TLPI 26-2

zombies.c

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>

/**
 * assuming grand parent, parent and child:
 *
 * * a.out shows that as soon as "parent" is a zombie, child is orphaned and
 *   adopted by pid 1.
 * * a.out zombies shows that since child is adopted, it doesn't stay a zombie,
 *   even if parent is.
 */
int main(int argc, char* argv[]) {
    int terminate_child = 0;

    setvbuf(stdout, NULL, _IONBF, 0);
    setvbuf(stderr, NULL, _IONBF, 0);

    if(argc > 1) {
        if(strcmp(argv[1], "zombies") == 0) {
            printf("Will terminate child promptly (to verify if it stays a zombie\n");
            terminate_child = 1;
        }
    }

    // block SIGCHLD, we'll never await anyone (this is inherited by children)
    sigset_t blocked;
    sigemptyset(&blocked);
    sigaddset(&blocked, SIGCHLD);

    sigprocmask(SIG_BLOCK, &blocked, NULL);

    printf("Welcome, i'm grand father (pid: %ld)\n", (long) getpid());

    pid_t parent = fork();
    if(parent == 0) {
        int pipefd[2];

        if(pipe(pipefd) == -1) {
            perror("Could not create pipe in parent, killing grand-parent and terminating self");
            kill(getppid(), SIGTERM);
            exit(0);
        }

        pid_t child = fork();
        if(child == 0) {
            printf("I'm child (pid: %ld)\n", (long) getpid());
            close(pipefd[1]);

            if(read(pipefd[0], NULL, 0) == 0) {
                sleep(1); /* pipe can be closed before the process actually
                             exited, this "ensures" that the parent is a zombie
                             alternatively reading /proc can help*/
                printf("parent terminated! PPID is now %ld\n", (long) getppid());
            }
            else {
                // Assume we never read a byte, it's -1
                perror("Could not read on pipe");
            }

            close(pipefd[0]);

            if(terminate_child) {
                printf("Terminating child\n");
                exit(0);
            }
            else {
                printf("Keeping child alive\n");
                pause();
            }
        }
        else {
            close(pipefd[0]);
            printf("I'm parent (pid: %ld)\n", (long) getpid());
            printf("parent going to terminate, now that child exists\n");
            exit(0); // this will close the write side of the pipe, thus notifying the child that we're dead.
        }
    }
    else {
        printf("Grand father awaiting forever\n");
        pause();
    }

    exit(0);
}