:mod:`!mmap` --- Memory-mapped file support

.. module:: mmap :synopsis: Interface to memory-mapped files for Unix and Windows.

.. include:: ../includes/wasm-notavail.rst

Memory-mapped file objects behave like both :class:bytearray and like :term:file objects <file object>. You can use mmap objects in most places where :class:bytearray are expected; for example, you can use the :mod:re module to search through a memory-mapped file. You can also change a single byte by doing obj[index] = 97, or change a subsequence by assigning to a slice: obj[i1:i2] = b'...'. You can also read and write data starting at the current file position, and :meth:seek through the file to different positions.

A memory-mapped file is created by the :class:~mmap.mmap constructor, which is different on Unix and on Windows. In either case you must provide a file descriptor for a file opened for update. If you wish to map an existing Python file object, use its :meth:~io.IOBase.fileno method to obtain the correct value for the fileno parameter. Otherwise, you can open the file using the :func:os.open function, which returns a file descriptor directly (the file still needs to be closed when done).

.. note:: If you want to create a memory-mapping for a writable, buffered file, you should :func:~io.IOBase.flush the file first. This is necessary to ensure that local modifications to the buffers are actually available to the mapping.

For both the Unix and Windows versions of the constructor, access may be specified as an optional keyword parameter. access accepts one of four values: :const:ACCESS_READ, :const:ACCESS_WRITE, or :const:ACCESS_COPY to specify read-only, write-through or copy-on-write memory respectively, or :const:ACCESS_DEFAULT to defer to prot. access can be used on both Unix and Windows. If access is not specified, Windows mmap returns a write-through mapping. The initial memory values for all three access types are taken from the specified file. Assignment to an :const:ACCESS_READ memory map raises a :exc:TypeError exception. Assignment to an :const:ACCESS_WRITE memory map affects both memory and the underlying file. Assignment to an :const:ACCESS_COPY memory map affects memory but does not update the underlying file.

.. versionchanged:: 3.7 Added :const:ACCESS_DEFAULT constant.

To map anonymous memory, -1 should be passed as the fileno along with the length.

.. class:: mmap(fileno, length, tagname=None,
access=ACCESS_DEFAULT, offset=0, *, trackfd=True)

(Windows version) Maps length bytes from the file specified by the file descriptor fileno, and creates a mmap object. If length is larger than the current size of the file, the file is extended to contain length bytes. If length is 0, the maximum length of the map is the current size of the file, except that if the file is empty Windows raises an exception (you cannot create an empty mapping on Windows).

tagname, if specified and not None, is a string giving a tag name for the mapping. Windows allows you to have many different mappings against the same file. If you specify the name of an existing tag, that tag is opened, otherwise a new tag of this name is created. If this parameter is omitted or None, the mapping is created without a name. Avoiding the use of the tagname parameter will assist in keeping your code portable between Unix and Windows.

offset may be specified as a non-negative integer offset. mmap references will be relative to the offset from the beginning of the file. offset defaults to 0. offset must be a multiple of the :const:ALLOCATIONGRANULARITY.

If trackfd is False, the file handle corresponding to fileno will not be duplicated, and the resulting :class:!mmap object will not be associated with the map's underlying file. This means that the :meth:~mmap.mmap.size and :meth:~mmap.mmap.resize methods will fail. This mode is useful to limit the number of open file handles. The original file can be renamed (but not deleted) after closing fileno.

.. versionchanged:: 3.15 The trackfd parameter was added.

.. audit-event:: mmap.new fileno,length,access,offset mmap.mmap

.. class:: mmap(fileno, length, flags=MAP_SHARED, prot=PROT_WRITE|PROT_READ,
access=ACCESS_DEFAULT, offset=0, *, trackfd=True) :noindex:

(Unix version) Maps length bytes from the file specified by the file descriptor fileno, and returns a mmap object. If length is 0, the maximum length of the map will be the current size of the file when :class:~mmap.mmap is called.

flags specifies the nature of the mapping. :const:MAP_PRIVATE creates a private copy-on-write mapping, so changes to the contents of the mmap object will be private to this process, and :const:MAP_SHARED creates a mapping that's shared with all other processes mapping the same areas of the file. The default value is :const:MAP_SHARED. Some systems have additional possible flags with the full list specified in :ref:MAP_* constants <map-constants>.

prot, if specified, gives the desired memory protection; the two most useful values are :const:PROT_READ and :const:PROT_WRITE, to specify that the pages may be read or written. prot defaults to :const:PROT_READ \| PROT_WRITE.

access may be specified in lieu of flags and prot as an optional keyword parameter. It is an error to specify both flags, prot and access. See the description of access above for information on how to use this parameter.

