# NAME IPC::Shareable - Use shared memory backed variables across processes
Coverage Status
# SYNOPSIS use IPC::Shareable qw(:lock); tie my %hash, 'IPC::Shareable', OPTIONS; tie my @array, 'IPC::Shareable', OPTIONS; tie my $scalar, 'IPC::Shareable', OPTIONS; # Lock, make changes, unlock tied(VARIABLE)->lock; # Do something with the variable tied(VARIABLE)->unlock; # Blocking lock attempt (a writer must have a LOCK_EX lock) tied(VARIABLE)->lock(LOCK_SH); my $val = VARIABLE->[5]; # Will wait to get value until writer releases LOCK_EX # Non-blocking lock attempt tied(VARIABLE)->lock(LOCK_SH|LOCK_NB) or print "Resource unavailable\n"; # Lock with a code reference, which will auto-unlock when the block finishes tied(VARIABLE)->lock(sub { print "hello!\n"; }); # Ensure only one instance of a script can be run at any time IPC::Shareable->singleton('UNIQUE SCRIPT LOCK STRING'); # SYNOPSIS - DEVELOPER/TROUBLESHOOTING # Get SYSV shared memory specifications of the system (if available) my $href = IPC::Shareable::sysv_info(); # Get the shared memory segment and semaphore objects directly my $segment = tied(VARIABLE)->seg; my $semaphore = tied(VARIABLE)->sem; # Get the shared memory segment and semaphores for a lower level my $seg = tied(%{ $hv{a}->{b} })->seg; my $sem = tied(%{ $hv{a}->{b} })->sem; # Fetch a printable string representation of the segment and semaphore # mapping for your data tied(VARIABLE)->seg_map; # Remove the shared memory segment and semaphore directly tied(VARIABLE)->remove; # Manual cleanup procedures (mainly used for unit testing etc) IPC::Shareable::clean_up; IPC::Shareable::clean_up_all; IPC::Shareable::clean_up_protected; # Get the actual IPC::Shareable tied object you can make method calls on # instead of using the tied object like the examples above my $knot = tied(VARIABLE); # Dereference first if using a tied reference # ...or get the knot at inception my $knot = tie my VARIABLE, 'IPC::Shareable', OPTIONS; my $sysv_info_href = $knot->sysv_info; # 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, 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. **Note**: When using nested data structures, each nested structure utilizes an additional shared memory segment. The entire structure is not squashed into a single segment. See ["DATA AND SEGMENT MAPPING"](#data-and-segment-mapping) for details. The association between variables in distinct processes is provided by GLUE (aka. a "key"). This is any arbitrary string or integer that serves as a common identifier for data across process space. Hence the statement: tie my %hash, 'IPC::Shareable', { key => 'GLUE STRING', create => 1 }; ...in program one and the statement tie my %thing, 'IPC::Shareable', { key => 'GLUE STRING' }; ...in program two will create and bind `%hash` the shared memory in program one and bind it to `%thing` 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, and the total number of segments allowed by the system (the exact values are system-dependent). The bound data structures are all linearized (using [JSON](https://metacpan.org/pod/JSON) by default or optionally [Storable](https://metacpan.org/pod/Storable)) 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. **Recommendation**: Utilizing the locking mechanisms is highly advised to ensure data consistency and integrity. See ["LOCKING"](#locking). **Recommendation**: If you're using JSON to serialize your data (the default), I would highly advise you to install the XS version ([JSON::XS](https://metacpan.org/pod/JSON%3A%3AXS)). We will automatically use it if available, and it is much faster than the pure Perl version ([JSON::PP](https://metacpan.org/pod/JSON%3A%3APP)). # OPTIONS Options are specified by passing a reference to a hash as the third argument to the `tie()` function that binds a variable. We also call these **attributes**. 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`. Note however, that going this route will not allow you to share your data across processes. The key can be specified as: - A text string (internally, a 32-bit CRC of the string is used as the key) - A hex string (eg. `'0xDEADBEEF'`), which we convert to integer form - A hex value (eg. `0xDEADBEEF`), used as-is as the integer key - An integer (eg. `1234`), used as-is as the integer key 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](https://metacpan.org/pod/IPC%3A%3AShareable) will create a new binding associated with GLUE as needed. If **create** is false, [IPC::Shareable](https://metacpan.org/pod/IPC%3A%3AShareable) 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()`. Default: **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"](#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. See ["warn"](#warn) to emit a warning before gracefully exiting when a collision occurs. 