[Linux Program] Linux System program by example - fork and dup

/* ch09-pipeline.c --- fork two processes into their own pipeline. 
   Minimal error checking for brevity. */  

#include <stdio.h>  
#include <errno.h>  
#include <sys/types.h>  
#include <sys/wait.h>  
#include <unistd.h>  

int pipefd[2];  

extern void left_child(void), right_child(void);  

/* main --- fork children, wait for them to finish */  

int main(int argc, char **argv)  
{  
    pid_t left_pid, right_pid;  
    pid_t ret;  
    int status;  

    if (pipe(pipefd) < 0) {      /* create pipe, very first thing */  
        perror("pipe");  
        exit(1);  
    }  

    if ((left_pid = fork()) < 0) {   /* fork left-hand child */  
        perror("fork");  
        exit(1);  
    } else if (left_pid == 0)  
        left_child();  

    if ((right_pid = fork()) < 0) {  /* fork right-hand child */  
        perror("fork");  
        exit(1);  
    } else if (right_pid == 0)  
        right_child();  

    close(pipefd[0]);       /* close parent's read copy of pipe */  
    close(pipefd[1]);       /* close parent's write copy of pipe */

    while ((ret = wait(& status)) > 0) { /* wait for children */  
        if (ret == left_pid)  /* If left_pid match then ...*/
            printf("left child terminated, status: %x\n", status);  
        else if (ret == right_pid) /* If right_pid match then ...*/  
            printf("right child terminated, status: %x\n", status);  
        else  
            printf("yow! unknown child %d terminated, status %x\n",  
              ret, status);  
    }  

    return 0;  
}  

/* 0 (read)  ---Pipe--> 1 (write) */


/* left_child --- do the work for the left child */  

void left_child(void)  
{  
/* fork - have parent source, so need to close some parameter */
    static char *left_argv[]  = { "echo", "hi", "there", NULL };  

    close(pipefd[0]);  /* Left_child don't need to read */
    close(1);  /* Close standard output */
    dup(pipefd[1]);  /* Copy left_child write to parent's write */
    close(pipefd[1]);  /* Copy parent's write*/ 

    execvp("echo", left_argv);  
    _exit(errno == ENOENT ? 127 : 126);  
}  

/* right_child --- do the work for the right child */  

void right_child(void)  
{  
/* fork - have parent source, so need to close some parameter */    
    static char *right_argv[] = { "sed", "s/hi/hello/g", NULL };  

    close(pipefd[1]);  /* right_child don't need to write */
    close(0);  /* Close standard input */
    dup(pipefd[0]);  
    close(pipefd[0]);  

    execvp("sed", right_argv);  
    _exit(errno == ENOENT ? 127 : 126);  
}  

Reference
1. Linux System programming by example (Example download)