getting "implicit declaration of function 'fcloseall' is invalid in C99" when compiling to gnu99

Question

Consider the following C code:

#include <stdio.h>
#include <stdlib.h>

void fatal(const char* message){
 /*
  Prints a message and terminates the program.
  Closes all open i/o streams before exiting.
 */
 printf("%s\n", message);
 fcloseall();
 exit(EXIT_FAILURE);
}

I'm using clang 2.8 to compile: clang -Wall -std=gnu99 -o <executable> <source.c>

And get: implicit declaration of function 'fcloseall' is invalid in C99

Which is true, but i'm explicitly compiling to gnu99 [which should support fcloseall()], and not to c99. Although the code runs, I don't like to have unresolved warnings when compiling. How can i solve this?

Edit: corrected tipo.

Answer 1

To include non-standard extensions when you include standard headers you need to define the appropriate feature test macro. In this case _GNU_SOURCE should work.

#define _GNU_SOURCE
#include <stdio.h>

This is independent of -std=gnu99 which enables language extensions, not library extensions.