forktest.c

/* The goal here is to create N forked processes and loop a list of 'jobs'
 * and submit each to a free fork when it becomes free */

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

#define MAXCHILD 8
pid_t children[MAXCHILD];

/* Sleep random time between 500 and 1000 msec */
int takesometime()
{
    int msec;
    srand(getpid());
    msec = rand() % 1000 + 1000;
    printf("[%05d] Doing \"work\" for %d milliseconds\n", getpid(), msec);
    fflush(stdout);
    return usleep(msec * 1000);
}

int childbypid(int pid)
{
    int child;

    for (child = 0; child < MAXCHILD; child++) {
        if (children[child] == pid) {
            children[child] = 0;
            return child;
        }
    }
    return -1;
}

int findfreechild()
{
    int child;
    for (child = 0; child < MAXCHILD; child++) {
        printf("Checking child %d\n", child);
        fflush(stdout);
        if (children[child] == 0) {
            printf("Child %d is free\n", child);
            fflush(stdout);
            return child;
        }
    }
    return -1;
}

void cleanchildren()
{
    for (int child = 0; child < MAXCHILD; child ++) {
        children[child] = 0;
    }
}

int countchildren()
{
    int count = 0;
    for (int child = 0; child < MAXCHILD; child ++) {
        if (children[child] > 0) {
            count++;
        }
    }
    return count;
}

int main(int argc, char *argv[]) 
{
    pid_t childpid;
    int event;
    int events = 16;
    int freeproc = -1;
    int status;

    cleanchildren();

    /* Start children. */
    for (event = 0; event < events; ++event) {
        /* Get next free child or wait for one to exit */
        if ((freeproc = findfreechild()) == -1) {
            freeproc = childbypid(wait(&status));
        }

        /* Fork process */
        if ((childpid = fork()) < 0) {
            perror("fork");
            abort();
        } else if (childpid == 0) {
            takesometime();
            exit(0);
        }
        children[freeproc] = childpid;
    }

    /* END: Wait for children to exit. */
    printf("Done jobs\n");
    while (countchildren() > 0) {
        childpid = wait(&status);
        printf("Child with PID %d exited with status 0x%x.\n", childpid, status);
        children[childbypid(childpid)] = 0;
    }

    return 0;
}