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Friday, January 8, 2010


Semaphores can best be described as counters used to control access to shared resources by multiple processes. They are most often used as a locking mechanism to prevent processes from accessing a particular resource while another process is performing operations on it.
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. A counting semaphore is a counter for a set of available resources, rather than a locked/unlocked flag of a single resource.
A semaphore S is an integer variable that, apart from initialization, is accessed only through two standard atomic operations : wait and signal. These operations were originally termed P(for wait) and V(for signal). The classical definitions of wait and signal are :
wait(S) : while S<=0 do no-op;

signal(S) : S:= S+1;

Modifications to the integer value of the semaphore in the wait and signal operations must be executed indivisibly.
- Semaphores can be used to deal with the n-process critical- section problem.
- Semaphores can be used to solve various synchronization problems.

A mutex is a binary semaphore (a semaphore with capacity of 1), usually including extra features like ownership, priority inversion protection or recursivity. The differences between mutexes and semaphores are operating system dependent, though mutexes are implemented by specialized and faster routines. Mutexes are meant to be used for mutual exclusion (post/release operation is restricted to thread which called pend/acquire) only and binary semaphores are meant to be used for event notification (post-ability from any thread) and mutual exclusion.

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