Sunday, January 04, 2009

Pretty printing wxStuff in gdb

I've recently stumbled upon a highly informative series of articles by Tom Tromey, one of gdb developers (among other things), about embedded Python in new gdb. The one immediately useful consequence of this is that you can now write pretty printers for custom data structures as explained here. Unlike the custom gdb commands, the custom pretty-printers apply to all occurrences of the objects, including as fields in the other data structures and even in the backtraces printed by gdb itself so they are much more useful.

So without further ado here is a pretty printer for wxString from svn trunk (it can also be trivially adapted for 2.8 version by using m_impl.m_pchData) shamelessly stolen from the examples above:

class wxStringPrinter:
def __init__(self, val):
self.val = val

def to_string(self):
return '"' + self.val['m_impl']['_M_dataplus']['_M_p'].string() + '"'

gdb.pretty_printers['^wxString$'] = wxStringPrinter

This is supposed to be put into a file sourced with source -p gdb command (there is apparently a better way to load this than simply inserting the command in .gdbinit but for now this is good enough for me).

And then:

[sunset:../src/build/wx-gtkud/samples/minimal]% gdb minimal
(gdb) b 149
Breakpoint 1 at 0x40cc65: file /usr/local/src/wx/HEAD/samples/minimal/minimal.cpp, line 149. (2 locations)
(gdb) r
Starting program: /usr/local/src/build/wx-gtkud/samples/minimal/minimal
[Thread debugging using libthread_db enabled]

Breakpoint 1, MyFrame (this=0x5751a0,title="Minimal wxWidgets App") at /usr/local/src/wx/HEAD/samples/minimal/minimal.cpp:149
149 SetIcon(wxICON(sample));
(gdb) bt 1
#0 MyFrame (this=0x5751a0, title="Minimal wxWidgets App") at /usr/local/src/wx/HEAD/samples/minimal/minimal.cpp:149
(gdb) p title
$1 = "Minimal wxWidgets App"

Of course, you do need a Python-enabled gdb for this to work. The easiest way is probably to get the package from Debian Experimental as I did. Otherwise you can always follow the instructions on Tom's blog and get gdb sources from git and built them yourself.

But this is really incredibly useful and will be even more so when we add pretty-printers for the other wxWidgets types (e.g. wxDateTime could definitely benefit). Any contributions to our custom pretty-printers would be welcome!


Anonymous said...

Hi Vadim,
this is very good news!
I was using wchar_print until now (
Now we only need a way to tell GDB how to not step into some functions to debug wx programs in a sane way.

Anonymous said...

gdb) source -p
Traceback (most recent call last):
File ".gdb/", line 21, in module
gdb.pretty_printers['^wxString$'] = wxStringPrinter
TypeError: list indices must be integers

Do you know what is happening here? i am kind of new to linux debugging:)

VZ said...

gdb Python API has changed since this post was written, now you need to write it like this