LXIII. 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|
max amount of shared memory segments on a system, normally 100 |
max amount of shared memory segments per process, normally 6 |
Note: These functions do not work on Windows systems.