[ python-Bugs-1341031 ] mmap does not accept length as 0

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Bugs item #1341031, was opened at 2005-10-28 13:26
Message generated for change (Comment added) made by josiahcarlson
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Category: Python Library
Group: Python 2.4
Status: Open
Resolution: None
Priority: 5
Submitted By: liturgist (liturgist)
Assigned to: Nobody/Anonymous (nobody)
Summary: mmap does not accept length as 0

Initial Comment:
Creating an mmap object does not accept a length
parameter of zero on Linux FC4 and Cygwin.  However, it
does on Windows XP.

$ ls -al t.dat
-rw-r--r--  1 pwatson mkgroup-l-d 64 Oct 28 10:13 t.dat

$ python
Python 2.4.1 (#1, May 27 2005, 18:02:40)
[GCC 3.3.3 (cygwin special)] on cygwin
Type "help", "copyright", "credits" or "license" for
more information.
>>> import mmap
>>> fp = open('t.dat', 'rb')
>>> b = mmap.mmap(fp.fileno(), 0, access=mmap.ACCESS_READ)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
EnvironmentError: [Errno 22] Invalid argument
>>> b = mmap.mmap(fp.fileno(), 64, access=mmap.ACCESS_READ)
>>> b.size()
64

=====
$ ls -al .profile
-rwxrwxr-x  1 pwatson pwatson 1920 Jul 22 06:57 .profile

$ python
Python 2.4.1 (#1, Jul 19 2005, 14:16:43)
[GCC 4.0.0 20050519 (Red Hat 4.0.0-8)] on linux2
Type "help", "copyright", "credits" or "license" for
more information.
>>> import mmap
>>> fp = open('.profile', 'rb')
>>> b = mmap.mmap(fp.fileno(), 0, access=mmap.ACCESS_READ)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
EnvironmentError: [Errno 22] Invalid argument
>>> b = mmap.mmap(fp.fileno(), 1920,
access=mmap.ACCESS_READ)
>>> b.size()
1920

----------------------------------------------------------------------

Comment By: Josiah Carlson (josiahcarlson)
Date: 2005-11-07 22:17

Message:
Logged In: YES 
user_id=341410

On Windows and Linux, I have been able to map positive,
nonzero portions of files for quite some time.  The only
use-case I can see in using an mmap of size 0 is if the file
was just created, and a user wanted to immediately resize
the mmap to be larger.

This can be worked-around with a simple fp.write('\0'),
followed by a mmap call with size 1.  Resize as
desired/necessary.

----------------------------------------------------------------------

Comment By: liturgist (liturgist)
Date: 2005-10-28 14:09

Message:
Logged In: YES 
user_id=197677

It would be helpful to creating cross-platform code for all
platforms to have the same requirements.  If this is marked
as Not-A-Bug, then how about changing it to a documentation
bug so that and example could be provided?

fp = open(fn, 'rb')
b = mmap.mmap(fp.fileno(), os.stat(fp.name).st_size,
access=mmap.ACCESS_READ)

----------------------------------------------------------------------

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