If trackfd is False, the file descriptor specified by fileno will not be duplicated, and the resulting :class:!mmap object will not be associated with the map's underlying file. This means that the :meth:~mmap.mmap.size and :meth:~mmap.mmap.resize methods will fail. This mode is useful to limit the number of open file descriptors.

To ensure validity of the created memory mapping the file specified by the descriptor fileno is internally automatically synchronized with the physical backing store on macOS.

.. versionchanged:: 3.13 The trackfd parameter was added.

This example shows a simple way of using :class:~mmap.mmap::

  import mmap

  # write a simple example file
  with open("hello.txt", "wb") as f:
      f.write(b"Hello Python!\n")

  with open("hello.txt", "r+b") as f:
      # memory-map the file, size 0 means whole file
      mm = mmap.mmap(f.fileno(), 0)
      # read content via standard file methods
      print(mm.readline())  # prints b"Hello Python!\n"
      # read content via slice notation
      print(mm[:5])  # prints b"Hello"
      # update content using slice notation;
      # note that new content must have same size
      mm[6:] = b" world!\n"
      # ... and read again using standard file methods
      mm.seek(0)
      print(mm.readline())  # prints b"Hello  world!\n"
      # close the map
      mm.close()

:class:~mmap.mmap can also be used as a context manager in a :keyword:with statement::

  import mmap

  with mmap.mmap(-1, 13) as mm:
      mm.write(b"Hello world!")

.. versionadded:: 3.2 Context manager support.

The next example demonstrates how to create an anonymous map and exchange data between the parent and child processes::

  import mmap
  import os

  mm = mmap.mmap(-1, 13)
  mm.write(b"Hello world!")

  pid = os.fork()

  if pid == 0:  # In a child process
      mm.seek(0)
      print(mm.readline())

      mm.close()

.. audit-event:: mmap.new fileno,length,access,offset mmap.mmap

Memory-mapped file objects support the following methods:

.. method:: close()

  Closes the mmap. Subsequent calls to other methods of the object will
  result in a ValueError exception being raised. This will not close
  the open file.

.. attribute:: closed

  ``True`` if the file is closed.

  .. versionadded:: 3.2

.. method:: find(sub[, start[, end]])

  Returns the lowest index in the object where the subsequence *sub* is
  found, such that *sub* is contained in the range [*start*, *end*].
  Optional arguments *start* and *end* are interpreted as in slice notation.
  Returns ``-1`` on failure.

  .. versionchanged:: 3.5
     Writable :term:`bytes-like object` is now accepted.

.. method:: flush([offset[, size]], *, flags=MS_SYNC)

  Flushes changes made to the in-memory copy of a file back to disk. Without
  use of this call there is no guarantee that changes are written back before
  the object is destroyed.  If *offset* and *size* are specified, only
  changes to the given range of bytes will be flushed to disk; otherwise, the
  whole extent of the mapping is flushed.  *offset* must be a multiple of the
  :const:`PAGESIZE` or :const:`ALLOCATIONGRANULARITY`.

  The *flags* parameter specifies the synchronization behavior.
  *flags* must be one of the :ref:`MS_* constants <ms-constants>` available
  on the system.

  On Windows, the *flags* parameter is ignored.

  ``None`` is returned to indicate success.  An exception is raised when the
  call failed.

  .. versionchanged:: 3.8
     Previously, a nonzero value was returned on success; zero was returned
     on error under Windows.  A zero value was returned on success; an
     exception was raised on error under Unix.

  .. versionchanged:: 3.15
     Allow specifying *offset* without *size*. Previously, both *offset*
     and *size* parameters were required together. Now *offset* can be
     specified alone, and the flush operation will extend from *offset*
     to the end of the mmap.

