Check if stdout is redirected to /dev/null

test-null-stdout.c

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <limits.h>
#include <stdlib.h>

int is_stdout_null() {
  char path[PATH_MAX], link[PATH_MAX];
  sprintf(path, "/proc/self/fd/%d", STDOUT_FILENO);

  ssize_t r;
  r = readlink(path, link, sizeof(link));
  if (r < 0) {
    return r;
  }

  link[r] = '\0';

  return strcmp(link, "/dev/null");
}

int main(void) {
  if (is_stdout_null() == 0) {
    fprintf(stderr, "stdout is redirected to /dev/null");
  }

  exit(EXIT_SUCCESS);
}