Chapter 51. Introduction to POSIX IPC

The POSIX.1b realtime extensions defined a set of IPC mechanisms that are analogous to the System V IPC mechanisms described in Chapter 45 to Chapter 48. (One of the POSIX.1b developers’ aims was to devise a set of IPC mechanisms that did not suffer the deficiencies of the System V IPC facilities.) These IPC mechanisms are collectively referred to as POSIX IPC. The three POSIX IPC mechanisms are the following:

• Message queues can be used to pass messages between processes. As with System V message queues, message boundaries are preserved, so that readers and writers communicate in units of messages (as opposed to the undelimited byte stream provided by a pipe). POSIX message queues permit each message to be assigned a priority, which allows high-priority messages to be queued ahead of low-priority messages. This provides some of the same functionality that is available via the type field of System V messages.

• Semaphores permit multiple processes to synchronize their actions. As with System V semaphores, a POSIX semaphore is a kernel-maintained integer whose value is never permitted to go below 0. POSIX semaphores are simpler to use than System V semaphores: they are allocated individually (as opposed to System V semaphore sets), and they are operated on individually using two operations that increase and decrease a semaphore’s value by one (as opposed to the ability of the semop() system call to atomically add or subtract arbitrary values from multiple semaphores in a System V semaphore set).

• Shared memory enables multiple processes to share the same region of memory. As with System V shared memory, POSIX shared memory provides fast IPC. Once one process has updated the shared memory, the change is immediately visible to other processes sharing the same region.