Running Function Inside Stub. Passing Function Pointer

Question

I'm working on creating a user-level thread library and what I want to do is run a function inside a stub and so I would like to pass the function pointer to the stub function.

Here is my stub function:

void _ut_function_stub(void (*f)(void), int id)
{
    (*f)();
    DeleteThread(id);
}

This is what the user calls. What I want to do is get pointer of _ut_function_stub to assign to pc and I've tried various different options including casting but the compiler keeps saying "invalid use of void expression".

int CreateThread (void (*f) (void), int weight)
{
    ... more code

    pc = (address_t)(_ut_function_stub(f, tcb->id));

    ... more code
}

Any help is appreciated. Thanks!

Answer 1

If you're interested in implementing your own user-level-threads library, I'd suggest looking into the (now deprecated) ucontext implementation. Specifically, looking at the definitions for the structs used in ucontext.h will help you see all the stuff you actually need to capture to get a valid snapshot of the thread state.

What you're really trying to capture with the erroneous (address_t) cast in your example is the current continuation. Unfortunately, C doesn't support first-class continuations, so you're going to be stuck doing something much more low-level, like swapping stacks and dumping registers (hence why I pointed you to ucontext as a reference—it's going to be kind of complicated if you really want to get this right).