Debugging: stepping through Python script using gdb?

Question

Very interesting question. Here’s my approach. Create signal_test.py:

import os
import signal

PID = os.getpid()

def do_nothing(*args):
    pass

def foo():
    print "Initializing..."
    a=10
    os.kill(PID, signal.SIGUSR1)
    print "Variable value is %d" % (a)
    print "All done!"

signal.signal(signal.SIGUSR1, do_nothing)

foo()

Then you can run it under gdb:

$ gdb --args python signal_test.py
GNU gdb (GDB) Red Hat Enterprise Linux (7.0.1-37.el5_7.1)
Copyright (C) 2009 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-redhat-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /net/gs/vol3/software/modules-sw/python/2.7/Linux/RHEL5/x86_64/bin/python...done.

And when you run it, it will go until you reach the call to kill():

(gdb) run
Starting program: /net/gs/vol3/software/modules-sw/python/2.7/Linux/RHEL5/x86_64/bin/python signal_test.py
warning: no loadable sections found in added symbol-file system-supplied DSO at 0x2aaaaaaab000
[Thread debugging using libthread_db enabled]
Initializing...

Program received signal SIGUSR1, User defined signal 1.
0x0000003d340306f7 in kill () from /lib64/libc.so.6

You can then look at a backtrace:

(gdb) backtrace
#0  0x0000003d340306f7 in kill () from /lib64/libc.so.6
#1  0x00000000004d82dd in posix_kill (self=<value optimized out>, args=<value optimized out>)
    at ./Modules/posixmodule.c:4047
#2  0x000000000049b574 in call_function (f=0x8aca30, throwflag=<value optimized out>)
    at Python/ceval.c:4012
#3  PyEval_EvalFrameEx (f=0x8aca30, throwflag=<value optimized out>) at Python/ceval.c:2665
#4  0x000000000049c5cd in call_function (f=0x8ac560, throwflag=<value optimized out>)
    at Python/ceval.c:4098
#5  PyEval_EvalFrameEx (f=0x8ac560, throwflag=<value optimized out>) at Python/ceval.c:2665
#6  0x000000000049d3bb in PyEval_EvalCodeEx (co=0x2aaaae224f30, globals=<value optimized out>, 
    locals=<value optimized out>, args=0x0, argcount=0, kws=0x0, kwcount=0, defs=0x0, defcount=0, 
    closure=0x0) at Python/ceval.c:3252
#7  0x000000000049d432 in PyEval_EvalCode (co=0x1a48, globals=0xa, locals=0x0) at Python/ceval.c:666
#8  0x00000000004bf321 in run_mod (fp=0x89ad60, filename=0x7fffffffb5b4 "signal_test.py", 
    start=<value optimized out>, globals=0x7e4680, locals=0x7e4680, closeit=1, flags=0x7fffffffaee0)
    at Python/pythonrun.c:1346
#9  PyRun_FileExFlags (fp=0x89ad60, filename=0x7fffffffb5b4 "signal_test.py", 
    start=<value optimized out>, globals=0x7e4680, locals=0x7e4680, closeit=1, flags=0x7fffffffaee0)
    at Python/pythonrun.c:1332
#10 0x00000000004bf5d8 in PyRun_SimpleFileExFlags (fp=<value optimized out>, 
    filename=0x7fffffffb5b4 "signal_test.py", closeit=1, flags=0x7fffffffaee0)
    at Python/pythonrun.c:936
#11 0x00000000004148cc in Py_Main (argc=<value optimized out>, argv=<value optimized out>)
    at Modules/main.c:599
#12 0x0000003d3401d994 in __libc_start_main () from /lib64/libc.so.6
#13 0x0000000000413b19 in _start ()

If you continue on, the rest of the program will run normally.

(gdb) continue
Continuing.
Variable value is 10
All done!

Program exited normally.

You can, instead, step through in the appropriate frame until you reach the statement you’re interested in. You’re probably going to want to run a debugging Python for this to make much sense.

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