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* doc/extend.texi (Function Attributes): Alphabetize. 

From-SVN: r83543
parent a52a0a7f
Showing with 474 additions and 457 deletions
gcc/ChangeLog
2004-06-23 Nathan Sidwell <nathan@codesourcery.com>
* doc/extend.texi (Function Attributes): Alphabetize.
2004-06-23 Richard Henderson <rth@redhat.com>
* c-gimplify.c (gimplify_decl_stmt): Update gimplify_type_sizes call.
......@@ -3107,6 +3111,7 @@
(expand_return): Don't check for possible tail recursion.
* tree.h (optimize_tail_recursion): Remove prototype.
>>>>>>> 2.4079
2004-06-02 Jan Hubicka <jh@suse.cz>
* tree-cfg.c (tree_find_edge_insert_loc): Allow inserting before
......
gcc/doc/extend.texi
......@@ -1913,90 +1913,39 @@ you may use @code{__noreturn__} instead of @code{noreturn}.
attributes.
@table @code
@cindex @code{noreturn} function attribute
@item noreturn
A few standard library functions, such as @code{abort} and @code{exit},
cannot return. GCC knows this automatically. Some programs define
their own functions that never return. You can declare them
@code{noreturn} to tell the compiler this fact. For example,
@smallexample
@group
void fatal () __attribute__ ((noreturn));
void
fatal (/* @r{@dots{}} */)
@{
/* @r{@dots{}} */ /* @r{Print error message.} */ /* @r{@dots{}} */
exit (1);
@}
@end group
@end smallexample
The @code{noreturn} keyword tells the compiler to assume that
@code{fatal} cannot return. It can then optimize without regard to what
would happen if @code{fatal} ever did return. This makes slightly
better code. More importantly, it helps avoid spurious warnings of
uninitialized variables.
The @code{noreturn} keyword does not affect the exceptional path when that
applies: a @code{noreturn}-marked function may still return to the caller
by throwing an exception.
Do not assume that registers saved by the calling function are
restored before calling the @code{noreturn} function.
@c Keep this table alphabetized by attribute name. Treat _ as space.
It does not make sense for a @code{noreturn} function to have a return
type other than @code{void}.
The attribute @code{noreturn} is not implemented in GCC versions
earlier than 2.5. An alternative way to declare that a function does
not return, which works in the current version and in some older
versions, is as follows:
@item alias ("@var{target}")
@cindex @code{alias} attribute
The @code{alias} attribute causes the declaration to be emitted as an
alias for another symbol, which must be specified. For instance,
@smallexample
typedef void voidfn ();
volatile voidfn fatal;
void __f () @{ /* @r{Do something.} */; @}
void f () __attribute__ ((weak, alias ("__f")));
@end smallexample
@cindex @code{noinline} function attribute
@item noinline
This function attribute prevents a function from being considered for
inlining.
declares @samp{f} to be a weak alias for @samp{__f}. In C++, the
mangled name for the target must be used.
Not all target machines support this attribute.
@cindex @code{always_inline} function attribute
@item always_inline
@cindex @code{always_inline} function attribute
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.
@cindex @code{pure} function attribute
@item pure
Many functions have no effects except the return value and their
return value depends only on the parameters and/or global variables.
Such a function can be subject
to common subexpression elimination and loop optimization just as an
arithmetic operator would be. These functions should be declared
with the attribute @code{pure}. For example,
@smallexample
int square (int) __attribute__ ((pure));
@end smallexample
@noindent
says that the hypothetical function @code{square} is safe to call
fewer times than the program says.
Some of common examples of pure functions are @code{strlen} or @code{memcmp}.
Interesting non-pure functions are functions with infinite loops or those
depending on volatile memory or other system resource, that may change between
two consecutive calls (such as @code{feof} in a multithreading environment).
@item cdecl
@cindex functions that do pop the argument stack on the 386
@opindex mrtd
On the Intel 386, the @code{cdecl} attribute causes the compiler to
assume that the calling function will pop off the stack space used to
pass arguments. This is
useful to override the effects of the @option{-mrtd} switch.
The attribute @code{pure} is not implemented in GCC versions earlier
than 2.96.
@cindex @code{const} function attribute
@item const
@cindex @code{const} function attribute
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} attribute above, since function is not
......@@ -2023,14 +1972,137 @@ extern const intfn square;
This approach does not work in GNU C++ from 2.6.0 on, since the language
specifies that the @samp{const} must be attached to the return value.
