Using the extern keyword.

[deltacoder1020]

no, do not define the variable in user_routines.h - this does not actually share the variable. instead, you get two copies, one in each file. (remember, putting something in an include file is just like inserting that text into each file in which you #include it). if you look at the user_routines.h file, you will notice that there is not a single actual variable defined there - there's a reason for that.

extern is probably the way to go here, just declare the variable in one file, then copy the definition to the other but tack "extern " on the front. if you're passing more than one thing back and forth, you might consider creating a struct to keep all of the info in one single variable that can be extern-ed (having more than one or two global variables looks messy )

if you wanted to be ambitious, you could modify main.c so that Process_Data_From_Local_IO() and Process_Data_From_Master_uP() could return and be called with a value, thus passing the info that way (using a struct)