Passing global variable to assembly

["nono240"]

Hi all, and thanks for reading.

I'm playing for days with the ARM port of GCC, and i need to access
global C variable from assembly.
Sorry for my newbie questions, but I'm relatively new to GCC and i've
tried to reach my global variable by using an underscore ( _ ) as
prefix without success.
I've one single C file and one single assembly file (.s).
Should i include some headers, or some directives for files to know
each other ?

Thank you in advance