/* Adapted from
   https://sourceware.org/pipermail/cygwin/2020-March/244219.html */

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

#define FIFO "/tmp/myfifo"
#define nsubproc  5
#define nmessages 4
#define pid_len   4

int
error (const int n, const char *name)
{
  fprintf (stderr, "\n%d\t%s:\t%d\t%s\n", getpid (), name, n, strerror (n));
  return n;
}

int
main ()
{
  if (mkfifo (FIFO, S_IRUSR | S_IWUSR | S_IWGRP) < 0
      && errno != EEXIST)
    return error (errno, "mkfifo");

  int rfd = open (FIFO, O_RDWR);
  if (rfd < 0)
    return error (errno, "open reader");

  for (int i = 0; i < nsubproc; i++)
    {
      pid_t pid = fork ();

      if (pid < 0)
	return error (errno, "fork");

      if (pid == 0)
	{
	  printf ("child %d\n", getpid ());
	  for (int j = 0; j < nmessages; j++)
	    {
	      char buf[pid_len + 2]; /* +1 for newline, +1 for nul */

	      int wfd = open (FIFO, O_WRONLY | O_NONBLOCK);
	      if (wfd < 0)
		_exit (error (errno, "open writer"));
	      sprintf (buf, "%d\n", getpid ());
	      ssize_t nwritten = write (wfd, buf, strlen (buf));
	      if (nwritten < 0)
		error (errno, "write");
	      /* printf ("i = %d, j = %d, nwritten = %zd\n", i, j, nwritten); */
	      close (wfd);
	    }
	  _exit (0);
	}
    }

  printf ("parent\n");
  char buf[pid_len + 2];
  for (int i = 0; i < nsubproc; i++)
    for (int j = 0; j < nmessages; j++)
      {
	if (read (rfd, buf, nsubproc) < 0)
	  error (errno, "read");
	buf[pid_len + 1] = '\0';
	printf ("%s", buf);
      }
  close (rfd);
}