C++ Pthreads. Arguments to pthread_create

Question

There are a couple of things that I don't understand about the function pthread_create .

here is the header

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

Firstly, I'm not familiar with the syntax of void *(*start_routine)(void*), . I know that the argument asked for here is the name of a function that returns void * , and takes one void * as its argument. Presumably pthread_create will refer to that funcion as start_routine . So I suppose this argument would be a function pointer? If so, what are the key syntax elements that make it so?

Secondly, why does pthread_create expect a function that has void * as its return type? What would pthread_create be able to do with data of unkown type?

Answer 1

The void *(*start_routine)(void*) should be read in the following order:

• start_routine - Name of the argument.
• *start_routine - So this argument is a pointer.
• (*start_routine)(...) - Aha, it's a pointer to a function.
• (*start_routine)(void*) - We now know the argument(s) of the function.
• void *(*start_routine)(void*) - And finally, this tells us the return type of the function.

The void* argument receives whatever is passed to arg - so if you need to pass any "input" to your new thread this is one way to do it.

The resulting void* is used as thread's exit status (as alternative to calling pthread_exit() explicitly). You can get this status through pthread_join() .

Answer 2

1. void *(*start_routine)(void*) is a function pointer. Everything about it is the 'key syntax', but in particular the syntax (*ptrname)(args) .

2. pthreads doesn't do anything with the void* that the start routine returns except to return it to you. See the man page for pthread_join to see how it comes back to you.