atfork.c

/*
 * atfork.c
 *
 * Demonstrate the use of "fork handlers" to protect data
 * invariants across a fork.
 */
#include <sys/types.h>
#include <pthread.h>
#include <sys/wait.h>
#include "errors.h"

pid_t self_pid;                         /* pid of current process */
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

/*
 * This routine will be called prior to executing the fork,
 * within the parent process.
 */
void fork_prepare (void)
{
    int status;

    /*
     * Lock the mutex in the parent before creating the child,
     * to ensure that no other thread can lock it (or change any
     * associated shared state) until after the fork completes.
     */
    status = pthread_mutex_lock (&mutex);
    if (status != 0)
        err_abort (status, "Lock in prepare handler");
}

/*
 * This routine will be called after executing the fork, within
 * the parent process
 */
void fork_parent (void)
{
    int status;

    /*
     * Unlock the mutex in the parent after the child has been created.
     */
    status = pthread_mutex_unlock (&mutex);
    if (status != 0)
        err_abort (status, "Unlock in parent handler");
}

/*
 * This routine will be called after executing the fork, within
 * the child process
 */
void fork_child (void)
{
    int status;

    /*
     * Update the file scope "self_pid" within the child process, and unlock
     * the mutex.
     */
    self_pid = getpid ();
    status = pthread_mutex_unlock (&mutex);
    if (status != 0)
        err_abort (status, "Unlock in child handler");
}

/*
 * Thread start routine, which will fork a new child process.
 */
void *thread_routine (void *arg)
{
    pid_t child_pid;
    int status;

    child_pid = fork ();
    if (child_pid == (pid_t)-1)
        errno_abort ("Fork");

    /*
     * Lock the mutex -- without the atfork handlers, the mutex will remain
     * locked in the child process and this lock attempt will hang (or fail
     * with EDEADLK) in the child.
     */
    status = pthread_mutex_lock (&mutex);
    if (status != 0)
        err_abort (status, "Lock in child");
    status = pthread_mutex_unlock (&mutex);
    if (status != 0)
        err_abort (status, "Unlock in child");
    printf ("After fork: %d (%d)\n", child_pid, self_pid);
    if (child_pid != 0) {
        if ((pid_t)-1 == waitpid (child_pid, (int*)0, 0))
            errno_abort ("Wait for child");
    }
    return NULL;
}

int main (int argc, char *argv[])
{
    pthread_t fork_thread;
    int atfork_flag = 1;
    int status;

    if (argc > 1)
        atfork_flag = atoi (argv[1]);
    if (atfork_flag) {
        status = pthread_atfork (fork_prepare, fork_parent, fork_child);
        if (status != 0)
            err_abort (status, "Register fork handlers");
    }
    self_pid = getpid ();
    status = pthread_mutex_lock (&mutex);
    if (status != 0)
        err_abort (status, "Lock mutex");
    /*
     * Create a thread while the mutex is locked. It will fork a process,
     * which (without atfork handlers) will run with the mutex locked.
     */
    status = pthread_create (&fork_thread, NULL, thread_routine, NULL);
    if (status != 0)
        err_abort (status, "Create thread");
    sleep (5);
    status = pthread_mutex_unlock (&mutex);
    if (status != 0)
        err_abort (status, "Unlock mutex");
    status = pthread_join (fork_thread, NULL);
    if (status != 0)
        err_abort (status, "Join thread");
    return 0;
}