"multiprocessing.shared_memory" —  Provides shared memory for direct access across processes
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**Source code:** Lib/multiprocessing/shared_memory.py

New in version 3.8.

======================================================================

This module provides a class, "SharedMemory", for the allocation and
management of shared memory to be accessed by one or more processes on
a multicore or symmetric multiprocessor (SMP) machine.  To assist with
the life-cycle management of shared memory especially across distinct
processes, a "BaseManager" subclass, "SharedMemoryManager", is also
provided in the "multiprocessing.managers" module.

In this module, shared memory refers to “System V style” shared memory
blocks (though is not necessarily implemented explicitly as such) and
does not refer to “distributed shared memory”.  This style of shared
memory permits distinct processes to potentially read and write to a
common (or shared) region of volatile memory.  Processes are
conventionally limited to only have access to their own process memory
space but shared memory permits the sharing of data between processes,
avoiding the need to instead send messages between processes
containing that data.  Sharing data directly via memory can provide
significant performance benefits compared to sharing data via disk or
socket or other communications requiring the
serialization/deserialization and copying of data.

class multiprocessing.shared_memory.SharedMemory(name=None, create=False, size=0)

   Creates a new shared memory block or attaches to an existing shared
   memory block.  Each shared memory block is assigned a unique name.
   In this way, one process can create a shared memory block with a
   particular name and a different process can attach to that same
   shared memory block using that same name.

   As a resource for sharing data across processes, shared memory
   blocks may outlive the original process that created them.  When
   one process no longer needs access to a shared memory block that
   might still be needed by other processes, the "close()" method
   should be called. When a shared memory block is no longer needed by
   any process, the "unlink()" method should be called to ensure
   proper cleanup.

   *name* is the unique name for the requested shared memory,
   specified as a string.  When creating a new shared memory block, if
   "None" (the default) is supplied for the name, a novel name will be
   generated.

   *create* controls whether a new shared memory block is created
   ("True") or an existing shared memory block is attached ("False").

   *size* specifies the requested number of bytes when creating a new
   shared memory block.  Because some platforms choose to allocate
   chunks of memory based upon that platform’s memory page size, the
   exact size of the shared memory block may be larger or equal to the
   size requested.  When attaching to an existing shared memory block,
   the "size" parameter is ignored.

   close()

      Closes access to the shared memory from this instance.  In order
      to ensure proper cleanup of resources, all instances should call
      "close()" once the instance is no longer needed.  Note that
      calling "close()" does not cause the shared memory block itself
      to be destroyed.

   unlink()

      Requests that the underlying shared memory block be destroyed.
      In order to ensure proper cleanup of resources, "unlink()"
      should be called once (and only once) across all processes which
      have need for the shared memory block.  After requesting its
      destruction, a shared memory block may or may not be immediately
      destroyed and this behavior may differ across platforms.
      Attempts to access data inside the shared memory block after
      "unlink()" has been called may result in memory access errors.
      Note: the last process relinquishing its hold on a shared memory
      block may call "unlink()" and "close()" in either order.

   buf

      A memoryview of contents of the shared memory block.

   name

      Read-only access to the unique name of the shared memory block.

   size

      Read-only access to size in bytes of the shared memory block.

The following example demonstrates low-level use of "SharedMemory"
instances:

   >>> from multiprocessing import shared_memory
   >>> shm_a = shared_memory.SharedMemory(create=True, size=10)
   >>> type(shm_a.buf)
   <class 'memoryview'>
   >>> buffer = shm_a.buf
   >>> len(buffer)
   10
   >>> buffer[:4] = bytearray([22, 33, 44, 55])  # Modify multiple at once
   >>> buffer[4] = 100                           # Modify single byte at a time
   >>> # Attach to an existing shared memory block
   >>> shm_b = shared_memory.SharedMemory(shm_a.name)
   >>> import array
   >>> array.array('b', shm_b.buf[:5])  # Copy the data into a new array.array
   array('b', [22, 33, 44, 55, 100])
   >>> shm_b.buf[:5] = b'howdy'  # Modify via shm_b using bytes
   >>> bytes(shm_a.buf[:5])      # Access via shm_a
   b'howdy'
   >>> shm_b.close()   # Close each SharedMemory instance
   >>> shm_a.close()
   >>> shm_a.unlink()  # Call unlink only once to release the shared memory

