Note: This manual is provided for your convenience only - this is not the official source and may not be 100% up to date.
CXLVI. Semaphore, Shared Memory and IPC Functions
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
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.
No external libraries are needed to build this extension.
Support for this functions are not enabled by default.
To enable System V semaphore support compile PHP with the option
The behaviour of these functions is affected by settings in php.ini.
For further details and definitions of the PHP_INI_* constants, see the Appendix H.
Table 2. Semaphore Configuration Options
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.