[Python-Dev] Disabling string interning for null and single-char causes segfaults (original) (raw)

Antoine Pitrou solipsis at pitrou.net
Sun Mar 3 00:36:36 CET 2013

On Sat, 2 Mar 2013 22:13:56 +0100 Stefan Bucur <stefan.bucur at gmail.com> wrote:

On Sat, Mar 2, 2013 at 4:31 PM, Antoine Pitrou <solipsis at pitrou.net> wrote: > On Fri, 1 Mar 2013 16:24:42 +0100 > Stefan Bucur <stefan.bucur at gmail.com> wrote: >> >> However, after applying this modification, when running "make test" I get a >> segfault in the test_all test case._ >> >> Before digging deeper into the issue, I wanted to ask here if there are any >> implicit assumptions about string identity and interning throughout the >> interpreter implementation. For instance, are two single-char strings >> having the same content supposed to be identical objects? > > From a language POV, no, but inside a specific interpreter such as > CPython it may be a reasonable expectation. > >> I'm assuming that it's either this, or some refcount bug in the interpreter >> that manifests only when certain strings are no longer interned and thus >> have a higher chance to get low refcount values. > > Indeed, if it's a real bug it would be nice to get it fixed :-)

By the way, in that case, what would be the best way to debug such type of ref count errors? I recently ran across this document [1], which kind of applies to debugging focused on newly introduced code.

That documents looks a bit outdated (1998!). I would suggest you enable core dumps (ulimit -c unlimited), then let Python crash and inspect the stack trace with gdb. You will get better results if using a debug build and the modern gdb inspection helpers: http://docs.python.org/devguide/gdb.html

Oh, by the way, it would be better to do your work on Python 3 rather than 2.7. Either the default branch or the 3.3 branch, I guess. See http://docs.python.org/devguide/setup.html#checkout

Regards

Antoine.

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