@cindex @code{nothrow} function attribute
@item nothrow
The @code{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 @code{qsort} and @code{bsearch} that
take function pointer arguments. The @code{nothrow} attribute is not
implemented in GCC versions earlier than 3.2.
@item constructor
@itemx destructor
@cindex @code{constructor} function attribute
@cindex @code{destructor} function attribute
The @code{constructor} attribute causes the function to be called
automatically before execution enters @code{main ()}. Similarly, the
@code{destructor} attribute causes the function to be called
automatically after @code{main ()} has completed or @code{exit ()} has
been called. Functions with these attributes are useful for
initializing data that will be used implicitly during the execution of
the program.
These attributes are not currently implemented for Objective-C@.
@item deprecated
@cindex @code{deprecated} attribute.
The @code{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:
@smallexample
int old_fn () __attribute__ ((deprecated));
int old_fn ();
int (*fn_ptr)() = old_fn;
@end smallexample
results in a warning on line 3 but not line 2.
The @code{deprecated} attribute can also be used for variables and
types (@pxref{Variable Attributes}, @pxref{Type Attributes}.)
@item dllexport
@cindex @code{__declspec(dllexport)}
On Microsoft Windows targets the @code{dllexport} 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} attribute. The pointer name
is formed by combining @code{_imp__} and the function or variable name.
Currently, the @code{dllexport}attribute is ignored for inlined
functions, but export can be forced by using the
@option{-fkeep-inline-functions} flag. The attribute is also ignored for
undefined symbols.
When applied to C++ classes. the attribute marks defined non-inlined
member functions and static data members as exports. Static consts
initialized in-class are not marked unless they are also defined
out-of-class.
On cygwin, mingw and arm-pe targets, @code{__declspec(dllexport)} is
recognized as a synonym for @code{__attribute__ ((dllexport))} for
compatibility with other Microsoft Windows compilers.
Alternative methods for including the symbol in the dll's export table
are to use a .def file with an @code{EXPORTS} section or, with GNU ld,
using the @option{--export-all} linker flag.
@item dllimport
@cindex @code{__declspec(dllimport)}
On Microsoft Windows targets, the @code{dllimport} attribute causes the
compiler to reference a function or variable via a global pointer to a
pointer that is set up by the Microsoft Windows dll library. The pointer
name is formed by combining @code{_imp__} and the function or variable
name. The attribute implies @code{extern} storage.
Currently, the attribute is ignored for inlined functions. If the
attribute is applied to a symbol @emph{definition}, an error is reported.
If a symbol previously declared @code{dllimport} is later defined, the
attribute is ignored in subsequent references, and a warning is emitted.
The attribute is also overridden by a subsequent declaration as
@code{dllexport}.
When applied to C++ classes, the attribute marks non-inlined
member functions and static data members as imports. However, the
attribute is ignored for virtual methods to allow creation of vtables
using thunks.
On cygwin, mingw and arm-pe targets, @code{__declspec(dllimport)} is
recognized as a synonym for @code{__attribute__ ((dllimport))} for
compatibility with other Microsoft Windows compilers.
The use of the @code{dllimport} 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} attribute on imported variables was
required on older versions of GNU ld, but can now be avoided by passing
the @option{--enable-auto-import} switch to ld. As with functions, using
the attribute for a variable eliminates a thunk in the dll.
One drawback to using this attribute is that a pointer to a function or
variable marked as dllimport cannot be used as a constant address. The
attribute can be disabled for functions by setting the
@option{-mnop-fun-dllimport} flag.
@item eightbit_data
@cindex eight bit data on the H8/300, H8/300H, and H8S
Use this attribute on the H8/300, H8/300H, and H8S to indicate that the specified
variable should be placed into the eight bit data section.
The compiler will generate more efficient code for certain operations
on data in the eight bit data area. Note the eight bit data area is limited to
256 bytes of data.
You must use GAS and GLD from GNU binutils version 2.7 or later for
this attribute to work correctly.
@item far
@cindex functions which handle memory bank switching
On 68HC11 and 68HC12 the @code{far} attribute causes the compiler to
use a calling convention that takes care of switching memory banks when
entering and leaving a function. This calling convention is also the
default when using the @option{-mlong-calls} option.
On 68HC12 the compiler will use the @code{call} and @code{rtc} instructions
to call and return from a function.
On 68HC11 the compiler will generate a sequence of instructions
to invoke a board-specific routine to switch the memory bank and call the
real function. The board-specific routine simulates a @code{call}.
At the end of a function, it will jump to a board-specific routine
instead of using @code{rts}. The board-specific return routine simulates
the @code{rtc}.
