Why does libc have two version numbers (on Ubuntu)?

Question

If I run this in Docker's ubuntu:latest:

root@4304dfbfa661:/# ls lib/x86_64-linux-gnu/libc* -l
-rwxr-xr-x 1 root root 1868984 Jan 15 02:51 lib/x86_64-linux-gnu/libc-2.23.so
lrwxrwxrwx 1 root root      12 Jan 15 02:51 lib/x86_64-linux-gnu/libc.so.6 -> libc-2.23.so

It seems that libc is numbered as both 6 and 2-23. Why are there two version numbers?

NB libc is (idiosyncratically) executable and running it gives

root@4304dfbfa661:/# ./lib/x86_64-linux-gnu/libc.so.6
GNU C Library (Ubuntu GLIBC 2.23-0ubuntu10) stable release version 2.23, by Roland McGrath et al.

So it's the libc.so.6 that's surprising. Does libc [or to be precise, glibc] have sonames that are unrelated to the version numbers?

Edit: to clarify, I understand the symlink. What confuses me is the existence of two numbering schemes. If you look at e.g. libstdc++, you find

lrwxrwxrwx 1 root root     19 Sep 11  2017 ./usr/lib64/libstdc++.so.6 -> libstdc++.so.6.0.19
-rwxr-xr-x 1 root root 995840 Aug  1  2017 ./usr/lib64/libstdc++.so.6.0.19

Having foo.so.6 as a symlink to foo.so.6.0.19 makes sense. Having foo.so.6 as a symlink to foo-2.23.so is confusing...

Answer 1

What confuses me is the existence of two numbering schemes.

Before the invention of GNU symbol versioning, any change to the ABI required that an entirely new version of the library was introduced, and two (or more) copies had to be present on the system.

External library versioning is described e.g. here.

With the introduction of per-symbol versioning (GNU symbol versioning), external library versioning became completely unnecessary: multiple ABIs could be supported in a single library.

This is why libc.so.6 stayed at version 6 since forever (late 1990s). There is no reason to have a symlink at all -- the library could simply be named libc.so.6. However, it is convenient to have the symlink and have it point to current library version, e.g. libc-2.27.so.

The libstdc++.so is also stuck at libstdc++.so.6, but it is different: maintaining multiple C++ ABIs is much more difficult. So the library keeps incrementing minor version with each GCC version (newer GCC requires newer versions of libstdc++.so).

But they don't change the .so.6 part because doing so would require having multiple libstdc++.so copies (which will share 99% of the code).