16.7. mmap — Memory-mapped file support
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 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).
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: ACCESS_READ, ACCESS_WRITE, or 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
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:
Changed in version 2.5: To map anonymous memory, -1 should be passed as the fileno along with the
length.
Changed in version 2.6:
Changed in version 2.6: mmap.mmap has formerly been a factory function creating mmap objects. Now
mmap.mmap is the class itself.
-
class
mmap.mmap(fileno, length[, flags[, prot[, access[, 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 the PAGESIZE or
ALLOCATIONGRANULARITY.
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("Hello Python!\n")
with open("hello.txt", "r+b") 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:
-
close()
Close the file. Subsequent calls to other methods of the object will
result in an exception being raised.
-
find(string[, start[, end]])
Returns the lowest index in the object where the substring string is
found, such that string is contained in the range [start, end].
Optional arguments start and end are interpreted as in slice notation.
Returns -1 on failure.
-
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.
-
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 throw a TypeError exception.
-
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.
-
read_byte()
Returns a string of length 1 containing the character at the current file
position, 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
throw a TypeError exception.
-
rfind(string[, start[, end]])
Returns the highest index in the object where the substring string is
found, such that string is contained in the range [start, end].
Optional arguments start and end are interpreted as in slice notation.
Returns -1 on failure.
-
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(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 ACCESS_READ, then
writing to it will throw a TypeError exception.
-
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 ACCESS_READ, then writing to it will
throw a TypeError exception.