POSIX Interprocess Communication

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POSIX interprocess communication (IPC) is a variation of System V interprocess communication. Like System V objects, POSIX IPC objects have read and write, but not execute, permissions for the owner, the owner's group, and for others. There is no way for the owner of a POSIX IPC object to assign a different owner. POSIX IPC includes the following features:

Messages allow processes to send formatted data streams to arbitrary processes.
Semaphores allow processes to synchronize execution.
Shared memory allows processes to share parts of their virtual address space.

Unlike the System V IPC interfaces, the POSIX IPC interfaces are all multithread safe.

POSIX Messages

The POSIX message queue interfaces are listed in the following table.

Table 7 POSIX Message Queue Interfaces

Interface Name	Purpose
`mq_open()`	Connects to and optionally creates a named message queue
`mq_close()`	Ends the connection to an open message queue
`mq_unlink()`	Ends the connection to an open message queue and causes the queue to be removed when the last process closes it
`mq_send()`	Places a message in the queue
`mq_receive()`	Receives (removes) the oldest, highest priority message from the queue
`mq_notify()`	Notifies a process or thread that a message is available in the queue
`mq_setattr()`	Set or get message queue attributes

POSIX Semaphores

POSIX semaphores are much lighter weight than are System V semaphores. A POSIX semaphore structure defines a single semaphore, not an array of up to 25 semaphores.

The POSIX semaphore interfaces are shown here:

sem_open(): Connects to, and optionally creates, a named semaphore
sem_init(): Initializes a semaphore structure (internal to the calling program, not a named semaphore)
sem_close(): Ends the connection to an open semaphore
sem_unlink(): Ends the connection to an open semaphore and causes the semaphore to be removed when the last process closes it
sem_destroy(): Initializes a semaphore structure (internal to the calling program, not a named semaphore)
sem_getvalue(): Copies the value of the semaphore into the specified integer
sem_wait(): Blocks while the semaphore is held by other processes or returns an error if the semaphore is held by another process

POSIX Shared Memory

POSIX shared memory is actually a variation of mapped memory (see Creating and Using Mappings). The major differences are:

You use shm_open() to open the shared memory object instead of calling open().
You use shm_unlink() to close and delete the object instead of calling close() which does not remove the object.

The options in shm_open() substantially fewer than the number of options provided in open().