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
You could us a while or do while loop taking input while there are still boxes to put x's and o's and no one has won the game. Once no boxes are left or someone has won you would end the loop and the game.