text: """Provides shared memory for direct access across processes.
  
  The API of this package is currently provisional. Refer to the
  documentation for details.
  """
  
  
  __all__ = [ 'SharedMemory', 'ShareableList' ]
  
  
  from functools import partial
  import mmap
  import os
  import errno
  import struct
  import secrets
  import types
  
  if os.name == "nt":
      import _winapi
      _USE_POSIX = False
  else:
      import _posixshmem
      _USE_POSIX = True
  
  
  _O_CREX = os.O_CREAT | os.O_EXCL
  
  # FreeBSD (and perhaps other BSDs) limit names to 14 characters.
  _SHM_SAFE_NAME_LENGTH = 14
  
  # Shared memory block name prefix
  if _USE_POSIX:
      _SHM_NAME_PREFIX = '/psm_' if os.sys.platform != 'OpenVMS' else 'psm_'
  else:
      _SHM_NAME_PREFIX = 'wnsm_'
  
  
  def _make_filename():
      "Create a random filename for the shared memory object."
      # number of random bytes to use for name
      nbytes = (_SHM_SAFE_NAME_LENGTH - len(_SHM_NAME_PREFIX)) // 2
      assert nbytes >= 2, '_SHM_NAME_PREFIX too long'
      name = _SHM_NAME_PREFIX + secrets.token_hex(nbytes)
      assert len(name) <= _SHM_SAFE_NAME_LENGTH
      return name
  
  
  class SharedMemory:
      """Creates a new shared memory block or attaches to an existing
      shared memory block.
  
      Every shared memory block is assigned a unique name.  This enables
      one process to create a shared memory block with a particular name
      so that 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."""
  
      # Defaults; enables close() and unlink() to run without errors.
      _name = None
      _fd = -1
      _mmap = None
      _buf = None
      _flags = os.O_RDWR
      _mode = 0o600
      _prepend_leading_slash = True if _USE_POSIX and os.sys.platform != 'OpenVMS' else False
  
      def __init__(self, name=None, create=False, size=0):
          if not size >= 0:
              raise ValueError("'size' must be a positive integer")
          if create:
              self._flags = _O_CREX | os.O_RDWR
              if size == 0:
                  raise ValueError("'size' must be a positive number different from zero")
          if name is None and not self._flags & os.O_EXCL:
              raise ValueError("'name' can only be None if create=True")
  
          if _USE_POSIX:
  
              # POSIX Shared Memory
  
              if name is None:
                  while True:
                      name = _make_filename()
                      try:
                          self._fd = _posixshmem.shm_open(
                              name,
                              self._flags,
                              mode=self._mode
                          )
                      except FileExistsError:
                          continue
                      self._name = name
                      break
              else:
                  name = "/" + name if self._prepend_leading_slash else name
                  self._fd = _posixshmem.shm_open(
                      name,
                      self._flags,
                      mode=self._mode
                  )
                  self._name = name
              try:
                  if create and size:
                      os.ftruncate(self._fd, size)
                  stats = os.fstat(self._fd)
                  size = stats.st_size
                  self._mmap = mmap.mmap(self._fd, size)
              except OSError:
                  self.unlink()
                  raise
  
              from .resource_tracker import register
              register(self._name, "shared_memory")
  
          else:
  
              # Windows Named Shared Memory
  
              if create:
                  while True:
                      temp_name = _make_filename() if name is None else name
                      # Create and reserve shared memory block with this name
                      # until it can be attached to by mmap.
                      h_map = _winapi.CreateFileMapping(
                          _winapi.INVALID_HANDLE_VALUE,
                          _winapi.NULL,
                          _winapi.PAGE_READWRITE,
                          (size >> 32) & 0xFFFFFFFF,
                          size & 0xFFFFFFFF,
                          temp_name
                      )
                      try:
                          last_error_code = _winapi.GetLastError()
                          if last_error_code == _winapi.ERROR_ALREADY_EXISTS:
                              if name is not None:
                                  raise FileExistsError(
                                      errno.EEXIST,
                                      os.strerror(errno.EEXIST),
                                      name,
                                      _winapi.ERROR_ALREADY_EXISTS
                                  )
                              else:
                                  continue
                          self._mmap = mmap.mmap(-1, size, tagname=temp_name)
                      finally:
                          _winapi.CloseHandle(h_map)
                      self._name = temp_name
                      break
  
              else:
                  self._name = name
                  # Dynamically determine the existing named shared memory
                  # block's size which is likely a multiple of mmap.PAGESIZE.
                  h_map = _winapi.OpenFileMapping(
                      _winapi.FILE_MAP_READ,
                      False,
                      name
                  )
                  try:
                      p_buf = _winapi.MapViewOfFile(
                          h_map,
                          _winapi.FILE_MAP_READ,
                          0,
                          0,
                          0
                      )
                  finally:
                      _winapi.CloseHandle(h_map)
                  size = _winapi.VirtualQuerySize(p_buf)
                  self._mmap = mmap.mmap(-1, size, tagname=name)
  
          self._size = size
          self._buf = memoryview(self._mmap)
  
      def __del__(self):
          try:
              self.close()
          except OSError:
              pass
  
      def __reduce__(self):
          return (
              self.__class__,
              (
                  self.name,
                  False,
                  self.size,
              ),
          )
  
      def __repr__(self):
          return f'{self.__class__.__name__}({self.name!r}, size={self.size})'
  
