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|
max amount of shared memory segments on a system,
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.