The following example demonstrates a practical use of the
"SharedMemory" class with NumPy arrays, accessing the same
"numpy.ndarray" from two distinct Python shells:

   >>> # In the first Python interactive shell
   >>> import numpy as np
   >>> a = np.array([1, 1, 2, 3, 5, 8])  # Start with an existing NumPy array
   >>> from multiprocessing import shared_memory
   >>> shm = shared_memory.SharedMemory(create=True, size=a.nbytes)
   >>> # Now create a NumPy array backed by shared memory
   >>> b = np.ndarray(a.shape, dtype=a.dtype, buffer=shm.buf)
   >>> b[:] = a[:]  # Copy the original data into shared memory
   >>> b
   array([1, 1, 2, 3, 5, 8])
   >>> type(b)
   <class 'numpy.ndarray'>
   >>> type(a)
   <class 'numpy.ndarray'>
   >>> shm.name  # We did not specify a name so one was chosen for us
   'psm_21467_46075'

   >>> # In either the same shell or a new Python shell on the same machine
   >>> import numpy as np
   >>> from multiprocessing import shared_memory
   >>> # Attach to the existing shared memory block
   >>> existing_shm = shared_memory.SharedMemory(name='psm_21467_46075')
   >>> # Note that a.shape is (6,) and a.dtype is np.int64 in this example
   >>> c = np.ndarray((6,), dtype=np.int64, buffer=existing_shm.buf)
   >>> c
   array([1, 1, 2, 3, 5, 8])
   >>> c[-1] = 888
   >>> c
   array([  1,   1,   2,   3,   5, 888])

   >>> # Back in the first Python interactive shell, b reflects this change
   >>> b
   array([  1,   1,   2,   3,   5, 888])

   >>> # Clean up from within the second Python shell
   >>> del c  # Unnecessary; merely emphasizing the array is no longer used
   >>> existing_shm.close()

   >>> # Clean up from within the first Python shell
   >>> del b  # Unnecessary; merely emphasizing the array is no longer used
   >>> shm.close()
   >>> shm.unlink()  # Free and release the shared memory block at the very end

class multiprocessing.managers.SharedMemoryManager([address[, authkey]])

   A subclass of "BaseManager" which can be used for the management of
   shared memory blocks across processes.

   A call to "start()" on a "SharedMemoryManager" instance causes a
   new process to be started. This new process’s sole purpose is to
   manage the life cycle of all shared memory blocks created through
   it.  To trigger the release of all shared memory blocks managed by
   that process, call "shutdown()" on the instance. This triggers a
   "SharedMemory.unlink()" call on all of the "SharedMemory" objects
   managed by that process and then stops the process itself.  By
   creating "SharedMemory" instances through a "SharedMemoryManager",
   we avoid the need to manually track and trigger the freeing of
   shared memory resources.

   This class provides methods for creating and returning
   "SharedMemory" instances and for creating a list-like object
   ("ShareableList") backed by shared memory.

   Refer to "multiprocessing.managers.BaseManager" for a description
   of the inherited *address* and *authkey* optional input arguments
   and how they may be used to connect to an existing
   "SharedMemoryManager" service from other processes.

   SharedMemory(size)

      Create and return a new "SharedMemory" object with the specified
      "size" in bytes.

   ShareableList(sequence)

      Create and return a new "ShareableList" object, initialized by
      the values from the input "sequence".

