Debug CPython with gdb

Python built in debug mode is ABI compatible with Python built in release mode since Python 3.8: Debug build uses the same ABI as release build.

Current Python thread state

Read tstate_current, if the GIL is held:

p ((PyThreadState*)_PyRuntime.gilstate.tstate_current)

Thread Local Storage (TLS):

p (PyThreadState *)PyThread_tss_get(&_PyRuntime.gilstate.autoTSSkey)

See also Python Thread State.

Put a breakpoint on the next exception

Put a breakpoint on the next exception: break _PyErr_SetObject (or break PyErr_SetObject in Python 3.7 and older).

Then use condition to only break at the expected exception.

Watch when reference count changes

Use a memory breakpoint like:

watch ((PyObject*)MEMORY_ADDRESS)->ob_refcnt

where MEMORY_ADDRESS is the address of a Python object.

Analyze a coredump

Current Python thread state, like PyThreadState_GET():

p ((PyThreadState*)_PyRuntime.gilstate.tstate_current)

Current error, like PyErr_Occurred():

p ((PyThreadState*)_PyRuntime.gilstate.tstate_current)->curexc_type

Load python-gdb.py

Load it manually:

(gdb) source /path/to/python-gdb.py

Add directory containing Python source code to “safe path”, to automatically load python-gdb.py when debugging Python. Add the following line to your ~/.gdbinit:

add-auto-load-safe-path ~/prog/

In my case, Python is in ~/prog/python/master, but I chose to allow to load any Python script from ~/prog/.

On Fedora, the script is provided as:

/usr/lib/debug/usr/lib64/libpython3.6m.so.1.0-3.6.6-1.fc28.x86_64.debug-gdb.py

Debug functions

You might want to call these functions in a running process from gdb:

  • _PyObject_Dump(obj)
  • _PyUnicode_Dump(obj): dump properties of the Unicode object, not it’s content
  • PyErr_Occurred(), _PyErr_Occurred(tstate) or tstate->curexc_type: get the current exception type, NULL if no exception was raised.
  • Check object consistency:
    • _PyDict_CheckConsistency()
    • _PyUnicode_CheckConsistency()
    • _PyObject_CheckConsistency()
    • _PyType_CheckConsistency()
    • _PyWideStringList_CheckConsistency()
  • if the gdb py-bt command is broken, try to call:
    • _Py_DumpTraceback(2, tstate)
    • _Py_DumpTracebackThreads(2, interp, tstate) where tstate can be NULL
    • Python 3.8: get tstate from _PyRuntime.gilstate.tstate_current and interp from _PyRuntime.gilstate.autoInterpreterState
    • 2 is the file descriptor 2: stderr