Writing C functions

This is a very quick question regarding C functions:

After you have written a functions.c file, how do you
compile it how postgres wants it with gcc?
The tutorial just said that here is an example, you
compile it as a .so file, then import it with these
sql commands. It didn't say how to make it a .so file
(it assumed we would know I guess).

Thanks (and please excuse my ignorance),
Jeff Davis
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