Introduction

These modules provide wrappers for the System V IPC family of functions. It includes semaphores, shared memory and inter-process messaging (IPC).

Semaphores may be used to provide exclusive access to resources on the current machine, or to limit the number of processes that may simultaneously use a resource.

This module provides also shared memory functions using System V shared memory. Shared memory may be used to provide access to global variables. Different httpd-daemons and even other programs (such as Perl, C, ...) are able to access this data to provide a global data-exchange. Remember, that shared memory is NOT safe against simultaneous access. Use semaphores for synchronization.

**Limits of Shared Memory by the Unix OS**
SHMMAX	max size of shared memory, normally 131072 bytes
SHMMIN	minimum size of shared memory, normally 1 byte
SHMMNI	max amount of shared memory segments on a system, normally 100
SHMSEG	max amount of shared memory segments per process, normally 6

The messaging functions may be used to send and receive messages to/from other processes. They provide a simple and effective means of exchanging data between processes, without the need for setting up an alternative using Unix domain sockets.

Note: Only the shared memory functions and ftok are available on Windows. Neither semaphores nor inter-process messaging functions are supported on that platform.