Overview of Semaphore Operations

Semaphore Operations

The semget() system call allocates an array containing a specified number of semaphores (see semget(2)). Assigning an array with only one semaphore in the set is usually the most sensible for keeping programs reasonably simple. Subsequent semaphore operations are performed atomically on the entire array; individual semaphores in the array are manipulated by an array of semaphore operations (see semop(2)). semctl() is used to ascertain or change a semaphore's permissions, change time, or owner (see semctl(2)).

Semaphores are typically incremented by a process to block other processes while it is performing a critical operation or using a shared resource. When finished, it decrements the value, allowing blocked processes to then access the resource. Semaphores can be configured as binary semaphores which have only two values: 0 and 1, or they can serve as general semaphores (or counters) where one process increments the semaphore and one or more cooperating processes decrement it. To prevent undetectable overflow conditions, the kernel imposes a maximum value limit beyond which semaphores cannot be incremented. This limit, defined by the semvmx kernel parameter, must not exceed the maximum value of 65535. Semaphores are not allowed to have negative (less than zero) values.

Semaphore Undo Operations

It may occasionally be necessary — when errors occur, a process must abort, a process dies, etc. — to change one or more semaphores to a new or previous value. This is called undoing a semaphore. Since the value of any semaphore when such conditions occur is unpredictable, the system enforces a limit on how much the value of a semaphore can change any undo operation. This limit is defined by the semaem kernel parameter.

For more information about System V IPC semaphore operation, consult an advanced UNIX programming text such as Advanced UNIX Programming by Marc J. Rochkind, Prentice-Hall, Inc., ISBN 0-13-011800-1.

Semaphore Limits

Configurable kernel parameters are available to limit the number of sets of semaphores that can exist simultaneously on the system and the total number of individual semaphores available to users that can exist simultaneously on the system. The number of semaphores that can exist in a set is not configurable. It is fixed at the POSIX standard value, 500.