GDB pretty printing support

[aaaaaaaaargh]

Debugging JSON objects with GDB is very tedious and in some cases (e.g. from within CLion) not even possible due to the complexity of the type. I've created a small pretty printer that takes care of this issue by utilising the libraries .dump() method.

I hope some of you may find that useful. Feel free to use this any way you like, including incorporation in the library codebase :)

# Pretty printing support for nlohmann::basic_json objects.

import gdb.printing

class NlohmannJsonPrinter(object):
    "Print a json"
    
    def __init__(self, val):
        self.val = val
        
    def to_string(self):
        eval_string = "(*("+str(self.val.type)+"*)("+str(self.val.address)+")).dump(2, ' ', false, nlohmann::detail::error_handler_t::strict).c_str()"
        result = gdb.parse_and_eval(eval_string)
        
        return result.string()
        
    def display_hint(self):
        return 'JSON'
        
def build_pretty_printer():
    pp = gdb.printing.RegexpCollectionPrettyPrinter(
        "nlohmann::basic_json")
    pp.add_printer('JSON', '^nlohmann::basic_json<.*>$', NlohmannJsonPrinter)
    return pp
 
def register_json_printer():
    gdb.printing.register_pretty_printer(
        gdb.current_objfile(),
        build_pretty_printer())

How to use:

• Save the code in a file somewhere. The exact location isn't that important, I use ~/Dev/.gdb.
• Add the path to your .gdbinit system paths, import and register the printer as usual:

sys.path.append('/home/your/path/to/the-file.gdb')

from nlohmannjson import register_json_printer
register_json_printer()

Have fun!

[abrownsword]

O.M.G., thank you!!!

(btw, I notice that in CLion it seems to include a bolded \n after each would-be line that gets pretty-printed... my python-fu is weak and I don't know how to eliminate the newlines from the Python's output?)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[aaaaaaaaargh]

You are very welcome! :)

[nlohmann]

Hi @aaaaaaaaargh! Sorry for not checking back sooner.

I tried to use the approach you described, but it seems my gdb knowledge is not sufficient.

What I tried:

$ cat ~/.gdbinit 
sys.path.append('/home/vagrant/foo/nlohmann-json.py')

from nlohmannjson import register_json_printer
register_json_printer()

/home/vagrant/foo/nlohmann-json.py is your script above.

Then I run gdb:

$ gdb ./main

GNU gdb (Ubuntu 8.3-0ubuntu1) 8.3
Copyright (C) 2019 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".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
/home/vagrant/.gdbinit:1: Error in sourced command file:
Undefined command: "sys".  Try "help".
Reading symbols from ./main...
(gdb) run
Starting program: /home/vagrant/foo/main 
terminate called after throwing an instance of 'nlohmann::detail::type_error'
  what():  [json.exception.type_error.305] cannot use operator[] with a string argument with number

Program received signal SIGABRT, Aborted.
__GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
50	../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb) print main::j
$1 = {m_type = nlohmann::detail::value_t::number_integer, m_value = {object = 0x1, array = 0x1, string = 0x1, boolean = true, number_integer = 1, 
    number_unsigned = 1, number_float = 4.9406564584124654e-324}}
(gdb) 

Any idea how to make this work?

[aaaaaaaaargh]

Hi Niels,

I think you need to do a couple of things.
First, you should import the 'sys' module. Also, I am not 100% sure about that, but I think you need to import the module directory, not the module itself.

Below is a copypasta of my very own .gdbinit, I hope this will help you get things running :)

$ cat ~/.gdbinit
python 
import sys

# This is where the python file lives
sys.path.append('/home/xyz/foo')

# JSON pretty printing.
from nlohmannjson import register_json_printer
register_json_printer()

[nlohmann]

Thanks for the quick response! I am still struggling though. Is there some kind of beginner's guide?

What I tried:

$ cat ~/.gdbinit 

python
import sys
sys.path.append('/home/vagrant/foo')

from nlohmannjson import register_json_printer
register_json_printer()

$ cat /home/vagrant/foo/nlohmannjson.py

# Pretty printing support for nlohmann::basic_json objects.

import gdb.printing

class NlohmannJsonPrinter(object):
    "Print a json"
    
    def __init__(self, val):
        self.val = val
        
    def to_string(self):
        eval_string = "(*("+str(self.val.type)+"*)("+str(self.val.address)+")).dump(2, ' ', false, nlohmann::detail::error_handler_t::strict).c_str()"
        result = gdb.parse_and_eval(eval_string)
        
        return result.string()
        
    def display_hint(self):
        return 'JSON'
        
def build_pretty_printer():
    pp = gdb.printing.RegexpCollectionPrettyPrinter(
        "nlohmann::basic_json")
    pp.add_printer('JSON', '^nlohmann::basic_json<.*>$', NlohmannJsonPrinter)
    return pp
 
def register_json_printer():
    gdb.printing.register_pretty_printer(
        gdb.current_objfile(),
        build_pretty_printer())

Yet I now get the following error in gdb:

(gdb) print main::j
Python Exception <class 'gdb.error'> No symbol "false" in current context.: 
$1 = 

[aaaaaaaaargh]

Huh.. that looks really strange. Can you please show the main method you use for testing?

I'm afraid as with GDB related things there isn't a ton of documentation about this. I did learn about this by a bit by looking at the Unreal Engine pretty printer and the ones shipped with GCC. But I'm sure we can find a way to solve this.

[aaaaaaaaargh]

Also please let me know what compiler you are using and whether you have any kind of optimization enabled (-O flags in GCC).

[abrownsword]

The only difference I see between your gdbinit and mine (for the pretty printing) is that I also register the stdc++ library pretty printers. Otherwise mine is working fine using gdb 8.2.1 and gcc 8.3 (although I do build those tools locally and there are more than a few available build config options on them).