mirror of
https://github.com/python/cpython.git
synced 2024-11-27 20:04:41 +08:00
8490f5acfe
codecs, that accepted only read-only bytes-like object now accept writable bytes-like object too.
280 lines
11 KiB
ReStructuredText
280 lines
11 KiB
ReStructuredText
: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 :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` 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).
|
|
|
|
.. 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 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.
|
|
|
|
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])
|
|
|
|
**(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=MAP_SHARED, prot=PROT_WRITE|PROT_READ, access=ACCESS_DEFAULT[, 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.
|
|
|
|
To ensure validity of the created memory mapping the file specified
|
|
by the descriptor *fileno* is internally automatically synchronized
|
|
with physical backing store on Mac OS X and OpenVMS.
|
|
|
|
This example shows a simple way of using :class:`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` can also be used as a context manager in a :keyword:`with`
|
|
statement.::
|
|
|
|
import mmap
|
|
|
|
with mmap.mmap(-1, 13) as mm:
|
|
mm.write("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()
|
|
|
|
|
|
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]])
|
|
|
|
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 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.
|
|
|
|
|
|
.. 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
|
|
raise a :exc:`TypeError` exception.
|
|
|
|
|
|
.. 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).
|
|
|
|
|
|
.. 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(bytes)
|
|
|
|
Write the bytes in *bytes* 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 raise a :exc:`TypeError` exception.
|
|
|
|
.. versionchanged: 3.5
|
|
Writable :term:`bytes-like object` is now accepted.
|
|
|
|
|
|
.. 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.
|