c++ using extern keyword

[snakec]

hi can any body explain working of extern function with example code? .I don't know how to use them ?how to declare them ?how to define them? please explain with source code.

[sasanet]

functions are all extern by default.
and variables are not.

define:
int test = 0; in some *cpp file globaly
then in another *.cpp file use:
extern int test; globaly too.