Yes the you can use the keyword extern like the following:
Code:
//user_routines_fast.c
int foo;
...
//user_routines.c
extern int foo;
void test()
{
foo=2;
}
But the more professional way of doing this is by declaring the variable in user_routines.h where than you don't have to use extern but rather only have to include the file user_routines.h (as it is already done in the default code)