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 and writable, `0600` is writable only by the effective UID of the process creating the shared variable, etc. Default: **0666** (world readable and writeable) ## size This field is used to specify the size of each shared memory segment allocated. **Note**: Each nested data structure requires a new shared memory segment. The `size` attribute is applied to the first, and all subsequent segments created, and does not reflect the overall size of memory to be used. The maximum size we allow for each segment by default is ~1GB. See the ["limit"](#limit) option to override this default. Default: `IPC::Shareable::SHM_BUFSIZ()` (ie. **65536**) ## protected The segments with this option set will persist even through all of our automatic and manual clean up procedures, less [clean\_up\_protected](#clean_up_protected-protect_key). Set this to a non-zero integer. The integer is persisted in the segment's associated semaphore set, so any process that later attaches to the same segment via `create => 0` will automatically have this attribute restored; it does not need to pass `protected` explicitly. The integer acts as a group key: all segments (including nested children) created under the same protected parent share the same value, so a single call to `clean_up_protected($key)` removes the entire group. 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. **Note**: The protect key is limited to values accepted by the system's semaphore implementation (typically 0-32767; 0 means unprotected). 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 cleanly. 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"](#protected) attribute set, it will not be removed upon program completion, even if `destroy` is set. Default: **false** ## serializer By default, we use [JSON](https://metacpan.org/pod/JSON) as the data serializer when writing to or reading from the shared memory segments we create. For cross-platform and cross-language interoperability this is the recommended choice. Alternatively, you can use [Storable](https://metacpan.org/pod/Storable) for richer data type support (eg. blessed objects). Send in either `json` or `storable` as the value to use the respective serializer. Default: **json** ## enforced\_write\_locking When enabled, writes from any knot are blocked while another knot holds `LOCK_EX` on the segment, or while there are active `LOCK_SH` readers. Pair with `violated_write_lock_warn` to also emit a warning when a write is blocked. **Note**: This protection system will never be reached if all callers use proper locking at all times. Default: **true** ## violated\_write\_lock\_warn When `enforced_write_locking` is enabled, and this attribute is set to true, we will emit a warning when a write violation occurs (a write attempted against a segment that another knot has locked with `LOCK_EX`, or a write attempted against a segment with active `LOCK_SH` readers). The warning includes the UUID of the object that caused the violation and the segment ID it occurred against. Default: **true** ## enforced\_read\_locking When enabled, an unlocked read against a segment that another knot has locked with `LOCK_EX` is detected. Reads are never **blocked**; this option only controls whether the check fires. Pair with `violated_read_lock_warn` to emit a warning when this happens. **Note**: Reads (fetches) are never blocked, even when a `LOCK_EX` is active. If a reader does not hold a `LOCK_SH` and reads while a writer holds `LOCK_EX`, the returned data may be stale or partially-written. To guarantee a coherent snapshot, acquire `LOCK_SH` before reading. **Note**: This protection system will never be reached if all callers use proper locking at all times. Default: **true** ## violated\_read\_lock\_warn When `enforced_read_locking` is enabled, and this attribute is set to true, we will emit a warning when an unlocked read is attempted against a segment that another knot has locked with `LOCK_EX`. The returned data may be stale or partially-written; the warning recommends acquiring `LOCK_SH` before reading to guarantee a coherent snapshot. The warning includes the UUID of the object that caused the violation and the segment ID it occurred against. Default: **true** ## 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, serializer => 'json', enforced_write_locking => 1, enforced_read_locking => 1, violated_write_lock_warn => 1, violated_read_lock_warn => 1, # METHODS - STANDARD USER These are typically the only methods a normal user will need in the course of their use of this distribution. ## new This `new()` call is not necessary and is a simple wrapper around `tie()`. It is capable only of returning a tied reference object (by default, a hash ref). 