes are automatically added to functions (not variable yet due to a current gold issue) that are accessed outside of LTO objects according to resolution file produced by gold. For other linkers that cannot generate resolution file, explicit externally_visible attributes are still necessary.

fd_arg, fd_arg (N)#

The fd_arg attribute may be applied to a function that takes an open file descriptor at referenced argument N.

It indicates that the passed filedescriptor must not have been closed. Therefore, when the analyzer is enabled with -fanalyzer, the analyzer may emit a -Wanalyzer-fd-use-after-close diagnostic if it detects a code path in which a function with this attribute is called with a closed file descriptor.

The attribute also indicates that the file descriptor must have been checked for validity before usage. Therefore, analyzer may emit -Wanalyzer-fd-use-without-check diagnostic if it detects a code path in which a function with this attribute is called with a file descriptor that has not been checked for validity.

fd_arg_read, fd_arg_read (N)#

The fd_arg_read is identical to fd_arg, but with the additional requirement that it might read from the file descriptor, and thus, the file descriptor must not have been opened as write-only.

The analyzer may emit a -Wanalyzer-access-mode-mismatch diagnostic if it detects a code path in which a function with this attribute is called on a file descriptor opened with O_WRONLY.

fd_arg_write, fd_arg_write (N)#: The fd_arg_write is identical to fd_arg_read except that the analyzer may emit a -Wanalyzer-access-mode-mismatch diagnostic if it detects a code path in which a function with this attribute is called on a file descriptor opened with O_RDONLY.

flatten#: Generally, inlining into a function is limited. For a function marked with this attribute, every call inside this function is inlined, if possible. Functions declared with attribute noinline and similar are not inlined. Whether the function itself is considered for inlining depends on its size and the current inlining parameters.

format (archetype, string-index, first-to-check)#

The format attribute specifies that a function takes printf, scanf, strftime or strfmon style arguments that 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, gnu_printf, gnu_scanf, gnu_strftime or strfmon. (You can also use __printf__, __scanf__, __strftime__ or __strfmon__.) On MinGW targets, ms_printf, ms_scanf, and ms_strftime are also present. archetype values such as printf refer to the formats accepted by the system’s C runtime library, while values prefixed with gnu_ always refer to the formats accepted by the GNU C Library. On Microsoft Windows targets, values prefixed with ms_ refer to the formats accepted by the msvcrt.dll library. 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 that 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.

For Objective-C dialects, NSString (or __NSString__) is recognized in the same context. Declarations including these format attributes are parsed for correct syntax, however the result of checking of such format strings is not yet defined, and is not carried out by this version of the compiler.

The target may also 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 one or more format strings 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). Multiple format_arg attributes may be applied to the same function, each designating a distinct parameter as a format 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.

In calls to a function declared with more than one format_arg attribute, each with a distinct argument value, the corresponding actual function arguments are checked against all format strings designated by the attributes. This capability is designed to support the GNU ngettext family of functions.

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 that 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.

For Objective-C dialects, the format-arg attribute may refer to an NSString reference for compatibility with the format attribute above.

The target may also allow additional types in format-arg attributes. See Format Checks Specific to Particular Target Machines.

gnu_inline#

This attribute should be used with a function that is also declared with the inline keyword. It directs GCC to treat the function as if it were defined in gnu90 mode even when compiling in C99 or gnu99 mode.

If the function is declared extern, 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 extern, in a library file. The definition in the header file causes most calls to the function to be inlined. If any uses of the function remain, they 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.

In C, if the function is neither extern nor static, then the function is compiled as a standalone function, as well as being inlined where possible.

This is how GCC traditionally handled functions declared inline. Since ISO C99 specifies a different semantics for inline, 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 __GNUC_GNU_INLINE__ or __GNUC_STDC_INLINE__ are defined. See An Inline Function is As Fast As a Macro.

