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 the process creates a new shared memory segment 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 we'll "croak()".

Defult: false

exclusive

If exclusive field is set to a true value, we will "croak()" if the 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.

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

Default: false

graceful

If exclusive is set to a true value, we normally "croak()" if a second process attempts to obtain the same shared memory segment. Set graceful to true and we'll "exit" silently and gracefully. This option does nothing if "exclusive" isn't set.

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

Default: false

warn

When set to a true value, graceful will output a warning if there are process collisions.

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)

size

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

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

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

protected

If set, the "clean_up()" and "clean_up_all()" routines will not remove the segments or semaphores related to the tied object.

Set this to a specific integer so we can pass the value to any child objects created under the main one.

To clean up protected objects, call "(tied %object)->clean_up_protected(integer)", where 'integer' is the value you set the "protected" option to. You can call this cleanup routine in the script you created the segment, or anywhere else, at any time.

Default: 0

limit

This field will allow you to set a segment size larger than the default maximum which is 1,073,741,824 bytes (approximately 1 GB). If set, we will "croak()" if a size specified is larger than the maximum. If it's set to a false value, we'll "croak()" if you send in a size larger than the total system RAM.

Default: true

destroy

If set to a true value, the shared memory segment underlying the data binding will be removed when the process that initialized the shared memory segment exits (gracefully)[1].

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.

NOTE: If the segment was created with its "protected" attribute set, it will not be removed upon program completion, even if "destroy" is set.

Default: false

tidy

For long running processes, set this to a true value to clean up unneeded segments from nested data structures. Comes with a slight performance hit.

Default: false

serializer

By default, we use Storable as the data serializer when writing to or reading from the shared memory segments we create. For cross-platform and cross-language purposes, you can optionally use JSON for this task.

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

Default: storable

Default Option Values

Default values for options are:

    key         => IPC_PRIVATE, # 0
    create      => 0,
    exclusive   => 0,
    mode        => 0666,
    size        => IPC::Shareable::SHM_BUFSIZ(), # 65536
    protected   => 0,
    limit       => 1,
    destroy     => 0,
    graceful    => 0,
    warn        => 0,
    tidy        => 0,
    serializer  => 'storable',

new

Instantiates and returns a reference to a hash backed by shared memory.

    my $href = IPC::Shareable->new(key => "testing", create => 1);
    $href=>{a} = 1;
    # Call tied() on the dereferenced variable to access object methods
    # and information
    tied(%$href)->ipcs;

Parameters:

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

Class method that ensures that only a single instance of a script can be run at any given time.

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

Returns the number of instantiated shared memory segments that currently exist on the system. This isn't precise; it simply does a "wc -l" line count on your system's "ipcs -m" call. It is guaranteed though to produce reliable results.

Return: Integer

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.

attributes

Retrieves the list of attributes that drive the IPC::Shareable object.

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.

Attributes are the "OPTIONS" that were used to create the object.

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

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.

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.

NOTE: If the segment was created with its "protected" attribute set, it will not be removed in the "END" block, even if "destroy" is set.

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.

This method will not clean up segments created with the "protected" option.

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.

This method will not clean up segments created with the "protected" option.

clean_up_protected($protect_key)

If a segment is created with the "protected" option, it, nor its children will be removed during calls of "clean_up()" or "clean_up_all()".

When setting "protected", you specified a lock key integer. When calling this method, you must send that integer in as a parameter so we know which segments to clean up.

    my $protect_key = 93432;
    IPC::Shareable->clean_up_protected($protect_key);
    # or
    tied($var)->clean_up_protected($protect_key;
    # or
    $knot->clean_up_protected($protect_key)

Parameters:

    $protect_key

Mandatory, Integer: The integer protect key you assigned wit the "protected" option

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, "END" 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).

For tied hashes, 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.