Why does pthread_exit use void*?

Question

I recently started using posix threads and the choice of argument types in the standard made me curious. I haven't been able to asnwer the question of why does pthread_exit use void* instead of int for the returning status of the thread? (the same as exit does).

The only advantage I see is that it lets the programmers define the status how they want (e.g. return a pointer to a complex structure), but I doubt it is widely used like this.

It seems that in most cases this choice has more overhead because of necessary casting.

Answer 1

This isn't just for status, it's the return value of the thread. Using a pointer allows the thread to return a pointer to a dynamically-allocated array or structure.

You can't really compare it with the exit() parameter, because that's for sending status to the operating system. This is intentionally very simple to allow portability with many OSes.

Answer 2

The only advantage I see is that it lets the programmers define the status how they want (e.g. return a pointer to a complex structure), but I doubt it is widely used like this.

Indeed, that's the reason. And it's probably not used that widely (e.g. you can communicate values via other means such as a pointer passed to thread function, global var with synchronisation, etc). But if you were to have a it like void pthread_exit(int);, the it takes away the ability to return pointers. So void pthread_exit(void*); is a more flexible design.

It seems that in most cases this choice has more overhead because of necessary casting.

In most cases, it's not used at all as the common way is to return nothing i.e. pthread_exit(NULL);. So it only matters when returning pointers (to structs and such) and in those cases a conversion to void * isn't necessary as any pointer type can be converted to void * without an explicit cast.