In C++, this attribute does not depend on extern in any way, but it still requires the inline keyword to enable its special behavior.

hot#

The hot attribute on a function is used to inform the compiler that the function is a hot spot of the compiled program. The function is optimized more aggressively and on many targets it is placed into a special subsection of the text section so all hot functions appear close together, improving locality.

When profile feedback is available, via -fprofile-use, hot functions are automatically detected and this attribute is ignored.

ifunc ("resolver")#

The ifunc attribute is used to mark a function as an indirect function using the STT_GNU_IFUNC symbol type extension to the ELF standard. This allows the resolution of the symbol value to be determined dynamically at load time, and an optimized version of the routine to be selected for the particular processor or other system characteristics determined then. To use this attribute, first define the implementation functions available, and a resolver function that returns a pointer to the selected implementation function. The implementation functions’ declarations must match the API of the function being implemented. The resolver should be declared to be a function taking no arguments and returning a pointer to a function of the same type as the implementation. For example:

void *my_memcpy (void *dst, const void *src, size_t len)
{
  ...
  return dst;
}

static void * (*resolve_memcpy (void))(void *, const void *, size_t)
{
  return my_memcpy; // we will just always select this routine
}

The exported header file declaring the function the user calls would contain:

extern void *memcpy (void *, const void *, size_t);

allowing the user to call memcpy as a regular function, unaware of the actual implementation. Finally, the indirect function needs to be defined in the same translation unit as the resolver function:

void *memcpy (void *, const void *, size_t)
     __attribute__ ((ifunc ("resolve_memcpy")));

In C++, the ifunc attribute takes a string that is the mangled name of the resolver function. A C++ resolver for a non-static member function of class C should be declared to return a pointer to a non-member function taking pointer to C as the first argument, followed by the same arguments as of the implementation function. G++ checks the signatures of the two functions and issues a -Wattribute-alias warning for mismatches. To suppress a warning for the necessary cast from a pointer to the implementation member function to the type of the corresponding non-member function use the -Wno-pmf-conversions option. For example:

class S
{
private:
  int debug_impl (int);
  int optimized_impl (int);

  typedef int Func (S*, int);

  static Func* resolver ();
public:

  int interface (int);
};

int S::debug_impl (int) { /* ... */ }
int S::optimized_impl (int) { /* ... */ }

S::Func* S::resolver ()
{
  int (S::*pimpl) (int)
    = getenv ("DEBUG") ? &S::debug_impl : &S::optimized_impl;

  // Cast triggers -Wno-pmf-conversions.
  return reinterpret_cast<Func*>(pimpl);
}

int S::interface (int) __attribute__ ((ifunc ("_ZN1S8resolverEv")));

Indirect functions cannot be weak. Binutils version 2.20.1 or higher and GNU C Library version 2.11.1 are required to use this feature.

interrupt, interrupt_handler#: Many GCC back ends support attributes to indicate that a function is an interrupt handler, which tells the compiler to generate function entry and exit sequences that differ from those from regular functions. The exact syntax and behavior are target-specific; refer to the following subsections for details.

leaf#

Calls to external functions with this attribute must return to the current compilation unit only by return or by exception handling. In particular, a leaf function is not allowed to invoke callback functions passed to it from the current compilation unit, directly call functions exported by the unit, or longjmp into the unit. Leaf functions might still call functions from other compilation units and thus they are not necessarily leaf in the sense that they contain no function calls at all.

The attribute is intended for library functions to improve dataflow analysis. The compiler takes the hint that any data not escaping the current compilation unit cannot be used or modified by the leaf function. For example, the sin function is a leaf function, but qsort is not.

Note that leaf functions might indirectly run a signal handler defined in the current compilation unit that uses static variables. Similarly, when lazy symbol resolution is in effect, leaf functions might invoke indirect functions whose resolver function or implementation function is defined in the current compilation unit and uses static variables. There is no standard-compliant way to write such a signal handler, resolver function, or implementation function, and the best that you can do is to remove the leaf attribute or mark all such static variables volatile. Lastly, for ELF-based systems that support symbol interposition, care should be taken that functions defined in the current compilation unit do not unexpectedly interpose other symbols based on the defined standards mode and defined feature test macros; otherwise an inadvertent callback would be added.