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)->seg_count; Parameters: Optional: See the ["OPTIONS"](#options) section for a list of all available options. Most often, you'll want to at minimum, 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. ## lock($flags, $code) Obtains a lock on the shared memory. `$flags` specifies the type of lock to acquire. If `$flags` is not specified, an exclusive read/write lock is obtained. Acceptable flags are: LOCK_EX - Exclusive; use when writing LOCK_SH - Shared; use when reading LOCK_EX|LOCK_NB - Exclusive, non-blocking LOCK_SH|LOCK_NB - Shared, non-blocking Parameters: $flags Optional, Integer: If this parameter is omitted, we default to `LOCK_EX`, an exclusive write lock. $code Optional, Code reference: If this parameter is sent in, and an exclusive lock is asked for, we will set the lock, execute the subroutine, and then call `unlock()` on the segment. The sub is called within an `eval`, so we will `unlock`, then `die` with whatever error your function threw. **Note**: Although the `$flags` and `$code` parameters appear positional, you can send in `$code` without sending in any `$flags`. When this occurs, `$flags` will automatically be set to `LOCK_EX`. Return: `true` on success, and `undef` on error. For non-blocking calls, 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` immediately if it would have had to wait to obtain the lock. **Note**: 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. **Note**: You can enforce a `LOCK_EX` lock at a software level by ensuring that the `enforced_write_locking` option is set to a true value (the default). This will prevent processes that decide not to implement the advisory locking from writing to the segment. The companion `enforced_read_locking` option (also true by default) enables detection of unlocked reads against an exclusively-locked segment; reads are never blocked, but a warning will be emitted if `violated_read_lock_warn` is also set. **Important**: Locks are inherited through forks, which can cause unintended and problematic side effects (particularly duplicated `LOCK_EX` locks). Don't `fork()` until all active locks have been released. 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"](#locking) for further details. ## unlock Removes a lock. Takes no parameters, returns `true` on success. This is equivalent to calling `shlock(LOCK_UN)`. See ["LOCKING"](#locking) for further details. ## 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** Return: `$$`. The process ID. **Note**: See [Script::Singleton](https://metacpan.org/pod/Script::Singleton). That library implements `singleton` for a script with a simple `use` line. # METHODS - OBJECT AND PROCESS These methods provide facilities for identifying information about the current object and the overall state information of the current processes. ## attributes Retrieves the list of attributes that drive the [IPC::Shareable](https://metacpan.org/pod/IPC%3A%3AShareable) 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/forks within the current process space. 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 only (ie. not including forks). The key is the memory segment ID, and the value is the segment and semaphore objects. ## uuid Returns the UUID of the object. # METHODS - MANUAL CLEANUP These methods are mainly for forced cleanup. `remove()` is used internally. These methods are generally never needed by a normal user, and are primarily for use in unit testing and other development work. ## clean\_up IPC::Shareable->clean_up; # or tied($var)->clean_up; # or $knot->clean_up; This is a class method that provokes [IPC::Shareable](https://metacpan.org/pod/IPC%3A%3AShareable) to remove all shared memory segments created by the process. Segments not created by the calling process are not removed. This method will not clean up segments created with the `protected` option. ## clean\_up\_all IPC::Shareable->clean_up_all; # or tied($var)->clean_up_all; # or $knot->clean_up_all This is a class method that provokes [IPC::Shareable](https://metacpan.org/pod/IPC%3A%3AShareable) to remove all shared memory segments encountered by the process. Segments are removed even if they were not created by the calling process. This method will not clean up segments created with the `protected` option. ## clean\_up\_protected($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"](#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. Because the protect key is stored in the segment's semaphore set, any process that attached to the segment (even without passing `protected` on tie) will have had its in-process attribute populated automatically. You can therefore call `clean_up_protected()` from any process that has attached to the segment, not only from the one that created it. 