Syntax-K

Know-How für Ihr Projekt

Perl Documentation

NAME

IPC::Shareable - share Perl variables between processes

SYNOPSIS

use IPC::Shareable (':lock');
tie SCALAR, 'IPC::Shareable', GLUE, OPTIONS;
tie ARRAY,  'IPC::Shareable', GLUE, OPTIONS;
tie HASH,   'IPC::Shareable', GLUE, OPTIONS;
(tied VARIABLE)->shlock;
(tied VARIABLE)->shunlock;
(tied VARIABLE)->shlock(LOCK_SH|LOCK_NB) 
       or print "resource unavailable\n";
(tied VARIABLE)->remove;
IPC::Shareable->clean_up;
IPC::Shareable->clean_up_all;

CONVENTIONS

The occurrence of a number in square brackets, as in [N], in the text of this document refers to a numbered note in the "NOTES".

DESCRIPTION

IPC::Shareable allows you to tie a variable to shared memory making it easy to share the contents of that variable with other Perl processes. Scalars, arrays, and hashes can be tied. The variable being tied may contain arbitrarily complex data structures - including references to arrays, hashes of hashes, etc.

The association between variables in distinct processes is provided by GLUE. This is an integer number or 4 character string[1] that serves as a common identifier for data across process space. Hence the statement

tie $scalar, 'IPC::Shareable', 'data';

in program one and the statement

tie $variable, 'IPC::Shareable', 'data';

in program two will bind $scalar in program one and $variable in program two.

There is no pre-set limit to the number of processes that can bind to data; nor is there a pre-set limit to the complexity of the underlying data of the tied variables[2]. The amount of data that can be shared within a single bound variable is limited by the system's maximum size for a shared memory segment (the exact value is system-dependent).

The bound data structures are all linearized (using Raphael Manfredi's Storable module) before being slurped into shared memory. Upon retrieval, the original format of the data structure is recovered. Semaphore flags can be used for locking data between competing processes.

OPTIONS

Options are specified by passing a reference to a hash as the fourth argument to the tie() function that enchants a variable. Alternatively you can pass a reference to a hash as the third argument; IPC::Shareable will then look at the field named key in this hash for the value of GLUE. So,

tie $variable, 'IPC::Shareable', 'data', \%options;

is equivalent to

tie $variable, 'IPC::Shareable', { key => 'data', ... };

Boolean option values can be specified using a value that evaluates to either true or false in the Perl sense.

NOTE: Earlier versions allowed you to use the word yes for true and the word no for false, but support for this "feature" is being removed. yes will still act as true (since it is true, in the Perl sense), but use of the word no now emits an (optional) warning and then converts to a false value. This warning will become mandatory in a future release and then at some later date the use of no will stop working altogether.

The following fields are recognized in the options hash.

Default values for options are

key       => IPC_PRIVATE,
create    => 0,
exclusive => 0,
destroy   => 0,
mode      => 0,
size      => IPC::Shareable::SHM_BUFSIZ(),

LOCKING

IPC::Shareable provides methods to implement application-level advisory locking of the shared data structures. These methods are called shlock() and shunlock(). To use them you must first get the object underlying the tied variable, either by saving the return value of the original call to tie() or by using the built-in tied() function.

To lock a variable, do this:

$knot = tie $sv, 'IPC::Shareable', $glue, { %options };
...
$knot->shlock;

or equivalently

tie($scalar, 'IPC::Shareable', $glue, { %options });
(tied $scalar)->shlock;

This will place an exclusive lock on the data of $scalar. You can also get shared locks or attempt to get a lock without blocking. IPC::Shareable makes the constants LOCK_EX, LOCK_SH, LOCK_UN, and LOCK_NB exportable to your address space with the export tags :lock, :flock, or :all. The values should be the same as the standard flock option arguments.

if ( (tied $scalar)->shlock(LOCK_SH|LOCK_NB) ) {
       print "The value is $scalar\n";
       (tied $scalar)->shunlock;
} else {
       print "Another process has an exlusive lock.\n";
}

If no argument is provided to shlock, it defaults to LOCK_EX. To unlock a variable do this:

$knot->shunlock;

or

(tied $scalar)->shunlock;

or

$knot->shlock(LOCK_UN);        # Same as calling shunlock

There are some pitfalls regarding locking and signals about which you should make yourself aware; these are discussed in "NOTES".

If you use the advisory locking, IPC::Shareable assumes that you know what you are doing and attempts some optimizations. When you obtain a lock, either exclusive or shared, a fetch and thaw of the data is performed. No additional fetch/thaw operations are performed until you release the lock and access the bound variable again. During the time that the lock is kept, all accesses are perfomed on the copy in program memory. If other processes do not honor the lock, and update the shared memory region unfairly, the process with the lock will not be in sync. In other words, IPC::Shareable does not enforce the lock for you.

A similar optimization is done if you obtain an exclusive lock. Updates to the shared memory region will be postponed until you release the lock (or downgrade to a shared lock).

Use of locking can significantly improve performance for operations such as iterating over an array, retrieving a list from a slice or doing a slice assignment.