The attribute has no effect on functions defined within the current compilation unit. This is to allow easy merging of multiple compilation units into one, for example, by using the link-time optimization. For this reason the attribute is not allowed on types to annotate indirect calls.

malloc, malloc (deallocator), malloc (deallocator, ptr-index)#

Attribute malloc indicates that a function is malloc -like, i.e., that the pointer P returned by the function cannot alias any other pointer valid when the function returns, and moreover no pointers to valid objects occur in any storage addressed by P. In addition, the GCC predicts that a function with the attribute returns non-null in most cases.

Independently, the form of the attribute with one or two arguments associates deallocator as a suitable deallocation function for pointers returned from the malloc -like function. ptr-index denotes the positional argument to which when the pointer is passed in calls to deallocator has the effect of deallocating it.

Using the attribute with no arguments is designed to improve optimization by relying on the aliasing property it implies. Functions like malloc and calloc have this property because they return a pointer to uninitialized or zeroed-out, newly obtained storage. However, functions like realloc do not have this property, as they may return pointers to storage containing pointers to existing objects. Additionally, since all such functions are assumed to return null only infrequently, callers can be optimized based on that assumption.

Associating a function with a deallocator helps detect calls to mismatched allocation and deallocation functions and diagnose them under the control of options such as -Wmismatched-dealloc. It also makes it possible to diagnose attempts to deallocate objects that were not allocated dynamically, by -Wfree-nonheap-object. To indicate that an allocation function both satisifies the nonaliasing property and has a deallocator associated with it, both the plain form of the attribute and the one with the deallocator argument must be used. The same function can be both an allocator and a deallocator. Since inlining one of the associated functions but not the other could result in apparent mismatches, this form of attribute malloc is not accepted on inline functions. For the same reason, using the attribute prevents both the allocation and deallocation functions from being expanded inline.

For example, besides stating that the functions return pointers that do not alias any others, the following declarations make fclose a suitable deallocator for pointers returned from all functions except popen, and pclose as the only suitable deallocator for pointers returned from popen. The deallocator functions must be declared before they can be referenced in the attribute.

int fclose (FILE*);
int pclose (FILE*);

__attribute__ ((malloc, malloc (fclose, 1)))
  FILE* fdopen (int, const char*);
__attribute__ ((malloc, malloc (fclose, 1)))
  FILE* fopen (const char*, const char*);
__attribute__ ((malloc, malloc (fclose, 1)))
  FILE* fmemopen(void *, size_t, const char *);
__attribute__ ((malloc, malloc (pclose, 1)))
  FILE* popen (const char*, const char*);
__attribute__ ((malloc, malloc (fclose, 1)))
  FILE* tmpfile (void);

The warnings guarded by -fanalyzer respect allocation and deallocation pairs marked with the malloc. In particular:

The analyzer will emit a -Wanalyzer-mismatching-deallocation diagnostic if there is an execution path in which the result of an allocation call is passed to a different deallocator.
The analyzer will emit a -Wanalyzer-double-free diagnostic if there is an execution path in which a value is passed more than once to a deallocation call.
The analyzer will consider the possibility that an allocation function could fail and return NULL. It will emit -Wanalyzer-possible-null-dereference and -Wanalyzer-possible-null-argument diagnostics if there are execution paths in which an unchecked result of an allocation call is dereferenced or passed to a function requiring a non-null argument. If the allocator always returns non-null, use __attribute__ ((returns_nonnull)) to suppress these warnings. For example:
```
char *xstrdup (const char *)
  __attribute__((malloc (free), returns_nonnull));
```
The analyzer will emit a -Wanalyzer-use-after-free diagnostic if there is an execution path in which the memory passed by pointer to a deallocation call is used after the deallocation.
The analyzer will emit a -Wanalyzer-malloc-leak diagnostic if there is an execution path in which the result of an allocation call is leaked (without being passed to the deallocation function).
The analyzer will emit a -Wanalyzer-free-of-non-heap diagnostic if a deallocation function is used on a global or on-stack variable.

