why zip64_limit defined as 1<<31 -1?

[Ian Kelly]

On Wed, Jan 28, 2015 at 2:36 PM, Chris Angelico <rosuav at gmail.com> wrote:
> On Thu, Jan 29, 2015 at 5:53 AM, jesse <chat2jesse at gmail.com> wrote:
>> should not it be 1<<32 -1(4g)?
>>
>> normal zip archive format should be able to support 4g file.
>>
>> thanks
>
> 1<<31-1 is the limit for a signed 32-bit integer. You'd have to look
> into the details of the zip file format to see whether that's the
> official limit or not; it might simply be that some (un)archivers have
> problems with >2GB files, even if the official stance is that it's
> unsigned.

The bug in which zip64 support was added indicates that the value was
indeed chosen as the limit of a signed 32-bit integer:

http://bugs.python.org/issue1446489