@item fastcall
@cindex functions that pop the argument stack on the 386
On the Intel 386, the @code{fastcall} attribute causes the compiler to
pass the first two arguments in the registers ECX and EDX. Subsequent
arguments are passed on the stack. The called function will pop the
arguments off the stack. If the number of arguments is variable all
arguments are pushed on the stack.
@item format (@var{archetype}, @var{string-index}, @var{first-to-check})
@cindex @code{format} function attribute
......@@ -2131,138 +2203,67 @@ requested by @option{-ansi} or an appropriate @option{-std} option, or
@option{-ffreestanding} is used. @xref{C Dialect Options,,Options
Controlling C Dialect}.
@item nonnull (@var{arg-index}, @dots{})
@cindex @code{nonnull} function attribute
The @code{nonnull} attribute specifies that some function parameters should
be non-null pointers. For instance, the declaration:
@item function_vector
@cindex calling functions through the function vector on the H8/300 processors
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.
@smallexample
extern void *
my_memcpy (void *dest, const void *src, size_t len)
__attribute__((nonnull (1, 2)));
@end smallexample
You must use GAS and GLD from GNU binutils version 2.7 or later for
this attribute to work correctly.
@noindent
causes the compiler to check that, in calls to @code{my_memcpy},
arguments @var{dest} and @var{src} are non-null. If the compiler
determines that a null pointer is passed in an argument slot marked
as non-null, and the @option{-Wnonnull} option is enabled, a warning
is issued. The compiler may also choose to make optimizations based
on the knowledge that certain function arguments will not be null.
@item interrupt
@cindex interrupt handler functions
Use this attribute on the ARM, AVR, C4x, M32R/D and Xstormy16 ports to indicate
that the specified function is an interrupt handler. The compiler will
generate function entry and exit sequences suitable for use in an
interrupt handler when this attribute is present.
If no argument index list is given to the @code{nonnull} attribute,
all pointer arguments are marked as non-null. To illustrate, the
following declaration is equivalent to the previous example:
Note, interrupt handlers for the m68k, H8/300, H8/300H, H8S, and SH processors
can be specified via the @code{interrupt_handler} attribute.
@smallexample
extern void *
my_memcpy (void *dest, const void *src, size_t len)
__attribute__((nonnull));
@end smallexample
@item no_instrument_function
@cindex @code{no_instrument_function} function attribute
@opindex finstrument-functions
If @option{-finstrument-functions} is given, profiling function calls will
be generated at entry and exit of most user-compiled functions.
Functions with this attribute will not be so instrumented.
@item section ("@var{section-name}")
@cindex @code{section} function attribute
Normally, the compiler places the code it generates in the @code{text} section.
Sometimes, however, you need additional sections, or you need certain
particular functions to appear in special sections. The @code{section}
attribute specifies that a function lives in a particular section.
For example, the declaration:
@smallexample
extern void foobar (void) __attribute__ ((section ("bar")));
@end smallexample
@noindent
puts the function @code{foobar} in the @code{bar} section.
Some file formats do not support arbitrary sections so the @code{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.
@item constructor
@itemx destructor
@cindex @code{constructor} function attribute
@cindex @code{destructor} function attribute
The @code{constructor} attribute causes the function to be called
automatically before execution enters @code{main ()}. Similarly, the
@code{destructor} attribute causes the function to be called
automatically after @code{main ()} has completed or @code{exit ()} has
been called. Functions with these attributes are useful for
initializing data that will be used implicitly during the execution of
the program.
These attributes are not currently implemented for Objective-C@.
@cindex @code{unused} attribute.
@item unused
This attribute, attached to a function, means that the function is meant
to be possibly unused. GCC will not produce a warning for this
function.
@cindex @code{used} attribute.
@item 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.
Note, on the AVR, interrupts will be enabled inside the function.
@cindex @code{deprecated} attribute.
@item deprecated
The @code{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:
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:
@smallexample
int old_fn () __attribute__ ((deprecated));
int old_fn ();
int (*fn_ptr)() = old_fn;
void f () __attribute__ ((interrupt ("IRQ")));
@end smallexample
results in a warning on line 3 but not line 2.
The @code{deprecated} attribute can also be used for variables and
types (@pxref{Variable Attributes}, @pxref{Type Attributes}.)
Permissible values for this parameter are: IRQ, FIQ, SWI, ABORT and UNDEF@.
@item warn_unused_result
@cindex @code{warn_unused_result} attribute
The @code{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
@code{realloc}.
