/ 
IPC-Shareable-1.00
River stage two • 8 direct dependents • 11 total dependents
/ IPC::Shareable

NAME

IPC::Shareable - Use shared memory backed variables across processes

Coverage Status

SYNOPSIS

   use IPC::Shareable qw(:lock);
   
   tie SCALAR, 'IPC::Shareable', OPTIONS;
   tie ARRAY,  'IPC::Shareable', OPTIONS;
   tie HASH,   'IPC::Shareable', OPTIONS;

   (tied VARIABLE)->lock;
   (tied VARIABLE)->unlock;
     
   (tied VARIABLE)->lock(LOCK_SH|LOCK_NB) 
       or print "Resource unavailable\n";

   my $segment   = (tied VARIABLE)->seg;
   my $semaphore = (tied VARIABLE)->sem;

   (tied VARIABLE)->remove;
              
   IPC::Shareable->clean_up;
   IPC::Shareable->clean_up_all;

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 and scripts.

Scalars, arrays, and hashes and even objects 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 (aka "key"). 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 my $scalar, 'IPC::Shareable', { key => 'GLUE', create => 1 };

...in program one and the statement

tie my $variable, 'IPC::Shareable', { key => 'GLUE' };

...in program two will create and bind $scalar the shared memory in program one and bind it to $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.

The following fields are recognized in the options hash:

key

key is the GLUE that is a direct reference to the shared memory segment that's to be tied to the variable.

If this option is missing, we'll default to using IPC_PRIVATE. This default key will not allow sharing of the variable between processes.

Default: IPC_PRIVATE

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.

Default: 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.

Default: 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.

Default: 0666 (world read and writeable)

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].

Only those memory segments that were created by the current process will be removed.

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.

Default: false

size

This field may be used to specify the size of the shared memory segment allocated.

Default: IPC::Shareable::SHM_BUFSIZ() (ie. 65536)

Default Option Values

Default values for options are:

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

METHODS

spawn(%opts)

Spawns a forked process running in the background that holds the shared memory segments backing your variable open.

Parameters:

Paremters are sent in as a hash.

key => $glue

Mandatory, String/Integer: The glue that you will be accessing your data as.

mode => 0666

Optional, Integer: The read/write permissions on the variable. Defaults to 0666.

Example:

use IPC::Shareable;

# following line sets things up and returns

IPC::Shareable->spawn(key => 'GLUE');

Now, either within the same script, or any other script on the system, your data will be available at the key/glue GLUE. Call unspawn() to remove it.

unspawn($key, $destroy)

This method will kill off the background process created with spawn().

Parameters:

$key

Mandatory, String/Integer: The glue (aka key) used in the call to spawn().

$destroy

Optional, Bool. If set to a true value, we will remove all semaphores and memory segments related to your data, thus removing the data in its entirety. If not set to a true value, we'll leave the memory segments in place, and you'll be able to re-attach to the data at any time. Defaults to false (0).

lock($flags)

Obtains a lock on the shared memory. $flags specifies the type of lock to acquire. If $flags is not specified, an exclusive read/write lock is obtained. Acceptable values for $flags are the same as for the flock() system call.

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:

tied(%var)->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 (read) lock:

tied(%var)->lock(LOCK_SH);

Multiple processes can hold a shared (read) 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:

tied(%var)->lock( LOCK_EX|LOCK_NB );
tied(%var)->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 using any of the following export tags:

use IPC::Shareable qw(:lock);
use IPC::Shareable qw(:flock);
use IPC::Shareable qw(:all);

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

See "LOCKING" for further details.

unlock

Removes a lock. Takes no parameters, returns true on success.

This is equivalent of calling shlock(LOCK_UN).

See "LOCKING" for further details.

seg

Called on either the tied variable or the tie object, returns the shared memory segment object currently in use.

sem

Called on either the tied variable or the tie object, returns the semaphore object related to the memory segment currently in use.

global_register

Returns a hash reference of hashes of all in-use shared memory segments across all processes. The key is the memory segment ID, and the value is the segment and semaphore objects.

process_register

Returns a hash reference of hashes of all in-use shared memory segments created by the calling process. The key is the memory segment ID, and the value is the segment and semaphore objects.

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 and subsequently unlock a variable, do this:

my $knot = tie my %hash, 'IPC::Shareable', { %options };

$knot->lock;
$hash{a} = 'foo';
$knot->unlock;

or equivalently, if you've decided to throw away the return of tie():

tie my %hash, 'IPC::Shareable', { %options };

tied(%hash)->lock;
$hash{a} = 'foo';
tied(%hash)->unlock;

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(%hash)->lock(LOCK_SH|LOCK_NB)){
    print "The value is $hash{a}\n";
    tied(%hash)->unlock;
} else {
    print "Another process has an exlusive lock.\n";
}

If no argument is provided to lock, it defaults to LOCK_EX.

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

Note that in the background, we perform lock optimization when reading and writing to the shared storage even if the advisory locks aren't being used.

Using the advisory locks can speed up processes that are doing several writes/ reads at the same time.

REFERENCES

Although references can reside within a shared data structure, the tied variable can not be a reference itself.

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 several options to control the timing of removal of shared memory segments.

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: The destruction is handled in an END block. Only those memory segments that are tied to the current process will be removed.

remove

tied($var)->remove;

# or

$knot->remove;

Calling remove() on the object underlying a tie()d variable removes the associated shared memory segments. 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;

# or

tied($var)->clean_up;

# or

$knot->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;

# or

tied($var)->clean_up_all;

# or

$knot->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.

RETURN VALUES

Calls to tie() that try to implement IPC::Shareable will return an instance of IPC::Shareable on success, and undef otherwise.

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, { key => 'GLUE', create => 1, 'destroy' => 1 };

    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...

General Notes

o

When using lock() 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.

Examples:

# List all semaphores and memory segments in use on the system

ipcs -a

# List all memory segments along with each one's associated process ID

ipcs -ap

# Remove *all* semaphores and memory segments

ipcrm -a
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 IPC::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@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>
Steve Bertrand      <steveb@cpan.org>

SEE ALSO

perltie, Storable, shmget, ipcs, ipcrm and other SysV IPC manual pages.