[Python-bugs-list] [ python-Bugs-801631 ] file.truncate fault on windows

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Bugs item #801631, was opened at 2003-09-06 09:08
Message generated for change (Comment added) made by tim_one
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Category: Windows
Group: Python 2.3
>Status: Closed
>Resolution: Fixed
Priority: 5
Submitted By: simon place (psi-man)
Assigned to: Tim Peters (tim_one)
Summary: file.truncate fault on windows

Initial Comment:
>manual says file.truncate leave the pointer unmoved, 



>>> f=file('test.dat', 'wb') to make things easier

>>> f.write('1234567890')

>>> f.close()

>>> f=file('test.dat','rb+')

>>> f.read(5)

>>>'12345'

>>> f.tell()

>>>5L

>>> f.truncate()

>>> f.tell()

>>>10L





>>>( PythonWin 2.3 (#46, Jul 29 2003, 18:54:32) [MSC

v.1200 32 bit (Intel)] on win32. )





Seems it's a Windows only bug.  On a 2.4.3 linux,

libc-2.2.5 the final

statement produces 5L (and the file is only 5 bytes

long).  Same on

another 2.4.19 libc-2.2.5, and a 2.4.21 libc-2.3.1

Linux, all with a

recent Python 2.4a0 .  

reproduce on Windows.

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

>Comment By: Tim Peters (tim_one)
Date: 2003-09-06 23:32

Message:
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user_id=31435

Repaired, via



Lib/test/test_file.py; new revision: 1.13

Misc/NEWS; new revision: 1.852

Objects/fileobject.c; new revision: 2.182

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

Comment By: Tim Peters (tim_one)
Date: 2003-09-06 12:29

Message:
Logged In: YES 
user_id=31435

Oh, fudge.  The Windows code here is trying very hard to 

preserve the file position, but it turns out that C fflush() on 

Windows changes the file position in this specific test case.



However, the C standard says (about fflush):



"""

If stream points to an output stream or an update stream in 

which the most recent operation was not input, the fflush 

function causes any unwritten data for that stream to be 

delivered to the host environment to be written to the file; 

otherwise, the behavior is undefined.

"""



The last operation performed by the test program before 

Python's internals call fflush() was an input operation, so the 

effect of calling fflush() is undefined.  If it varies across 

platforms (as appears to be the case here), that's fine by the 

standard, and Python is relying on undefined behavior.



If you stick, e.g., f.seek(5) after the f.tell() call, then on 

Windows this test program works as intended (prints 5 at the 

end, and the file is truncated to 5 bytes).  Then the last 

operation performed on the stream opened for update is 

*not* an input operation, so the effect of fflush() is defined.

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

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