REFERENCES

When a reference to a non-tied scalar, hash, or array is assigned to a tie()d variable, IPC::Shareable will attempt to tie() the thingy being referenced[4]. This allows disparate processes to see changes to not only the top-level variable, but also changes to nested data. This feature is intended to be transparent to the application, but there are some caveats to be aware of.

First of all, IPC::Shareable does not (yet) guarantee that the ids shared memory segments allocated automagically are unique. The more automagical tie()ing that happens, the greater the chance of a collision.

Secondly, since a new shared memory segment is created for each thingy being referenced, the liberal use of references could cause the system to approach its limit for the total number of shared memory segments allowed.

OBJECTS

IPC::Shareable implements tie()ing objects to shared memory too. Since an object is just a reference, the same principles (and caveats) apply to tie()ing objects as other reference types.

DESTRUCTION

perl(1) will destroy the object underlying a tied variable when then tied variable goes out of scope. Unfortunately for IPC::Shareable, this may not be desirable: other processes may still need a handle on the relevant shared memory segment. IPC::Shareable therefore provides an interface to allow the application to control the timing of removal of shared memory segments. The interface consists of three methods - remove(), clean_up(), and clean_up_all() - and the destroy option to tie().

EXAMPLES

In a file called server:

#!/usr/bin/perl -w
use strict;
use IPC::Shareable;
my $glue = 'data';
my %options = (
    create    => 'yes',
    exclusive => 0,
    mode      => 0644,
    destroy   => 'yes',
);
my %colours;
tie %colours, 'IPC::Shareable', $glue, { %options } or
    die "server: tie failed\n";
%colours = (
    red => [
        'fire truck',
        'leaves in the fall',
    ],
    blue => [
        'sky',
        'police cars',
    ],
);
((print "server: there are 2 colours\n"), sleep 5)
    while scalar keys %colours == 2;
print "server: here are all my colours:\n";
foreach my $c (keys %colours) {
    print "server: these are $c: ",
        join(', ', @{$colours{$c}}), "\n";
}
exit;

In a file called client

#!/usr/bin/perl -w
use strict;
use IPC::Shareable;
my $glue = 'data';
my %options = (
    create    => 0,
    exclusive => 0,
    mode      => 0644,
    destroy   => 0,
    );
my %colours;
tie %colours, 'IPC::Shareable', $glue, { %options } or
    die "client: tie failed\n";
foreach my $c (keys %colours) {
    print "client: these are $c: ",
        join(', ', @{$colours{$c}}), "\n";
}
delete $colours{'red'};
exit;

And here is the output (the sleep commands in the command line prevent the output from being interrupted by shell prompts):

bash$ ( ./server & ) ; sleep 10 ; ./client ; sleep 10
server: there are 2 colours
server: there are 2 colours
server: there are 2 colours
client: these are blue: sky, police cars
client: these are red: fire truck, leaves in the fall
server: here are all my colours:
server: these are blue: sky, police cars

RETURN VALUES

Calls to tie() that try to implement IPC::Shareable will return true if successful, undef otherwise. The value returned is an instance of the IPC::Shareable class.

AUTHOR

Benjamin Sugars <bsugars@canoe.ca>

NOTES

Footnotes from the above sections

  1. If GLUE is longer than 4 characters, only the 4 most significant characters are used. These characters are turned into integers by unpack()ing them. If GLUE is less than 4 characters, it is space padded.

  2. IPC::Shareable provides no pre-set limits, but the system does. Namely, there are limits on the number of shared memory segments that can be allocated and the total amount of memory usable by shared memory.

  3. If the process has been smoked by an untrapped signal, the binding will remain in shared memory. If you're cautious, you might try

    $SIG{INT} = \&catch_int;
    sub catch_int {
        die;
    }
    ...
    tie $variable, IPC::Shareable, 'data', { 'destroy' => 'Yes!' };

    which will at least clean up after your user hits CTRL-C because IPC::Shareable's END method will be called. Or, maybe you'd like to leave the binding in shared memory, so subsequent process can recover the data...

  4. This behaviour is markedly different from previous versions of IPC::Shareable. Older versions would sometimes tie() referenced thingies, and sometimes not. The new approach is more reliable (I think) and predictable (certainly) but uses more shared memory segments.

General Notes

CREDITS

Thanks to all those with comments or bug fixes, especially

Maurice Aubrey      <maurice@hevanet.com>
Stephane Bortzmeyer <bortzmeyer@pasteur.fr>
Doug MacEachern     <dougm@telebusiness.co.nz>
Robert Emmery       <roberte@netscape.com>
Mohammed J. Kabir   <kabir@intevo.com>
Terry Ewing         <terry@intevo.com>
Tim Fries           <timf@dicecorp.com>
Joe Thomas          <jthomas@women.com>
Paul Makepeace      <Paul.Makepeace@realprogrammers.com>
Raphael Manfredi    <Raphael_Manfredi@pobox.com>
Lee Lindley         <Lee.Lindley@bigfoot.com>
Dave Rolsky         <autarch@urth.org>

BUGS

Certainly; this is beta software. When you discover an anomaly, send an email to me at bsugars@canoe.ca.

SEE ALSO

perl(1), perltie(1), Storable(3), shmget(2), ipcs(1), ipcrm(1) and other SysV IPC man pages.