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.
Table 1. 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.
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.
Table 2. Semaphore Configuration Options
| Name | Default | Changeable |
|---|
| sysvmsg.value | "42" | PHP_INI_ALL |
| sysvmsg.string | "foobar" | PHP_INI_ALL |
For further details and definition of the PHP_INI_* constants see
ini_set().