@item interrupt_handler
@cindex interrupt handler functions on the m68k, H8/300 and SH processors
Use this attribute on the m68k, H8/300, H8/300H, H8S, and SH to indicate that
the specified function is an interrupt handler. The compiler will generate
function entry and exit sequences suitable for use in an interrupt
handler when this attribute is present.
@smallexample
int fn () __attribute__ ((warn_unused_result));
int foo ()
@{
if (fn () < 0) return -1;
fn ();
return 0;
@}
@end smallexample
@item long_call/short_call
@cindex indirect calls on ARM
This attribute specifies how a particular function is called on
ARM@. Both attributes override the @option{-mlong-calls} (@pxref{ARM Options})
command line switch and @code{#pragma long_calls} settings. The
@code{long_call} attribute causes the compiler to always call the
function by first loading its address into a register and then using the
contents of that register. The @code{short_call} attribute always places
the offset to the function from the call site into the @samp{BL}
instruction directly.
results in warning on line 5.
@item longcall/shortcall
@cindex functions called via pointer on the RS/6000 and PowerPC
On the RS/6000 and PowerPC, the @code{longcall} attribute causes the
compiler to always call this function via a pointer, just as it would if
the @option{-mlongcall} option had been specified. The @code{shortcall}
attribute causes the compiler not to do this. These attributes override
both the @option{-mlongcall} switch and the @code{#pragma longcall}
setting.
@item weak
@cindex @code{weak} attribute
The @code{weak} attribute causes the declaration to be emitted as a weak
symbol rather than a global. This is primarily useful in defining
library functions which can be overridden in user code, though it can
also be used with non-function declarations. Weak symbols are supported
for ELF targets, and also for a.out targets when using the GNU assembler
and linker.
@xref{RS/6000 and PowerPC Options}, for more information on whether long
calls are necessary.
@item malloc
@cindex @code{malloc} attribute
......@@ -2276,166 +2277,228 @@ long as the old pointer is never referred to (including comparing it
to the new pointer) after the function returns a non-@code{NULL}
value.
@item alias ("@var{target}")
@cindex @code{alias} attribute
The @code{alias} attribute causes the declaration to be emitted as an
alias for another symbol, which must be specified. For instance,
@item model (@var{model-name})
@cindex function addressability on the M32R/D
@cindex variable addressability on the IA-64
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} is one of @code{small}, @code{medium}, or
@code{large}, representing each of the code models.
Small model objects live in the lower 16MB of memory (so that their
addresses can be loaded with the @code{ld24} instruction), and are
callable with the @code{bl} instruction.
Medium model objects may live anywhere in the 32-bit address space (the
compiler will generate @code{seth/add3} instructions to load their addresses),
and are callable with the @code{bl} instruction.
Large model objects may live anywhere in the 32-bit address space (the
compiler will generate @code{seth/add3} instructions to load their addresses),
and may not be reachable with the @code{bl} instruction (the compiler will
generate the much slower @code{seth/add3/jl} instruction sequence).
On IA-64, use this attribute to set the addressability of an object.
At present, the only supported identifier for @var{model-name} is
@code{small}, indicating addressability via ``small'' (22-bit)
addresses (so that their addresses can be loaded with the @code{addl}
instruction). Caveat: such addressing is by definition not position
independent and hence this attribute must not be used for objects
defined by shared libraries.
@item naked
@cindex function without a prologue/epilogue code
Use this attribute on the ARM, AVR, C4x and IP2K ports to indicate that the
specified function does not need prologue/epilogue sequences generated by
the compiler. It is up to the programmer to provide these sequences.
@item near
@cindex functions which do not handle memory bank switching on 68HC11/68HC12
On 68HC11 and 68HC12 the @code{near} attribute causes the compiler to
use the normal calling convention based on @code{jsr} and @code{rts}.
This attribute can be used to cancel the effect of the @option{-mlong-calls}
option.
@item no_instrument_function
@cindex @code{no_instrument_function} function attribute
@opindex finstrument-functions
If @option{-finstrument-functions} is given, profiling function calls will
be generated at entry and exit of most user-compiled functions.
Functions with this attribute will not be so instrumented.
@item noinline
@cindex @code{noinline} function attribute
This function attribute prevents a function from being considered for
inlining.
@item nonnull (@var{arg-index}, @dots{})
@cindex @code{nonnull} function attribute
The @code{nonnull} attribute specifies that some function parameters should
be non-null pointers. For instance, the declaration:
@smallexample
void __f () @{ /* @r{Do something.} */; @}
void f () __attribute__ ((weak, alias ("__f")));
extern void *
my_memcpy (void *dest, const void *src, size_t len)
__attribute__((nonnull (1, 2)));
@end smallexample
declares @samp{f} to be a weak alias for @samp{__f}. In C++, the
mangled name for the target must be used.
