Using standard C++ library debug symbols? Ubuntu / Linux / libstdc++6-8-dbg? - c++

Question

There is a package called libstdc++6-8-dbg on Ubuntu Linux (latest version at time of writing).
It is described as:
GNU Standard C++ Library v3 (debugging files)
This package contains the shared library of libstdc++ compiled with
debugging symbols.
Among other things it contains these files:
/usr/lib/x86_64-linux-gnu/debug/libstdc++.a
/usr/lib/x86_64-linux-gnu/debug/libstdc++.so.6.0.25
/usr/lib/x86_64-linux-gnu/debug/libstdc++fs.a
Normally to compile a (single translation unit) C++ program with gcc you can write:
$ g++ myprogram.cc
To add generation of debug symbols of user code you pass -g:
$ g++ -g myprogram.cc
But this doesn't include the debug versions of the standard library.
What extra options do you need to pass to g++ to tell it to use the debug versions of the standard library provided by libstdc++6-8-dbg?

Answer 1

The OP wants to resolve C++ standard library's symbols in the backtrace correctly. John's answer correctly states that this could be achieved by linking against the debug version of the standard libraries.
However, Ubuntu also provides debug symbol packages that, once installed, allow GDB to resolve symbols in standard libraries whose debug symbols have been stripped i.e. in the release version of the standard library. We provide a how-to example below (I'm using Ubuntu 20.04):
Suppose the generated binary is named a.out. We first find the version of libstdc++ it links against:
$ ldd a.out
...
libstdc++.so.6 => /lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007ff8dc6f7000)
...
We search for the package that provides the shared library file (/lib is a symbolic link to /usr/lib. The full path must be used here.):
$ dpkg -S /usr/lib/x86_64-linux-gnu/libstdc++.so.6
libstdc++6:amd64: /usr/lib/x86_64-linux-gnu/libstdc++.so.6
Follow the instructions to add the repo for debug symbol packages and then update the package index. The link also describes how one may search for debug symbol packages, but I directly searched with the package name:
$ apt list libstdc++6\*
...
libstdc++6-dbgsym/focal-updates 10.2.0-5ubuntu1~20.04 amd64
...
There will be tons of results, but be sure to look out for dbgsym, rather than dbg! After installing libstdc++6-dbgsym, GDB should be able to resolve the symbols, even if your binary isn't linked against the debug library.
The texts above should answer the OP's question. Now I point out an issue with John's answer.
GDB automatically reads in the debug symbols once you've installed the package. You don't need to compile your program any differently.
This statement is 100% correct, but the figure included is irrelevant and doesn't prove the statement. There are three closely-releated concepts at play here:
Debug library package: The package libstdc++6-8-dbg provides a version of the libstdc++ library with debug symbols.
Debug symbol pagkage: The package libstdc++6-dbgsym provides the debug symbols for the libstdc++ library. That is, it doesn't include any machine instructions for functions like printf, only the debug symbols. libstdc++6-8-dbg bundles code and debug symbols together into one library.
Release library package: The packages libstdc++6-8 and libstdc++6 provide the release versions of the standard library, meaning that they don't carry debug symbols, only the code. Debug symbol packages should be used together with the release libraries.
GDB will automatically read the debug symbols in the debug symbol package, but not in the debug library. The auto-load in John's figure simply states that the Python script /usr/share/gdb/auto-load/usr/lib/x86_64-linux-gnu/debug/libstdc++.so.6.0.25-gdb.py will be run automatically when the debug library is loaded and has nothing to do with the auto-loading of debug symbols.

Answer 2

GDB automatically reads in the debug symbols once you've installed the package. You don't need to compile your program any differently.
If you want your program to load the debug version your best bet is to adjust the library search path. You could do that by setting LD_LIBRARY_PATH temporarily:
$ LD_LIBRARY_PATH=/usr/lib/x86_64-linux-gnu/debug/
$ ldd test
libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/debug/libstdc++.so.6 (0x00007efcef670000)
...
Or permanently:
$ export LD_LIBRARY_PATH=/usr/lib/x86_64-linux-gnu/debug/
$ ldd test
libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/debug/libstdc++.so.6 (0x00007efcef670000)
...
Or you could make it a system-wide change. You can do that in Ubuntu by adding a config entry to /etc/ld.so.conf.d/ and running ldconfig to update the cache.
$ sudoedit /etc/ld.so.conf.d/debug.conf
$ cat /etc/ld.so.conf.d/debug.conf
/usr/lib/x86_64-linux-gnu/debug
$ sudo ldconfig
$ ldd test
libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/debug/libstdc++.so.6 (0x00007f3aced53000)
...
The config files are searched alphabetically so just make sure the one you write (debug.conf above) comes earlier than the default one (x86_64-linux-gnu.conf on my system).