// test fork() and pipe()
//
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>

void error( const char *msg) { perror(msg); exit(1); }

int main( void) {

  int fd[2]; if( pipe(fd) < 0) perror("pipe");
// 0 stdin, 1 stdout
// fd[0] read side, fd[1] write side

  int pid = fork(); if( pid < 0) error("fork");

  if( pid != 0) { // parent

    dup2(fd[1],1); // connect stdout to pipe
    close(fd[0]); close(fd[1]); // close unneeded descriptors
    // printf( "hi from parent\n"); fflush(stdout); // goes into the pipe
    execlp( "ls", "ls", NULL);
    error( "ls");

  } else { // child

    dup2(fd[0],0); // connect stdin to pipe
    close(fd[0]); 
    close(fd[1]); // close unneeded descriptors
    // char msg[BUFSIZ]; fgets(msg,BUFSIZ,stdin); // read from the pipe
    // printf( "child got: %s", msg); // write to terminal
    sleep(1);
    execlp( "cat", "cat", "-n", NULL);
    error( "cat");
  }
}

/* sample run:

$ ./a.out
$      1        NOTES
     2  a.out
     3  a9
     4  fork2.c
     5  pthread.c
     6  pthread2.c

*/