Communicating via pipes in C

Question

I am trying to write this little program, where the parent and the child communicate with each other via pipes, the code here works, unless you 'uncomment' the commented lines, than it comes to some sort of deadlock, and I cannot figure it out why? Any ideas?

int main(int argc, char **argv){

  int fd[2];
  int fd2[2];
  pid_t pid;
  pipe(fd);
  pipe(fd2);
  pid = fork();

  if(pid==0){
      close(fd[1]);
      dup2(fd[0],fileno(stdin));
      close(fd2[0]);
      FILE *output = fdopen(fd2[1],"w");
      char buffer[255];
      while(fgets(buffer,255,stdin)!=NULL)
          printf("child: %s",buffer);
  //  fprintf(output,"%s",buffer);
  } else {
      close(fd[0]);
      close(fd2[1]);
      FILE *output = fdopen(fd[1],"w");
      char buffer[255];
      while(fgets(buffer,255,stdin)!=NULL)
          fprintf(output,"%s",buffer);
      //FILE *input = fdopen(fd2[0],"r");
      //while(fgets(buffer,255,input)!=NULL)
      //  printf("Parent: %s",buffer);
  }

  return 0;
}

Which process do you think should terminate first, the parent or the child? — David Schwartz, Jan 20 '16 at 21:20
@DavidSchwartz the child. But also if I wait for the child process, then it is a deadlock again. — TimNeutron, Jan 20 '16 at 21:21
How can the child terminate? Its `while` loop waits for its `stdin` to close. Where does the parent close it? Where is the `close(fd[1])` in the parent? — David Schwartz, Jan 20 '16 at 21:23
@DavidSchwartz it terminates when it reaches EOF (ctrl + D), then it outputs what it has read — TimNeutron, Jan 20 '16 at 21:25
There is no EOF. The parent never does `close(fd[1])` which would indicate end of file. If you want to send an EOF to the child, the parent needs to close its end of the pipe. — David Schwartz, Jan 20 '16 at 21:25
You do *what* manually? You close the parent's side of the pipe to the child? How the heck could you do that?! — David Schwartz, Jan 20 '16 at 21:27
Standard C11 or C99 does not have pipes. But POSIX and Linux (and MacOSX) have them. So you should at least edit your question to add some tags — Basile Starynkevitch, Jan 20 '16 at 21:32
`nanomsg` may be interesting to you... http://tim.dysinger.net/posts/2013-09-16-getting-started-with-nanomsg.html — Mark Setchell, Jan 20 '16 at 22:16

score 2 · Accepted Answer · answered Jan 20 '16 at 21:29

2

The parent needs to close its side of the pipe to the child so that the child will detect end-of-file and terminate.

  while(fgets(buffer,255,stdin)!=NULL)
      fprintf(output,"%s",buffer);
  fclose(output); // does close(fd[1]);
  FILE *input = fdopen(fd2[0],"r");
  while(fgets(buffer,255,input)!=NULL)
    printf("Parent: %s",buffer);

answered Jan 20 '16 at 21:29

David Schwartz

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Thank you now there is no more deadlock!! But the output is gibberish...any idea why? – TimNeutron Jan 20 '16 at 21:35
Oh okay...I am super tired and I did close(fd[0]) instead of fclose(output). Thanks sir, you are like a debugging ninja – TimNeutron Jan 20 '16 at 21:38

score 2 · Answer 2 · edited May 23 '17 at 11:59

When you have both input and output pipes between two (single-threaded) processes, you can have some deadlock, so you need to have an event loop using a multiplexing syscall (generally poll(2)...) and you will either read or write, depending on what is possible. Of course you need to buffer! BTW, in that case, you'll better use low level syscalls(2) without using <stdio.h> (and if you still do use stdio, don't forget to fflush(3)....). See also this answer.

^{(of course I am supposing a POSIX or Linux system)}

score 1 · Answer 3 · answered Jan 20 '16 at 21:34

1

Make sure everything gets closed. After

dup2(fd[0],fileno(stdin));

you should do:

close(fd[0]);

answered Jan 20 '16 at 21:34

William Pursell

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Communicating via pipes in C

3 Answers3