The analyzer assumes that deallocators can gracefully handle the NULL pointer. If this is not the case, the deallocator can be marked with __attribute__((nonnull)) so that -fanalyzer can emit a -Wanalyzer-possible-null-argument diagnostic for code paths in which the deallocator is called with NULL.

no_icf#: This function attribute prevents a functions from being merged with another semantically equivalent function.

no_instrument_function#: If any of -finstrument-functions, -p, or -pg are given, profiling function calls are generated at entry and exit of most user-compiled functions. Functions with this attribute are not so instrumented.

no_profile_instrument_function#: The no_profile_instrument_function attribute on functions is used to inform the compiler that it should not process any profile feedback based optimization code instrumentation.

no_reorder#: Do not reorder functions or variables marked no_reorder against each other or top level assembler statements the executable. The actual order in the program will depend on the linker command line. Static variables marked like this are also not removed. This has a similar effect as the -fno-toplevel-reorder option, but only applies to the marked symbols.

no_sanitize ("sanitize_option")#

The no_sanitize attribute on functions is used to inform the compiler that it should not do sanitization of any option mentioned in sanitize_option. A list of values acceptable by the -fsanitize option can be provided.

void __attribute__ ((no_sanitize ("alignment", "object-size")))
f () { /* Do something. */; }
void __attribute__ ((no_sanitize ("alignment,object-size")))
g () { /* Do something. */; }

no_sanitize_address, no_address_safety_analysis#: The no_sanitize_address attribute on functions is used to inform the compiler that it should not instrument memory accesses in the function when compiling with the -fsanitize=address option. The no_address_safety_analysis is a deprecated alias of the no_sanitize_address attribute, new code should use no_sanitize_address.

no_sanitize_thread#: The no_sanitize_thread attribute on functions is used to inform the compiler that it should not instrument memory accesses in the function when compiling with the -fsanitize=thread option.

no_sanitize_undefined#: The no_sanitize_undefined attribute on functions is used to inform the compiler that it should not check for undefined behavior in the function when compiling with the -fsanitize=undefined option.

no_sanitize_coverage#: The no_sanitize_coverage attribute on functions is used to inform the compiler that it should not do coverage-guided fuzzing code instrumentation (-fsanitize-coverage).

no_split_stack#: If -fsplit-stack is given, functions have a small prologue which decides whether to split the stack. Functions with the no_split_stack attribute do not have that prologue, and thus may run with only a small amount of stack space available.

no_stack_limit#: This attribute locally overrides the -fstack-limit-register and -fstack-limit-symbol command-line options; it has the effect of disabling stack limit checking in the function it applies to.

noclone#: This function attribute prevents a function from being considered for cloning—a mechanism that produces specialized copies of functions and which is (currently) performed by interprocedural constant propagation.

noinline#

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

If the function does not have side effects, there are optimizations other than inlining that cause function calls to be optimized away, although the function call is live. To keep such calls from being optimized away, put

asm ("");

(see Extended Asm - Assembler Instructions with C Expression Operands) in the called function, to serve as a special side effect.

noipa#: Disable interprocedural optimizations between the function with this attribute and its callers, as if the body of the function is not available when optimizing callers and the callers are unavailable when optimizing the body. This attribute implies noinline, noclone and no_icf attributes. However, this attribute is not equivalent to a combination of other attributes, because its purpose is to suppress existing and future optimizations employing interprocedural analysis, including those that do not have an attribute suitable for disabling them individually. This attribute is supported mainly for the purpose of testing the compiler.

nonnull, nonnull (arg-index, ...)#

The nonnull attribute may be applied to a function that takes at least one argument of a pointer type. It indicates that the referenced arguments must be non-null pointers. For instance, the declaration:

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

informs the compiler that, in calls to my_memcpy, arguments dest and src must be non-null.