Not all target machines support this attribute.
@noindent
causes the compiler to check that, in calls to @code{my_memcpy},
arguments @var{dest} and @var{src} are non-null. If the compiler
determines that a null pointer is passed in an argument slot marked
as non-null, and the @option{-Wnonnull} option is enabled, a warning
is issued. The compiler may also choose to make optimizations based
on the knowledge that certain function arguments will not be null.
@item visibility ("@var{visibility_type}")
@cindex @code{visibility} attribute
The @code{visibility} attribute on ELF targets causes the declaration
to be emitted with default, hidden, protected or internal visibility.
If no argument index list is given to the @code{nonnull} attribute,
all pointer arguments are marked as non-null. To illustrate, the
following declaration is equivalent to the previous example:
@smallexample
void __attribute__ ((visibility ("protected")))
f () @{ /* @r{Do something.} */; @}
int i __attribute__ ((visibility ("hidden")));
extern void *
my_memcpy (void *dest, const void *src, size_t len)
__attribute__((nonnull));
@end smallexample
See the ELF gABI for complete details, but the short story is:
@item noreturn
@cindex @code{noreturn} function attribute
A few standard library functions, such as @code{abort} and @code{exit},
cannot return. GCC knows this automatically. Some programs define
their own functions that never return. You can declare them
@code{noreturn} to tell the compiler this fact. For example,
@table @dfn
@item default
Default visibility is the normal case for ELF. This value is
available for the visibility attribute to override other options
that may change the assumed visibility of symbols.
@smallexample
@group
void fatal () __attribute__ ((noreturn));
@item hidden
Hidden visibility indicates that the symbol will not be placed into
the dynamic symbol table, so no other @dfn{module} (executable or
shared library) can reference it directly.
void
fatal (/* @r{@dots{}} */)
@{
/* @r{@dots{}} */ /* @r{Print error message.} */ /* @r{@dots{}} */
exit (1);
@}
@end group
@end smallexample
@item protected
Protected visibility indicates that the symbol will be placed in the
dynamic symbol table, but that references within the defining module
will bind to the local symbol. That is, the symbol cannot be overridden
by another module.
The @code{noreturn} keyword tells the compiler to assume that
@code{fatal} cannot return. It can then optimize without regard to what
would happen if @code{fatal} ever did return. This makes slightly
better code. More importantly, it helps avoid spurious warnings of
uninitialized variables.
@item internal
Internal visibility is like hidden visibility, but with additional
processor specific semantics. Unless otherwise specified by the psABI,
GCC defines internal visibility to mean that the function is @emph{never}
called from another module. Note that hidden symbols, while they cannot
be referenced directly by other modules, can be referenced indirectly via
function pointers. By indicating that a symbol cannot be called from
outside the module, GCC may for instance omit the load of a PIC register
since it is known that the calling function loaded the correct value.
@end table
The @code{noreturn} keyword does not affect the exceptional path when that
applies: a @code{noreturn}-marked function may still return to the caller
by throwing an exception.
Not all ELF targets support this attribute.
Do not assume that registers saved by the calling function are
restored before calling the @code{noreturn} function.
@item regparm (@var{number})
@cindex @code{regparm} attribute
@cindex functions that are passed arguments in registers on the 386
On the Intel 386, the @code{regparm} attribute causes the compiler to
pass up to @var{number} integer arguments in registers EAX,
EDX, and ECX instead of on the stack. Functions that take a
variable number of arguments will continue to be passed all of their
arguments on the stack.
It does not make sense for a @code{noreturn} function to have a return
type other than @code{void}.
Beware that on some ELF systems this attribute is unsuitable for
global functions in shared libraries with lazy binding (which is the
default). Lazy binding will send the first call via resolving code in
the loader, which might assume EAX, EDX and ECX can be clobbered, as
per the standard calling conventions. Solaris 8 is affected by this.
GNU systems with GLIBC 2.1 or higher, and FreeBSD, are believed to be
safe since the loaders there save all registers. (Lazy binding can be
disabled with the linker or the loader if desired, to avoid the
problem.)
The attribute @code{noreturn} is not implemented in GCC versions
earlier than 2.5. An alternative way to declare that a function does
not return, which works in the current version and in some older
versions, is as follows:
@item stdcall
@cindex functions that pop the argument stack on the 386
On the Intel 386, the @code{stdcall} attribute causes the compiler to
assume that the called function will pop off the stack space used to
pass arguments, unless it takes a variable number of arguments.
