CXLVI. 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	Changelog
sysvmsg.value	"42"	PHP_INI_ALL
sysvmsg.string	"foobar"	PHP_INI_ALL

For further details and definitions of the PHP_INI_* constants, see the Appendix I.

Resource Types

Predefined Constants

The constants below are defined by this extension, and will only be available when the extension has either been compiled into PHP or dynamically loaded at runtime.

Table 3. System V message constants

Constant	Type	Changelog
MSG_IPC_NOWAIT	integer
MSG_EAGAIN	integer	As of 5.2.0
MSG_ENOMSG	integer	As of 5.2.0
MSG_NOERROR	integer
MSG_EXCEPT	integer

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