Why is libtool's "current" used as "SOVERSION" on BSD, rather than "major"?

Question

On Unix-like systems dynamic shared libraries (.so files) have an SONAME. It can be extracted for example with: readelf -a libfoo.so.0.3.2 | grep SONAME -> libfoo.so.0. The last part is also called the SOVERSION. That version number marks versions of the library that are binary compatible (ABI). So when a program links to one version of the library it can also use a later version if the SOVERSION doesn't change.

Libtool has a -version-info mechanism to decide such a SOVERSION with current, revision and age. The clue is: current is increased also on compatible changes and major = current - age is used on most systems to set the SOVERSION.

However, on BSD (FreeBSD, NetBSD, OpenBSD) the SOVERSION is set to current, which is mentioned in a couple of places like the suse-wiki, and I also tested this on the mentioned platforms.

The question is: Why is libtool doing this on BSD? Why is this considered to be "the way BSD does this"?

That means every compatible change for Linux/Darwin/SunOS is an incompatible change for the BSDs, because the SOVERSION changes.

Answer 1

According to at least one developer who posted on the GNU mailing list, this could be considered a bug:

https://lists.gnu.org/archive/html/bug-libtool/2011-05/msg00007.html

That email is also full of useful references to the library versioning policies to various BSDs, which do appear to contradict libtool's behaviour