How to make parent process invoke wait when child exits with parent unblocked in Linux?

Here is an example, waitpid system call will block parent process until child process exits:

    pid_t pid = fork();
    if (pid < 0) {
        printf("errorn");
        return -1;
    } else if (pid == 0) {
        printf("childn");
        do_sonmething_in_child();
    } else {
        int status;
        waitpid(pid, &status, 0);
        printf("waitpid returned,do something elsen");
        do_something_else();
    }

What I want to achieve is parent process won’t be blocked, this can be done by using signal, so I changed my code:

void handler(int n) {
    printf("handler %dn", n);
    int status;
    wait(&status);
}
pid_t pid = fork();
if (pid < 0) {
    printf("errorn");
    return -1;
} else if (pid == 0) {
    printf("childn");
    do_sonmething_in_child();
} else {
    int status;
    signal(SIGCHLD, handler);
    do_something_else();
}

But another problem comes, if child exit before signal(SIGCHLD, handler), child will be a zombie, we can’t expect the child exit after signal(SIGCHLD, handler) is invoked, so, how to solve my problem?

Answer

You can set the signal handler before the fork () call.