pthread_join wrong type of argument

Question

I've been trying to create a simple thread that reads from stdin and stores input to a linked list. When creating and joining the thread I get the following error:

warning: passing argument 1 of ‘pthread_join’ makes integer from pointer without a cast [-Wint-conversion]
   pthread_join(prod_thread, NULL);

I'm passing a thread_t* argument to pthread_join but it seems that it expects an int which confuses me. Can anyone explain why this is happening? Here is the code:

pair_t* head = malloc(sizeof(pair_t));
pthread_t* prod_thread = malloc(sizeof(pthread_t));
pthread_create(prod_thread, NULL, prod_func, head);
pthread_join(prod_thread, NULL);

The prod_func function looks like this:

void* prod_func(void* head) {
  ...
}

I've also tried calling pthread_join(&prod_thread, NULL); but then I get the same error.

Answer 1

I'm passing a thread_t* argument to pthread_join but it seems that it expects an int which confuses me.

pthread_join expects its first argument to be a pthread_t (not a pthread_t *). Exactly what type pthread_t actually is varies from implementation to implementation, but in your implementation it is an integer type. You are passing a pointer instead.

I've also tried calling pthread_join(&prod_thread, NULL); but then I get the same error.

Of course you do. if prod_thread has type pthread_t * then its address, &prod_thread, has type pthread_t **. That's going the wrong direction (and the result is still a pointer). As you have written your declarations, what you actually want is

pthread_join(*prod_thread, NULL);