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.
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 --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.
The behaviour of these functions is affected by settings in php.ini.
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.