Question

This code below is for executing ls -l | wc -l.
In the code, if I comment close(p[1]) in parent then the program just hangs, waiting for some input. Why it is so? The child writes output of ls on p1 and parent should have taken that output from p0.

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

main ()
{
  int i;
  int p[2];
  pid_t ret;
  pipe (p);
  ret = fork ();

  if (ret == 0)
    {
      close (1);
      dup (p[1]);
      close (p[0]);
      execlp ("ls", "ls", "-l", (char *) 0);
    }

  if (ret > 0)
    {
      close (0);
      dup (p[0]);
      //Doubt, Commenting the line below does not work WHy?
      close (p[1]);
      wait (NULL);
      execlp ("wc", "wc", "-l", (char *) 0);
    }
}

Answer 1

pipe + fork creates 4 file descriptors, two are inputs

Before the fork you have a single pipe with one input and one output.

After the fork you will have a single pipe with two inputs and two outputs.

If you have two inputs for the pipe (that a proc writes to) and two outputs (that a proc reads from), you need to close the other input or the reader will also have a pipe input which never gets closed.

In your case the parent is the reader, and in addition to the output end of the pipe, it has an open other end, or input end, of the pipe that stuff could, in theory, be written to. As a result, the pipe never sends an eof, because when the child exits the pipe is still open due to the parent’s unused fd.

So the parent deadlocks, waiting forever for it to write to itself.