Question

I have a question about pthread_kill() behavior.

Here’s a small code I’m trying out:

void my_handler1(int sig)
{
    printf("my_handle1: Got signal %d, tid: %lu\n",sig,pthread_self());
    //exit(0);
}

void *thread_func1(void *arg)
{
    struct sigaction my_action;
    my_action.sa_handler = my_handler1;
    my_action.sa_flags = SA_RESTART;
    sigaction(SIGUSR1, &my_action, NULL);
    printf("thread_func1 exit\n");
}


void *thread_func2(void *arg)
{
    int s;
    s = pthread_kill(tid1_g,SIGUSR1);
    if(s)
            handle_error(s,"tfunc2: pthread_kill");

    printf("thread_func2 exit\n");
}


int main()
{
    int s = 0;
    pthread_t tid1;

    s = pthread_create(&tid1,NULL,thread_func1,NULL);
    if(s)
            handle_error(s,"pthread_create1");

    tid1_g = tid1;
    printf("tid1: %lu\n",tid1);
    s = pthread_join(tid1,NULL);
    if(s)
            handle_error(s, "pthread_join");

    printf("After join tid1\n");

    pthread_t tid3;
    s = pthread_create(&tid3,NULL,thread_func2,NULL);
    if(s)
            handle_error(s,"pthread_create3");

    s = pthread_join(tid3,NULL);
    if(s)
            handle_error(s, "pthread_join3");

    printf("After join tid3\n");
    return 0;
}

The output I’m getting is:

tid1: 140269627565824
thread_func1 exit
After join tid1
my_handle1: Got signal 10, tid: 140269627565824
thread_func2 exit
After join tid3

So, even though I’m calling pthread_kill() on a thread that has already finished, the handler for that thread is still getting called. Isn’t pthread_kill() supposed to return error(ESRCH) in case the thread doesn’t exist?

Answer 1

Any use (*) of the pthread_t for a thread after its lifetime (i.e. after pthread_join successfully returns, or after the thread terminates in the detached state) results in undefined behavior. You should only expect ESRCH if the pthread_t is still valid, i.e. if you haven’t joined the thread yet. Otherwise all bets are off.

Note: By “use” (*), I mean passing it to a pthread_ function in the standard library. As far as I can tell, merely assigning it to another pthread_t variable or otherwise passing it around between your own functions without “using” it doesn’t result in UB.