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IPC::Shareable(3pm) User Contributed Perl Documentation IPC::Shareable(3pm)
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.
key The key field is used to determine the GLUE when using the three-argument form of the
call to tie(). This argument is then, in turn, used as the KEY argument in subsequent
calls to shmget() and semget().
The default value is IPC_PRIVATE, meaning that your variables cannot be shared with
other processes.
create
create is used to control whether calls to tie() create new shared memory segments or
not. If create is set to a true value, IPC::Shareable will create a new binding
associated with GLUE as needed. If create is false, IPC::Shareable will not attempt
to create a new shared memory segment associated with GLUE. In this case, a shared
memory segment associated with GLUE must already exist or the call to tie() will fail
and return undef. The default is false.
exclusive
If exclusive field is set to a true value, calls to tie() will fail (returning undef)
if a data binding associated with GLUE already exists. If set to a false value, calls
to tie() will succeed even if a shared memory segment associated with GLUE already
exists. The default is false
mode
The mode argument is an octal number specifying the access permissions when a new data
binding is being created. These access permission are the same as file access
permissions in that 0666 is world readable, 0600 is readable only by the effective UID
of the process creating the shared variable, etc. The default is 0666 (world readable
and writable).
destroy
If set to a true value, the shared memory segment underlying the data binding will be
removed when the process calling tie() exits (gracefully)[3]. Use this option with
care. In particular you should not use this option in a program that will fork after
binding the data. On the other hand, shared memory is a finite resource and should be
released if it is not needed. The default is false
size
This field may be used to specify the size of the shared memory segment allocated.
The default is IPC::Shareable::SHM_BUFSIZ().
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().
destroy option
As described in "OPTIONS", specifying the destroy option when tie()ing a variable
coerces IPC::Shareable to remove the underlying shared memory segment when the process
calling tie() exits gracefully. Note that any related shared memory segments created
automagically by the use of references will also be removed.
remove()
(tied $var)->remove;
Calling remove() on the object underlying a tie()d variable removes the associated
shared memory segment. The segment is removed irrespective of whether it has the
destroy option set or not and irrespective of whether the calling process created the
segment.
clean_up()
IPC::Shareable->clean_up;
This is a class method that provokes IPC::Shareable to remove all shared memory
segments created by the process. Segments not created by the calling process are not
removed.
clean_up_all()
IPC::Shareable->clean_up_all;
This is a class method that provokes IPC::Shareable to remove all shared memory
segments encountered by the process. Segments are removed even if they were not
created by the calling process.
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 AT 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
o When using shlock() to lock a variable, be careful to guard against signals. Under
normal circumstances, IPC::Shareable's END method unlocks any locked variables when
the process exits. However, if an untrapped signal is received while a process holds
an exclusive lock, DESTROY will not be called and the lock may be maintained even
though the process has exited. If this scares you, you might be better off
implementing your own locking methods.
One advantage of using "flock" on some known file instead of the locking implemented
with semaphores in IPC::Shareable is that when a process dies, it automatically
releases any locks. This only happens with IPC::Shareable if the process dies
gracefully. The alternative is to attempt to account for every possible calamitous
ending for your process (robust signal handling in Perl is a source of much debate,
though it usually works just fine) or to become familiar with your system's tools for
removing shared memory and semaphores. This concern should be balanced against the
significant performance improvements you can gain for larger data structures by using
the locking mechanism implemented in IPC::Shareable.
o There is a program called ipcs(1/8) (and ipcrm(1/8)) that is available on at least
Solaris and Linux that might be useful for cleaning moribund shared memory segments or
semaphore sets produced by bugs in either IPC::Shareable or applications using it.
o This version of IPC::Shareable does not understand the format of shared memory
segments created by versions prior to 0.60. If you try to tie to such segments, you
will get an error. The only work around is to clear the shared memory segments and
start with a fresh set.
o Iterating over a hash causes a special optimization if you have not obtained a lock
(it is better to obtain a read (or write) lock before iterating over a hash tied to
Shareable, but we attempt this optimization if you do not). The fetch/thaw operation
is performed when the first key is accessed. Subsequent key and and value accesses
are done without accessing shared memory. Doing an assignment to the hash or fetching
another value between key accesses causes the hash to be replaced from shared memory.
The state of the iterator in this case is not defined by the Perl documentation.
Caveat Emptor.
CREDITS
Thanks to all those with comments or bug fixes, especially
Maurice Aubrey <maurice AT hevanet.com>
Stephane Bortzmeyer <bortzmeyer AT pasteur.fr>
Doug MacEachern <dougm AT telebusiness.nz>
Robert Emmery <roberte AT netscape.com>
Mohammed J. Kabir <kabir AT intevo.com>
Terry Ewing <terry AT intevo.com>
Tim Fries <timf AT dicecorp.com>
Joe Thomas <jthomas AT women.com>
Paul Makepeace <Paul.Makepeace AT realprogrammers.com>
Raphael Manfredi <Raphael_Manfredi AT pobox.com>
Lee Lindley <Lee.Lindley AT bigfoot.com>
Dave Rolsky <autarch AT urth.org>
BUGS
Certainly; this is beta software. When you discover an anomaly, send an email to me at
bsugars AT canoe.ca.
SEE ALSO
perl(1), perltie(1), Storable(3), shmget(2), ipcs(1), ipcrm(1) and other SysV IPC man
pages.
perl v5.10.0 2008-12-03 IPC::Shareable(3pm)
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On Apache
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