Passing function pointers

TraverseWith takes as one of its arguments a pointer to a function that returns an int and takes 3 parameters. Can someone tell me (in terms of asm) what the compiler does with the code int (*callback)(int index, int item, void *param).

Code:

int traverseWith(int array[], size_t length, 
                 int (*callback)(int index, int item, void *param), 
                 void *param)

Thanks alot!

It is a pointer like any other. The compiler loads it into a register and calls it like any other function.

Originally Posted by brewbuck

It is a pointer like any other.

*Like any other function pointer.

Originally Posted by Dave_Sinkula

*Like any other function pointer.

True.

What is this code: "int (*callback)(int index, int item, void *param)" really saying?

What would the asm look like?

Your compiler will give you the best answer. It's not the same everywhere.

Originally Posted by keira

What is this code: "int (*callback)(int index, int item, void *param)" really saying?

What would the asm look like?

The "prototype" that you have for

Code:

int (*callback)(int index, int item, void *param)

is there so that the compiler can check that you have a sensible function as the callback.

In this case, it says "callback" is a pointer, to a function that returns an in, and the function takes two integer and one void * parameter.

It really makes no difference to the compiler as such if you were to pass a function pointer to a function that takes one parameter, or 15 parameters - but because it's probably not the right thing to do, we have taught the compiler to tell us when we get it wrong.

callback itself is just a pointer - it happens to be a pointer to some function, and when it is being used later on in the code.

But for the call to "traverseWith", all it will do is pick up a 32 (or 64) bit pointer to the function you want to use, and store that so that the traverseWith function can use it to call the callback function.

Inside traversewith, you'll find some code that looks something like this:

Code:

      .... 
       mov  param[esp], eax
       push eax
       mov  index[esp], eax
       push eax
       mov  item[esp], eax
       push eax
       mov  callback[esp], eax
       call    eax
     ....

Appologies for unaligned spacing on the assembler code.

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