What are these statements for?

Hi

Code:

static void *l_alloc (void *ud, void *ptr, size_t osize, size_t nsize) {
  (void)ud;
  (void)osize;
  if (nsize == 0) {
    free(ptr);
    return NULL;
  }
  else
    return realloc(ptr, nsize);
}

in the function above: Could anybody tell me what the red statements are for? They should have absolutely no effect to the program logic.
The only thing I can think of is that the compiler is used to check something about the condition of the passed arguments.
But why casting a void* to void? I didn't even know that this is legal in C/C++.

Thank you!

to hide the warning

warning C4100: 'ud' : unreferenced formal parameter

Ah thanks.

Code:

static void *l_alloc (void */*ud*/, void *ptr, size_t osize, size_t nsize)

would be a bit nicer in my eyes.

But: Why can I cast to void but not declare something as void, e.g.

Code:

void a; //error
void* b;
(void)b; //legal

Originally Posted by pheres

Ah thanks.

Code:

static void *l_alloc (void */*ud*/, void *ptr, size_t osize, size_t nsize)

would be a bit nicer in my eyes.

we are talking C here?

error C2055: expected formal parameter list, not a type list

Why can I cast to void but not declare something as void, e.g.

void is not a type

void* is type

I wasn't aware of the fact that one can cast to anything else than a type.
For whats reason is that possible or what could someone possibly do with the result?

I think that is just a way to signal that the line should be a no-op (meaning something like: ignore the result). Without (void) the compiler would warn that the statement has no effect. Now it sees that it is not supposed to have an effect.