path: root/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 :func:`mmap` function, 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 function, *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 readonly, 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.

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


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

   **(Windows version)** Maps *length* bytes from the file specified by the file
   handle *fileno*, and returns 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.


.. function:: 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 :func:`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 :func:`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:: mmap.close()

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


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

   Returns the lowest index in the object where the substring *string* is found.
   Returns ``-1`` on failure.  *start* is the index at which the search begins, and
   defaults to zero.


.. method:: mmap.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.


.. method:: mmap.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:: mmap.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:: mmap.read_byte()

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


.. method:: mmap.readline()

   Returns a single line, starting at the current file position and up to the next
   newline.


.. method:: mmap.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:: mmap.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:: mmap.size()

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


.. method:: mmap.tell()

   Returns the current position of the file pointer.


.. method:: mmap.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:: mmap.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.