Type cast void*(*)(void*) to void(*)(void)

Question

As a part of an assignment, I am trying to create a user level thread library like pthreads.

For handling the context switching between the threads, I am using 'swapcontext' function. Before using it I have to make a context using 'makecontext' function. 'makecontext' expects a function pointer with return type void and argument type (void) .

Whereas, the thread function has to be of the type void* thread_func (void*)

Is there a way to do a typecasting? Or is there some other way to do context switching at the user level?

Answer 1

It is illegal to invoke a function with an incompatible prototype by casting the address of the function to a different prototype and invoking it through the resulting pointer:

void *my_callback(void *arg) { ... }

void (*broken)(void *) = (void (*)(void *)) my_callback;
broken(some_arg);   // incorrect, my_callback returns a `void *`

What you can do is pass to makecontext your own callback which will call thread_func and ignore its return value. A small function that only serves to call another function is sometimes called a trampoline .

/* return type is compatible with the prototype of the callback received
   by makecontext; simply calls the real callback */
static void trampoline(int cb, int arg)
{
  void *(*real_cb)(void *) = (void *(*)(void *)) cb;
  void *real_arg = arg;
  real_cb(real_arg);
}

int my_pthread_create(void *(*cb)(void *), void *arg)
{
  ucontext_t *ucp;
  ...
  /* For brevity treating `void *` as the same size as `int` -
     DO NOT USE AS-IS.
     makecontext exposes an annoyingly inconvenient API that only
     accepts int arguments; correct code would deconstruct each
     pointer into two ints (on architectures where pointer is
     larger than int) and reconstruct them in the trampoline. */
  makecontext(ucp, trampoline, 2, (int) cb, (int) arg);
  ...
}

For bonus points, you can modify the trampoline to store the void * value returned by the callback function on the stack and have your equivalent of pthread_join() retrieve it.

Answer 2

In principle, you can always cast any type of pointer to any other kind of pointer, but for function pointers, I would strongly suggest against .

Your thread_func will expect an argument on the stack which will not be provided if invoked after your miscast. Even worse, thread_func will write a return value somewhere where it shouldn't, thus corrupting your stack.

A solution would be to wrap the invocation in its own function of appropriate type.

Answer 3

You can typecast a function pointer just like a variable. The syntax is more awkward, but it's certainly possible (whether it's a good idea is another discussion entirely).

In this case, though, it's probably not what you want to do. From the man page for swapcontext :

Before invoking makecontext(), the caller must allocate a new stack for this context and assign its address to ucp->uc_stack,

Your thread function takes an argument. Pass that argument to the new context via the stack you created. The function that you pass to makecontext() can be a wrapper function that retrieves the value from the stack and passes it to the thread function as an argument. A typecast alone wouldn't provide a method for getting the data in the argument down to the function in the new context.