It sounds like your professor is trying to get you all to understand the scope of variables and loops.
You can build a tic tac.toe without a self defined function, you will still have to use main however.
Use a for loop which sets the variable to check for a win
Set your array array of nine to zero
Accept input with all your error checks
Check for win or tie
Break out of loop
A C program usually at least has the main function as an entry point to the program.
There are certain exceptions though. You can look here on this link
I just did a simple Google search btw