What is needed to use gdb 7's support for debugging Python programs?

Question

I'd like to use gdb 7's support for debugging Python "inferior processes".

What do I need to be able to do that?

For example:

• What flags does the inferior Python's configure script need to have been run with?

• Does the inferior Python process have to be Python 2.7 or newer (I see that's when the part of the gdb support for this that's in Python source tree was committed)? Or is Python 2.7 only needed by the gdb process itself?

• What files need to have been installed that might not be packaged by all distributions? For example, on packages.ubuntu.com I don't get any hits for python-gdb.py, which I believe is needed.

It would be very handy to know what's needed on particular distributions. I am particularly interested in what's needed for Ubuntu and Centos.

Answer 1

Python seems to need to have been compiled with --with-pydebug (on Ubuntu 12.04, package python-dbg contains a suitable Python executable, itself called python-dbg). The inferior Python does not need to be Python 2.7 -- 2.6 loads the 2.7 gdb extensions successfully (see the debugging session below). At least on Ubuntu 12.04, the file that gets installed that defines the gdb extensions is called libpython.py, not python-gdb.py (for some reason, building Python yields a build directory containing both those files -- they are identical).

However, I don't think it's currently possible to debug using production core files: it looks like the gdb extensions for Python inferior processes try to get hold of variables that get optimized away in a production binary (for example, the f variable in PyEval_EvalFrameEx). It seems Linux / gdb, and Python has yet to reach the level of awesomeness achieved for JavaScript on Illumos which Bryan Cantrill reports here is able to debug production core dumps in this way:

http://www.infoq.com/presentations/Debugging-Production-Systems

Here's the debug session on Ubuntu 12.04 showing gdb running a Python 2.6 inferior process to debug a segfault, using Python 2.7's gdb extensions. First the code to cause the segfault:

~/Downloads/Python-2.6.4$ cat ~/bin/dumpcore.py
class Foo:

    def bar(self):
        from ctypes import string_at
        string_at(0xDEADBEEF) # this code will cause Python to segfault


def main():
    f = Foo()
    f.someattr = 42
    f.someotherattr = {'one':1, 'two':2L, 'three':[(), (None,), (None, None)]}
    f.bar()


if __name__ == "__main__":
    main()

and the debugging session:

~/Downloads/Python-2.6.4$ gdb --args ./python ~/bin/dumpcore.py
GNU gdb (Ubuntu/Linaro 7.4-2012.04-0ubuntu2.1) 7.4-2012.04
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
For bug reporting instructions, please see:
<http://bugs.launchpad.net/gdb-linaro/>...
Reading symbols from /home/john/Downloads/Python-2.6.4/python...done.
(gdb) run
Starting program: /home/john/Downloads/Python-2.6.4/python /home/john/bin/dumpcore.py
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Program received signal SIGSEGV, Segmentation fault.
0x0000000000468d67 in PyString_FromString (str=0xdeadbeef <Address 0xdeadbeef out of bounds>) at Objects/stringobject.c:116
116             size = strlen(str);
(gdb) py-bt
Undefined command: "py-bt".  Try "help".
(gdb) python
>import sys
>sys.path.insert(0, "/home/john/Downloads/Python-2.7/Tools/gdb")
>import libpython
>(gdb) py-bt
#10 Frame 0x8f0f90, for file /home/john/Downloads/Python-2.6.4/Lib/ctypes/__init__.py, line 496, in string_at (ptr=3735928559, size=-1)
    return _string_at(ptr, size)
#14 Frame 0x8ebf90, for file /home/john/bin/dumpcore.py, line 5, in bar (self=<Foo(someattr=42, someotherattr={'three': [(), (None,), (None, None)], 'two': 2L, 'one': 1}) at remote 0x7ffff6e03240>, string_at=<function at remote 0x7ffff6e1c990>)
        string_at(0xDEADBEEF) # this code will cause Python to segfault
#17 Frame 0x8ebd80, for file /home/john/bin/dumpcore.py, line 12, in main (f=<Foo(someattr=42, someotherattr={'three': [(), (None,), (None, None)], 'two': 2L, 'one': 1}) at remote 0x7ffff6e03240>)
    f.bar()
#20 Frame 0x8eb680, for file /home/john/bin/dumpcore.py, line 16, in <module> ()
    main()
(gdb)