Question

This question comes up in my mind when I look at the synopsis of pthread_create, which is:

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

I don’t understand why the third parameter has to be that complex. Its purpose is just for passing in a function address. So why don’t they use

void (*start_routine)(void *)

or even

void (start_routine)(void *)

In general, in what situations should we use a function signature like the third parameter above?

Answer 1

The start_routine takes a void* to allow the thread starter to pass some arbitrary data to the new thread. It returns a void* so that the thread can return a meaningful exit code to the caller (instead of explicitly having to invoke pthread_exit from the thread function).

Note that your second suggestion (void (start_routine)(void*) is not a function pointer, and so isn’t an option for a parameter to pthread_create or any other function at all.