Question

In c, we create a thread like so:

void * run(void * arg){
    printf("hello world\n");
}

int main(){
    pthread_t thread;
    int a = pthread_create(&thread, NULL, run, (void*)0);
}

But it will not work if I declare run as

void run(){}

On the other hand, if I cast it to (void *) in the parameter of pthread_create, it works fine. So it only accepts functions with return types of (void *).

Why?

Thanks !

Answer 1

On my system, man pthread_create says:

If start_routine returns, the effect is as if there was an implicit call to pthread_exit(), using the return value of start_routine as the exit status.

This return value is available through the pthread_join() function:

On return from a successful pthread_join() call with a non-NULL value_ptr argument, the value passed to pthread_exit() by the terminating thread is stored in the location referenced by value_ptr.