Debugging Python SegFault with gdb

Resources

• here's a tutorial (PDF, 21 pages) via U. Maryland ⠶ Samuel Huang — CMSC212: Introduction to Low-Level Programming Concepts
  • gives an idea of the program [but more intended toward use with C rather than Python]
• "How to interpret a gdb backtrace
  • StackOverflow question (about C++, not Python[/Cython])
• Official Python dev guide: gdb Support
• wiki.python.org ⠶ DebuggingWithGdb

Brief workflow

• make a python file e.g. segger.py which causes the segfault
• [in a terminal] run gdb python
  • N.B. - this just worked for me, there's mention online of needing to recompile with gdb support on some systems
• in the interactive gdb session, type run segger.py [or whatever you called your script], to give something like:

Starting program: /home/myusername/anaconda3/bin/python segger.py
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff12ab700 (LWP 12345)]
[New Thread 0x7ffff89yz700 (LWP 12346)]
…
[New Thread 0x7ffff89yz700 (LWP 12350)]
Program received signal SIGSEGV, Segmentation fault.
Capitalised_Snake_Case_Ref_1 (a=0x123f4567890a1b23, b=0x7fffabfcdf00 <sipVoidPtr_Type>) at Path/to.c:1343
1234    Path/to.c: {Error message goes here}.
(gdb) 

• where:
  • anaconda3 may differ if you're not using an anaconda3 installation
  • number of threads may vary based on your system
  • Capitalised_Snake_Case_Ref_1 and sipVoidPtr_Type refer to a function in cpython and variable in C respectively
    • you can search for the function in github.com/python/cpython/ e.g. PyType_IsSubtype at Objects/typeobject.c ⠶ line 1386 though note that line numbers on the repo may differ since your installation was released [releases are tagged]
    • N.B. the "sip" refers to "Sessions Initiation Protocol" (SIP), see riverbankcomputing.com ⠶ intro and wiki.python.org ⠶ SIP