Demonstration of creating new processes and waiting for them to terminate

fork_demo.c

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

void fork_demo() {
    pid_t pid;
    printf("Before fork. \tme =%8d\tchild =     n/a\tparent = %8d\n", getpid(), getppid());
    pid = fork();
    printf("After fork.  \tme =%8d\tchild =%8d\tparent = %8d\n", getpid(), pid, getppid());
    pid = fork();
    printf("Another fork.\tme =%8d\tchild =%8d\tparent = %8d\n", getpid(), pid, getppid());
}

wait_demo.c

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

static void reap_children();
static void sleep_and_exit(int exit_value);

void wait_demo() {
    if (fork()) {   // the parent process
        if (fork()) {
            reap_children();
        } else {
            sleep_and_exit(1);
        }
    } else {        // the child process
        sleep_and_exit(0);
    }
}

static void reap_children() {
    pid_t pid;
    int status;
    while ((pid = wait(&status)) > 0) {
        if (WIFEXITED(status)) {
            printf("Process %d terminated with exit status %d.\n", pid, WEXITSTATUS(status));
        } else {
            printf("Process %d terminated abnormally.\n", pid);
        }
    }
    if (errno == ECHILD) {
        printf("No more children.\n");
    } else {
        printf("Unexpected error returned by wait().\n");
    }
}

static void sleep_and_exit(int exit_value) {
    sleep(1);
    exit(exit_value);
}

waitpid_demo.c

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
#include <stdlib.h>

static void sleep_and_exit(int exit_value);

void waitpid_demo() {
    bool is_parent = true;
    int number_of_children = 4;
    int status;
    pid_t pid, pids[number_of_children];
    for (int i = 0; i < number_of_children; i++) {
        pid = fork();
        if (pid == 0) {
            is_parent = false;
        } else {
            pids[i] = pid;
        }
    }
    if (!is_parent) {
        sleep_and_exit(0);
    } else {
        long counter = 1;
        while ((pid = waitpid(pids[0], &status, WNOHANG)) == 0) {
            counter++;
        }
        printf("Process %d terminated after %ld calls to waitpid().\n", pid, counter);
        pid = waitpid(pids[1], &status, 0);
        printf("Process %d terminated.\n", pid);
        counter = 1;
        while ((pid = waitpid(-1, &status, WNOHANG)) == 0) {
            counter++;
        }
        printf("Process %d terminated after %ld calls to waitpid().\n", pid, counter);
        pid = waitpid(-1, &status, 0);
        printf("Process %d terminated.\n", pid);
    }
}

static void sleep_and_exit(int exit_value) {
    sleep(1);
    exit(exit_value);
}