System V IPC

System V IPC is the name given to three interprocess communication mechanisms that are widely available on UNIX systems: message queues, semaphore, and shared memory.

For System V IPC APIs visit sysvipc(7)

Although POSIX provides alternative APIs for each mechanism, the functionality is same in both SysV and POSIX APIs.

Message queues

System V message queues allow data to be exchanged in units called messages. Each message can have an associated priority.
See POSIX message queue mq_overview(7)

Semaphores

System V semaphores allow processes to synchronise their actions. System V semaphores are allocated in groups called sets; each semaphore in a set is a counting semaphore.

See POSIX semaphores sem_overview(7).

A semaphore is an integer whose value is never allowed to fall below zero. Two operations can be performed on semaphores:
increment the semaphore value by one (sem_post(3)); and decrement the semaphore value by one (sem_wait(3)).
If the value of a semaphore is currently zero, then a sem_wait(3) operation will block until the value becomes greater than zero.

Shared memory segments

System V shared memory allows processes to share a region a memory (a “segment”). See POSIX shared memory shm_overview(7)