void pointer to pointer

Let's say we have a function func that that takes a void ** argument and changes what *arg points to.

Code:

void func(void **arg);

I have to cast the argument to void ** so the compiler doesn't complain, which I'm guessing is a sign that I'm doing something that I shouldn't be doing.

Code:

int *ptr;
func((void **)&ptr);

It doesn't look like you are doing anything wrong. Is there an actual problem with your code? Most likely you are using some C++ compiler which requires explicit casting.

or perhaps you forgot to #include stdlib.h?

I'm guessing it has something to do with this:

Chapter 22: Pointers to Pointers

One side point about pointers to pointers and memory allocation: although the void * type, as returned by malloc, is a ``generic pointer,'' suitable for assigning to or from pointers of any type, the hypothetical type void ** is not a ``generic pointer to pointer.''

Originally Posted by Memloop

I'm guessing it has something to do with this:

Chapter 22: Pointers to Pointers

Good find!

Pointer types can only convert to void * without a cast. void ** is not the same as void *. You can relieve the caller of the need to cast by doing it yourself:

Code:

void func(void *arg)
{
    void **realarg = (void **)arg;
    /* whatever */
}

EDIT: To expand a bit, "void *" means "a pointer to anything," which is very general. "void **" means "a pointer to a void *" which is much more specific.