System V vs Posix IPC

 System V vs Posix IPC

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 twenty five semaphores.

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.

System V Semaphores

Permissions for Messages, Semaphores, and Shared Memory

Messages, semaphores, and shared memory have read and write permissions, but no execute permission, for the owner, group, and others, which is similar to ordinary files. Like files, the creating process identifies the default owner. Unlike files, the creating process can assign ownership of the facility to another user or revoke an ownership assignment.

System V Semaphores versus POSIX Semaphores

The current Unix environment comes with two types of semaphores: System V and POSIX. In general, the older Unix-based systems uses the System V version and the current Linux-based systems use the POSIX version. However, the general behavior and technology of semaphores does not change irrespective of the version used. Let’s look at the interfaces, and how they work on these two different versions of semaphores.

There are a number of differences between System V and POSIX semaphores.

  • One marked difference between the System V and POSIX semaphore implementations is that in System V you can control how much the semaphore count can be increased or decreased; whereas in POSIX, the semaphore count is increased and decreased by 1.
  • POSIX semaphores do not allow manipulation of semaphore permissions, whereas System V semaphores allow you to change the permissions of semaphores to a subset of the original permission.
  • Initialization and creation of semaphores is atomic (from the user’s perspective) in POSIX semaphores.
  • From a usage perspective, System V semaphores are clumsy, while POSIX semaphores are straight-forward
  • The scalability of POSIX semaphores (using unnamed semaphores) is much higher than System V semaphores. In a user/client scenario, where each user creates her own instances of a server, it would be better to use POSIX semaphores.
  • System V semaphores, when creating a semaphore object, creates an array of semaphores whereas POSIX semaphores create just one. Because of this feature, semaphore creation (memory footprint-wise) is costlier in System V semaphores when compared to POSIX semaphores.
  • It has been said that POSIX semaphore performance is better than System V-based semaphores.
  • POSIX semaphores provide a mechanism for process-wide semaphores rather than system-wide semaphores. So, if a developer forgets to close the semaphore, on process exit the semaphore is cleaned up. In simple terms, POSIX semaphores provide a mechanism for non-persistent semaphores.


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