# Apache::Session::Lock::Semaphore - phpMan

## NAME
    [Apache::Session::Lock::Semaphore] - Provides mutual exclusion through
    semaphores

## SYNOPSIS
     use [Apache::Session::Lock::Semaphore];

     my $locker = [Apache::Session::Lock::Semaphore]->new;
     die "no semaphores" unless $locker;

     $locker->acquire_read_lock($ref);
     $locker->acquire_write_lock($ref);
     $locker->release_read_lock($ref);
     $locker->release_write_lock($ref);
     $locker->release_all_locks($ref);

## DESCRIPTION
    [Apache::Session::Lock::semaphore] fulfills the locking interface of
    [Apache::Session]. Mutual exclusion is achieved through system semaphores
    and the [IPC::Semaphore] module.

## CONFIGURATION
    The module must know how many semaphores to use, and what semaphore key
    to use. The number of semaphores has an impact on performance. More
    semaphores means less lock contention. You should use the maximum number
    of semaphores that your platform will allow. On stock NetBSD, OpenBSD,
    and Solaris systems, this is probably 16. On Linux 2.2, this is 32. This
    module tries to guess the number based on your operating system, but it
    is safer to configure it yourself.

    To set the number of semaphores, you need to pass an argument in the
    usual [Apache::Session] style. The name of the argument is NSems, and the
    value is an integer power of 2. For example:

     tie %s, '[Apache::Session::Blah]', $id, {NSems => 16};

    You may also need to configure the semaphore key that this package uses.
    By default, it uses key 31818. You can change this using the argument
    SemaphoreKey:

     tie %s, '[Apache::Session::Blah]', $id, {NSems => 16, SemaphoreKey => 42};

## PROBLEMS
    There are a few problems that people frequently encounter when using
    this package.

    If you get an invalid argument message, that usually means that the
    system is unhappy with the number of semaphores that you requested. Try
    decreasing the number of semaphores. The semaphore blocks that this
    package creates are persistent until the system is rebooted, so if you
    request 8 semaphores one time and 16 semaphores the next, it won't work.
    Use the system commands ipcs and ipcrm to inspect and remove unwanted
    semphore blocks.

  Cygwin
    IPC on Cygwin requires running cygserver. Without it, program will exit
    with "Bad System call" message. It cannot be intercepted with eval.

    Read /usr/share/doc/Cygwin/cygserver.README for more information.

  Darwin/MacOS X
    Darwin and MacOS X may not have semaphores, see
    <<http://sysnet.ucsd.edu/~bellardo/darwin/sysvsem.html>>

  *BSD
    Error "No space left on device" means that maximum number of semaphores
    is reached. See
    <<http://www.postgresql.org/docs/7.3/static/kernel-resources.html>> for
    more information.

## AUTHOR
    This module was written by Jeffrey William Baker <<jwbaker@acm.org>>.

## SEE ALSO
    [Apache::Session]