Can't make gdb pretty-printer work with nlohmann_json v3.9.0 & Python 3.6.5

[jjalowie]

Hi, I'm trying to inspect an nlohmann_json object in gdb. I'm using this old version of the pretty-printer as I suspect that with the old versions of nlohmann_json library and Python that I'm using (nlohmann_json v3.9.0, Python v3.6.5) it should be working (I can't migrate to newer versions because I'm working with some legacy code).

Here is the log of what I am doing:

[root@machine dir]# gdb --args ./program --arg0 --arg1 --arg2
GNU gdb (GDB) Red Hat Enterprise Linux 11.2-3.el8
Copyright (C) 2022 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".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://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"...
Finished enabling pretty-printer for nlohmann::basic_json
Reading symbols from ./program...

(gdb) 
(gdb) info pretty-printer
global pretty-printers:
  builtin
    mpx_bound128
  json_lookup_function
(gdb) b main.cpp:108
Breakpoint 1 at 0x926088: file /root/dir/main.cpp, line 108.
(gdb) r
Starting program: /root/dir/program --arg0 --arg1 --arg2
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib64/libthread_db.so.1".
[New Thread 0x7ffff725e700 (LWP 3476729)]
[New Thread 0x7ffff6a5d700 (LWP 3476730)]
[New Thread 0x7ffff625c700 (LWP 3476731)]
[New Thread 0x7ffff5a5b700 (LWP 3476732)]
[New Thread 0x7ffff525a700 (LWP 3476733)]
[New Thread 0x7ffff4a59700 (LWP 3476734)]
[New Thread 0x7ffff4258700 (LWP 3476735)]
[New Thread 0x7ffff3a57700 (LWP 3476736)]
[New Thread 0x7ffff3256700 (LWP 3476737)]
[New Thread 0x7ffff2a55700 (LWP 3476738)]
[New Thread 0x7ffff2254700 (LWP 3476739)]
[New Thread 0x7ffff1a53700 (LWP 3476740)]
[New Thread 0x7ffff1252700 (LWP 3476741)]
[New Thread 0x7ffff0a51700 (LWP 3476742)]
[New Thread 0x7ffff0250700 (LWP 3476743)]
[Detaching after vfork from child process 3476744]
<some output from the program>
Thread 1 "main" hit Breakpoint 1, <hidden> /root/dir/main.cpp:108
108         data_json["some_key_1"] = some_value_1;
Missing separate debuginfos, use: dnf debuginfo-install glibc-2.28-211.el8.x86_64
(gdb) enable pretty-printer json_lookup_function
0 printers enabled
2 of 2 printers enabled
(gdb) p -pretty on -array on -- data_json
$2 = {
  m_type = nlohmann::detail::value_t::object,
  m_value = {
    object = 0x7fffef945500,
    array = 0x7fffef945500,
    string = 0x7fffef945500,
    binary = 0x7fffef945500,
    boolean = false,
    number_integer = 140737212863744,
    number_unsigned = 140737212863744,
    number_float = 6.953342196742267e-310
  }
}
(gdb) p -pretty on -array on -- data_json["some_key_0"]
Invalid cast.

Quick summary:

• the json_lookup_function pretty-printer was succesfully register and I explicitly enabled it yet:
• p -pretty on -array on -- data_json outputs hexadecimal values instead of key-value pairs
• p -pretty on -array on -- data_json["some_key_0"] returns an error Invalid cast. wheres it should print a valid value as some_key_0 is a valid key in the data_json object.

What am I doing wrong?

[jjalowie]

I came to the conclusion that the problem I was facing was caused by statically linking libstdc++. If I link it dynamically then gdb automatically loads pretty-printers for STL containers defined in libstdc++. I suspect those pretty-printers are required for nlohmann_json pretty-printers to work. Could someone with better knowledge of gdb confirm it? If that is the case then I think that the gdb pretty-printer docs should mention that loading STL pretty-printers is required in order for the nlohmann_json pretty-printer to work.

The pretty-printers I loaded in the end:

