C code.

Read and Write process communication using shared memory in C.

How do you use a flag in shmget() (a flag stored in shared memory so that all processes can access them) so that a write process knows that a string has been displayed, so it can write a new string. And so that a read process knows when a string has been written into memory, so it can display that string (prevent displaying the same string again). Please provide a code example.