Need help in Global variable in c

Hi,

I read this article about global variable in c:
http://www.phim.unibe.ch/comp_doc/c_manual/C/SYNTAX/glo_int_vars.html

But I have a few questions
1. how can I declare the global variable so that it can be set/get by
other files?
2. If I have multiple instance of the same program running in the same
machine (linux), will each program instance has its own of global
variable?

Thank you.

.