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


        use IPC::Shareable qw(:lock);
        my $href = IPC::Shareable->new(%options);
        # ...or
        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;
        # Ensure only one instance of a script can be run at any time
        IPC::Shareable->singleton('UNIQUE SCRIPT LOCK STRING');


    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, 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 any arbitrary string or integer that serves
    as a common identifier for data across process space. Hence the

        tie my $scalar, 'IPC::Shareable', { key => 'GLUE STRING', create => 1 }; program one and the statement

        tie my $variable, 'IPC::Shareable', { key => 'GLUE STRING' }; 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. 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 or optionally JSON) 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 are specified by passing a reference to a hash as the third
    argument to the tie() function that enchants a variable.

    The following fields are recognized in the options hash:


    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 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


    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


    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


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

    Default: false


    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)


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

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

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


    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


    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

    Default: false


    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


    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

    Default: storable

 Default Option Values

    Default values for options are:

        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',



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


    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

    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.



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


    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

    Default: false


    Returns the number of instantiated shared memory segments that
    currently exist on the system.

    Return: Integer


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


    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.


        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)

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



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


    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).


    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:


    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

    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.


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

    This is equivalent of calling shlock(LOCK_UN).

    See "LOCKING" for further details.


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


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


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



    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.


    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.


    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.


    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 };
        $hash{a} = 'foo';

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

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

    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";
        } 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.


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


    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.


        # or

    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.


        # or
        # or

    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.


        # or
        # or

    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.


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


    Benjamin Sugars <>


    Steve Bertrand <>


 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 {
       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


      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


      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.


          # 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


      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.


      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.


    Thanks to all those with comments or bug fixes, especially

        Maurice Aubrey      <>
        Stephane Bortzmeyer <>
        Doug MacEachern     <>
        Robert Emmery       <>
        Mohammed J. Kabir   <>
        Terry Ewing         <>
        Tim Fries           <>
        Joe Thomas          <>
        Paul Makepeace      <>
        Raphael Manfredi    <>
        Lee Lindley         <>
        Dave Rolsky         <>
        Steve Bertrand      <>


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