IPC: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 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.

Controlling Semaphores

semctl() changes permissions and other characteristics of a semaphore set. It is prototyped as follows:

int semctl(int semid, int semnum, int cmd, union semun arg);

It must be called with a valid semaphore ID, semid. The semnum value selects a semaphore within an array by its index. The cmd argument is one of the following control flags:

GETVAL
— Return the value of a single semaphore.
SETVAL
— Set the value of a single semaphore. In this case, arg is taken as arg.val, an int.
GETPID
— Return the PID of the process that performed the last operation on the semaphore or array.
GETNCNT
— Return the number of processes waiting for the value of a semaphore to increase.
GETZCNT
— Return the number of processes waiting for the value of a particular semaphore to reach zero.
GETALL
— Return the values for all semaphores in a set. In this case, arg is taken as arg.array, a pointer to an array of unsigned shorts (see below).
SETALL
— Set values for all semaphores in a set. In this case, arg is taken as arg.array, a pointer to an array of unsigned shorts.
IPC_STAT
— Return the status information from the control structure for the semaphore set and place it in the data structure pointed to by arg.buf, a pointer to a buffer of type semid_ds.

IPC_SET
— Set the effective user and group identification and permissions. In this case, arg is taken as arg.buf.
IPC_RMID
— Remove the specified semaphore set.