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<book xml:id="book.sem" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
 <?phpdoc extension-membership="bundled" ?>
 <title>Semaphore, Shared Memory and IPC </title>
 <titleabbrev>Semaphore</titleabbrev>
 
 <!-- {{{ preface -->
 <preface xml:id="intro.sem">
  &reftitle.intro;
  <para>
   These modules provide wrappers for the System V IPC family of functions.
   It includes semaphores, shared memory and inter-process messaging (IPC).
  </para>
  <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.
  </para>
  <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>
  <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.
  </para>
  <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>

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