write() not reaching child process

wasp · 07-07-2006, 03:33 AM

Code:

void die()
{
  perror("fatal");
  _exit(112);
}

int main()
{
  int pfds[2];
  int pid;
  int stat;
  char *args[5];

  if (pipe(pfds) == -1) die();

  switch (pid = fork()) {
    case -1:
      die();
      break;
    case 0:
      if (close(0) == -1) _exit(112);
      if (dup2(pfds[0], 0) == -1) _exit(112);
      args[0] = "/usr/bin/sort";
      args[1] = 0;
      if (execve(args[0], args, 0) == -1) _exit(112);
      break;
   default:
      if (write(pfds[1], "a\nb\nc\n", 6) == -1) die();
      if (waitpid(pid, &stat, 0) == -1) die();
      break;
  }

  return 0;
}

The write() call is sucessful, but no output is seen from
/usr/bin/sort and the program hangs. I'm guessing that
sort waits for input on stdin but for some reason never
receives it. The parent process sits at the waitpid()
call forever.

What did I do wrong?

debulu · 07-07-2006, 06:16 AM

Code:

  Try this modified code:

void die()
{
  perror("fatal");
  exit(112);
}

int main()
{
  int pfds[2];
  int pid;
  int stat;
  char *args[5];

  if (pipe(pfds) == -1) die();

  switch (pid = fork()) {
    case -1:
      die();
      break;
    case 0:
      if (close(0) == -1) die();
      if (dup2(pfds[0], 0) == -1) die();
      close(pfds[0]);
      close(pfds[1]);
      args[0] = "/bin/sort";
      args[1] = 0;
      if (execve(args[0], args, 0) == -1) die();
   default:
      close(pfds[0]);
      if (write(pfds[1], "a\nb\nc\n", 6) == -1) die();
      close(pfds[1]);
      if (waitpid(pid, &stat, 0) == -1) die();
      break;
  }

  return 0;
}

From the code you must be understanding that You have to close the unused Fds. In child You have to close both read and write as you are duplicating the read as stdin.In parent you have to close read fd first and then close the write fd.
NB: My machine sort is in /bin
enjoy

wasp · 07-07-2006, 04:01 PM

Thanks, it works.