Contents|Index|Previous|Next
Declaring
Attributes of Functions
In GNU C, you declare certain
things about functions called in your program which help the compiler optimize
function calls and check your code more carefully.
The keyword, __attribute__,
allows you to specify special attributes when making a declaration. This
keyword is followed by an attribute specification inside double parentheses.
Eight attributes, noreturn,
const,
format,
section,
constructor,
destructor,
unused
and weak
are currently defined for functions. Other attributes, including section
are supported for variables declarations (see Specifying
Attributes of Variables) and for types (see Specifying
Attributes of Types).
You may also specify attributes
with ‘__’
preceding and following each keyword. This allows you to use them in header
files without being concerned about a possible macro of the same name.
For example, you may use
__noreturn__ instead
of noreturn.
noreturn
A few standard library functions, such as abort
and exit,
cannot return. GNU CC knows this automatically. Some programs define their
own functions that never return. You can declare them noreturn
to tell the compiler this fact as the following example shows.
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.
Do not assume that registers
saved by the calling function are restored before calling the noreturn
function. It does not make sense for a noreturn
function to have a return type other than void.
The attribute noreturn
is not implemented in GNU C versions earlier than 2.5.
An alternative way to declare
that a function does not return, which works in the current version and
in some older versions, is as follows:
typedef void voidfn ();
volatile voidfn fatal;
const
Many functions
do not examine any values except their arguments, and have no effects except
the return value. 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, const.
For example, the following says that the hypothetical function, square,
is safe to call fewer times than the program says.
int square (int) __attribute__ ((const));
The attribute, const,
is not implemented in GNU C versions earlier than 2.5. An alternative way
to declare that a function has no side effects, which works in the current
version and in some older versions, is as follows.
typedef int intfn ();
extern const intfn square;
This approach does not work
in GNU C++ from 2.6.0 on, since the language specifies that const
must be attached to the return value.
- Note:
A function that has pointer arguments and examines the data pointed to
must not be declared const.
Likewise, a function that calls a non-const
function usually must not be const.
It does not make sense for a const
function to return void.
format (archetype, string-index,
first-to-check)
The format
attribute specifies that a function takes printf
or scanf
style arguments which should be type-checked against a format string. For
example, the following declaration causes the compiler to check the arguments
in calls to my_ printf
for consistency with the printf
style format string argument, my_format.
extern int
my_printf (void *my_object, const char *my_format, ...)
__attribute__ ((format (printf, 2, 3)));
The parameter, archetype,
determines how the format string is interpreted, and should be either printf
or scanf.
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.
In the previous example,
the format
string (my_format)is
the second argument of the function my_print,
and the arguments to check start with the third argument, so the correct
parameters for the format
attribute are 2 and 3.
The format
attribute allows you to identify your own functions which take format
strings as arguments, so that GNU CC can check the calls to these functions
for errors. The compiler always checks formats for the ANSI library functions
printf,
fprintf,
sprintf,
scanf,
fscanf,
sscanf,
vprintf,
vfprintf
and vsprintf
whenever such warnings are re-quested (using ‘-Wformat’),
so there is no need to modify the header file, ‘stdio.h’.
format_arg (string-index)
The format_arg
attribute specifies that a function takes printf
or scanf
style arguments, modifies it (for example, to translate it into another
language), and passes it to a printf
or scanf
style function. For example, the following declaration causes the compiler
to check the arguments in calls to my_
dgettext, whose
result is passed to a printf
or scanf
type function for consistency with the printf
style format string argument, my_format.
extern char *
my_dgettext (char *my_domain, const char *my_format)
__attribute__ ((format_arg (2)));
The parameter, string-index,
specifies which argument is the format string argument (starting from 1).
The format-arg
attribute allows you to identify your own functions which modify format
strings, so that GNU CC can check the calls to printf
and scanf
function whose operands are a call to one of your own function. The compiler
always treats gettext,
dgettext,
and dcgettext
in this manner.
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 following declaration puts the function, foobar,
in the bar
section.
extern void foobar (void) __attribute__
\
((section
("bar")));
Some file formats do not
support arbitrary sections so the section
attribute is not available on all platforms. If you need to map the entire
contents of a module to a particular section, consider using the facilities
of the linker instead.
constructor
destructor
The constructor
attribute causes the function to be called automatically before execution
enters main().
