Problems with pthreads

Question

I'm really new to threads, and I have to do an assignment. I have a graph of 6 nodes and I should create threads that move from the first node to the last one. I have everything set up except the part that needs to be done in threads. I have read some tutorials on the internet but they aren't enough, I'm having some problems and I don't understand why. Anyway, here's the code:

int main (void) {

    pthread_t threads[NUM_THREADS];
    int i = 0, rc, a = 0;
    creatGraph();
    rc = pthread_create(&threads[i], NULL, buscarExp(i,0), NULL);
    if (rc) {
        printf("ERROR al crear el funcionari %d\n,",i);
        exit(-1);
    }
    pthread_exit(NULL);
    return 0;
}

And here are the two main rutines I need:

    void buscarDespatx(int i, int actual) {
        if(llista[actual].id == 0) {    // ja hem arribat al despatx
            pthread_exit(NULL);
            //buscarExp(i,actual); // el funcionari ha deixat l'expedient i va a buscar-ne un altre
        } else {    // no ha arribat al despatx
            int seguent = rand() % llista[actual].Npares; // trio el node pare de tots els nodes pares q tindra
            llista[actual].Proces[llista[actual].membres] = 0; // trec el proces del node actual
            llista[actual].membres--; // decremento el nombre de processos al noded actual
            llista[llista[actual].pares[seguent]].membres++; // incremento el nombre de processos del node pare al que anira el proces actual
            llista[llista[actual].pares[seguent]].Proces[llista[llista[actual].pares[seguent]].membres] = i; // afegeixo el proces actual a la llista de processos del node pare que anira el proces
            buscarDespatx(i,llista[llista[actual].pares[seguent]].id);
            printf("BUSCAR EXP: El node %d ha estat modificat i ha marxat el proces %d\n",actual,i);
        }
    }

void buscarExp(int i, int actual) {
    if(llista[actual].id == -1) {   // ja hem arribat al expedient
        buscarDespatx(i,actual); // el funcionari te l'expedient i el va a deixar al despatx
    } else {    // no ha arribat a l'expedient
        int node = 0;
        if(llista[actual].dret != NULL) {   // aquest node te dos fils
            int seg = rand();
            if(seg%2 == 0) {    // avança pel fill esq
                llista[actual].esq->membres++;  // sumo un membre al seguent node
                llista[actual].esq->Proces[llista[actual].esq->membres] = i;    // poso el proces al seguent node
            } else { // avanço pel fill dret
                node = 1;
                llista[actual].dret->membres++; // sumo un membre al seguent node
                llista[actual].dret->Proces[llista[actual].dret->membres] = i;  // poso el proces al seguent node
            }
        } else {    // nomes te fill esquerra
            llista[actual].esq->membres++;  // sumo un membre al seguent node
            llista[actual].esq->Proces[llista[actual].esq->membres] = i;    // poso el proces al seguent node
        }
        llista[actual].Proces[llista[actual].membres] = 0;      // elimino el funcionari del node actual
        llista[actual].membres--;
        printf("BUSCAR EXP: El node %d ha estat modificat i ha marxat el proces %d\n",actual,i);
        if(node == 1) { // ha passat pel dret
            buscarExp(i,llista[actual].dret->id);
        } else { // passa per l'esquerra
            buscarExp(i,llista[actual].esq->id);
        }
    }
}

So, if I understood how threads works, this should do the following: The 'main' creates the pthread and it starts doing the routing "buscarExp(i,0)", then in "buscarExp(i,0)" it keeps doing this routine recursively untill it reaches the bottom of the graph if(llista[actual].id == -1) and then it goes back to the first node with the routine "buscarDespatx(i,actual)". When it fins the initial node, I use pthread_exit(NULL); to terminate the thread.

Is this how the thread I create in the main would behave if the code was 100% correct?

Thanks!

"it isn't working" is terribly insufficient. What isn't working? What are you seeing that you didn't expect? Have you added debug print messages? Have you stepped through your code in a debugger? — Jonathon Reinhart
– Jonathon Reinhart, Commented Dec 7, 2014 at 16:16
You're right... I know I should be more specific, I was trying to confirm if I understood the flow of the threads or not, I'll edit the main post — p. bosch
– p. bosch, Commented Dec 7, 2014 at 16:35

Jonathon Reinhart · Accepted Answer · 2014-12-08 14:21:23Z

rc = pthread_create(&threads[i], NULL, buscarExp(i,0), NULL);

Is not going to work. If you look at the documentation for pthread_create, you'll see that the third parameter (start_routine) is supposed to be a function pointer. However, your code is first calling buscarExp(i,0), then trying to pass that (incorrectly-typed) result as the thread function to pthread_create.

You need to pass a function with a compatible signature to pthread_create, using the void* to pass it any additional parameters.

Also, your main thread is exiting immediately, which is a problem, because the data I passed lives on the stack of main(). Most likely, you want to pthread_join with all of your threads after you start them.

Something like this:

struct thread_data {
    int thread_num;
    // Other things you want to pass here
}

void *thread_func(void *_data) {
    struct thread_data *data = _data;

    buscarExp(data->thread_num, 0)
}

int main (void) {

    pthread_t threads[NUM_THREADS];
    struct thread_data thread_data[NUM_THREADS];

    int i = 0, rc, a = 0;

    creatGraph();

    /* Start all threads */
    for (i=0; i<NUM_THREADS; ++i) {
        thread_data[0].thread_num = i;
        rc = pthread_create(&threads[i], NULL, thread_func, &thread_data[i]);
        if (rc) {
            printf("ERROR al crear el funcionari %d\n,",i);
            exit(-1);
        }
    }

    /* Wait for all threads to finish */
    for (i=0; i<NUM_THREADS; ++i) {
        pthread_join(&threads[i], NULL);
    }
    return 0;
}

Okay this is what I was looking for. Thank you! One last question though, if I understood your code right, this will create NUM_THREADS and run buscarExp on each of them, when they are done with buscarExp they will finish and everything will be over, right?
Okay I fully understood (I think) how pthreads work, Thanks Jonathon!
You're welcome. Welcome to the wonderful, terrible world of multithreading!

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