Function pointers and callback functions

[kikivenkat]

Hi,

I understand the concept of function pointers.
I am a little confused with the call back functions.

1.If a function is invoked using a function pointer, then does it mean the function invoked is a callback function?

2. Some say that the calling convention used for a callback function is different(__stdcall,PASCAL).However, I see that such calling conventions are used only for callback functions registered with the OS.I dont see it being used for application level callbacks.

3.Some say that any function invoked using a function pointer, would automaticaly be a callback only when the call is happening between a dll and third party.

Which of the above statements are correct?

Any help in this regard would be really appreciated.

Reagrds,
kikivenkat

[weaknessforcats]

OK, here you go:

1) all functions are called by address.
2) the name of the function is the address of the function.
3) a function pointer is just a pointer that can contain the address of a function.
4) when you use the function pointer to call the function:

fptr(3);        ??
fptr(3,4);     ???
fptr(3,4,"Hello"); ???
 

the function whose address is in the function pointer better have the correct arguments and return type. That is why when you initialize a function pointer you can only use functions with arguments and return type that match rthe pointer definition:

void MyFunction(int, int);
void (*fptr)(int, int);
 
fptr = MyFunction;   //OK
 
fptr(3,4);     //calls MyFunction
fptr(3);        //COMPILER ERROR - incorect arguments
fptr(3, 4, "Hello");        //COMPILER ERROR - incorect arguments
 

Now a callback function is just passing a function pointer to a function that does the callback. That means using a function pointer as a function argument. And that means the arguiments and return type have to be known at the time the function doing the callback was written - perhaps years before you come along. Therefore, the callback function pointer has pre-defined arguments and return types. That means your function, to be used as a callback, must conform to the callback function spec.

Here is one from one of the Windows APIs:

HRESULT __stdcall myCallbackFunctionName(
DRM_STATUS_MSG msg,
HRESULT hr,
VOID* pvParam,
VOID* pvContext
);

That means your function must have these arguments, return type, and calling convention as this pattern. Otherwise, you can't use it as a callback.