Code hay quá (không phải của tôi) nên lưu lại để nhớ

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

#define FIRST_PRIME_NUMBER 2
#define MAX_PRIME_NUMBER 1000

void
child_process (int parentfd)
{
  pid_t pid;
  int buf, divider = 0, len, pipefd[2], childfd = -1;

  while (1)
    {
      len = read (parentfd, &buf, sizeof (buf));
      if (divider == 0)
        {
          divider = buf;
          printf ("%d\n", divider);
        }
      else if (buf < 0)
        {
          if (childfd != -1)
            write (childfd, &buf, sizeof (buf));
          break;
        }
      else if (buf % divider != 0)
        {
          if (childfd == -1)
            {
              if (pipe (pipefd) < 0)
                break;
              pid = fork ();
              if (pid == 0)
                {
                  parentfd = pipefd[0];
                  close (pipefd[1]);
                  divider = 0;
                }
              else
                {
                  childfd = pipefd[1];
                  close (pipefd[0]);
                }
            }
          if (childfd != -1)
            write (childfd, &buf, sizeof (buf));
        }
    }

  return;
}

int
main ()
{
  pid_t pid;
  int pipefd[2], i;

  if (pipe (pipefd) < 0)
    {
      fprintf (stderr, "failed to initialize pipe");
      return -1;
    }

  pid = fork ();
  if (pid == 0)
    {
      close (pipefd[1]);
      child_process (pipefd[0]);
    }
  else
    {
      close (pipefd[0]);
      for (i = FIRST_PRIME_NUMBER; i <= MAX_PRIME_NUMBER; i++)
        {
          write (pipefd[1], &i, sizeof (i));
        }

      i = -1;
      write (pipefd[1], &i, sizeof (i));
    }

  wait (NULL);

  return 0;
}