mmap — Memory-mapped file support — Python 3.14.3 documentation
Description: Availability: not WASI. This module does not work or is not available on WebAssembly. See WebAssembly platforms for more information. Memory-mapped file objects behave like both bytearray and like ...

# `mmap` — Memory-mapped file support

Availability: not WASI.

This module does not work or is not available on WebAssembly. See WebAssembly platforms for more information.

Memory-mapped file objects behave like both `bytearray` and like file objects. You can use mmap objects in most places where `bytearray` are expected; for example, you can use the `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 `seek()` through the file to different positions.

A memory-mapped file is created by the `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 `fileno()` method to obtain the correct value for the fileno parameter. Otherwise, you can open the file using the `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 `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: `ACCESS_READ`, `ACCESS_WRITE`, or `ACCESS_COPY` to specify read-only, write-through or copy-on-write memory respectively, or `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 `ACCESS_READ` memory map raises a `TypeError` exception. Assignment to an `ACCESS_WRITE` memory map affects both memory and the underlying file. Assignment to an `ACCESS_COPY` memory map affects memory but does not update the underlying file.

Changed in version 3.7: Added `ACCESS_DEFAULT` constant.

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

class mmap.mmap(fileno, length, tagname=None, access=ACCESS_DEFAULT, offset=0)

  (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 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 `ALLOCATIONGRANULARITY`.

  Raises an auditing event `mmap.__new__` with arguments `fileno`, `length`, `access`, `offset`.

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

  (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 `mmap` is called.

  flags specifies the nature of the mapping. `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 `MAP_SHARED` creates a mapping that’s shared with all other processes mapping the same areas of the file. The default value is `MAP_SHARED`. Some systems have additional possible flags with the full list specified in MAP_* constants.

  prot, if specified, gives the desired memory protection; the two most useful values are `PROT_READ` and `PROT_WRITE`, to specify that the pages may be read or written. prot defaults to `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 `ALLOCATIONGRANULARITY` which is equal to `PAGESIZE` on Unix systems.

  If trackfd is `False`, the file descriptor specified by fileno will not be duplicated, and the resulting `mmap` object will not be associated with the map’s underlying file. This means that the `size()` and `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.

  Changed in version 3.13: The trackfd parameter was added.

  This example shows a simple way of using `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()

  `mmap` can also be used as a context manager in a `with` statement:

      import mmap

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

  Added in version 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()

  Raises an auditing event `mmap.__new__` with arguments `fileno`, `length`, `access`, `offset`.

  Memory-mapped file objects support the following methods:

  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.

  closed

    `True` if the file is closed.

    Added in version 3.2.

  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.

    Changed in version 3.5: Writable bytes-like object is now accepted.

  flush()
  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. offset must be a multiple of the `PAGESIZE` or `ALLOCATIONGRANULARITY`.

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

    Changed in version 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.

  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 MADV_* 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 `PAGESIZE`.

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

    Added in version 3.8.

  move(dest, src, count)

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

  read([n])

    Return a `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.

    Changed in version 3.3: Argument can be omitted or `None`.

  read_byte()

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

  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.

  resize(newsize)

    Resizes the map and the underlying file, if any.

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

    On Windows: Resizing the map will raise an `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.

    Changed in version 3.11: Correctly fails if attempting to resize when another map is held Allows resize against an anonymous map on Windows

  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.

    Changed in version 3.5: Writable bytes-like object is now accepted.

  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).

    Changed in version 3.13: Return the new absolute position instead of `None`.

  seekable()

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

    Added in version 3.13.

  size()

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

  tell()

    Returns the current position of the file pointer.

  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 `ValueError` will be raised). The file position is updated to point after the bytes that were written. If the mmap was created with `ACCESS_READ`, then writing to it will raise a `TypeError` exception.

    Changed in version 3.5: Writable bytes-like object is now accepted.

    Changed in version 3.6: The number of bytes written is now returned.

  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 `ACCESS_READ`, then writing to it will raise a `TypeError` exception.

## MADV_* Constants

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

  These options can be passed to `mmap.madvise()`. Not every option will be present on every system.

  Availability: Systems with the madvise() system call.

  Added in version 3.8.

## MAP_* Constants

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

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

  Changed in version 3.10: Added `MAP_POPULATE` constant.

  Added in version 3.11: Added `MAP_STACK` constant.

  Added in version 3.12: Added `MAP_ALIGNED_SUPER` and `MAP_CONCEAL` constants.

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