@smallexample
typedef void voidfn ();
@item fastcall
@cindex functions that pop the argument stack on the 386
On the Intel 386, the @code{fastcall} attribute causes the compiler to
pass the first two arguments in the registers ECX and EDX. Subsequent
arguments are passed on the stack. The called function will pop the
arguments off the stack. If the number of arguments is variable all
arguments are pushed on the stack.
volatile voidfn fatal;
@end smallexample
@item cdecl
@cindex functions that do pop the argument stack on the 386
@opindex mrtd
On the Intel 386, the @code{cdecl} attribute causes the compiler to
assume that the calling function will pop off the stack space used to
pass arguments. This is
useful to override the effects of the @option{-mrtd} switch.
@item nothrow
@cindex @code{nothrow} function attribute
The @code{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 @code{qsort} and @code{bsearch} that
take function pointer arguments. The @code{nothrow} attribute is not
implemented in GCC versions earlier than 3.2.
@item longcall/shortcall
@cindex functions called via pointer on the RS/6000 and PowerPC
On the RS/6000 and PowerPC, the @code{longcall} attribute causes the
compiler to always call this function via a pointer, just as it would if
the @option{-mlongcall} option had been specified. The @code{shortcall}
attribute causes the compiler not to do this. These attributes override
both the @option{-mlongcall} switch and the @code{#pragma longcall}
setting.
@item pure
@cindex @code{pure} function attribute
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}. For example,
@xref{RS/6000 and PowerPC Options}, for more information on whether long
calls are necessary.
@smallexample
int square (int) __attribute__ ((pure));
@end smallexample
@item long_call/short_call
@cindex indirect calls on ARM
This attribute specifies how a particular function is called on
ARM@. Both attributes override the @option{-mlong-calls} (@pxref{ARM Options})
command line switch and @code{#pragma long_calls} settings. The
@code{long_call} attribute causes the compiler to always call the
function by first loading its address into a register and then using the
contents of that register. The @code{short_call} attribute always places
the offset to the function from the call site into the @samp{BL}
instruction directly.
@noindent
says that the hypothetical function @code{square} is safe to call
fewer times than the program says.
@item function_vector
@cindex calling functions through the function vector on the H8/300 processors
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.
Some of common examples of pure functions are @code{strlen} or @code{memcmp}.
Interesting non-pure functions are functions with infinite loops or those
depending on volatile memory or other system resource, that may change between
two consecutive calls (such as @code{feof} in a multithreading environment).
You must use GAS and GLD from GNU binutils version 2.7 or later for
this attribute to work correctly.
The attribute @code{pure} is not implemented in GCC versions earlier
than 2.96.
@item interrupt
@cindex interrupt handler functions
Use this attribute on the ARM, AVR, C4x, M32R/D and Xstormy16 ports to indicate
that the specified function is an interrupt handler. The compiler will
generate function entry and exit sequences suitable for use in an
interrupt handler when this attribute is present.
@item regparm (@var{number})
@cindex @code{regparm} attribute
@cindex functions that are passed arguments in registers on the 386
On the Intel 386, the @code{regparm} attribute causes the compiler to
pass up to @var{number} integer arguments in registers EAX,
EDX, and ECX instead of on the stack. Functions that take a
variable number of arguments will continue to be passed all of their
arguments on the stack.
Note, interrupt handlers for the m68k, H8/300, H8/300H, H8S, and SH processors
can be specified via the @code{interrupt_handler} attribute.
Beware that on some ELF systems this attribute is unsuitable for
global functions in shared libraries with lazy binding (which is the
default). Lazy binding will send the first call via resolving code in
the loader, which might assume EAX, EDX and ECX can be clobbered, as
per the standard calling conventions. Solaris 8 is affected by this.
GNU systems with GLIBC 2.1 or higher, and FreeBSD, are believed to be
safe since the loaders there save all registers. (Lazy binding can be
disabled with the linker or the loader if desired, to avoid the
problem.)
Note, on the AVR, interrupts will be enabled inside the function.
@item saveall
@cindex save all registers on the H8/300, H8/300H, and H8S
Use this attribute on the H8/300, H8/300H, and H8S to indicate that
all registers except the stack pointer should be saved in the prologue
regardless of whether they are used or not.
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:
@item section ("@var{section-name}")
@cindex @code{section} function attribute
Normally, the compiler places the code it generates in the @code{text} section.
Sometimes, however, you need additional sections, or you need certain
particular functions to appear in special sections. The @code{section}
attribute specifies that a function lives in a particular section.
