U need the extern statement. That plus the function prototype tell the compiler/likerhow to set up the call, what to return from it, what the storage spaces required are and also. extern tells the compiler it will find the function (or a variable) in another file.
One way of doing this is to create a main.h, decalre this variable as extern and then include the main.h in the init.c file. This is needed if the compiler enforces ansi stds strictly. Otherwise, make sure that u link both files after compilation, use the extern keyword directly in your init.c file.