  .. versionchanged:: 3.15
     Added *flags* parameter to control synchronization behavior.

.. method:: madvise(option[, start[, length]])

  Send advice *option* to the kernel about the memory region beginning at
  *start* and extending *length* bytes.  *option* must be one of the
  :ref:`MADV_* constants <madvise-constants>` available on the system.  If
  *start* and *length* are omitted, the entire mapping is spanned.  On
  some systems (including Linux), *start* must be a multiple of the
  :const:`PAGESIZE`.

  Availability: Systems with the ``madvise()`` system call.

  .. versionadded:: 3.8

.. method:: move(dest, src, count)

  Copy the *count* bytes starting at offset *src* to the destination index
  *dest*.  If the mmap was created with :const:`ACCESS_READ`, then calls to
  move will raise a :exc:`TypeError` exception.

.. method:: read([n])

  Return a :class:`bytes` containing up to *n* bytes starting from the
  current file position. If the argument is omitted, ``None`` or negative,
  return all bytes from the current file position to the end of the
  mapping. The file position is updated to point after the bytes that were
  returned.

  .. versionchanged:: 3.3
     Argument can be omitted or ``None``.

.. method:: read_byte()

  Returns a byte at the current file position as an integer, and advances
  the file position by 1.

.. method:: readline()

  Returns a single line, starting at the current file position and up to the
  next newline. The file position is updated to point after the bytes that were
  returned.

.. method:: resize(newsize)

  Resizes the map and the underlying file, if any.

  Resizing a map created with *access* of :const:`ACCESS_READ` or
  :const:`ACCESS_COPY`, will raise a :exc:`TypeError` exception.
  Resizing a map created with *trackfd* set to ``False``,
  will raise a :exc:`ValueError` exception.

  **On Windows**: Resizing the map will raise an :exc:`OSError` if there are other
  maps against the same named file. Resizing an anonymous map (ie against the
  pagefile) will silently create a new map with the original data copied over
  up to the length of the new size.

  Availability: Windows and systems with the ``mremap()`` system call.

  .. versionchanged:: 3.11
     Correctly fails if attempting to resize when another map is held
     Allows resize against an anonymous map on Windows

.. method:: rfind(sub[, start[, end]])

  Returns the highest index in the object where the subsequence *sub* is
  found, such that *sub* is contained in the range [*start*, *end*].
  Optional arguments *start* and *end* are interpreted as in slice notation.
  Returns ``-1`` on failure.

  .. versionchanged:: 3.5
     Writable :term:`bytes-like object` is now accepted.

.. method:: seek(pos[, whence])

  Set the file's current position.  *whence* argument is optional and
  defaults to ``os.SEEK_SET`` or ``0`` (absolute file positioning); other
  values are ``os.SEEK_CUR`` or ``1`` (seek relative to the current
  position) and ``os.SEEK_END`` or ``2`` (seek relative to the file's end).

  .. versionchanged:: 3.13
     Return the new absolute position instead of ``None``.

.. method:: seekable()

  Return whether the file supports seeking, and the return value is always ``True``.

  .. versionadded:: 3.13

.. method:: set_name(name, /)

  Annotate the memory mapping with the given *name* for easier identification
  in ``/proc/<pid>/maps`` if the kernel supports the feature and :option:`-X dev <-X>` is passed
  to Python or if Python is built in :ref:`debug mode <debug-build>`.
  The length of *name* must not exceed 67 bytes including the ``'\0'`` terminator.

  .. availability:: Linux >= 5.17 (kernel built with ``CONFIG_ANON_VMA_NAME`` option)

  .. versionadded:: 3.15

.. method:: size()

  Return the length of the file, which can be larger than the size of the
  memory-mapped area.
  For an anonymous mapping, return its size.

  .. versionchanged:: 3.15
     Anonymous mappings are now supported on Unix.

.. method:: tell()

  Returns the current position of the file pointer.

