semaphores

Semaphores are a programming construct designed by
E. W. Dijkstra in the late 1960s. Dijkstra's model was the operation of
railroads: consider a stretch of railroad in which there is a single
track over which only one train at a time is allowed. Guarding this track
is a semaphore. A train must wait before entering the single track until
the semaphore is in a state that permits travel. When the train enters the
track, the semaphore changes state to prevent other trains from entering
the track. A train that is leaving this section of track must again
change the state of the semaphore to allow another train to enter. In the
computer version, a semaphore appears to be a simple integer. A
process (or a thread) waits for permission to proceed by waiting for
the integer to become 0. The signal if it proceeds signals that this by
performing incrementing the integer by 1.
When it is finished, the process changes the
semaphore's value by subtracting one from it.

Semaphores let processes query or alter status information. They are
often used to monitor and control the availability of system resources
such as shared memory segments.

Semaphores can be operated on as individual units or as elements in a
set. Because System V IPC semaphores can be in a large array, they are
extremely heavy weight. Much lighter weight semaphores are available in
the threads library (see man semaphore and also
Chapter 30.3) and POSIX semaphores (see below briefly).
Threads library semaphores must be used with mapped memory . A semaphore
set consists of a control structure and an array of individual
semaphores. A set of semaphores can contain up to 25 elements.

In a similar fashion to message queues, the semaphore set must be
initialized using semget(); the semaphore creator can change its
ownership or permissions using semctl(); and semaphore operations
are performed via the semop() function