Demonstrate that stdin/stdout/stderr are only conventions.

test.c

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

// Demonstrate, that stdin, stdout, stderr are only conventional. We run a child
// process which doesn't have them open and open a random file to demonstrate,
// that it will just use one of those numbers.
//
// Works only under linux (but same principles apply on other unices) and needs
// to be started without parameters.
int main(int argc, char **argv) {
  if (argc > 1) {
    // Let's just open any file we know exists.
    int fd = open("/tmp", O_RDONLY);
    // We need to somehow signal what this new opened file has as an fd. We
    // can't print, as stdout is potentially closed, so we use the exit status
    // instead to signal an int to the parent, which will then print it.
    return fd;
  }

  // Fork off a child process.
  if (fork() == 0) {
    // Close stdin, stdout, stderr. You can see the effect of closing only some
    // of them by commenting these lines out partly.
    close(0);
    close(1);
    close(2);

    // Self-exec (only works under linux). Pass a parameter, so that the child
    // knows it's the child
    execl("/proc/self/exe", argv[0], "-child", NULL);
    // Should never be reached. If exit code is 255, something went wrong.
    return 255;
  }

  // Wait for child to exit and print exit status.
  int status;
  wait(&status);
  printf("%d\n", WEXITSTATUS(status));
  return 0;
}