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

static void
fatal (register char const *const msg)
{
  fputs (msg, stderr);
  fputc ('\n', stderr);
  exit (2);
}

static void
fatal_perror (register char const *const msg)
{
  perror (msg);
  exit (2);
}

static int
get_fd_flags (register int const fd)
{
  register int result;

  if ((result = fcntl (fd, F_GETFL)) == -1)
    fatal_perror ("Error in fcntl()");
  return result;
}

static void
set_fd_flags (register int const fd, register int const flags)
{
  if (fcntl (fd, F_SETFL, flags) == -1)
    fatal_perror ("Error in fcntl()");
}

int
main (void)
{
  register int fd;
  register int flags;

  {
    enum { fdmode = 0600 };
    auto char fd_path[80];

    sprintf (fd_path, "/tmp/pid-%u-up.mq", getpid ());

#if defined(TRY_FIFO)
    if (mkfifo (fd_path, fdmode) == -1)
      fatal_perror ("Error in mkfifo()");
    enum { fdflags = O_RDWR | O_NONBLOCK };
#else
    enum { fdflags = O_CREAT | O_EXCL | O_RDWR | O_NONBLOCK };
#endif
    if ((fd = open (fd_path, fdflags, fdmode)) == -1)
      fatal_perror ("Error in open()");

    if (unlink (fd_path) == -1)
      fatal_perror ("Error in unlink()");
  }

  if (((flags = get_fd_flags (fd)) & O_NONBLOCK) == 0)
    fatal ("O_NONBLOCK magically unset before fork");

  if (!fork ())
    {
      /* child */
      set_fd_flags (fd, 0);
      sleep (5);
    }
  else
    {
      /* parent */
      sleep (1);
      if (((flags = get_fd_flags (fd)) & O_NONBLOCK) == 0)
        fatal ("O_NONBLOCK magically unset in parent");
    }

  return 0;
}