#include <errno.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
 
int main(void) {
   pid_t childpid;
   int fd[2];
 
   if ((pipe(fd) == -1) || ((childpid = fork()) == -1)) {
      perror("Failed to setup pipeline");
      return 1;
   }
      
   if (childpid == 0) {                                  /* ls is the child */
      if (dup2(fd[1], STDOUT_FILENO) == -1) 
         perror("Failed to redirect stdout of ls");
      else if ((close(fd[0]) == -1) || (close(fd[1]) == -1)) 
         perror("Failed to close extra pipe descriptors on ls");
      else { 
         execl("/bin/ls", "ls", "-l", NULL);
         perror("Failed to exec ls");
      }
      return 1; 
   } 
   if (dup2(fd[0], STDIN_FILENO) == -1)               /* sort is the parent */
       perror("Failed to redirect stdin of sort");
   else if ((close(fd[0]) == -1) || (close(fd[1]) == -1)) 
       perror("Failed to close extra pipe file descriptors on sort"); 
   else {
      execl("/bin/sort", "sort", "-n", "+4", NULL);
      perror("Failed to exec sort");
   }
   return 1; 
}