For example, the declaration:
@smallexample
void f () __attribute__ ((interrupt ("IRQ")));
extern void foobar (void) __attribute__ ((section ("bar")));
@end smallexample
Permissible values for this parameter are: IRQ, FIQ, SWI, ABORT and UNDEF@.
@noindent
puts the function @code{foobar} in the @code{bar} section.
@item interrupt_handler
@cindex interrupt handler functions on the m68k, H8/300 and SH processors
Use this attribute on the m68k, H8/300, H8/300H, H8S, and SH to indicate that
the specified function is an interrupt handler. The compiler will generate
function entry and exit sequences suitable for use in an interrupt
handler when this attribute is present.
Some file formats do not support arbitrary sections so the @code{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.
@item short_call
See long_call/short_call.
@item shortcall
See longcall/shortcall.
@item signal
@cindex signal handler functions on the AVR processors
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.
@item sp_switch
Use this attribute on the SH to indicate an @code{interrupt_handler}
......@@ -2449,21 +2512,11 @@ void f () __attribute__ ((interrupt_handler,
sp_switch ("alt_stack")));
@end smallexample
@item trap_exit
Use this attribute on the SH for an @code{interrupt_handler} to return using
@code{trapa} instead of @code{rte}. This attribute expects an integer
argument specifying the trap number to be used.
@item eightbit_data
@cindex eight bit data on the H8/300, H8/300H, and H8S
Use this attribute on the H8/300, H8/300H, and H8S to indicate that the specified
variable should be placed into the eight bit data section.
The compiler will generate more efficient code for certain operations
on data in the eight bit data area. Note the eight bit data area is limited to
256 bytes of data.
You must use GAS and GLD from GNU binutils version 2.7 or later for
this attribute to work correctly.
@item stdcall
@cindex functions that pop the argument stack on the 386
On the Intel 386, the @code{stdcall} attribute causes the compiler to
assume that the called function will pop off the stack space used to
pass arguments, unless it takes a variable number of arguments.
@item tiny_data
@cindex tiny data section on the H8/300H and H8S
......@@ -2473,139 +2526,98 @@ 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.
@item saveall
@cindex save all registers on the H8/300, H8/300H, and H8S
Use this attribute on the H8/300, H8/300H, and H8S to indicate that
all registers except the stack pointer should be saved in the prologue
regardless of whether they are used or not.
@item signal
@cindex signal handler functions on the AVR processors
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.
@item naked
@cindex function without a prologue/epilogue code
Use this attribute on the ARM, AVR, C4x and IP2K ports to indicate that the
specified function does not need prologue/epilogue sequences generated by
the compiler. It is up to the programmer to provide these sequences.
@item model (@var{model-name})
@cindex function addressability on the M32R/D
@cindex variable addressability on the IA-64
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} is one of @code{small}, @code{medium}, or
@code{large}, representing each of the code models.
Small model objects live in the lower 16MB of memory (so that their
addresses can be loaded with the @code{ld24} instruction), and are
callable with the @code{bl} instruction.
Medium model objects may live anywhere in the 32-bit address space (the
compiler will generate @code{seth/add3} instructions to load their addresses),
and are callable with the @code{bl} instruction.
Large model objects may live anywhere in the 32-bit address space (the
compiler will generate @code{seth/add3} instructions to load their addresses),
and may not be reachable with the @code{bl} instruction (the compiler will
generate the much slower @code{seth/add3/jl} instruction sequence).
@item trap_exit
Use this attribute on the SH for an @code{interrupt_handler} to return using
@code{trapa} instead of @code{rte}. This attribute expects an integer
argument specifying the trap number to be used.
On IA-64, use this attribute to set the addressability of an object.
At present, the only supported identifier for @var{model-name} is
@code{small}, indicating addressability via ``small'' (22-bit)
addresses (so that their addresses can be loaded with the @code{addl}
instruction). Caveat: such addressing is by definition not position
independent and hence this attribute must not be used for objects
defined by shared libraries.
@item unused
@cindex @code{unused} attribute.
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.
@item far
@cindex functions which handle memory bank switching
On 68HC11 and 68HC12 the @code{far} attribute causes the compiler to
use a calling convention that takes care of switching memory banks when
entering and leaving a function. This calling convention is also the
default when using the @option{-mlong-calls} option.
@item used
@cindex @code{used} attribute.
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.
On 68HC12 the compiler will use the @code{call} and @code{rtc} instructions
to call and return from a function.
@item visibility ("@var{visibility_type}")
@cindex @code{visibility} attribute
The @code{visibility} attribute on ELF targets causes the declaration
to be emitted with default, hidden, protected or internal visibility.
