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# XCVIII.Semaphore, Shared Memory and IPC Functions

# 介紹

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.

> 備注:此延伸不適用於 Windows 平台。

# 需求

建立這個延伸不需要任何外部程式庫。

# 安裝

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.

# 執行時期組態

`php.ini`中的設定會影響這些函數的行為。

表格 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().

# 資源類型

# 預先定義常數

以下的常數由此延伸定義，只在這個延伸被編譯成PHP或實行時期被動態載入時有效。

表格 3.System V message constants

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

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