cpython/Doc/library/mmap.rst

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

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


Memory-mapped file objects behave like both strings and like file objects.
Unlike normal string objects, however, these are mutable.  You can use mmap
objects in most places where strings are expected; for example, you can use
the :mod:`re` module to search through a memory-mapped file.  Since they're
mutable, you can change a single character by doing ``obj[index] = 'a'``, or
change a substring by assigning to a slice: ``obj[i1:i2] = '...'``.  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` 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:`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).

For both the Unix and Windows versions of the constructor, *access* may be
specified as an optional keyword parameter. *access* accepts one of three
values: :const:`ACCESS_READ`, :const:`ACCESS_WRITE`, or :const:`ACCESS_COPY`
to specify read-only, write-through or copy-on-write memory respectively.
*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:: 2.5
   To map anonymous memory, -1 should be passed as the fileno along with the
   length.

.. versionchanged:: 2.6
   mmap.mmap has formerly been a factory function creating mmap objects. Now
   mmap.mmap is the class itself.

.. class:: mmap(fileno, length[, tagname[, access[, offset]]])

   **(Windows version)** Maps *length* bytes from the file specified by the
   file handle *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 tag 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 ALLOCATIONGRANULARITY.


.. class:: mmap(fileno, length[, flags[, prot[, access[, offset]]]])
   :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` 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`.

   *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.

   *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 PAGESIZE or
   ALLOCATIONGRANULARITY.
   
   This example shows a simple way of using :class:`mmap`::

      import mmap

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

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


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

      import mmap
      import os

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

      pid = os.fork()

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

          map.close()


   Memory-mapped file objects support the following methods:


   .. method:: close()

      Close the file.  Subsequent calls to other methods of the object will
      result in an exception being raised.


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

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


   .. method:: flush([offset, size])

      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.

      **(Windows version)** A nonzero value returned indicates success; zero
      indicates failure.

      **(Unix version)** A zero value is returned to indicate success. An
      exception is raised when the call failed.


   .. 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 throw a :exc:`TypeError` exception.


   .. method:: read(num)

      Return a string containing up to *num* bytes starting from the current
      file position; the file position is updated to point after the bytes that
      were returned.


   .. method:: read_byte()

      Returns a string of length 1 containing the character at the current file
      position, 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.


   .. method:: resize(newsize)

      Resizes the map and the underlying file, if any. If the mmap was created
      with :const:`ACCESS_READ` or :const:`ACCESS_COPY`, resizing the map will
      throw a :exc:`TypeError` exception.


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

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


   .. 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).


   .. method:: size()

      Return the length of the file, which can be larger than the size of the
      memory-mapped area.


   .. method:: tell()

      Returns the current position of the file pointer.


   .. method:: write(string)

      Write the bytes in *string* into memory at the current position of the
      file pointer; 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 throw a :exc:`TypeError` exception.


   .. method:: write_byte(byte)

      Write the single-character string *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
      throw a :exc:`TypeError` exception.