global pretty-printers:
  builtin
    mpx_bound128
  objfile /usr/lib64/libstdc++.so.6 pretty-printers:
  libstdc++-v6
    __gnu_cxx::_Slist_iterator
    __gnu_cxx::__8::_Slist_iterator
    __gnu_cxx::__8::__normal_iterator
    __gnu_cxx::__8::slist
    __gnu_cxx::__normal_iterator
    __gnu_cxx::slist
    __gnu_debug::_Safe_iterator
    std::_Deque_const_iterator
    std::_Deque_iterator
    std::_Fwd_list_const_iterator
    std::_Fwd_list_iterator
    std::_List_const_iterator
    std::_List_iterator
    std::_Node_handle
    std::_Rb_tree_const_iterator
    std::_Rb_tree_iterator
    std::__8::_Deque_const_iterator
    std::__8::_Deque_iterator
    std::__8::_Fwd_list_const_iterator
    std::__8::_Fwd_list_iterator
    std::__8::_List_const_iterator
    std::__8::_List_iterator
    std::__8::_Node_handle
    std::__8::_Rb_tree_const_iterator
    std::__8::_Rb_tree_iterator
    std::__8::__cxx11::__cxx1998::list
    std::__8::__cxx11::basic_string
    std::__8::__cxx11::list
    std::__8::__cxx1998::_Deque_const_iterator
    std::__8::__cxx1998::_Deque_iterator
    std::__8::__cxx1998::_Fwd_list_const_iterator
    std::__8::__cxx1998::_Fwd_list_iterator
    std::__8::__cxx1998::_List_const_iterator
    std::__8::__cxx1998::_List_iterator
    std::__8::__cxx1998::bitset
    std::__8::__cxx1998::deque
    std::__8::__cxx1998::forward_list
    std::__8::__cxx1998::list
    std::__8::__cxx1998::map
    std::__8::__cxx1998::multimap
    std::__8::__cxx1998::multiset
    std::__8::__cxx1998::set
    std::__8::__cxx1998::unordered_map
    std::__8::__cxx1998::unordered_multimap
    std::__8::__cxx1998::unordered_multiset
    std::__8::__cxx1998::unordered_set
    std::__8::__cxx1998::vector
    std::__8::any
    std::__8::basic_string
    std::__8::basic_string_view
    std::__8::bitset
    std::__8::deque
    std::__8::experimental::filesystem::v1::__cxx11::path
    std::__8::experimental::filesystem::v1::path
    std::__8::experimental::fundamentals_v1::any
    std::__8::experimental::fundamentals_v1::basic_string_view
    std::__8::experimental::fundamentals_v1::optional
    std::__8::filesystem::__cxx11::path
    std::__8::filesystem::path
    std::__8::forward_list
    std::__8::list
    std::__8::map
    std::__8::multimap
    std::__8::multiset
    std::__8::optional
    std::__8::pair
    std::__8::priority_queue
    std::__8::queue
    std::__8::set
    std::__8::shared_ptr
    std::__8::stack
    std::__8::tr1::shared_ptr
    std::__8::tr1::unordered_map
    std::__8::tr1::unordered_multimap
    std::__8::tr1::unordered_multiset
    std::__8::tr1::unordered_set
    std::__8::tr1::weak_ptr
    std::__8::tuple
    std::__8::unique_ptr
    std::__8::unordered_map
    std::__8::unordered_multimap
    std::__8::unordered_multiset
    std::__8::unordered_set
    std::__8::variant
    std::__8::vector
    std::__8::weak_ptr
    std::__cxx11::__cxx1998::list
    std::__cxx11::basic_string
    std::__cxx11::list
    std::__cxx1998::_Deque_const_iterator
    std::__cxx1998::_Deque_iterator
    std::__cxx1998::_Fwd_list_const_iterator
    std::__cxx1998::_Fwd_list_iterator
    std::__cxx1998::_List_const_iterator
    std::__cxx1998::_List_iterator
    std::__cxx1998::bitset
    std::__cxx1998::deque
    std::__cxx1998::forward_list
    std::__cxx1998::list
    std::__cxx1998::map
    std::__cxx1998::multimap
    std::__cxx1998::multiset
    std::__cxx1998::set
    std::__cxx1998::unordered_map
    std::__cxx1998::unordered_multimap
    std::__cxx1998::unordered_multiset
    std::__cxx1998::unordered_set
    std::__cxx1998::vector
    std::__debug::bitset
    std::__debug::deque
    std::__debug::forward_list
    std::__debug::list
    std::__debug::map
    std::__debug::multimap
    std::__debug::multiset
    std::__debug::priority_queue
    std::__debug::queue
    std::__debug::set
    std::__debug::stack
    std::__debug::unique_ptr
    std::__debug::unordered_map
    std::__debug::unordered_multimap
    std::__debug::unordered_multiset
    std::__debug::unordered_set
    std::__debug::vector
    std::any
    std::basic_string
    std::basic_string_view
    std::bitset
    std::deque
    std::experimental::filesystem::v1::__cxx11::path
    std::experimental::filesystem::v1::path
    std::experimental::fundamentals_v1::any
    std::experimental::fundamentals_v1::basic_string_view
    std::experimental::fundamentals_v1::optional
    std::filesystem::__cxx11::path
    std::filesystem::path
    std::forward_list
    std::list
    std::map
    std::multimap
    std::multiset
    std::optional
    std::pair
    std::priority_queue
    std::queue
    std::set
    std::shared_ptr
    std::stack
    std::tr1::shared_ptr
    std::tr1::unordered_map
    std::tr1::unordered_multimap
    std::tr1::unordered_multiset
    std::tr1::unordered_set
    std::tr1::weak_ptr
    std::tuple
    std::unique_ptr
    std::unordered_map
    std::unordered_multimap
    std::unordered_multiset
    std::unordered_set
    std::variant
    std::vector
    std::weak_ptr