[ python-Bugs-1274069 ] bz2module.c compiler warning

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Bugs item #1274069, was opened at 2005-08-26 16:25
Message generated for change (Comment added) made by loewis
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Category: Extension Modules
Group: Python 2.5
Status: Open
Resolution: None
Priority: 5
Submitted By: Tim Peters (tim_one)
Assigned to: Nobody/Anonymous (nobody)
Summary: bz2module.c compiler warning

Initial Comment:
On Python HEAD, there's a new warning while 
compiling bz2module.c (MSVC 7.1):

python\Modules\bz2module.c(1095) : warning 
C4244: '=' :
   conversion from 'Py_off_t' to 'int', possible loss of data

Looks like a legitimate complaint to me.  The line in 
question:

	readsize = offset-bytesread;

Those have types int, Py_off_t, and int respectively.  Is 
it true that offset-bytesread _necessarily_ fits in an int?  
If so, a comment should explain why, and a cast should 
be added to squash the warning.  Or If the difference 
doesn't necessarily fit in an int, the code is wrong.

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>Comment By: Martin v. Löwis (loewis)
Date: 2005-08-27 12:30

Message:
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Clearly, the difference is bound by buffersize (since, if it
is larger, readsize is set to buffersize). buffersize is
initialized to SMALLCHUNK, which is 8192, and never changed,
so yes, the difference fits to an int an all supported
platforms.

OTOH, the condition of the if statement is bogus:

    if ((size_t)offset-bytesread > buffersize)

offset is of type Py_off_t, which is a 64-bit type on most
platforms. Casting it to size_t causes truncation on 32-bit
platforms.

Furthermore, I think self->pos should be of type Py_off_t,
as it will wrap around for files larger >2GB on 32-bit
platforms; the same for self->size.

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Comment By: Reinhold Birkenfeld (birkenfeld)
Date: 2005-08-26 17:05

Message:
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user_id=1188172

I think declaring readsize as size_t should fix the problem.

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