Exit a child process after forking

Question

I have this code and it's goal is to create N child processes and print out each PID and process number.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

void childProcess(int num)
{
    pid_t pid = fork();

    printf("Hello! I am process no. %d! my PID is %d!\n", num, pid);
}

int main()
{
    int i = 0;

    for(i = 1; i <= 5; i++)
    {
        childProcess(i);
        _exit(3);
    }
    exit(0);
}

However after trying multiple ways: e.g exit vs _exit, recursion in childProcess, pid = wait(), I am still having trouble creating only 5 processes. With this code so far my output is:

Hello! I am process no. 1! my PID is 96196!
Hello! I am process no. 1! my PID is 0!

I'm not sure how to properly exit from a child process. Without the exit the code creates N! processes.

Answer 1

You are correctly exiting child process, it is just that you do it at wrong time. After fork() every child process keeps running same loop as parent, creating another child processes and that's why you end up having a lot of them.

Correct code would be something like this:

for (int i = 0; i < 5; i++) {
  if (fork() == 0) {
    // child process
    printf("I'm child %d, my pid is %d\n", i, getpid());
    exit(0);
  }
  // parent process keeps running the loop
}

Answer 2

fork() splits your process into two processes, a parent and a child. You need some sort of test of its return value inside childProcess() to figure out if you're in the parent or the child. If pid is 0 then you're in the child; if non-zero then you're in the parent. Based on that you'll want to conditionally exit the program.