IPC::ShareLite(3) User Contributed Perl Documentation IPC::ShareLite(3)NAMEIPC::ShareLite - Lightweight interface to shared memory
VERSION
This document describes IPC::ShareLite version 0.17
SYNOPSIS
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;
DESCRIPTIONIPC::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
purpose.
An exclusive lock is obtained by calling the lock() method:
$share->lock();
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:
$share->unlock;
METHODS
"new($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
'no'.
$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
import.
$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 );
"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
finished.
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
that.
The constants LOCK_EX, LOCK_SH, LOCK_NB, and LOCK_UN are available for
import:
use IPC::ShareLite qw( :lock );
Or, just use the flock constants available in the Fcntl module.
"unlock"
Releases any locks. This is actually equivalent to:
$share->lock( LOCK_UN );
The method returns true on success and undef on error.
"version"
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;
"key"
Get a share's key.
my $key = $share->key;
"create"
Get a share's create flag.
"exclusive"
Get a share's exclusive flag.
"flags"
Get a share's flag.
"mode"
Get a share's mode.
"size"
Get a share's segment size.
"num_segments"
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
boundary.
"destroy"
Get or set the share's destroy flag.
PERFORMANCE
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.
PORTABILITY
The module should compile on any system with SysV IPC and an ANSI C
compiler, and should compile cleanly with the -pedantic and -Wall
flags.
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.
AUTHOR
Copyright 1998-2002, Maurice Aubrey <maurice@hevanet.com>. All rights
reserved.
This release by Andy Armstrong <andy@hexten.net>.
This module is free software; you may redistribute it and/or modify it
under the same terms as Perl itself.
CREDITS
Special thanks to Benjamin Sugars for developing the IPC::Shareable
module.
See the Changes file for other contributors.
SEE ALSO
IPC::Shareable, ipc(2), shmget(2), semget(2), perl.
perl v5.14.1 2009-03-11 IPC::ShareLite(3)