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: $protect_key Mandatory, Integer: The integer protect key you assigned with the `protected` option ## remove($key) Parameters: $key Optional, see ["key"](#key) for valid values. Preferably, an integer or a hex string prefixed with `0x`. **Note**: If the `$key` parameter is sent in, we will delete that segment only and return immediately thereafter. tied($var)->remove; # or $knot->remove; # Remove a specific segment by key (can remove non C # segments). If key is sent in, the caller can be the module or the object. IPC::Shareable->remove('0xdeadbeef'); # hex string IPC::Shareable->remove(0xdeadbeef); # hex integer IPC::Shareable->remove(1234); # integer tied($var)->remove('Test'); # string **Note**: 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. # METHODS - SYSTEM AND SHARED MEMORY These methods are for very low level diagnostic, troubleshooting, investigation, informational and fact finding situations. **Note**: Both ["seg"](#seg) and ["sem"](#sem) are external objects and have their own methods and data that can be used for analysis. This is particularly true with ["seg"](#seg). Each of their respective documentation sections link to their corresponding documentation. ## seg Called on either a tied variable or on the tie object, returns the shared memory segment object currently in use. tie my %h, ...; $h{a}->{b}{c} = 10; my $top_level_seg = tied(%h)->seg; my $bot_level_seg = tied(%{ $h{a}->{b} })->seg; See [IPC::Shareable::SharedMem](https://metacpan.org/pod/IPC%3A%3AShareable%3A%3ASharedMem) documentation for details and available methods. ## sem Called on either a tied variable or on the tie object, returns the semaphore object related to the memory segment currently in use. tie my %h, ...; $h{a}->{b}{c} = 10; my $top_level_sem = tied(%h)->sem; my $bot_level_sem = tied(%{ $h{a}->{b} })->sem; See [IPC::Semaphore](https://metacpan.org/pod/IPC%3A%3ASemaphore) documentation. ## seg\_count Returns the number of shared memory segments that currently exist on the system, by counting data lines in your system's `ipcs -m` output. It is guaranteed to produce consistent results. Return: Integer ## sem\_count Returns the number of semaphore sets that currently exist on the system, by parsing `ipcs -s`. Since each [IPC::Shareable](https://metacpan.org/pod/IPC%3A%3AShareable) segment is associated with exactly one semaphore set (same SysV key), this count moves in lockstep with ["seg\_count"](#seg_count) when [IPC::Shareable](https://metacpan.org/pod/IPC%3A%3AShareable) segments are the only semaphore users on the system and are created and destroyed cleanly. Return: Integer ## shm\_segments($key) my $ipc_shareable_segments = IPC::Shareable->shm_segments; # Filtered to one variable's segments only my $segs = IPC::Shareable->shm_segments('my_key'); my $segs = IPC::Shareable->shm_segments('0xDEADBEEF'); Class/object method. Scans all existing shared memory segments on the system and returns a hash reference mapping the hex key string (eg. `'0xdeadbeef'`) to the raw literal contents of that segment. Only loads segments that were created by [IPC::Shareable](https://metacpan.org/pod/IPC%3A%3AShareable). Segments created with `IPC_PRIVATE` (key `0x00000000`) are skipped because they cannot be looked up by key. Parameters: $key Optional, String or Int: If sent in, we will restrict the result to only the segments related to the variable the `$key` reflects. Without this parameter, all [IPC::Shareable](https://metacpan.org/pod/IPC%3A%3AShareable) segments on the system are returned. Return: Hash reference where each key is the SHM key in hex format. Field descriptions: **known**: `1` if this segment is currently tied in the calling process, `0` if not. A value of `0` includes segments legitimately persisted by another process (`destroy => 0`), not just crashed leftovers. See ["unknown\_segments"](#unknown_segments) for important caveats. **local\_process**: `1` if created by the same process this method is being run, and `0` if not. **content**: The actual raw content of the shared memory segment. **child\_keys**: Nested data structures each require their own segment. Keys within this array reference map to child segments. Here's an example data structure, and what the return value of `shm_segments` would look like for it using the JSON serializer. Note that the top-level structure is a hash, and it contains two nested hashes (keys 'c; and 'd'), which are each stored in their own segments. It also has two scalar values (keys 'a' and 'b'), which are stored in the top-level segment. # Actual data { a => 1, b => 'hello', c => { x => 10, y => 20, }, d => { p => 'foo', q => 'bar', }, } # Call return (JSON content strings will be on one line; separated for # clarity) { '0x2abc0001' => { known => 1, local_process => 1, content => 'IPC::Shareable{ "a": 1, "b": "hello", "c": { "__ics__": { "child_key_hex": "0x000e1b1d", "child_key": "924445", "type": "HASH" } }, "d": { "__ics__": { "child_key_hex": "0x000097af", "child_key": "38831", "type": "HASH" } } }', child_keys => [ '0x000e1b1d', '0x000097af' ], }, '0x000e1b1d' => { known => 1, local_process => 1, content => 'IPC::Shareable{"y":20,"x":10}', child_keys => [], }, '0x000097af' => { known => 1, local_process => 1, content => 'IPC::Shareable{"p":"foo","q":"bar"}', child_keys => [], } } ## unknown\_segments my @unknown_segments = IPC::Shareable->unknown_segments; for my $key (@unknown_segments) { print "Unknown segment: $key\n"; IPC::Shareable->remove($key); } Class/object method. Returns a list of hex key strings (eg. `'0xdeadbeef'`) for all shared memory segments that were created by [IPC::Shareable](https://metacpan.org/pod/IPC%3A%3AShareable) but are not currently tied in the calling process. **Important**: this method has no way to distinguish between a segment that was left behind by a crashed process and one that is legitimately persisted by another running process (`destroy => 0`). Both will appear in the returned list. Only call `remove` on entries you are certain belong to your own application and are no longer in use. Return: List of hex key strings. ## seg\_map # Show all IPC::Shareable segments visible on the system print IPC::Shareable->seg_map; # Show only the segment tree rooted at this object print $knot->seg_map; print tied(%hash)->seg_map; When called as a **class method**, returns a human-readable string showing all [IPC::Shareable](https://metacpan.org/pod/IPC%3A%3AShareable) shared memory segments visible on the current system, organised as a tree (root segments at the top, nested children indented below their parent). When called as an **object method**, the output is filtered to just the segment tree rooted at that object (the segment itself plus any nested children). For each segment the output includes: - The hex key and OS segment ID - Status tags: `known` (tied in this process) or `unknown`, and `owner` if this process created the segment - Semaphore information: OS semaphore ID (`sem_id`), `SEM_MARKER`, read-lock counter, write-lock counter, and `PROTECTED` (the integer stored in `SEM_PROTECTED`) - The list of child segment hex keys, or `(none)` - The segment's current content. Reference values that are child segments are shown as `` rather than being recursed into. Segments not tied in this process show `(not accessible)`. Example: tie my %h, 'IPC::Shareable', { key => 0x1a2b, create => 1, destroy => 1 }; $h->{nested} = { x => 1, y => 2 }; my $mapping = tied(%h)->seg_map; print $mapping; Output: IPC::Shareable Segment Map ========================== [known, owner] key: 0x00001a2b seg_id: 1890844693 Semaphores: sem_id: 1272774674 1: SEM_MARKER=1 2: READERS=0 3: WRITERS=0 4: PROTECTED=0 Children: 0x00018373 Content: { nested => } [known, owner] key: 0x00018373 seg_id: 1888682002 Semaphores: sem_id: 1300234259 1: SEM_MARKER=1 2: READERS=0 3: WRITERS=0 4: PROTECTED=0 Children: (none) Content: { x => "1", y => "2" } ## sysv\_info my $sysv_info = IPC::Shareable->sysv_info; print "Max segment size: $sysv_info->{shmmax}\n"; print "Max segments (system): $sysv_info->{shmmni}\n"; Class method. Returns a hash reference containing the kernel's SysV shared memory configuration parameters for the current platform. Returns `undef` if the platform is not supported or no data could be read. On MacOS, reads from `sysctl kern.sysv`. Example return value: { shmmax => 4194304, # Maximum size of a single segment (bytes) shmmin => 1, # Minimum size of a single segment (bytes) shmmni => 32, # Maximum number of segments system-wide shmseg => 8, # Maximum number of segments per process shmall => 1024, # Maximum total shared memory (pages) } On Linux, reads from `/proc/sys/kernel/`. Example return value: { shmmax => 18446744073692774399, # Maximum size of a single segment (bytes) shmmin => 1, # Minimum size of a single segment (bytes) shmmni => 4096, # Maximum number of segments system-wide shmall => 18446744073692774399, # Maximum total shared memory (pages) } Note: Linux has no per-process segment limit (`shmseg`); only the system-wide `shmmni` applies. On FreeBSD, reads from `sysctl kern.ipc`. Example return value: { shmmax => 536870912, # Maximum size of a single segment (bytes) shmmin => 1, # Minimum size of a single segment (bytes) shmmni => 192, # Maximum number of segments system-wide shmseg => 128, # Maximum number of segments per process shmall => 131072, # Maximum total shared memory (pages) } On Solaris (including OmniOS/illumos), the kernel's SysV shared memory configuration is not yet read programmatically. This method returns `undef` on Solaris; use system tools such as `prctl` or `mdb -k` to inspect the kernel IPC limits instead. Return: Hash reference, or `undef` if the platform is not supported or no data could be read. # LOCKING IPC::Shareable provides methods to implement application-level advisory and enforced locking of the shared data structures. These methods are `lock()` and `unlock()`. 