XCIII. Semaphore, Shared Memory and IPC Functions
Introduction
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.
Installation
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.
Runtime Configuration
The behaviour of these functions is affected by settings in php.ini.
Table 2. Semaphore Configuration Options
| Name | Default | Changeable |
|---|---|---|
| sysvmsg.value | "42" | PHP_INI_ALL |
| sysvmsg.string | "foobar" | PHP_INI_ALL |
- Table of Contents
- ftok -- Convert a pathname and a project identifier to a System V IPC key
- msg_get_queue -- Create or attach to a message queue
- msg_receive -- Receive a message from a message queue
- msg_remove_queue -- Destroy a message queue
- msg_send -- Send a message to a message queue
- msg_set_queue -- Set information in the message queue data structure
- msg_stat_queue -- Returns information from the message queue data structure
- sem_acquire -- Acquire a semaphore
- sem_get -- Get a semaphore id
- sem_release -- Release a semaphore
- sem_remove -- Remove a semaphore
- shm_attach -- Creates or open a shared memory segment
- shm_detach -- Disconnects from shared memory segment
- shm_get_var -- Returns a variable from shared memory
- shm_put_var -- Inserts or updates a variable in shared memory
- shm_remove_var -- Removes a variable from shared memory
- shm_remove -- Removes shared memory from Unix systems