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I was looking through some code from the SDL library and came across a function declared like this:
LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
Now, I'm a Delphi coder. No hablo C muy bien, senor. But I remember enough syntax from my college courses to read it like this:
Function name is WndProc. Argument list is pretty self-explanatory. Function return type is LRESULT. But what in the world is that "CALLBACK" doing there? In Delphi, any function can be used as a callback; you just need to pass the right type of function pointer. Is there any particular reason why C doesn't work that way? Or does it mean something different?
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A callback's primary purpose is to execute code in response to an event. These events might be user-initiated, such as mouse clicks or typing. With a callback, you may instruct your application to "execute this code every time the user clicks a key on the keyboard." button.
A custom callback function can be created by using the callback keyword as the last parameter. It can then be invoked by calling the callback() function at the end of the function. The typeof operator is optionally used to check if the argument passed is actually a function. console.
Example: Using a Callback FunctionThe sayName() function is passed as an argument to the greet() function. The setTimeout() method executes the greet() function only after 2 seconds. However, the sayName() function waits for the execution of the greet() function.
Simply put: A callback is a function that is to be executed after another function has finished executing — hence the name 'call back'. More complexly put: In JavaScript, functions are objects. Because of this, functions can take functions as arguments, and can be returned by other functions.
The "CALLBACK" is a calling convention. There are other kinds of calling conventions. CALLBACK is the same as __stdcall.
http://www.codeguru.com/cpp/cpp/cpp_mfc/callbacks/article.php/c10557
Some more information at Raymond Chen's blog:
http://blogs.msdn.com/oldnewthing/archive/2004/01/08/48616.aspx
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