LIX. 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

SHMMAXmax size of shared memory, normally 131072 bytes
SHMMINminimum size of shared memory, normally 1 byte
SHMMNImax amount of shared memory segments, normally 100
SHMSEGmax amount of shared memory per process, normally 6

Note: These functions do not work on Windows systems.

Table of Contents
sem_get — Get a semaphore id
sem_acquire — Acquire a semaphore
sem_release — Release a semaphore
shm_attach — Creates or open a shared memory segment
shm_detach — Disconnects from shared memory segment
shm_remove — Removes shared memory from Unix systems
shm_put_var — Inserts or updates a variable in shared memory
shm_get_var — Returns a variable from shared memory
shm_remove_var — Removes a variable from shared memory