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       IPC::ShareLite - Lightweight interface to shared memory


       This document describes IPC::ShareLite version 0.17


           use IPC::ShareLite;

           my $share = IPC::ShareLite->new(
               -key     => 1971,
               -create  => 'yes',
               -destroy => 'no'
           ) or die $!;

           $share->store( "This is stored in shared memory" );
           my $str = $share->fetch;


       IPC::ShareLite provides a simple interface to shared memory, allowing
       data to be efficiently communicated between processes. Your operating
       system must support SysV IPC (shared memory and semaphores) in order to
       use this module.

       IPC::ShareLite provides an abstraction of the shared memory and
       semaphore facilities of SysV IPC, allowing the storage of arbitrarily
       large data; the module automatically acquires and removes shared memory
       segments as needed. Storage and retrieval of data is atomic, and
       locking functions are provided for higher-level synchronization.

       In many respects, this module is similar to IPC::Shareable. However,
       IPC::ShareLite does not provide a tied interface, does not
       (automatically) allow the storage of variables, and is written in C for
       additional speed.

       Construct an IPC::ShareLite object by calling its constructor:

           my $share = IPC::ShareLite->new(
               -key     => 1971,
               -create  => 'yes',
               -destroy => 'no'
           ) or die $!;

       Once an instance has been created, data can be written to shared memory
       by calling the store() method:

               $share->store("This is going in shared memory");

       Retrieve the data by calling the fetch() method:

               my $str = $share->fetch();

       The store() and fetch() methods are atomic; any processes attempting to
       read or write to the memory are blocked until these calls finish.
       However, in certain situations, you'll want to perform multiple
       operations atomically.  Advisory locking methods are available for this

       An exclusive lock is obtained by calling the lock() method:


       Happily, the lock() method also accepts all of the flags recognized by
       the flock() system call.  So, for example, you can obtain a shared lock
       like this:

               $share->lock( LOCK_SH );

       Or, you can make either type of lock non-blocking:

               $share->lock( LOCK_EX|LOCK_NB );

       Release the lock by calling the unlock() method:



   "new($key, $create, $destroy, $exclusive, $mode, $flags, $size)"
       This is the constructor for IPC::ShareLite.  It accepts both the
       positional and named parameter calling styles.

       $key is an integer value used to associate data between processes.  All
       processes wishing to communicate should use the same $key value.  $key
       may also be specified as a four character string, in which case it will
       be converted to an integer value automatically.  If $key is undefined,
       the shared memory will not be accessible from other processes.

       $create specifies whether the shared memory segment should be created
       if it does not already exist.  Acceptable values are 1, 'yes', 0, or

       $destroy indicates whether the shared memory segments and semaphores
       should be removed from the system once the object is destroyed.
       Acceptable values are 1, 'yes', 0, or 'no'.

       If $exclusive is true, instantiation will fail if the shared memory
       segment already exists. Acceptable values are 1, 'yes', 0, or 'no'.

       $mode specifies the permissions for the shared memory and semaphores.
       The default value is 0666.

       $flags specifies the exact shared memory and semaphore flags to use.
       The constants IPC_CREAT, IPC_EXCL, and IPC_PRIVATE are available for

       $size specifies the shared memory segment size, in bytes. The default
       size is 65,536 bytes, which is fairly portable. Linux, as an example,
       supports segment sizes of 4 megabytes.

       The constructor croaks on error.

   "store( $scalar )"
       This method stores $scalar into shared memory.  $scalar may be
       arbitrarily long.  Shared memory segments are acquired and released
       automatically as the data length changes.  The only limits on the
       amount of data are the system-wide limits on shared memory pages
       (SHMALL) and segments (SHMMNI) as compiled into the kernel.

       The method raises an exception on error.

       Note that unlike IPC::Shareable, this module does not automatically
       allow references to be stored. Serializing all data is expensive, and
       is not always necessary. If you need to store a reference, you should
       employ the Storable module yourself. For example:

           use Storable qw( freeze thaw );
               $hash = { red => 1, white => 1, blue => 1 };
           $share->store( freeze( $hash ) );
           $hash = thaw( $share->fetch );

       This method returns the data that was previously stored in shared
       memory.  The empty string is returned if no data was previously stored.