The attribute has an effect both on functions calls and function definitions.

For function calls:

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. See Options to Request or Suppress Warnings.
The -fisolate-erroneous-paths-attribute option can be specified to have GCC transform calls with null arguments to non-null functions into traps. See Options That Control Optimization.
The compiler may also perform optimizations based on the knowledge that certain function arguments cannot be null. These optimizations can be disabled by the -fno-delete-null-pointer-checks option. See Options That Control Optimization.

For function definitions:

If the compiler determines that a function parameter that is marked with nonnull is compared with null, and -Wnonnull-compare option is enabled, a warning is issued. See Options to Request or Suppress Warnings.
The compiler may also perform optimizations based on the knowledge that nonnul parameters cannot be null. This can currently not be disabled other than by removing the nonnull attribute.

If no arg-index 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));

noplt#

The noplt attribute is the counterpart to option -fno-plt. Calls to functions marked with this attribute in position-independent code do not use the PLT.

/* Externally defined function foo.  */
int foo () __attribute__ ((noplt));

int
main (/* ... */)
{
  /* ... */
  foo ();
  /* ... */
}

The noplt attribute on function foo tells the compiler to assume that the function foo is externally defined and that the call to foo must avoid the PLT in position-independent code.

In position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT to use the GOT instead.

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.

In order to preserve backtraces, GCC will never turn calls to noreturn functions into tail calls.

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.

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.

optimize (level, ...), optimize (string, ...)#

optimize (string, ...)#

The optimize attribute is used to specify that a function is to be compiled with different optimization options than specified on the command line. The optimize attribute arguments of a function behave behave as if appended to the command-line.

Valid arguments are constant non-negative integers and strings. Each numeric argument specifies an optimization level. Each string argument consists of one or more comma-separated substrings. Each substring that begins with the letter O refers to an optimization option such as -O0 or -Os. Other substrings are taken as suffixes to the -f prefix jointly forming the name of an optimization option. See Options That Control Optimization.

#pragma GCC optimize can be used to set optimization options for more than one function. See Function Specific Option Pragmas, for details about the pragma.

Providing multiple strings as arguments separated by commas to specify multiple options is equivalent to separating the option suffixes with a comma (,) within a single string. Spaces are not permitted within the strings.

Not every optimization option that starts with the -f prefix specified by the attribute necessarily has an effect on the function. The optimize attribute should be used for debugging purposes only. It is not suitable in production code.

patchable_function_entry#

In case the target’s text segment can be made writable at run time by any means, padding the function entry with a number of NOPs can be used to provide a universal tool for instrumentation.

The patchable_function_entry function attribute can be used to change the number of NOPs to any desired value. The two-value syntax is the same as for the command-line switch -fpatchable-function-entry=N,M, generating N NOPs, with the function entry point before the M th NOP instruction. M defaults to 0 if omitted e.g. function entry point is before the first NOP.

If patchable function entries are enabled globally using the command-line option -fpatchable-function-entry=N,M, then you must disable instrumentation on all functions that are part of the instrumentation framework with the attribute patchable_function_entry (0) to prevent recursion.

pure#

Calls to functions that have no observable effects on the state of the program other than to return a value may lend themselves to optimizations such as common subexpression elimination. Declaring such functions with the pure attribute allows GCC to avoid emitting some calls in repeated invocations of the function with the same argument values.

The pure attribute prohibits a function from modifying the state of the program that is observable by means other than inspecting the function’s return value. However, functions declared with the pure attribute can safely read any non-volatile objects, and modify the value of objects in a way that does not affect their return value or the observable state of the program.

For example,

int hash (char *) __attribute__ ((pure));

tells GCC that subsequent calls to the function hash with the same string can be replaced by the result of the first call provided the state of the program observable by hash, including the contents of the array itself, does not change in between. Even though hash takes a non-const pointer argument it must not modify the array it points to, or any other object whose value the rest of the program may depend on. However, the caller may safely change the contents of the array between successive calls to the function (doing so disables the optimization). The restriction also applies to member objects referenced by the this pointer in C++ non-static member functions.