      @property
      def buf(self):
          "A memoryview of contents of the shared memory block."
          return self._buf
  
      @property
      def name(self):
          "Unique name that identifies the shared memory block."
          reported_name = self._name
          if _USE_POSIX and self._prepend_leading_slash:
              if self._name.startswith("/"):
                  reported_name = self._name[1:]
          return reported_name
  
      @property
      def size(self):
          "Size in bytes."
          return self._size
  
      def close(self):
          """Closes access to the shared memory from this instance but does
          not destroy the shared memory block."""
          if self._buf is not None:
              self._buf.release()
              self._buf = None
          if self._mmap is not None:
              self._mmap.close()
              self._mmap = None
          if _USE_POSIX and self._fd >= 0:
              os.close(self._fd)
              self._fd = -1
  
      def unlink(self):
          """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 access
          to the shared memory block."""
          if _USE_POSIX and self._name:
              from .resource_tracker import unregister
              _posixshmem.shm_unlink(self._name)
              unregister(self._name, "shared_memory")
  
  
  _encoding = "utf8"
  
  class ShareableList:
      """Pattern for a mutable list-like object shareable via a shared
      memory block.  It differs from the built-in list type in that these
      lists can not change their overall length (i.e. no append, insert,
      etc.)
  
      Because values are packed into a memoryview as bytes, the struct
      packing format for any storable value must require no more than 8
      characters to describe its format."""
  
      # The shared memory area is organized as follows:
      # - 8 bytes: number of items (N) as a 64-bit integer
      # - (N + 1) * 8 bytes: offsets of each element from the start of the
      #                      data area
      # - K bytes: the data area storing item values (with encoding and size
      #            depending on their respective types)
      # - N * 8 bytes: `struct` format string for each element
      # - N bytes: index into _back_transforms_mapping for each element
      #            (for reconstructing the corresponding Python value)
      _types_mapping = {
          int: "q",
          float: "d",
          bool: "xxxxxxx?",
          str: "%ds",
          bytes: "%ds",
          None.__class__: "xxxxxx?x",
      }
      _alignment = 8
      _back_transforms_mapping = {
          0: lambda value: value,                   # int, float, bool
          1: lambda value: value.rstrip(b'\x00').decode(_encoding),  # str
          2: lambda value: value.rstrip(b'\x00'),   # bytes
          3: lambda _value: None,                   # None
      }
  
      @staticmethod
      def _extract_recreation_code(value):
          """Used in concert with _back_transforms_mapping to convert values
          into the appropriate Python objects when retrieving them from
          the list as well as when storing them."""
          if not isinstance(value, (str, bytes, None.__class__)):
              return 0
          elif isinstance(value, str):
              return 1
          elif isinstance(value, bytes):
              return 2
          else:
              return 3  # NoneType
  
      def __init__(self, sequence=None, *, name=None):
          if name is None or sequence is not None:
              sequence = sequence or ()
              _formats = [
                  self._types_mapping[type(item)]
                      if not isinstance(item, (str, bytes))
                      else self._types_mapping[type(item)] % (
                          self._alignment * (len(item) // self._alignment + 1),
                      )
                  for item in sequence
              ]
              self._list_len = len(_formats)
              assert sum(len(fmt) <= 8 for fmt in _formats) == self._list_len
              offset = 0
              # The offsets of each list element into the shared memory's
              # data area (0 meaning the start of the data area, not the start
              # of the shared memory area).
              self._allocated_offsets = [0]
              for fmt in _formats:
                  offset += self._alignment if fmt[-1] != "s" else int(fmt[:-1])
                  self._allocated_offsets.append(offset)
              _recreation_codes = [
                  self._extract_recreation_code(item) for item in sequence
              ]
              requested_size = struct.calcsize(
                  "q" + self._format_size_metainfo +
                  "".join(_formats) +
                  self._format_packing_metainfo +
                  self._format_back_transform_codes
              )
  
              self.shm = SharedMemory(name, create=True, size=requested_size)
          else:
              self.shm = SharedMemory(name)
  
          if sequence is not None:
              _enc = _encoding
              struct.pack_into(
                  "q" + self._format_size_metainfo,
                  self.shm.buf,
                  0,
                  self._list_len,
                  *(self._allocated_offsets)
              )
              struct.pack_into(
                  "".join(_formats),
                  self.shm.buf,
                  self._offset_data_start,
                  *(v.encode(_enc) if isinstance(v, str) else v for v in sequence)
              )
              struct.pack_into(
                  self._format_packing_metainfo,
                  self.shm.buf,
                  self._offset_packing_formats,
                  *(v.encode(_enc) for v in _formats)
              )
              struct.pack_into(
                  self._format_back_transform_codes,
                  self.shm.buf,
                  self._offset_back_transform_codes,
                  *(_recreation_codes)
              )
  
          else:
              self._list_len = len(self)  # Obtains size from offset 0 in buffer.
              self._allocated_offsets = list(
                  struct.unpack_from(
                      self._format_size_metainfo,
                      self.shm.buf,
                      1 * 8
                  )
              )
  
      def _get_packing_format(self, position):
          "Gets the packing format for a single value stored in the list."
          position = position if position >= 0 else position + self._list_len
          if (position >= self._list_len) or (self._list_len < 0):
              raise IndexError("Requested position out of range.")
  