       The method raises an exception on error.

   "lock( $type )"
       Obtains a lock on the shared memory.  $type specifies the type of lock
       to acquire.  If $type is not specified, an exclusive read/write lock is
       obtained.  Acceptable values for $type are the same as for the flock()
       system call.  The method returns true on success, and undef on error.
       For non-blocking calls (see below), the method returns 0 if it would
       have blocked.

       Obtain an exclusive lock like this:

               $share->lock( LOCK_EX ); # same as default

       Only one process can hold an exclusive lock on the shared memory at a
       given time.

       Obtain a shared lock this this:

               $share->lock( LOCK_SH );

       Multiple processes can hold a shared lock at a given time.  If a
       process attempts to obtain an exclusive lock while one or more
       processes hold shared locks, it will be blocked until they have all

       Either of the locks may be specified as non-blocking:

               $share->lock( LOCK_EX|LOCK_NB );
               $share->lock( LOCK_SH|LOCK_NB );

       A non-blocking lock request will return 0 if it would have had to wait
       to obtain the lock.

       Note that these locks are advisory (just like flock), meaning that all
       cooperating processes must coordinate their accesses to shared memory
       using these calls in order for locking to work.  See the flock() call
       for details.

       Locks are inherited through forks, which means that two processes
       actually can possess an exclusive lock at the same time.  Don't do

       The constants LOCK_EX, LOCK_SH, LOCK_NB, and LOCK_UN are available for

               use IPC::ShareLite qw( :lock );

       Or, just use the flock constants available in the Fcntl module.

       Releases any locks.  This is actually equivalent to:

               $share->lock( LOCK_UN );

       The method returns true on success and undef on error.

       Each share has a version number that incrementents monotonically for
       each write to the share. When the share is initally created its version
       number will be 1.

           my $num_writes = $share->version;

       Get a share's key.

           my $key = $share->key;

       Get a share's create flag.

       Get a share's exclusive flag.

       Get a share's flag.

       Get a share's mode.

       Get a share's segment size.

       Get the number of segments in a share. The memory usage of a share can
       be approximated like this:

           my $usage = $share->size * $share->num_segments;

       $usage will be the memory usage rounded up to the next segment

       Get or set the share's destroy flag.


       For a rough idea of the performance you can expect, here are some
       benchmarks.  The tests were performed using the Benchmark module on a
       Cyrix PR166+ running RedHat Linux 5.2 with the 2.0.36 kernel, perl
       5.005_02 using perl's malloc, and the default shared memory segment
       size.  Each test was run 5000 times.

               DATA SIZE (bytes)       TIME (seconds)  Op/Sec

        store  16384                   2               2500
        fetch  16384                   2               2500

        store  32768                   3               1666
        fetch  32768                   3               1666

        store  65536                   6               833
        fetch  65536                   5               1000

        store  131072                  12              416
        fetch  131072                  12              416

        store  262144                  28              178
        fetch  262144                  27              185

        store  524288                  63              79
        fetch  524288                  61              81

       Most of the time appears to be due to memory copying.  Suggestions for
       speed improvements are welcome.


       The module should compile on any system with SysV IPC and an ANSI C
       compiler, and should compile cleanly with the -pedantic and -Wall

       The module has been tested under Solaris, FreeBSD, and Linux.  Testing
       on other platforms is needed.

       If you encounter a compilation error due to the definition of the semun
       union, edit the top of sharestuff.c and undefine the semun definition.
       And then please tell me about it.

       I've heard rumors that a SysV IPC interface has been constructed for
       Win32 systems.  Support for it may be added to this module.

       IPC::ShareLite does not understand the shared memory data format used
       by IPC::Shareable.


       Copyright 1998-2002, Maurice Aubrey <>.  All rights

       This release by Andy Armstrong <>.

       This module is free software; you may redistribute it and/or modify it
       under the same terms as Perl itself.


       Special thanks to Benjamin Sugars for developing the IPC::Shareable

       See the Changes file for other contributors.


       IPC::Shareable, ipc(2), shmget(2), semget(2), perl.

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