Help using the GDB C++ STL pretty-printers / xmethods

Jonathan Wakely jwakely.gcc@gmail.com
Sun May 8 08:16:13 GMT 2022


On Sun, 8 May 2022, 09:13 Jonathan Wakely, <jwakely.gcc@gmail.com> wrote:

>
>
> On Sun, 8 May 2022, 00:09 Paul Smith, <paul@mad-scientist.net> wrote:
>
>> On Sat, 2022-05-07 at 20:51 +0100, Jonathan Wakely wrote:
>> > On Sat, 7 May 2022 at 20:07, Paul Smith <paul@mad-scientist.net>
>> > wrote:
>> > GCC's 'make install' should do everything needed. That installs
>> > $prefix/lib64/libstdc++.so.6.0.30-gdb.py alongside the .so file, and
>> > gdb auto-loads that when debugging a process linked to the
>> > libstdc++.so.6.0.30 library. That python script imports the
>> > register_libstdcxx_printers function and runs it.
>> >
>> > Maybe you're only linking statically to libstdc++.a?
>>
>> Ah.  Yes I'm linking statically.
>>
>> > Hmm, that's reminiscent of
>> > https://sourceware.org/bugzilla/show_bug.cgi?id=25234
>>
>> I checked and when I first attach I do see:
>>
>>   (gdb) show lang
>>   The current source language is "auto; currently c".
>>
>> and things don't work, then after I change to a C++ frame I see:
>>
>>   (gdb) show lang
>>   The current source language is "auto; currently c++".
>>
>> and things work.
>>
>> I discovered that if I add:
>>
>>   set language c++
>>
>> to my init, that it all works properly.  For my purposes this is a
>> sufficient workaround.
>>
>> It's a bit strange (confusing) that the C++ pretty-printers work
>> without having to do that, but the C++ xmethods do not.
>>
>> Also just a data point, my previous GDB (10.2) didn't require this:
>> when I attach with that version, GDB chose the auto language as "c++"
>> immediately.  I suppose it's worth a bugzilla report.
>>
>> > All the logic to do that in Python is already present in the
>> > printers,
>>
>> I figured so I'd hoped there was something here already.  I get what
>> you're saying of course.  Maybe I'll find some time to dig into this...
>> at some point...
>>
>> > > (b) Ways to access the contents of containers like unique_ptr,
>> > > shared_ptr, etc. from python functions.  So if in my class I have
>> > > "std::unique_ptr<Foo> fooPtr" and in my python functions I have a
>> > > variable "fooPtr" which refers to this object, I would like a way
>> > > to retrieve a gdb.Value containing its pointer.
>> >
>> > The UniquePtrGetWorker Xmethod already does that. You should be able
>> > to just do:
>> >
>> >  py ptr = gdb.parse_and_eval('uniqptr.get()')
>>
>> xmethods don't help me (IIUC) because I'm in the middle of some Python
>> function and the value I want to retrieve is in a Python variable, not
>> in a GDB variable, so I can't easily access it with parse_and_eval().
>>
>> For instance in my examples here I'd have a python method:
>>
>>   def find_obj(val):
>>       if val['mgr']['initialized']:
>>           return val['mgr']
>>       return val['otherMgr']
>>
>> or whatever, but of course I can't do this because val['mgr'] is a
>> std::unique_ptr and I don't know how to dig out the object it points
>> to.  The above doesn't need to work as-is: something like:
>>
>>   def find_obj(val):
>>       mgr = StdUnique(val['mgr']).get()
>>       if mgr['initialized']:
>>           return mgr
>>       return val['otherMgr']
>>
>> or whatever would be fine.
>>
>
>
> gdb.set_convenience_variable('mgr', val['mgr'])
> init = gdb.parse_and_eval('$mgr->initialized')
>
> This will use the xmethod to evaluate the expression.
>

And then:

if init:
    return gdb.parse_and_eval('*$mgr')


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