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}.
At the end of a function, it will jump to a board-specific routine
instead of using @code{rts}. The board-specific return routine simulates
the @code{rtc}.
@smallexample
void __attribute__ ((visibility ("protected")))
f () @{ /* @r{Do something.} */; @}
int i __attribute__ ((visibility ("hidden")));
@end smallexample
@item near
@cindex functions which do not handle memory bank switching on 68HC11/68HC12
On 68HC11 and 68HC12 the @code{near} attribute causes the compiler to
use the normal calling convention based on @code{jsr} and @code{rts}.
This attribute can be used to cancel the effect of the @option{-mlong-calls}
option.
See the ELF gABI for complete details, but the short story is:
@item dllimport
@cindex @code{__declspec(dllimport)}
On Microsoft Windows targets, the @code{dllimport} attribute causes the compiler
to reference a function or variable via a global pointer to a pointer
that is set up by the Microsoft Windows dll library. The pointer name is formed by
combining @code{_imp__} and the function or variable name. The attribute
implies @code{extern} storage.
@table @dfn
@c keep this list of visibilies in alphabetical order.
Currently, the attribute is ignored for inlined functions. If the
attribute is applied to a symbol @emph{definition}, an error is reported.
If a symbol previously declared @code{dllimport} is later defined, the
attribute is ignored in subsequent references, and a warning is emitted.
The attribute is also overridden by a subsequent declaration as
@code{dllexport}.
@item default
Default visibility is the normal case for ELF. This value is
available for the visibility attribute to override other options
that may change the assumed visibility of symbols.
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.
@item hidden
Hidden visibility indicates that the symbol will not be placed into
the dynamic symbol table, so no other @dfn{module} (executable or
shared library) can reference it directly.
On cygwin, mingw and arm-pe targets, @code{__declspec(dllimport)} is
recognized as a synonym for @code{__attribute__ ((dllimport))} for
compatibility with other Microsoft Windows compilers.
@item internal
Internal visibility is like hidden visibility, but with additional
processor specific semantics. Unless otherwise specified by the psABI,
GCC defines internal visibility to mean that the function is @emph{never}
called from another module. Note that hidden symbols, while they cannot
be referenced directly by other modules, can be referenced indirectly via
function pointers. By indicating that a symbol cannot be called from
outside the module, GCC may for instance omit the load of a PIC register
since it is known that the calling function loaded the correct value.
The use of the @code{dllimport} 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} attribute on imported variables was
required on older versions of GNU ld, but can now be avoided by passing
the @option{--enable-auto-import} switch to ld. As with functions, using
the attribute for a variable eliminates a thunk in the dll.
@item protected
Protected visibility indicates that the symbol will be placed in the
dynamic symbol table, but that references within the defining module
will bind to the local symbol. That is, the symbol cannot be overridden
by another module.
One drawback to using this attribute is that a pointer to a function or
variable marked as dllimport cannot be used as a constant address. The
attribute can be disabled for functions by setting the
@option{-mnop-fun-dllimport} flag.
@end table
@item dllexport
@cindex @code{__declspec(dllexport)}
On Microsoft Windows targets the @code{dllexport} 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} attribute. The pointer name is
formed by combining @code{_imp__} and the function or variable name.
Not all ELF targets support this attribute.
Currently, the @code{dllexport}attribute is ignored for inlined
functions, but export can be forced by using the
@option{-fkeep-inline-functions} flag. The attribute is also ignored for
undefined symbols.
@item warn_unused_result
@cindex @code{warn_unused_result} attribute
The @code{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
@code{realloc}.
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.
@smallexample
int fn () __attribute__ ((warn_unused_result));
int foo ()
@{
if (fn () < 0) return -1;
fn ();
return 0;
@}
@end smallexample
On cygwin, mingw and arm-pe targets, @code{__declspec(dllexport)} is
recognized as a synonym for @code{__attribute__ ((dllexport))} for
compatibility with other Microsoft Windows compilers.
results in warning on line 5.
Alternative methods for including the symbol in the dll's export table
are to use a .def file with an @code{EXPORTS} section or, with GNU ld,
using the @option{--export-all} linker flag.
@item weak
@cindex @code{weak} attribute
The @code{weak} attribute causes the declaration to be emitted as a weak
symbol rather than a global. This is primarily useful in defining
library functions which can be overridden in user code, though it can
also be used with non-function declarations. Weak symbols are supported
for ELF targets, and also for a.out targets when using the GNU assembler
and linker.
@end table
......
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