Compiler directive for definition of popen?

[Svante Signell <svante.signell@telia.com>]

Which compiler directive to use to get the definition of popen included
in the file. It does not seem to be included by default, and
-D__USE_POSIX2 etc does not work either. Have been googling around but
not found anything usable so far.

popen is NOT defined: gcc -ansi -Wall -pedantic test.c
test.c: In function 'open_gnuplot':
test.c:25: warning: implicit declaration of function 'popen'

popen is defined: gcc -Wall -pedantic test.c

In the test case below no runtime error occurs but in the real
application is does!

Different compilers have been tried: 2.95,3.3,3.4,4.0,4.1

test.c:
#include <stdio.h>
                                                                                 
#define SIX 1 + 5
#define NINE 8 + 1
                                                                                 
int main(void)
{
  FILE  *open_gnuplot();
  static FILE *GNU = {0}; /* Pointer to gnuplot commands */
  GNU = open_gnuplot();
  fprintf(GNU,"plot sin(x)/x\n");
  printf( "The meaning of life: %d\n", SIX * NINE );
  getchar();
  return 0;
}

FILE *open_gnuplot()
{
  FILE *fileptr;
  if(!(fileptr=(FILE *)popen("gnuplot -persist","w"))) {
    printf("Unable to open pipe to gnuplot for writing\n");
    return NULL;
  }
    return fileptr;
}

Please cc: me since I'm not subscribed to this list.

Thanks,
-- 
Svante Signell <svante.signell@telia.com>