# Semaphore, Shared Memory and IPC Functions

## Introduction

This module provides 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: This extension is not available on Windows platforms.

## Requirements

No external libraries are needed to build this extension.

## Installation

Support for this functions are not enabled by default. To enable System V semaphore support compile PHP with the option --enable-sysvsem. To enable the System V shared memory support compile PHP with the option --enable-sysvshm. To enable the System V messages support compile PHP with the option --enable-sysvmsg.

## Runtime Configuration

The behaviour of these functions is affected by settings in php.ini.

Semaphore Configuration Options
Name | Default | Changeable | Changelog
--- | --- | --- | ---
sysvmsg.value | "42" | PHP_INI_ALL |
sysvmsg.string | "foobar" | PHP_INI_ALL |

For further details and definitions of the PHP_INI_* constants, see the php.ini directives.

## Resource Types

## Predefined Constants

The constants below are defined by this extension, and will only be available when the extension has either been compiled into PHP or dynamically loaded at runtime.

System V message constants
Constant | Type | Changelog
--- | --- | ---
MSG_IPC_NOWAIT | integer |
MSG_EAGAIN | integer | As of 5.2.0
MSG_ENOMSG | integer | As of 5.2.0
MSG_NOERROR | integer |
MSG_EXCEPT | integer |

## Table of Contents

- ftok — Convert a pathname and a project identifier to a System V IPC key
- msg_get_queue — Create or attach to a message queue
- msg_receive — Receive a message from a message queue
- msg_remove_queue — Destroy a message queue
- msg_send — Send a message to a message queue
- msg_set_queue — Set information in the message queue data structure
- msg_stat_queue — Returns information from the message queue data structure
- sem_acquire — Acquire a semaphore
- sem_get — Get a semaphore id
- sem_release — Release a semaphore
- sem_remove — Remove a semaphore
- shm_attach — Creates or open a shared memory segment
- shm_detach — Disconnects from shared memory segment
- shm_get_var — Returns a variable from shared memory
- shm_put_var — Inserts or updates a variable in shared memory
- shm_remove_var — Removes a variable from shared memory
- shm_remove — Removes shared memory from Unix systems