Question

Here is an example of thread creation code that is often seen. pthread_create uses a lot of pointers/addresses and I was wondering why this is so.

    pthread_t threads[NUM_THREADS];
    long t;
      for(t=0; t<NUM_THREADS; t++){
          rc = pthread_create(&threads[t], NULL, &someMethod, (void *)t);
      }

Is there a major advantage or difference for using the ‘&’ to refer to the variable array ‘threads’ as well as ‘someMethod’ (as opposed to just ‘threads’ and just ‘someMethod’)? And also, why is ‘t’ usually passed as a void pointer instead of just ‘t’?

Answer 1

int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
    void *(*start_routine)(void*), void *arg);

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You need to pass a pointer to a pthread_t variable to pthread_create. &threads[t] and threads+t achieve this. threads[t] does not. pthread_create requires a pointer so it can return a value through it.

someMethod is a suitable expression for the third argument, since it’s the address of the function. I think &someMethod is redundantly equivalent, but I’m not sure.

You are casting t to void * in order to jam a long into a void *. I don’t think a long is guaranteed to fit in a void *. It’s definitely a suboptimal solution even if the guarantee exists. You should be passing a pointer to t (&t, no cast required) for clarity and to ensure compatibility with the expected void *. Don’t forget to adjust someMethod accordingly.

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pthread_t threads[NUM_THREADS];
long t;
for (t=0; t<NUM_THREADS; t++) {
    rc = pthread_create(&threads[t], NULL, someMethod, &t);
}