execlp "sort" with input from pipe stucks, why?

Question

sort is waiting, but what? I tried execlp("head", "head", "-n", "3", NULL); instead sort and it works fine.

#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>
#include <assert.h>
int main()
{
  int p[2], cat_pid, sort_pid;
  if (pipe(p) < 0) { assert(0 && "pipe fail"); }
  if ((cat_pid = fork()) == 0) { dup2(p[1], 1); execlp("cat", "cat", "text", NULL); assert(0 && "cat fail"); }
  if ((sort_pid = fork()) == 0) { dup2(p[0], 0); execlp("sort", "sort", NULL); assert(0 && "sort fail"); }
  waitpid(sort_pid, NULL, 0);
}

input text is:

hello
world
foo
bar

Answer 1

As sort is waiting for EOF you need to close pipe' write end. One is closed once cat is done and another one is in parent process. Close pipe' write end in parent and everything should go fine.

man 7 pipe

If all file descriptors referring to the write end of a pipe have been closed, then an attempt to read(2) from the pipe will see end-of-file (read(2) will return 0).