          v = struct.unpack_from(
              "8s",
              self.shm.buf,
              self._offset_packing_formats + position * 8
          )[0]
          fmt = v.rstrip(b'\x00')
          fmt_as_str = fmt.decode(_encoding)
  
          return fmt_as_str
  
      def _get_back_transform(self, position):
          "Gets the back transformation function for a single value."
  
          if (position >= self._list_len) or (self._list_len < 0):
              raise IndexError("Requested position out of range.")
  
          transform_code = struct.unpack_from(
              "b",
              self.shm.buf,
              self._offset_back_transform_codes + position
          )[0]
          transform_function = self._back_transforms_mapping[transform_code]
  
          return transform_function
  
      def _set_packing_format_and_transform(self, position, fmt_as_str, value):
          """Sets the packing format and back transformation code for a
          single value in the list at the specified position."""
  
          if (position >= self._list_len) or (self._list_len < 0):
              raise IndexError("Requested position out of range.")
  
          struct.pack_into(
              "8s",
              self.shm.buf,
              self._offset_packing_formats + position * 8,
              fmt_as_str.encode(_encoding)
          )
  
          transform_code = self._extract_recreation_code(value)
          struct.pack_into(
              "b",
              self.shm.buf,
              self._offset_back_transform_codes + position,
              transform_code
          )
  
      def __getitem__(self, position):
          position = position if position >= 0 else position + self._list_len
          try:
              offset = self._offset_data_start + self._allocated_offsets[position]
              (v,) = struct.unpack_from(
                  self._get_packing_format(position),
                  self.shm.buf,
                  offset
              )
          except IndexError:
              raise IndexError("index out of range")
  
          back_transform = self._get_back_transform(position)
          v = back_transform(v)
  
          return v
  
      def __setitem__(self, position, value):
          position = position if position >= 0 else position + self._list_len
          try:
              item_offset = self._allocated_offsets[position]
              offset = self._offset_data_start + item_offset
              current_format = self._get_packing_format(position)
          except IndexError:
              raise IndexError("assignment index out of range")
  
          if not isinstance(value, (str, bytes)):
              new_format = self._types_mapping[type(value)]
              encoded_value = value
          else:
              allocated_length = self._allocated_offsets[position + 1] - item_offset
  
              encoded_value = (value.encode(_encoding)
                               if isinstance(value, str) else value)
              if len(encoded_value) > allocated_length:
                  raise ValueError("bytes/str item exceeds available storage")
              if current_format[-1] == "s":
                  new_format = current_format
              else:
                  new_format = self._types_mapping[str] % (
                      allocated_length,
                  )
  
          self._set_packing_format_and_transform(
              position,
              new_format,
              value
          )
          struct.pack_into(new_format, self.shm.buf, offset, encoded_value)
  
      def __reduce__(self):
          return partial(self.__class__, name=self.shm.name), ()
  
      def __len__(self):
          return struct.unpack_from("q", self.shm.buf, 0)[0]
  
      def __repr__(self):
          return f'{self.__class__.__name__}({list(self)}, name={self.shm.name!r})'
  
      @property
      def format(self):
          "The struct packing format used by all currently stored items."
          return "".join(
              self._get_packing_format(i) for i in range(self._list_len)
          )
  
      @property
      def _format_size_metainfo(self):
          "The struct packing format used for the items' storage offsets."
          return "q" * (self._list_len + 1)
  
      @property
      def _format_packing_metainfo(self):
          "The struct packing format used for the items' packing formats."
          return "8s" * self._list_len
  
      @property
      def _format_back_transform_codes(self):
          "The struct packing format used for the items' back transforms."
          return "b" * self._list_len
  
      @property
      def _offset_data_start(self):
          # - 8 bytes for the list length
          # - (N + 1) * 8 bytes for the element offsets
          return (self._list_len + 2) * 8
  
      @property
      def _offset_packing_formats(self):
          return self._offset_data_start + self._allocated_offsets[-1]
  
      @property
      def _offset_back_transform_codes(self):
          return self._offset_packing_formats + self._list_len * 8
  
      def count(self, value):
          "L.count(value) -> integer -- return number of occurrences of value."
  
          return sum(value == entry for entry in self)
  
      def index(self, value):
          """L.index(value) -> integer -- return first index of value.
          Raises ValueError if the value is not present."""
  
          for position, entry in enumerate(self):
              if value == entry:
                  return position
          else:
              raise ValueError(f"{value!r} not in this container")
  
      __class_getitem__ = classmethod(types.GenericAlias)
  