.. method:: write(bytes)

  Write the bytes in *bytes* into memory at the current position of the
  file pointer and return the number of bytes written (never less than
  ``len(bytes)``, since if the write fails, a :exc:`ValueError` will be
  raised).  The file position is updated to point after the bytes that
  were written.  If the mmap was created with :const:`ACCESS_READ`, then
  writing to it will raise a :exc:`TypeError` exception.

  .. versionchanged:: 3.5
     Writable :term:`bytes-like object` is now accepted.

  .. versionchanged:: 3.6
     The number of bytes written is now returned.

.. method:: write_byte(byte)

  Write the integer *byte* into memory at the current
  position of the file pointer; the file position is advanced by ``1``. If
  the mmap was created with :const:`ACCESS_READ`, then writing to it will
  raise a :exc:`TypeError` exception.

.. _madvise-constants:

MADV_* Constants ++++++++++++++++

.. data:: MADV_NORMAL MADV_RANDOM MADV_SEQUENTIAL MADV_WILLNEED MADV_DONTNEED MADV_REMOVE MADV_DONTFORK MADV_DOFORK MADV_HWPOISON MADV_MERGEABLE MADV_UNMERGEABLE MADV_SOFT_OFFLINE MADV_HUGEPAGE MADV_NOHUGEPAGE MADV_DONTDUMP MADV_DODUMP MADV_FREE MADV_NOSYNC MADV_AUTOSYNC MADV_NOCORE MADV_CORE MADV_PROTECT MADV_FREE_REUSABLE MADV_FREE_REUSE

These options can be passed to :meth:mmap.madvise. Not every option will be present on every system.

Availability: Systems with the madvise() system call.

.. versionadded:: 3.8

.. _map-constants:

MAP_* Constants +++++++++++++++

.. data:: MAP_SHARED MAP_PRIVATE MAP_32BIT MAP_ALIGNED_SUPER MAP_ANON MAP_ANONYMOUS MAP_CONCEAL MAP_DENYWRITE MAP_EXECUTABLE MAP_HASSEMAPHORE MAP_JIT MAP_NOCACHE MAP_NOEXTEND MAP_NORESERVE MAP_POPULATE MAP_RESILIENT_CODESIGN MAP_RESILIENT_MEDIA MAP_STACK MAP_TPRO MAP_TRANSLATED_ALLOW_EXECUTE MAP_UNIX03

These are the various flags that can be passed to :meth:`mmap.mmap`.  :data:`MAP_ALIGNED_SUPER`
is only available at FreeBSD and :data:`MAP_CONCEAL` is only available at OpenBSD.  Note
that some options might not be present on some systems.

.. versionchanged:: 3.10
   Added :data:`MAP_POPULATE` constant.

.. versionadded:: 3.11
   Added :data:`MAP_STACK` constant.

.. versionadded:: 3.12
   Added :data:`MAP_ALIGNED_SUPER` and :data:`MAP_CONCEAL` constants.

.. versionadded:: 3.13
   Added :data:`MAP_32BIT`, :data:`MAP_HASSEMAPHORE`, :data:`MAP_JIT`,
   :data:`MAP_NOCACHE`, :data:`MAP_NOEXTEND`, :data:`MAP_NORESERVE`,
   :data:`MAP_RESILIENT_CODESIGN`, :data:`MAP_RESILIENT_MEDIA`,
   :data:`MAP_TPRO`, :data:`MAP_TRANSLATED_ALLOW_EXECUTE`, and
   :data:`MAP_UNIX03` constants.

.. _ms-constants:

MS_* Constants ++++++++++++++

.. data:: MS_SYNC MS_ASYNC MS_INVALIDATE

These flags control the synchronization behavior for :meth:`mmap.flush`:

* :data:`MS_SYNC` - Synchronous flush: writes are scheduled and the call
  blocks until they are physically written to storage.
* :data:`MS_ASYNC` - Asynchronous flush: writes are scheduled but the call
  returns immediately without waiting for completion.
* :data:`MS_INVALIDATE` - Invalidate cached data: invalidates other mappings
  of the same file so they can see the changes.

.. versionadded:: 3.15

:mod:`!mmap` --- Memory-mapped file support

:mod:!mmap --- Memory-mapped file support

:mod:`!mmap` --- Memory-mapped file support