Accessing Python Objects in a Core Dump

Question

Is there anyway to discover the python value of a PyObject* from a corefile in gdb

Answer 1

It's lots of work, but of course it can be done, especially if you have all the symbols. Look at the header files for the specific version of Python (and compilation options in use to build it): they define PyObject as a struct which includes, first and foremost, a pointer to a type. Lots of macros are used, so you may want to run the compile of that Python from sources again, with exactly the same flags but in addition a -E to stop after preprocessing, so you can refer to the specific C code that made the bits you're seeing in the core dump.

A type object has, among many other things, a string (array of char) that's its name, and from it you can infer what exactly objects of that type contain -- be it content directly, or maybe some content (such as a length, i.e. number of items) and a pointer to the actual data.

I've done such super-advanced post-mortem debugging a couple of times (starting with VERY precise knowledge of the Python versions involved and all the prepared preprocessed sources &c) and each time it took me a day or two (were I still a freelance and charging by the hour, if I had to bid on such a task I'd say at least 20 hours -- at my not-cheap hourly rates!-).

IOW, it's worth it only if it's really truly the only way out of some very costly pickle. On the plus side, it WILL teach you more about Python's internals than you ever thought was there, even after memorizing every line of the sources. Good luck, you'll need some!!!