Signal handlers - please help to understand

[Igor Gorbounov <igorbounov topazelectro ru>]

I need a help in understanding this thing: what is actually done when 
somewhere
in a program some signal is emitted? Where are data originated from? Does
it all behave as a calling procedure and is stack used here? Well, here is a
situation: some data come into commport, my app receives a SIGIO signal and
emits a custom signal (from a sigc++ library), that delivers the receved 
data
to other parts of a program. So what is going here - is this SIGIO 
signal handler
participating in further data propagation or it looks like this handler 
just raised
some flag and exited? Is there any "halvanic" connection between the SIGIO
handler and the parts of program, that use those data? Where do those 
data belong?
Or, what is their scope?
Sorry if this question appeared to be somewhat offtopic...
   Igor Gorbounov