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.
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