Some 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 consecutive calls (such as the standard C feof function in a multithreading environment).

The pure attribute imposes similar but looser restrictions on a function’s definition than the const attribute: pure allows the function to read any non-volatile memory, even if it changes in between successive invocations of the function. Declaring the same function with both the pure and the const attribute is diagnosed. Because a pure function cannot have any observable side effects it does not make sense for such a function to return void. Declaring such a function is diagnosed.

returns_nonnull#

The returns_nonnull attribute specifies that the function return value should be a non-null pointer. For instance, the declaration:

extern void *
mymalloc (size_t len) __attribute__((returns_nonnull));

lets the compiler optimize callers based on the knowledge that the return value will never be null.

returns_twice#: The returns_twice attribute tells the compiler that a function may return more than one time. The compiler ensures that all registers are dead before calling such a function and emits 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.

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, sentinel (position)#

This function attribute indicates that an argument in a call to the function is expected to be an explicit NULL. The attribute is only valid on variadic functions. By default, the sentinel is expected to be the last argument of the function call. If the optional position argument is specified to the attribute, the sentinel must be located at position 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 object pointer type. If your system defines the NULL macro with an integer type then you need to add an explicit cast. During installation GCC replaces the system <stddef.h> header with a copy that redefines NULL appropriately.

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

simd, simd("mask")#

This attribute enables creation of one or more function versions that can process multiple arguments using SIMD instructions from a single invocation. Specifying this attribute allows compiler to assume that such versions are available at link time (provided in the same or another translation unit). Generated versions are target-dependent and described in the corresponding Vector ABI document. For x86_64 target this document can be found here.

The optional argument mask may have the value notinbranch or inbranch, and instructs the compiler to generate non-masked or masked clones correspondingly. By default, all clones are generated.

If the attribute is specified and #pragma omp declare simd is present on a declaration and the -fopenmp or -fopenmp-simd switch is specified, then the attribute is ignored.

stack_protect#: This attribute adds stack protection code to the function if flags -fstack-protector, -fstack-protector-strong or -fstack-protector-explicit are set.

no_stack_protector#: This attribute prevents stack protection code for the function.

target (string, ...)#

Multiple target back ends implement the target attribute to specify that a function is to be compiled with different target options than specified on the command line. The original target command-line options are ignored. One or more strings can be provided as arguments. Each string consists of one or more comma-separated suffixes to the -m prefix jointly forming the name of a machine-dependent option. See Machine-Dependent Options.

The target attribute can be used for instance to have a function compiled with a different ISA (instruction set architecture) than the default. #pragma GCC target can be used to specify target-specific options for more than one function. See Function Specific Option Pragmas, for details about the pragma.

For instance, on an x86, you could declare one function with the target("sse4.1,arch=core2") attribute and another with target("sse4a,arch=amdfam10"). This is equivalent to compiling the first function with -msse4.1 and -march=core2 options, and the second function with -msse4a and -march=amdfam10 options. It is up to you to make sure that a function is only invoked on a machine that supports the particular ISA it is compiled for (for example by using cpuid on x86 to determine what feature bits and architecture family are used).

int core2_func (void) __attribute__ ((__target__ ("arch=core2")));
int sse3_func (void) __attribute__ ((__target__ ("sse3")));

The options supported are specific to each target; refer to x86 Function Attributes, PowerPC Function Attributes, ARM Function Attributes, AArch64 Function Attributes, Nios II Function Attributes, and S/390 Function Attributes for details.

symver ("name2@nodename")#

On ELF targets this attribute creates a symbol version. The name2 part of the parameter is the actual name of the symbol by which it will be externally referenced. The nodename portion should be the name of a node specified in the version script supplied to the linker when building a shared library. Versioned symbol must be defined and must be exported with default visibility.

