How do i use testandset() to synchronise two critical sections

Question

I have to write two .c programs, both using the same shared memory. While one cycles from a to z, the other one has to print the same char. The task is to print the alphabet using testandset(). I'm not sure how to utilise the function so that it's completed.

this is the printing(consumer) program I tried this is the cycling(producer) program I tried