Setting up four processes with fork() using 1 pipe

Question

Trying to write a simple program that uses pipe/fork to create/manage 1 parent and 4 kid processes. The parent process is supposed to display a main menu like so.

Main Menu:
1. Display children states
2. Kill a child
3. Signal a child
4. Reap a child
5. Kill and reap all children

The code I'm writing now is supposed to create 4 child processes. I'm unsure if I'm setting up the four child processes correctly. I understand fork returns 0 to the child and the PID to the parent. How do I access the parent for these PID values? Where exactly do I set up the menu for the parent process?

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

#define BUFSIZE 1024
#define KIDS    4
main()
{
    //create unnamed pipe
    int fd[2];
    char buf[BUFSIZE];

    if (pipe(fd) < 0) 
    {
        perror("pipe failed");
        exit (1);
    }

    //array of child pids
    size_t child_pid[4]; 

    //create 4 proccesses 
    int i = 0;
    for (i = 0; i < KIDS; i++) {
        child_pid[i] = fork();
        if (child_pid[i]) {
            continue;
        } else if (child_pid[i] == 0) {
            close(fd[0]);
            printf("Child %d: pid: %zu", i+1, child_pid[i]);
            break;
        } else {
            printf("fork error\n");
            exit(1);
        }

    } 

}

My output is:

Child 1: pid: 0
Child 2: pid: 0
Child 3: pid: 0
Child 4: pid: 0

Was there any error? How many times does it loop? and where is this declared "pid"? — artsylar
Sorry I was changing around that loop so much I messed that up, it's supposed to be child_pid[i]. I am editing in the output I'm getting. — tdwig
The printf("Child %d: pid: %zu", i+1, child_pid[i]); works just fine. It is only executed when child_pid[i] == 0, and this is exactly what it does (ie it prints a zero). The error is in your concept. — sokin

Armali Armali · Accepted Answer · 2016-01-19T11:57:26

I'm unsure if I'm setting up the four child processes correctly.

Right, you shouldn't let the children break out of their code block, so change

            break;

to

            sleep(99);  // or whatever you want the child to do
            exit(0);

How do I access the parent …?

There's getppid(), if for some reason you need it.

Where exactly do I set up the menu for the parent process?

Do that after the for loop before the end of main, that's where the parent continues execution.

Setting up four processes with fork() using 1 pipe

1 Answers