The following example demonstrates the basic mechanisms of a
"SharedMemoryManager":

   >>> from multiprocessing.managers import SharedMemoryManager
   >>> smm = SharedMemoryManager()
   >>> smm.start()  # Start the process that manages the shared memory blocks
   >>> sl = smm.ShareableList(range(4))
   >>> sl
   ShareableList([0, 1, 2, 3], name='psm_6572_7512')
   >>> raw_shm = smm.SharedMemory(size=128)
   >>> another_sl = smm.ShareableList('alpha')
   >>> another_sl
   ShareableList(['a', 'l', 'p', 'h', 'a'], name='psm_6572_12221')
   >>> smm.shutdown()  # Calls unlink() on sl, raw_shm, and another_sl

The following example depicts a potentially more convenient pattern
for using "SharedMemoryManager" objects via the "with" statement to
ensure that all shared memory blocks are released after they are no
longer needed:

   >>> with SharedMemoryManager() as smm:
   ...     sl = smm.ShareableList(range(2000))
   ...     # Divide the work among two processes, storing partial results in sl
   ...     p1 = Process(target=do_work, args=(sl, 0, 1000))
   ...     p2 = Process(target=do_work, args=(sl, 1000, 2000))
   ...     p1.start()
   ...     p2.start()  # A multiprocessing.Pool might be more efficient
   ...     p1.join()
   ...     p2.join()   # Wait for all work to complete in both processes
   ...     total_result = sum(sl)  # Consolidate the partial results now in sl

When using a "SharedMemoryManager" in a "with" statement, the shared
memory blocks created using that manager are all released when the
"with" statement’s code block finishes execution.

class multiprocessing.shared_memory.ShareableList(sequence=None, *, name=None)

   Provides a mutable list-like object where all values stored within
   are stored in a shared memory block.  This constrains storable
   values to only the "int", "float", "bool", "str" (less than 10M
   bytes each), "bytes" (less than 10M bytes each), and "None" built-
   in data types. It also notably differs from the built-in "list"
   type in that these lists can not change their overall length (i.e.
   no append, insert, etc.) and do not support the dynamic creation of
   new "ShareableList" instances via slicing.

   *sequence* is used in populating a new "ShareableList" full of
   values. Set to "None" to instead attach to an already existing
   "ShareableList" by its unique shared memory name.

   *name* is the unique name for the requested shared memory, as
   described in the definition for "SharedMemory".  When attaching to
   an existing "ShareableList", specify its shared memory block’s
   unique name while leaving "sequence" set to "None".

   count(value)

      Returns the number of occurrences of "value".

   index(value)

      Returns first index position of "value".  Raises "ValueError" if
      "value" is not present.

   format

      Read-only attribute containing the "struct" packing format used
      by all currently stored values.

   shm

      The "SharedMemory" instance where the values are stored.

The following example demonstrates basic use of a "ShareableList"
instance:

>>> from multiprocessing import shared_memory
>>> a = shared_memory.ShareableList(['howdy', b'HoWdY', -273.154, 100, None, True, 42])
>>> [ type(entry) for entry in a ]
[<class 'str'>, <class 'bytes'>, <class 'float'>, <class 'int'>, <class 'NoneType'>, <class 'bool'>, <class 'int'>]
>>> a[2]
-273.154
>>> a[2] = -78.5
>>> a[2]
-78.5
>>> a[2] = 'dry ice'  # Changing data types is supported as well
>>> a[2]
'dry ice'
>>> a[2] = 'larger than previously allocated storage space'
Traceback (most recent call last):
  ...
ValueError: exceeds available storage for existing str
>>> a[2]
'dry ice'
>>> len(a)
7
>>> a.index(42)
6
>>> a.count(b'howdy')
0
>>> a.count(b'HoWdY')
1
>>> a.shm.close()
>>> a.shm.unlink()
>>> del a  # Use of a ShareableList after call to unlink() is unsupported

The following example depicts how one, two, or many processes may
access the same "ShareableList" by supplying the name of the shared
memory block behind it:

>>> b = shared_memory.ShareableList(range(5))         # In a first process
>>> c = shared_memory.ShareableList(name=b.shm.name)  # In a second process
>>> c
ShareableList([0, 1, 2, 3, 4], name='...')
>>> c[-1] = -999
>>> b[-1]
-999
>>> b.shm.close()
>>> c.shm.close()
>>> c.shm.unlink()
