Newbie about using extern in C.

[James Ng Yuen Sum <james_nghk@yahoo.com.hk>]

Hello,

I am a newbie in using linux and C language. I want to ask how to use
"extern". In C language, we can have two .c file, in one file the
variable(s) can be open for other .c file use if extern is the storage
class of the variable.

But my teacher has told me that in gcc, I have to use Makefile to do it.
But I do not know how to do that in gcc.

Thank you for any suggestion.

Regards,
James Ng