XXXVII. Semaphore and Shared Memory Functions

Table of Contents
sem_get
sem_acquire
sem_release
shm_attach
shm_detach
shm_remove
shm_put_var
shm_get_var
shm_remove_var

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