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One of the best (but little known) features of GNU C is the __attribute__ mechanism, which allows a developer to attach characteristics to function declarations to allow the compiler to perform more error checking. It was designed in a way to be compatible with non-GNU implementations, and we've been using this for years in highly portable code with very good results.

Table of Contents
  1. __attribute unused
  2. __attribute__ format
  3. __attribute__ noreturn
  4. __attribute__ const
  5. Putting them together
  6. Compatibility with non-GNU compilers
  7. Other References

Note that __attribute__ spelled with two underscores before and two after, and there are always two sets of parentheses surrounding the contents. There is a good reason for this - see below. Gnu CC needs to use the -Wall compiler directive to enable this (yes, there is a finer degree of warnings control available, but we are very big fans of max warnings anyway).


__attribute__ unused

One of the easiest attributes to use, this marks a variable as intentionally being possibly unused. Not only does this quiet the compiler from issuing an unused-variable warning, it tells the human the same thing: this is intentional.

Of course, it's a good idea to actually remove variables that you're not using, this is not always possible. A common case of the unused int argc parameter to main() is one, as are variables sometimes excluded by conditional compilation.

int main(int argc, char **argv)
{
   /* code that uses argv, but not argc */
}

$ gcc -W test.c
test.c:1: warning: unused parameter 'argc'

The __attribute__ is added just after the variable name, and though it can appear unwieldy, it's a style you can get used to:

int main(int argc __attribute__((unused)), char **argv)
{ ...

Additional uses shown, each with a comment showing the compiler warning it might have generated. Here we assume the code is being compiled without the DEBUG macro being defined, which excludes the actual use of mypid.

/* warning: 'someFunction' declared 'static' but never defined */
static int someFunction() __attribute__((unused));

int main(int argc __attribute__((unused)), char **argv)
{
/* warning: unused variable 'mypid' */
int	mypid __attribute__((unused)) = getpid();

#ifdef DEBUG
	printf("My PID = %d\n", mypid);
#endif

	return 0;
}

__attribute__ format

This __attribute__ allows assigning printf-like or scanf-like characteristics to the declared function, and this enables the compiler to check the format string against the parameters provided throughout the code. This is exceptionally helpful in tracking down hard-to-find bugs.

There are two flavors:

but in practice we use the first one much more often.

The (m) is the number of the "format string" parameter, and (n) is the number of the first variadic parameter. To see some examples:

/* like printf() but to standard error only */
extern void eprintf(const char *format, ...)
	__attribute__((format(printf, 1, 2)));  /* 1=format 2=params */

/* printf only if debugging is at the desired level */
extern void dprintf(int dlevel, const char *format, ...)
	__attribute__((format(printf, 2, 3)));  /* 2=format 3=params */

With the functions so declared, the compiler will examine the argument lists

$ cat test.c
1  extern void eprintf(const char *format, ...)
2               __attribute__((format(printf, 1, 2)));
3
4  void foo()
5  {
6      eprintf("s=%s\n", 5);             /* error on this line */
7
8      eprintf("n=%d,%d,%d\n", 1, 2);    /* error on this line */
9  }

$ cc -Wall -c test.c
test.c: In function `foo':
test.c:6: warning: format argument is not a pointer (arg 2)
test.c:8: warning: too few arguments for format

Note that the "standard" library functions - printf and the like - are already understood by the compiler by default.

__attribute__ noreturn

This attribute tells the compiler that the function won't ever return, and this can be used to suppress errors about code paths not being reached. The C library functions abort() and exit() are both declared with this attribute:

extern void exit(int)   __attribute__((noreturn));
extern void abort(void) __attribute__((noreturn));

Once tagged this way, the compiler can keep track of paths through the code and suppress errors that won't ever happen due to the flow of control never returning after the function call.

In this example, two nearly-identical C source files refer to an "exitnow()" function that never returns, but without the __attribute__ tag, the compiler issues a warning. The compiler is correct here, because it has no way of knowing that control doesn't return.

$ cat test1.c
extern void exitnow();

int foo(int n)
{
        if ( n > 0 )
	{
                exitnow();
		/* control never reaches this point */
	}
        else
                return 0;
}

$ cc -c -Wall test1.c
test1.c: In function `foo':
test1.c:9: warning: this function may return with or without a value

But when we add __attribute__, the compiler suppresses the spurious warning:

$ cat test2.c
extern void exitnow() __attribute__((noreturn));

int foo(int n)
{
        if ( n > 0 )
                exitnow();
        else
                return 0;
}

$ cc -c -Wall test2.c
no warnings!

__attribute__ const

This attribute marks the function as considering only its numeric parameters. This is mainly intended for the compiler to optimize away repeated calls to a function that the compiler knows will return the same value repeatedly. It applies mostly to math functions that have no static state or side effects, and whose return is solely determined by the inputs.

In this highly-contrived example, the compiler normally must call the square() function in every loop even though we know that it's going to return the same value each time:

extern int square(int n) __attribute__((const));

...
	for (i = 0; i < 100; i++ )
	{
		total += square(5) + i;
	}

By adding __attribute__((const)), the compiler can choose to call the function just once and cache the return value.

In virtually every case, const can't be used on functions that take pointers, because the function is not considering just the function parameters but also the data the parameters point to, and it will almost certainly break the code very badly in ways that will be nearly impossible to track down.

Furthermore, the functions so tagged cannot have any side effects or static state, so things like getchar() or time() would behave very poorly under these circumstances.

Putting them together

Multiple __attributes__ can be strung together on a single declaration, and this is not uncommon in practice. You can either use two separate __attribute__s, or use one with a comma-separated list:

/* send printf-like message to stderr and exit */
extern void die(const char *format, ...)
	__attribute__((noreturn))
	__attribute__((format(printf, 1, 2)));

/*or*/

extern void die(const char *format, ...)
	__attribute__((noreturn, format(printf, 1, 2)));

If this is tucked away safely in a library header file, all programs that call this function receive this checking.

Compatibility with non-GNU compilers

Fortunately, the __attribute__ mechanism was cleverly designed in a way to make it easy to quietly eliminate them if used on platforms other than GNU C. Superficially, __attribute__ appears to have multiple parameters (which would typically rule out using a macro), but the two sets of parentheses effectively make it a single parameter, and in practice this works very nicely.

/* If we're not using GNU C, elide __attribute__ */
#ifndef __GNUC__
#  define  __attribute__(x)  /*NOTHING*/
#endif

Note that __attribute__ applies to function declarations, not definitions, and we're not sure why this is. So when defining a function that merits this treatment, an extra declaration must be used (in the same file):

/* function declaration */
void die(const char *format, ...) __attribute__((noreturn))
                                  __attribute__((format(printf,1,2)));

void die(const char *format, ...)
{
	/* function definition */
}

Other References

We'll note that there are many more attributes available, including those for variables and types, and they are not covered here: we have chosen to just touch on the high points. Those wishing more information can find it in the GNU online documentation at http://gcc.gnu.org:

GCC 4.0
GCC 4.0 Function Attributes
GCC 4.0 Variable Attributes
GCC 4.0 Type Attributes
GCC 3.2
GCC 3.2 Function Attributes
GCC 3.2 Variable Attributes
GCC 3.2 Type Attributes
GCC 3.1
GCC 3.1 Function Attributes
GCC 3.1 Variable Attributes
GCC 3.1 Type Attributes
GCC 3.0.4
Function Attributes
Variable Attributes
Type Attributes
GCC 2.95.3
Function Attributes
Variable Attributes
Type Attributes