What is a semaphore? Explain busy waiting semaphores.

A semaphore is a protected variable or abstract data type which constitutes the classic method for restricting access to shared resources such as shared memory in a parallel programming environment.

 

Weak, Busy-wait Semaphores:
• The simplest way to implement semaphores.
• Useful when critical sections last for a short time, or we have lots of CPUs.
• S initialized to positive value (to allow someone in at the beginning).
• S is an integer variable that, apart from initialization, can only be accessed through 2
atomic and mutually exclusive operations: wait(s):

                              while (s.value != 0);

                              s.value--;
                        signal(s):

s.value++;
All happens atomically i.e. wrap pre and post protocols.