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. See [lock()](#lock-flags-code) for flag combinations allowed. ## Lock and unlock To lock and subsequently unlock a variable, do this: tie my %hash, 'IPC::Shareable', { %options }; tied(%hash)->lock; $hash{a}->{b} = 1; tied(%hash)->unlock; This will place an exclusive lock on the data of `%hash`, including all nested data below the parent. You can also get shared locks or attempt to get a lock without blocking. [IPC::Shareable](https://metacpan.org/pod/IPC%3A%3AShareable) makes the constants `LOCK_EX`, `LOCK_SH`, `LOCK_NB`, and `LOCK_UN` 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. When attempting to get a blocking lock (eg. `LOCK_EX` or `LOCK_SH`) while another process has an exclusive write lock (`LOCK_EX`), your call will block and wait until the other process releases its exclusive lock. The same thing happens if you attempt to get a `LOCK_EX` if there are any other processes that hold a `LOCK_SH`. Here is an example of how to manage a non-blocking lock: if (tied(%hash)->lock(LOCK_SH|LOCK_NB)){ print "The value is $hash{a}\n"; tied(%hash)->unlock; } else { print "Another process has an exclusive lock.\n"; } If no argument is provided to `lock`, it defaults to `LOCK_EX`. ## Enforced write and read locking Additional safeguards are in place to protect your locked data from processes that don't bother to implement locking explicitly. ### Violating an enforced write lock By default, the `enforced_write_locking` option is set to true, which means that if a tied variable sets a `LOCK_EX`, all writes from all other processes will fail, and their data will not be updated. If the offending process has `violated_write_lock_warn` set to true (also default), it will receive a warning regarding the issue. ### Violating an enforced read lock Also enabled by default, the `enforced_read_locking` will catch instances where a process attempts a read of data that is currently locked with `LOCK_EX` by another process. Unlike write protection, read protection does not prevent the read; it simply sets the stage for you to be able to warn the user that they are receiving stale data. To have the user warned that they are in fault, the `violated_read_lock_warn` option must be set to true, which it is by default. The warning advises the user that the data they have received is stale, and that they should refactor their code to implement proper locking. ### Important 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 (ie. transactions). 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 a lock, `END` will not be called. This is _not_ a deadlock risk: all semaphore lock operations in `IPC::Shareable` use the `SEM_UNDO` flag, which causes the kernel to automatically reverse any semaphore operations when the process exits, regardless of the cause of death (including `SIGKILL` and hardware faults). Other processes waiting for the lock will be unblocked. # LOCKING BEHAVIOR MATRIX The following matrix describes what happens to a second object (B) when a first object (A) holds `LOCK_EX` on a segment, across all combinations of the four lock-control attributes: - EW = `enforced_write_locking` - ER = `enforced_read_locking` - WW = `violated_write_lock_warn` - WR = `violated_read_lock_warn` ## Lock acquisition (attribute-independent) `semop` runs at the kernel level; none of the four flags affect whether a lock is granted. +--------------------------+----------------------------------------------+ | B's attempt | Lock result while A holds LOCK_EX | +--------------------------+----------------------------------------------+ | LOCK_EX | Blocks, then acquires once A unlocks | | LOCK_EX | LOCK_NB | Returns 0 immediately | | LOCK_SH | Blocks, then acquires once A unlocks | | LOCK_SH | LOCK_NB | Returns 0 immediately | | (no lock) | N/A | +--------------------------+----------------------------------------------+ ## Behavior after lock state is established ### Case 1: B successfully holds LOCK\_EX (blocking attempts complete after A unlocks) All flags are irrelevant; `FETCH` uses the cache (skipping the read check), and the write check bypasses on `LOCK_EX` ownership. +----------+--------------+-----------+--------------+ | Read | Read warn? | Write | Write warn? | +----------+--------------+-----------+--------------+ | cache | never | succeeds | never | +----------+--------------+-----------+--------------+ ### Case 2: B successfully holds LOCK\_SH (after A unlocks) `FETCH` uses cache (no read warn possible). Writes go through the write check, which sees `SEM_READERS > 0` from B's own `LOCK_SH`. +----+----+------------------------------------+--------------+ | EW | WW | Write outcome | Write warn? | +----+----+------------------------------------+--------------+ | 0 | * | succeeds (enforcement off) | no | | 1 | 0 | blocked ("active readers") | no | | 1 | 1 | blocked ("active readers") | YES | +----+----+------------------------------------+--------------+ ### Case 3: B is unlocked (NB attempt returned 0, or B never attempted a lock); A still holds LOCK\_EX, so SEM\_WRITERS = 1 +----+----+----+----+-------------------+--------------+-------------------+---------------+ | EW | ER | WW | WR | Read | Read warn? | Write | Write warn? | +----+----+----+----+-------------------+--------------+-------------------+---------------+ | 0 | 0 | 0 | 0 | raw shmem (stale) | no | succeeds (race) | no | | 0 | 0 | 0 | 1 | raw shmem | no | succeeds | no | | 0 | 0 | 1 | 0 | raw shmem | no | succeeds | no | | 0 | 0 | 1 | 1 | raw shmem | no | succeeds | no | | 0 | 1 | 0 | 0 | raw shmem | no | succeeds | no | | 0 | 1 | 0 | 1 | raw shmem | YES | succeeds | no | | 0 | 1 | 1 | 0 | raw shmem | no | succeeds | no | | 0 | 1 | 1 | 1 | raw shmem | YES | succeeds | no | | 1 | 0 | 0 | 0 | raw shmem | no | blocked | no | | 1 | 0 | 0 | 1 | raw shmem | no | blocked | no | | 1 | 0 | 1 | 0 | raw shmem | no | blocked | YES | | 1 | 0 | 1 | 1 | raw shmem | no | blocked | YES | | 1 | 1 | 0 | 0 | raw shmem | no | blocked | no | | 1 | 1 | 0 | 1 | raw shmem | YES | blocked | no | | 1 | 1 | 1 | 0 | raw shmem | no | blocked | YES | | 1 | 1 | 1 | 1 | raw shmem | YES | blocked | YES | +----+----+----+----+-------------------+--------------+-------------------+---------------+ ## When A holds LOCK\_SH instead of LOCK\_EX When A holds a shared lock, `SEM_READERS > 0` and `SEM_WRITERS = 0`. This collapses the matrix in three significant ways: - **Lock acquisition diverges.** B's `LOCK_SH` and `LOCK_SH | LOCK_NB` both succeed immediately; multiple readers can hold `LOCK_SH` concurrently. Only the `LOCK_EX` attempts still block (or return 0 for the NB variant). +--------------------------+----------------------------------------------+ | B's attempt | Lock result while A holds LOCK_SH | +--------------------------+----------------------------------------------+ | LOCK_EX | Blocks, then acquires once A unlocks | | LOCK_EX | LOCK_NB | Returns 0 immediately | | LOCK_SH | Acquires immediately (concurrent readers OK) | | LOCK_SH | LOCK_NB | Acquires immediately | | (no lock) | N/A | +--------------------------+----------------------------------------------+ - **Read warnings never fire.** The read check tests `SEM_WRITERS > 0`, which is false. ER and WR become irrelevant; unlocked reads return raw shmem but never warn. The data is also genuinely fresher: A is reading, not writing, so there is no stale-write risk. - **Write warnings carry a different message.** Unlocked writes are still blocked when `EW = 1`, but via the `SEM_READERS > 0` branch of the write check. The warning text becomes: "...has active readers (enforced write locking enabled)" rather than the "exclusively locked" variant. Write outcome and warn behavior across (EW, WW) are otherwise identical to Case 3 above. ## Rules distilled from the matrix - **Lock acquisition** is governed only by SysV semaphores; the four flags do not participate. - **Read result** is always raw shmem when unlocked, always cached when locked; the four flags only affect whether a warning is emitted, never the value returned. - **Read warns** iff `ER = 1` AND `WR = 1` AND another process holds `LOCK_EX`. - **Write blocks** iff `EW = 1` AND (another process holds `LOCK_EX` OR has active `LOCK_SH` readers OR the caller itself holds only `LOCK_SH`). - **Write warns** iff the write was blocked AND `WW = 1`. - `LOCK_EX` ownership bypasses every check in the write path and never reaches the read check, so the four flags never fire for the lock holder. # DATA AND SEGMENT MAPPING For simple data (none of the values are references), a single segment is used throughout. However, with nested data, each value that is a reference is stored in its own, separate shared memory segment (the key is auto-generated). Consider a three-level hash: $h{a}{b}{c} = 1; This creates three segments: Root segment (SysV key 0xABCD) stored data: { a => } | v Child segment (SysV key 11111) stored data: { b => } | v Grandchild segment (SysV key 22222) stored data: { c => 1 } Each segment only knows about its direct children. The chain is followed lazily, one level at a time, as you `FETCH` down into the structure. (See the [shm\_segments()](#shm_segments-key) documentation to gather this structure within code). When you replace a child with a new reference where the previous value was also a reference, a new segment is created and the new data is stored there. The old segment is automatically removed. When a value that is a reference is deleted from the data, the memory segment that held that data is automatically cleaned up and freed. ## Storable The child knot object (which holds \_key, \_type, etc.) is frozen in-place inside the parent's serialized byte blob. On thaw, the child knot is reconstructed from those bytes and re-attached to the existing child