====== تابع ()fprintf و ()dprintf برای نوشتن در stdout و stderr ======

<code C>
// man 3 printf
/* fprintf() write output to the given stdio(3) output stream */
int fprintf(FILE *restrict stream, const char *restrict format, ...);

/* dprintf() write output to the given file descriptor */
int dprintf(int fd, const char *restrict format, ...);

dprintf():
    Since glibc 2.10:
        _POSIX_C_SOURCE >= 200809L
    Before glibc 2.10:
        _GNU_SOURCE
</code>

<code C>
/* output.c */
#define _POSIX_C_SOURCE 200809L
#include <stdio.h>

int main(void)
{
   printf("A regular message on stdout\n");

   /* Using streams with fprintf() */
   fprintf(stdout, "Also a regular message on "
      "stdout\n");
   fprintf(stderr, "An error message on stderr\n");

   /* Using file descriptors with dprintf().
    * This requires _POSIX_C_SOURCE 200809L
    * (man 3 dprintf)*/

   /* /dev/stdout -> /proc/self/fd/1 */
   dprintf(1, "A regular message, printed to "
      "fd 1\n");

   /* /dev/stderr -> /proc/self/fd/2 */
   dprintf(2, "An error message, printed to "
      "fd 2\n");

   return 0;
}
</code>

  $ make output
  cc     output.c   -o output

  $ ./output
  A regular message on stdout
  Also a regular message on stdout
  An error message on stderr
  A regular message, printed to fd 1
  An error message, printed to fd 2

  $ ./output  2> /dev/null
  A regular message on stdout
  Also a regular message on stdout
  A regular message, printed to fd 1

  $ ./output  > /dev/null
  An error message on stderr
  An error message, printed to fd 2

  $ ./output  &> /dev/null