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# LXXXVIII. Semaphore and Shared Memory Functions

This module provides semaphore functions using System V semaphores. 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

> Note: These functions do not work on Windows systems.

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