[Python-Dev] [Python-checkins] cpython (2.7): Issue #10276: test_zlib checks that inputs of 2 GB are handled correctly by

[Ethan Furman]

Victor Stinner wrote:
> Le mercredi 04 mai 2011 à 15:40 -0700, Ethan Furman a écrit :
>> Victor Stinner wrote:
>>> Le mardi 03 mai 2011 à 16:22 +0200, Nadeem Vawda a écrit :
>>>> On Tue, May 3, 2011 at 3:19 PM, victor.stinner
>>>> <python-checkins at python.org> wrote:
>>>>> 
>>>>> +    int_max = 0x7FFFFFFF
>>>>> 
>>>>> +            with open(TESTFN, "wb+") as f:
>>>>> +                f.seek(self.int_max-4)
>>>>> +                f.write("asdf")
>>>>> +                f.flush()
>>>> 
>>>> 0x7FFFFFFF is (2G-1) bytes. For a 2GB buffer, int_max should be
>>>> 0x80000000. However, if you make this change, crc32() and adler32()
>>>> raise OverflowErrors (see changeset a0681e7a6ded).
 >>>
>>> I don't want to check OverflowError: the test is supposed to compute the
>>> checksum of a buffer of 0x7FFFFFFF bytes
 >>
>> The comment says 'check that inputs of 2 GB are handled correctly' but 
>> the file created is 1 byte short of 2Gb.  Is the test wrong, or just 
>> wrongly commented?  Or am I not understanding?
> 
> If you write a byte after 2 GB of zeros, the file size is 2 GB+the few
> bytes. This trick is to create quickly a large file: some OSes support
> sparse files, zeros are not written on disk. But on Mac OS X and
> Windows, you really write 2 GB+some bytes.

True, but that's not what's happening -- four bytes are being written at
int_max - 4, and int_max is one less that 2GB; hence the resulting file 
is one less than 2GB.

~Ethan~