[Python-Dev] ints not overflowing into longs? (original) (raw)

Antoine Pitrou solipsis at pitrou.net
Thu Nov 3 12:30:02 CET 2011

On Wed, 2 Nov 2011 19:41:30 -0700 Guido van Rossum <guido at python.org> wrote:

Apparently Macports is still using a buggy compiler.

If I understand things correctly, this is technically not a buggy compiler but Python making optimistic assumptions about the C standard. (from issue11149: "clang (as with gcc 4.x) assumes signed integer overflow is undefined. But Python depends on the fact that signed integer overflow wraps")

I'd happily call that a buggy C standard, though :-)

Regards

Antoine.

I reported a similar issue before and got this reply from Ned Delly:

""" Thanks for the pointer. That looks like a duplicate of Issue11149 (and Issue12701). Another manifestation of this was reported in Issue13061 which also originated from MacPorts. I'll remind them that the configure change is likely needed for all Pythons. It's still safest to stick with good old gcc-4.2 on OS X at the moment. """ (Those issues are on bugs.python.org.) --Guido On Wed, Nov 2, 2011 at 7:32 PM, Derek Shockey <derek.shockey at gmail.com> wrote: > I just found an unexpected behavior and I'm wondering if it is a bug. > In my 2.7.2 interpreter on OS X, built and installed via MacPorts, it > appears that integers are not correctly overflowing into longs and > instead are yielding bizarre results. I can only reproduce this when > using the exponent operator with two ints (declaring either operand > explicitly as long prevents the behavior). > >>>> 2**100 > 0 >>>> 2**100L > 1267650600228229401496703205376L > >>>> 20**20 > -2101438300051996672 >>>> 20L**20 > 104857600000000000000000000L > >>>> 10**20 > 7766279631452241920 >>>> 10L**20L > 100000000000000000000L > > To confirm I'm not crazy, I tried in the 2.7.1 and 2.6.7 installations > included in OS X 10.7, and also a 2.7.2+ (not sure what the + is) on > an Ubuntu machine and didn't see this behavior. This looks like some > kind of truncation error, but I don't know much about the internals of > Python and have no idea what's going on. I assume since it's only in > my MacPorts installation, it must be build configuration issue that is > specific to OS X, perhaps only 10.7, or MacPorts. > > Am I doing something wrong, and is there a way to fix it before I > compile? I could find any references to this problem as a known issue. > > Thanks, > Derek _> ________________________ > Python-Dev mailing list > Python-Dev at python.org > http://mail.python.org/mailman/listinfo/python-dev > Unsubscribe: http://mail.python.org/mailman/options/python-dev/guido%40python.org >

-- --Guido van Rossum (python.org/~guido)

More information about the Python-Dev mailing list