__attribute__ ((__symver__ ("foo@VERS_1"))) int
foo_v1 (void)
{
}

Will produce a .symver foo_v1, foo@VERS_1 directive in the assembler output.

One can also define multiple version for a given symbol (starting from binutils 2.35).

__attribute__ ((__symver__ ("foo@VERS_2"), __symver__ ("foo@VERS_3")))
int symver_foo_v1 (void)
{
}

This example creates a symbol name symver_foo_v1 which will be version VERS_2 and VERS_3 of foo.

If you have an older release of binutils, then symbol alias needs to be used:

__attribute__ ((__symver__ ("foo@VERS_2")))
int foo_v1 (void)
{
  return 0;
}

__attribute__ ((__symver__ ("foo@VERS_3")))
__attribute__ ((alias ("foo_v1")))
int symver_foo_v1 (void);

Finally if the parameter is "name2@@nodename" then in addition to creating a symbol version (as if "name2@nodename" was used) the version will be also used to resolve name2 by the linker.

tainted_args#

The tainted_args attribute is used to specify that a function is called in a way that requires sanitization of its arguments, such as a system call in an operating system kernel. Such a function can be considered part of the ‘attack surface’ of the program. The attribute can be used both on function declarations, and on field declarations containing function pointers. In the latter case, any function used as an initializer of such a callback field will be treated as being called with tainted arguments.

The analyzer will pay particular attention to such functions when both -fanalyzer and -fanalyzer-checker=taint are supplied, potentially issuing warnings guarded by -Wanalyzer-tainted-allocation-size, -Wanalyzer-tainted-array-index, -Wanalyzer-tainted-divisor, -Wanalyzer-tainted-offset, and -Wanalyzer-tainted-size.

target_clones (options)#

The target_clones attribute is used to specify that a function be cloned into multiple versions compiled with different target options than specified on the command line. The supported options and restrictions are the same as for target attribute.

For instance, on an x86, you could compile a function with target_clones("sse4.1,avx"). GCC creates two function clones, one compiled with -msse4.1 and another with -mavx.

On a PowerPC, you can compile a function with target_clones("cpu=power9,default"). GCC will create two function clones, one compiled with -mcpu=power9 and another with the default options. GCC must be configured to use GLIBC 2.23 or newer in order to use the target_clones attribute.

It also creates a resolver function (see the ifunc attribute above) that dynamically selects a clone suitable for current architecture. The resolver is created only if there is a usage of a function with target_clones attribute.

Note that any subsequent call of a function without target_clone from a target_clone caller will not lead to copying (target clone) of the called function. If you want to enforce such behaviour, we recommend declaring the calling function with the flatten attribute?

unused#: This attribute, attached to a function, means that the function is meant to be possibly unused. GCC does 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.

When applied to a member function of a C++ class template, the attribute also means that the function is instantiated if the class itself is instantiated.

retain#

For ELF targets that support the GNU or FreeBSD OSABIs, this attribute will save the function from linker garbage collection. To support this behavior, functions that have not been placed in specific sections (e.g. by the section attribute, or the -ffunction-sections option), will be placed in new, unique sections.

This additional functionality requires Binutils version 2.36 or later.

visibility ("visibility_type")#

This attribute affects the linkage of the declaration to which it is attached. It can be applied to variables (see Common Variable Attributes) and types (see Common Type Attributes) as well as functions.

There are four supported visibility_type values: default, hidden, protected or internal visibility.

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

The possible values of visibility_type correspond to the visibility settings in the ELF gABI.

default

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.

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.

On Darwin, default visibility means that the declaration is visible to other modules.

Default visibility corresponds to ‘external linkage’ in the language.

hidden

Hidden visibility indicates that the entity declared has a new form of linkage, which we call ‘hidden linkage’. Two declarations of an object with hidden linkage refer to the same object if they are in the same shared object.

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 a function is never 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.

protected

Protected visibility is like default visibility except that it indicates that references within the defining module bind to the definition in that module. That is, the declared entity cannot be overridden by another module.

