<?xml version="1.0" encoding="utf-8"?>

<book xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="book.sem">

<?phpdoc extension-membership="bundled" ?>

<title>Semaphore, Shared Memory and IPC</title>

<titleabbrev>Semaphore</titleabbrev>

<!-- {{{ preface -->

<preface xml:id="intro.sem">

&reftitle.intro;

<simpara>

These modules provide wrappers for the System V IPC family of functions.

It includes semaphores, shared memory and inter-process messaging (IPC).

</simpara>

<simpara>

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.

</simpara>

<para>

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>

<title>Limits of Shared Memory by the Unix OS</title>

<tgroup cols="2">

<tbody>

<row>

<entry>SHMMAX</entry>

<entry>max size of shared memory, normally 131072 bytes</entry>

</row>

<row>

<entry>SHMMIN</entry>

<entry>minimum size of shared memory, normally 1 byte</entry>

</row>

<row>

<entry>SHMMNI</entry>

<entry>

max amount of shared memory segments on a system,

normally 100

</entry>

</row>

<row>

<entry>SHMSEG</entry>

<entry>

max amount of shared memory segments per process, normally 6

</entry>

</row>

</tbody>

</tgroup>

</table>

</para>

<simpara>

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.

</simpara>

<note>

<simpara>

Only the shared memory functions and <function>ftok</function> are available

on Windows. Neither semaphores nor inter-process messaging functions are

supported on that platform.

</simpara>

</note>

</preface>

<!-- }}} -->

&reference.sem.setup;

&reference.sem.constants;

&reference.sem.reference;

&reference.sem.sysvmessagequeue;

&reference.sem.sysvsemaphore;

&reference.sem.sysvsharedmemory;

</book>