Similarly, the destructor attribute causes the function to be called automatically
after main()
has completed or exit()
has been called. Functions with these attributes are useful for initializing
data that will be used implicitly during the execution of the program.
These attributes are not currently implemented for Objective C.
unused
This attribute,
attached to a function, means that the function is meant to be possibly
unused. GNU CC will not produce a warning for this function. GNU C++ does
not currently support this attribute as definitions without parameters
are valid in C++.
weak
The weak
attribute causes the declaration to be emitted as a weak symbol rather
than a global. This is primarily useful in defining library functions which
can be overridden in user code, though it can also be used with non-function
declarations. Weak symbols are supported for ELF targets, and also for
a.out targets when using the GNU assembler and linker.
alias ("target")
The alias attribute
causes the declaration to be emitted as an alias for another symbol, which
must be specified. For instance, the following declares ‘f’
to be a weak alias for ‘__f’.
In C++, the mangled name for the target must be used.
void __f () { /* do something */; }
void f () __attribute__ ((weak, alias ("__f")));
Not all target machines
support this attribute.
regparm (number)
On the Intel 386,
the regparm
attribute causes the compiler to pass up to number
integer arguments in registers EAX,
EDX,
and ECX
instead of on the stack. Functions that take a variable number of arguments
will continue to be passed all of their arguments on the stack.
stdcall
On the Intel 386,
the stdcall
attribute causes the compiler to assume that the called function will pop
off the stack space used to pass arguments, unless it takes a variable
number of arguments. The PowerPC compiler for Windows NT currently ignores
the stdcall
attribute.
cdecl
On the Intel 386,
the cdecl
attribute causes the compiler to assume that the called function will pop
off the stack space used to pass arguments. This is useful to override
the effects of the switch,
‘-mrtd’.
The PowerPC compiler for Windows NT currently ignores the cdecl
attribute.
longcall
On the RS/6000
and PowerPC, the longcall
attribute causes the compiler to always call the function via a pointer,
so that functions which reside further than 64 megabytes (67,108,864 bytes)
from the current location can be called.
dllimport
On the PowerPC
running Windows NT, the dllimport
attribute causes the compiler to call the function via a global pointer
to the function pointer that is set up by the Windows NT dll library. The
pointer name is formed by combining __imp_
and the function name.
dllexport
On the PowerPC
running Windows NT, the dllexport
attribute causes the compiler to provide a global pointer to the function
pointer, so that it can be called with the dllimport
attribute. The pointer name is formed by combining __imp_
and the function name.
exception (except-func[,
except-arg])
On the PowerPC running Windows NT, the exception
attribute causes the compiler to modify the structured exception table
entry it emits for the declared function. The string or identifier, except-func,
is placed in the third entry of the structured exception table. It represents
a function which is called by the exception handling mechanism if an exception
occurs. If it was specified, the string or identifier, except-arg,
is placed in the fourth entry of the structured exception table.
function_vector
Use this option on the H8/300 and H8/300H 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 shares space with the interrupt vector.
You must use GAS and GLD
from GNU binutils version 2.7 or later for this option to work correctly.
interrupt_handler
Use this option on the H8/300 and H8/300H 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.
eightbit_data
Use this option on the H8/300 and H8/300H 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 option to work correctly.
tiny_data
Use this option on the H8/300H to indicate that the specified variable
should be placed into the tiny data section. The compiler will generate
more efficient code for loads and stores on data in the tiny data section.
Note the tiny data area is limited to slightly under 32kbytes of data.
You can specify multiple
attributes in a declaration by separating them by commas within the double
parentheses or by immediately following an attribute declaration with another
attribute declaration.
Some people object to the
__attribute__
feature, suggesting that ANSI C’s #pragma
should be used instead. There are two reasons for not doing this:
- It is impossible to generate
#pragma
commands from a macro.
- There is no telling what
the same #pragma
might mean in another compiler.
These two reasons apply
to almost any application that might be proposed for #pragma.
It is basically a mistake to use #pragma
for anything.