About the concept of the callback

[libra]

Hello everybody:

I am tracing the gdb source code now.

In the gdb source code,there use many callback function.

I don't understand why it uses callback.

Now, i know how to implement the callback function.

But, i still don't know why it must use the callback. 

Anyone can tell me where is the document about the concept of the callback

that i can get.

                          Thank a lot!!!