what why how ？
http://tedfelix.com/software/c++-callbacks.html
Using a C++ class member function (cannot be static) as a C callback function
http://stackoverflow.com/questions/4881896/using-a-c-class-member-function-cannot-be-static-as-a-c-callback-function
Callbacks in C++
http://tedfelix.com/software/c++-callbacks.html
The Function Pointer Tutorials
http://www.newty.de/fpt/intro.html
http://www.cprogramming.com/tutorial/function-pointers.html