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Debugging a Mixed Python and C Language Stack

developer.nvidia.com

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Re: Debugging a Mixed Python and C Language Stack

#21

Post by jebarker »

jaza wrote: post #9

An inspiring, albeit daunting, write-up for someone like me, who has been a python coder for over 10 years, but who has never professionally coded in C or C++. I'd be pleasantly surprised with myself if I could one day debug like that. Also somewhat depressing that, yet again, the GIL was to blame, and that after all that impressive investigatory work, the fix was (spoiler alert!) "rewrite the offending function in C…

I predominantly develop in Python but somewhat frequently peer into the C++ and CUDA underlying Pytorch. I am not a competent C or C++ programmer but often am able to debug and fix bugs that originate in the lower levels of the stack. That ability developed just through exposure and focused prodding when I found issues, but I don't think it has much relation or translates to being a professional coder in C/C++/CUDA. Coding and debugging are different skill sets IMO.

Re: Debugging a Mixed Python and C Language Stack

#22

Post by fmajid »

winter_blue wrote: post #3
fmajid wrote: post #2

GDB can decode Python stack traces as Python code, as long as you have have python-gdb.py from the Python source distribution in the same place as your Python executable.

Wow, that's pretty amazing. I wonder how it's implemented, and if there are any tutorials on implementing something similar, for programming language designers/creators.

GDB itself is fully scriptable in Python:

https://sourceware.org/gdb/current/onlinedocs/gdb.html/Pytho...

Re: Debugging a Mixed Python and C Language Stack

#23

Post by planede »

MereInterest wrote: post #17
PaulHoule wrote: post #6

That reminds me of the time I debugged some code (also neural network) that was in both Java and C++ and I was able to attach both gdb and jdb to the same process but had to disable the segfault trap on gdb because the jvm segfaults all the time in normal operation.

As a C++ developer, I'm struggling to understand how and why segfaults would ever be part of normal operation. In my mental model, the presence of a segfault means that a program has gone so far off the expected path that no guarantees can be made about its state whatsoever, so the only safe thing to do is to let the program crash. Is there a reason why the JVM regularly segfaults?

Segfaults are unexpected and shouldn't happen in a conforming C++ program that doesn't evaluate operations with undefined behavior.

But in non-portable C or C++ program doing manual memory management with OS primitives targeting a specific OS the conditions for segfault are well documented, and you can also rely on the programs behavior when that happens (the OS raises a signal for you, that you can handle).

There are not many programs that should be written this way, but I assume the JVM might fall into this category. I'm still not sure if handling page faults this way in regular operation is the best strategy, but I would worry about performance more than correctness.

Re: Debugging a Mixed Python and C Language Stack

#25

Post by PaulHoule »

MereInterest wrote: post #17
PaulHoule wrote: post #6

That reminds me of the time I debugged some code (also neural network) that was in both Java and C++ and I was able to attach both gdb and jdb to the same process but had to disable the segfault trap on gdb because the jvm segfaults all the time in normal operation.

As a C++ developer, I'm struggling to understand how and why segfaults would ever be part of normal operation. In my mental model, the presence of a segfault means that a program has gone so far off the expected path that no guarantees can be made about its state whatsoever, so the only safe thing to do is to let the program crash. Is there a reason why the JVM regularly segfaults?

I think Java catches the segfault and throws a NullPointerException or something like that.
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