Question

I’m trying to write a multi-threaded program, the number of threads based on command-line input, and so I can’t hard-code pre-declared threads. Is this a valid way of doing it?

int threads = 5; // (dynamic, not hard-coded)
int i = 0;
pthread_t * thread = malloc(sizeof(pthread_t)*threads);

for (i = 0; i < threads; i++) {
    pthread_t foobar;
    thread[i] = foobar; // will this cause a conflict?
}

for (i = 0; i < threads; i++) {

    int ret = pthread_create(&thread[i], NULL, (void *)&foobar_function, NULL);

    if(ret != 0) {
        printf ("Create pthread error!\n");
        exit (1);
    }
}

---

Here’s my result from modifications suggested below. Seems to work just fine.

int threads = 5;
int i;

pthread_t * thread = malloc(sizeof(pthread_t)*threads);

for (i = 0; i < threads; i++) {

    int ret = pthread_create(&thread[i], NULL, &foobar_function, NULL);

    if(ret != 0) {
        printf ("Create pthread error!\n");
        exit (1);
    }
    // pthread_join(thread[i], NULL); // don't actually want this here :)
}

sleep(1);     // main() will probably finish before your threads do,
free(thread); // so we'll sleep for illustrative purposes

Answer 1

What’s in the first cycle? Does it set the array elements to uninitialized value?

So i think that’s what you need:

int threads = 5, i = 0, ret = -1;

pthread_t * thread = malloc(sizeof(pthread_t)*threads);

for (i = 0; i < threads; i++) {

    ret = pthread_create(&thread[i], NULL, &foobar_function, NULL);

    if(ret != 0) {
        printf ("Create pthread error!\n");
        exit (1);
    }
}

It spawns threads threads, starting foobar_function in each. And you have (if everything goes well:)) their ids in thread array. So for example you can cancel second thread by calling pthread_cancel(thread[1]) etc.