how to call Functions implemented in other file?

[houanito]

hello!

i have a problem using gcc c Complier in Unix.
I want to use a function which is implemented in a file in another path.This function is  written by the distirbuters of the system and so the system uses this function.But i want to use that in order to do one part of my homework.
Because the file that contains this function is *.c and not *.h, adding #include *.c to my code,generates problem..

any help??thanks!