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# CXXXIX. Semaphore, Shared Memory and IPC Functions

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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.

表格 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.

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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.

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表格 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 附錄 G.

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表格 3. System V message constants

Constant | Type
--- | ---
MSG_IPC_NOWAIT | integer
MSG_NOERROR | integer
MSG_EXCEPT | integer

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