php-doc-en/reference/sem/reference.xml

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 <reference id="ref.sem">
  <title>Semaphore, Shared Memory and IPC Functions</title>
  <titleabbrev>Semaphore</titleabbrev>

  <partintro>
   <para>
    This module provides 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>
     These functions do not work on Windows systems.
    </simpara>
   </note>
  </partintro>

&reference.sem.functions;

</reference>
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