"mmap" --- 内存映射文件支持
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内存映射（mmap）文件对象的行为既像 "bytearray" 又像 *文件对象*。 你可
以在大部分接受 "bytearray" 的地方使用 mmap 对象；例如，你可以使用 "re"
模块来搜索一个内存映射文件。 你也可以通过执行 "obj[index] = 97" 来修改
单个字节，或者通过对切片赋值来修改一个子序列: "obj[i1:i2] = b'...'"。
你还可以在文件的当前位置开始读取和写入数据，并使用 "seek()" 前往另一个
位置。

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).

備註: 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.

3.7 版更變: 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])

   **(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".

   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])

   **(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".

   *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.

   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 "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!")

   3.2 版新加入: 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.

      3.2 版新加入.

   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.

      3.5 版更變: 现在支持可写的 *字节类对象*。

   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.

      3.8 版更變: 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.

      3.8 版新加入.

   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.

      3.3 版更變: 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.

   resize(newsize)

      Resizes the map and the underlying file, if any. If the mmap was
      created with "ACCESS_READ" or "ACCESS_COPY", resizing the map
      will raise a "TypeError" exception.

   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.

      3.5 版更變: 现在支持可写的 *字节类对象*。

   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).

   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.

      3.5 版更變: 现在支持可写的 *字节类对象*。

      3.6 版更變: 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

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

   Availability: Systems with the madvise() system call.

   3.8 版新加入.
