   #PHP Manual Function Reference sql_regcase ftok

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XCIII. Semaphore, Shared Memory and IPC Functions

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.

Requirements

   No external libraries are needed to build this extension.

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().

Resource Types

Predefined Constants

   This extension has no constants defined.

   Table of Contents
   ftok --  Convert a pathname and a project identifier to a System V IPC
          key

   msg_get_queue --  Create or attach to a message queue
   msg_receive --  Receive a message from a message queue
   msg_remove_queue --  Destroy a message queue
   msg_send --  Send a message to a message queue
   msg_set_queue --  Set information in the message queue data structure
   msg_stat_queue --  Returns information from the message queue data
          structure

   sem_acquire -- Acquire a semaphore
   sem_get -- Get a semaphore id
   sem_release -- Release a semaphore
   sem_remove -- Remove a semaphore
   shm_attach -- Creates or open a shared memory segment
   shm_detach -- Disconnects from shared memory segment
   shm_get_var -- Returns a variable from shared memory
   shm_put_var -- Inserts or updates a variable in shared memory
   shm_remove_var -- Removes a variable from shared memory
   shm_remove -- Removes shared memory from Unix systems
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