what is actually the use and meaning when lets say you specify CALLBACK to a function some say that its for calling Convention, some say its when using function pointers im confused pls advise

Just for reference, because I didn't realize this at first, he's talking about the C macro CALLBACK not what callbacks are (i think)

and I have no freaking clue, sounds like a win32 thing.

In essence on most wi32 platforms CALLBCK is just __stdcall.

Quote:From MSDN:
Microsoft Specific

The __stdcall calling convention is used to call Win32 API functions. The
callee cleans the stack, so the compiler makes vararg functions __cdecl.
Functions that use this calling convention require a function prototype.

return-type __stdcall function-name[(argument-list)]

The following list shows the implementation of this calling convention.

Element                 |          Implementation ------------------------+--------------------------------------------------Argument-passing        |         order Right to left. Argument-passing        | convention By value, unless a pointer or reference type is passed.                         |Stack-maintenance       | responsibility Called function pops its own arguments from the stack.                         |Name-decoration         | convention An underscore (_) is prefixed to the name.                         | The name is followed by the at sign (@) followed by                         | the number of bytes (in decimal) in the argument                         | list. Therefore, the function declared as                         | int func( int a, double b ) is decorated as follows: _func@12 Case-translation        | convention None 

The /Gz compiler option specifies __stdcall for all functions not explicitly
declared with a different calling convention.

Functions declared using the __stdcall modifier return values the same way as
functions declared using __cdecl.

END Microsoft Specific

[edit]
Formatting...
[/edit]

I'm not sure about a true definition but the way I see it is that callback is usually used for functions. Many API's including the Windows API will use these functions when a certain event occures: in other words, it will 'callback' to them. The Windows API however, is usually more focuesd on using messages than using callback functions. Often, the function itself will be passed as a parameter and the API being used will call the function passed to it when a certain event occures or a certain condition or set of conditions is true.

This is only my interpretation of what a callback is used for and it could be wrong.

Quote:Original post by yuppies
what is actually the use and meaning when lets say you specify CALLBACK to a function
some say that its for calling Convention, some say its when using function pointers
im confused

pls advise

To answer your question directly: both is right. CALLBACK is used to define a common calling convention, esp. to avoid stack corruption when using funktion pointers. Without a common calling convention within the API (which is sperad across many DLLs), name lookup as well as function arguement retrieval and stack constistency could be messed up when DLL A calls code from DLL B (be it a function call from within the API or a function pointer provided by a user program).

Since Microsoft introduced other calling convention, such as __fastcall, which passes as many parameters using registers as possible, there had to be a common standard in order to keep the stack usage and argument retrievial consistent between multiple processes.

Hope that help,
Pat.