F_SETPIPE_SZ uneclared

I have included the following headers:

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

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>

      

        
        
        
      

    

I also tried to use

#define _GNU_SOURCE

      

        
        
        
      

    

before #include <unistd.h>

, but that doesn't help either.

I'm trying to use fcntl

and pass it F_SETPIPE_SZ

as the second argument, but I keep getting this error message:

error: 'F_SETPIPE_SZ uneclared (use in this function first)

I actually found out that I don't need this, but I'm just wondering why I can't use it.

Thank.

So here's the solution, thanks to Chrono Kitsune: Placed

 #define _GNU_SOURCE

      

        
        
        
      

    

before turning on.