Question

In the following code:

int main (int argc, const char * argv[]) {
    // insert code here...

    pthread_t t1, t2;
    int sp1, sp2;

    sp1 = pthread_create( &t1, NULL, getScalarProduct, NULL);
    sp2 = pthread_create( &t2, NULL, getScalarProduct, NULL);
    pthread_join( t1, NULL);
    pthread_join( t2, NULL);

    printf("Seperate scalars: %d %d\n", sp1, sp2);
    finalScalarProd = sp1 + sp2;


    printf("Result: %d\n", finalScalarProd);

    return 0;
}

I’ve been unable to get anything back other than zero for the finalScalarProduct, and both sp1 and sp2 are zero also. I believe it’s something to do with the NULL argument being passed in pthread_join. I don’t really understand what this argument is for.

Any help appreciated!

Answer 1

That’s because pthread_create returns zero upon success.
The value is not the result of the main thread function, but the result of the thread creation (that might fail in some cases).

void * threadMainFunc(void * arg) {
    // modify arg like this
    int * ip = (int *)arg;
    *ip = 3; // this is the "return value"
    return NULL;
}

pthread_create( &t1, NULL, threadMainFunc, &sp1);