key

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

Defult: false

exclusive

See "graceful" for a silent, non-exception exit if a second process attempts to obtain an in-use "exclusive" segment.

Default: false

graceful

Useful for ensuring only a single process is running at a time.

Default: false

warn

Default: false

mode

Default: 0666 (world read and writeable)

size

The maximum size we allow by default is ~1GB. See the "limit" option to override this default.

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

limit

Default: true

destroy

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

tidy

Default: false

serializer

Send in either "json" or "storable" as the value to use the respective serializer.

Default: storable

Default Option Values

    key         => IPC_PRIVATE,
    create      => 0,
    exclusive   => 0,
    mode        => 0,
    size        => IPC::Shareable::SHM_BUFSIZ(),
    limit       => 1,
    destroy     => 0,
    graceful    => 0,
    warn        => 0,
    tidy        => 0,
    serializer  => 'storable',

new

Parameters:

Hash, Optional: See the "OPTIONS" section for a list of all available options. Most often, you'll want to send in the key, create and destroy options.

It is possible to get a reference to an array or scalar as well. Simply send in either "var = > 'ARRAY'" or "var => 'SCALAR'" to do so.

Return: A reference to a hash (or array or scalar) which is backed by shared memory.

singleton($glue, $warn)

Parameters:

    $glue

Mandatory, String: The key/glue that identifies the shared memory segment.

    $warn

Optional, Bool: Send in a true value to have subsequent processes throw a warning that there's been a shared memory violation and that it will exit.

Default: false

ipcs

Return: Integer

spawn(%opts)

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;

    # The following line sets things up and returns

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

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

unspawn($key, $destroy)

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)

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

This is equivalent of calling "shlock(LOCK_UN)".

See "LOCKING" for further details.

seg

sem

attributes

Parameters:

    $attribute

Optional, String: The name of the attribute. If sent in, we'll return the value of this specific attribute. Returns "undef" if the attribute isn't found.

Returns: A hash reference of all attributes if $attributes isn't sent in, the value of the specific attribute if it is.

global_register

process_register

destroy Option

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.

Footnotes from the above sections

1.

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 and semaphores along with each one's associated process ID

    ipcs -ap

    # List just the shared memory segments

    ipcs -m

    # List the details of an individual memory segment

    ipcs -i 12345678

    # 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.