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<title>Introduction</title>
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<h1 class="title">Introduction</h1>
<p class="para">
This module provides wrappers for the System V IPC family of functions.
It includes semaphores, shared memory and inter-process messaging (IPC).
</p>
<p class="para">
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.
</p>
<p class="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 class="doctable table">
<caption><strong>Limits of Shared Memory by the Unix OS</strong></caption>
<tbody class="tbody">
<tr>
<td>SHMMAX</td>
<td>max size of shared memory, normally 131072 bytes</td>
</tr>
<tr>
<td>SHMMIN</td>
<td>minimum size of shared memory, normally 1 byte</td>
</tr>
<tr>
<td>SHMMNI</td>
<td>
max amount of shared memory segments on a system,
normally 100
</td>
</tr>
<tr>
<td>SHMSEG</td>
<td>
max amount of shared memory segments per process, normally 6
</td>
</tr>
</tbody>
</table>
</p>
<p class="para">
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.
</p>
<blockquote class="note"><p><strong class="note">Note</strong>: <span class="simpara">This extension is not
available on Windows platforms.</span></p></blockquote>
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