All visibilities are supported on many, but not all, ELF targets (supported when the assembler supports the .visibility pseudo-op). Default visibility is supported everywhere. Hidden visibility is supported on Darwin targets.

The visibility attribute should be applied only to declarations that 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.

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.

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.

A C++ namespace declaration can also have the visibility attribute.

namespace nspace1 __attribute__ ((visibility ("protected")))
{ /* Do something. */; }

This attribute applies only to the particular namespace body, not to other definitions of the same namespace; it is equivalent to using #pragma GCC visibility before and after the namespace definition (see Visibility Pragmas).

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.

If both the template and enclosing class have explicit visibility, the visibility from the template is used.

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 a declaration of an external symbol to be emitted as a weak symbol rather than a global. This is primarily useful in defining library functions that can be overridden in user code, though it can also be used with non-function declarations. The overriding symbol must have the same type as the weak symbol. In addition, if it designates a variable it must also have the same size and alignment as the weak symbol. 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. Alternatively, target may be given as an argument to weakref itself, naming the target definition of the alias. The target must have the same type as the declaration. In addition, if it designates a variable it must also have the same size and alignment as the declaration. In either form of the declaration weakref implicitly marks the declared symbol as weak. Without a target given as an argument to weakref or to alias, weakref is equivalent to weak (in that case the declaration may be extern).

/* Given the declaration: */
extern int y (void);

/* the following... */
static int x (void) __attribute__ ((weakref ("y")));

/* is equivalent to... */
static int x (void) __attribute__ ((weakref, alias ("y")));

/* or, alternatively, to... */
static int x (void) __attribute__ ((weakref));
static int x (void) __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 it becomes a weak undefined symbol. If it is directly referenced, however, then such strong references prevail, and a definition is 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 link with relocatable output (i.e. ld -r) on them.

A declaration to which weakref is attached and that is associated with a named target must be static.

zero_call_used_regs ("choice")#

The zero_call_used_regs attribute causes the compiler to zero a subset of all call-used registers (1) at function return. This is used to increase program security by either mitigating Return-Oriented Programming (ROP) attacks or preventing information leakage through registers.

In order to satisfy users with different security needs and control the run-time overhead at the same time, the choice parameter provides a flexible way to choose the subset of the call-used registers to be zeroed. The three basic values of choice are:

skip doesn’t zero any call-used registers.
used only zeros call-used registers that are used in the function. A ‘used’ register is one whose content has been set or referenced in the function.
all zeros all call-used registers.

In addition to these three basic choices, it is possible to modify used or all as follows:

Adding -gpr restricts the zeroing to general-purpose registers.
Adding -arg restricts the zeroing to registers that can sometimes be used to pass function arguments. This includes all argument registers defined by the platform’s calling conversion, regardless of whether the function uses those registers for function arguments or not.

The modifiers can be used individually or together. If they are used together, they must appear in the order above.

The full list of choice s is therefore:

skip: doesn’t zero any call-used register.
used: only zeros call-used registers that are used in the function.
used-gpr: only zeros call-used general purpose registers that are used in the function.
used-arg: only zeros call-used registers that are used in the function and pass arguments.
used-gpr-arg: only zeros call-used general purpose registers that are used in the function and pass arguments.
all: zeros all call-used registers.
all-gpr: zeros all call-used general purpose registers.
all-arg: zeros all call-used registers that pass arguments.
all-gpr-arg: zeros all call-used general purpose registers that pass arguments.

Of this list, used-arg, used-gpr-arg, all-arg, and all-gpr-arg are mainly used for ROP mitigation.

The default for the attribute is controlled by -fzero-call-used-regs.

1: A ‘call-used’ register is a register whose contents can be changed by a function call; therefore, a caller cannot assume that the register has the same contents on return from the function as it had before calling the function. Such registers are also called ‘call-clobbered’, ‘caller-saved’, or ‘volatile’.