segment. ## JSON JSON can't serialize blessed objects, so each child pointer is written as an explicit marker: { "__ics__" => { type => "HASH", child_key => 11111, child_key_hex => "0x00002b67" } } The raw JSON in the root segment looks like: {"a":{"__ics__":{"type":"HASH","child_key":11111,"child_key_hex":"0x00002b67"}}} The raw JSON in the child segment (key 11111) looks like: {"b":{"__ics__":{"type":"HASH","child_key":22222,"child_key_hex":"0x000056ce"}}} Finally, the value in the child is not a reference, so it's stored as literal data: {"c": 1} On decode, any `__ics__` marker is spotted and a tie with `create => 0` is used to re-attach to the existing child segment by that key; no new segment is created, it simply reconnects. # SEMAPHORES Each memory segment that we utilize comes with it a semaphore set of four individual semaphores. These semaphores keep state information about the segment itself, and manages the locking aspects. ## SEM\_MARKER Semaphore slot ID 0. Signals whether the associated shared memory segment has been initialized and is ready for use. `1` if it is, `0` if it isn't. ## SEM\_READERS Semaphore slot ID 1. Specifies the current number of readers holding a `LOCK_SH`. A write lock (`LOCK_EX` can't be obtained until this value is reduced to `0`. ## SEM\_WRITERS Semaphore slot ID 2. Value is `1` if a process has a `LOCK_EX` write lock, and `0` if not. ## SEM\_PROTECTED Semaphore slot ID 3. Used to keep track of the `protected` option value for protected segments. See ["protected"](#protected). # DESTRUCTION perl will destroy the object underlying a tied variable when then tied variable goes out of scope. Unfortunately for [IPC::Shareable](https://metacpan.org/pod/IPC%3A%3AShareable), this may not be desirable: other processes may still need a handle on the relevant shared memory segment. [IPC::Shareable](https://metacpan.org/pod/IPC%3A%3AShareable) therefore provides several options to control the timing of removal of shared memory segments. ## destroy Option As described in ["OPTIONS"](#options), specifying the **destroy** option when `tie()`ing a variable coerces [IPC::Shareable](https://metacpan.org/pod/IPC%3A%3AShareable) to remove the underlying shared memory segment when the process calling `tie()` exits gracefully. ## Notes **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"](#protected) attribute set, it will not be removed in the `END` block, even if `destroy` is set. **Note**: The `END` block only runs on a _clean_ exit (normal program end, `die`, or `exit`). It does **not** run for untrapped signals (`SIGTERM`, `SIGINT`, etc.) or for `SIGKILL`. If your process may be terminated by a signal and you want `destroy` cleanup to run, install signal handlers that call `exit`: $SIG{INT} = $SIG{TERM} = $SIG{HUP} = sub { exit }; This causes the `END` block to fire on those signals. `SIGKILL` cannot be caught; any segments left behind by it can be recovered with `IPC::Shareable->clean_up_all`. **Note**: Advisory locks (`lock()`/`unlock()`) are _always_ released automatically when a process dies, even on `SIGKILL`, because the underlying semaphore operations use `SEM_UNDO`. Lock release is therefore not a concern; only shared memory _segment_ data requires the signal handler precaution above. ## See also See ["METHODS - MANUAL CLEANUP"](#methods-manual-cleanup) for further information. # EXPORTS We do not export anything by default. You must request an item individually, or by tag. ## Tags ### :lock Aliases: `:flock` Includes: `LOCK_EX`, `LOCK_SH`, `LOCK_NB` and `LOCK_UN`. ### :flock Simple legacy alias for `:lock`. ### :semaphores Includes: `SEM_MARKER`, `SEM_READERS`, `SEM_WRITERS` and `SEM_PROTECTED`. ### :all Includes [":lock"](#lock) and [":semaphores"](#semaphores). # AUTHORS Benjamin Sugars Steve Bertrand (since 2016) # NOTES ## Important Notes - o In v1.14, we changed our default serializer from `Storable` to `JSON`. For backward compatibility, there is a process whereby if you have existing segments saved in `Storable` format and the JSON serializer can't process it, we'll automatically fall back to `Storable` for you. You should however recreate the segments with the `JSON` serializer. ## General Notes - o This distribution has minor parts of it developed in C/XS, but these components are only built if we can determine that you've got the proper build tools installed. If not, we simply skip the XS build and fall back to our pure Perl code. - 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](https://metacpan.org/pod/IPC%3A%3AShareable), but we attempt this optimization if you do not). - o 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. # CREDITS 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 # SEE ALSO [perltie](https://metacpan.org/pod/perltie), [Storable](https://metacpan.org/pod/Storable), `shmget`, `ipcs`, `ipcrm` and other SysV IPC manual pages.