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 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 seek()
through the file to different positions.
A memory-mapped file is created by the 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
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).
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: ACCESS_READ,
ACCESS_WRITE, or 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
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 2.5:
To map anonymous memory,
-1 should be passed as the fileno along with the length.
mmap( |
fileno, length[, tagname[, access]]) |
-
(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.
mmap( |
fileno, length[, flags[,
prot[, access]]]) |
-
(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.
Memory-mapped file objects support the following methods:
-
Close the file. Subsequent calls to other methods of the object
will result in an exception being raised.
-
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.
-
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.
-
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 throw a
TypeError exception.
-
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.
-
Returns a string of length 1 containing the character at the current
file position, and advances the file position by 1.
-
Returns a single line, starting at the current file position and up to
the next newline.
-
Resizes the map and the underlying file, if any.
If the mmap was created with ACCESS_READ or
ACCESS_COPY, resizing the map will throw a TypeError exception.
-
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).
-
Return the length of the file, which can be larger than the size of
the memory-mapped area.
-
Returns the current position of the file pointer.
-
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
ACCESS_READ, then writing to it will throw a
TypeError exception.
-
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 ACCESS_READ,
then writing to it will throw a TypeError exception.
Release 2.5.2, documentation updated on 21st February, 2008.
See About this document... for information on suggesting changes.
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