For what is{Composite,Damage,XFixes}ClientCallback?

[Carlos Eduardo Rodrigues Diógenes]

Hi,

What is the need to do what is done in
{Composite,Damage,XFixes}ClientCallback?

I read the "Definition of the Porting Layer for the X v11 Sample Server"
that comes with the xserver docs and comments about the "Callback
Manager", but can't realize for what is the code in the functions above.

Thanks,
-- 
Carlos Eduardo Rodrigues Diógenes
Projeto xLupa - http://www.unioeste.br/projetos/xlupa