Difference between library file version and library version?

[jwdonal]

I have been using linux for ages but I don't know the answer to this question and Google was not being helpful (possibly because I'm not using the proper terminology?).

I was dealing with a library dependency issue for a program when I came up with the following question.

What is the difference between the library version number appended to the .so files and the version of the library itself? Moreover, why do they not match?

For one example, I have libfreetype6 library installed. The .so file that is linked to is:

/usr/lib/libfreetype.so.6 -> libfreetype.so.6.16.1

So the version is 6.16.1......but it's really not. Because the actual version of libfreetype that is installed is 2.9.1-r3.

There is no freetype version 6.16.1 on the freetype.org website. The latest on the website is 2.10.1.

So will/can someone please tell me why there are two different version numbers?

Why wouldn't the .so file just be named libfreetype.so.2.9.1??? That would make much more sense to me.

And this is not specific to the freetype library either, it seems to be prevalent across many (all?) libraries.

Where do the 6.16.1 numbers come from? Who decides what these numbers are? Are they even related to the 2.9.1 library version at all or do they reference something totally different? Perhaps the C library version that the freetype source was compiled with or something?

Could someone please enlighten me? It's really bugging me that I don't know this.

Thanks!

[ct85711]

Version numbering in general is a complete jumbled mess as a whole, to where you shouldn't be too worried about the version numbers too closely. Some of the times, the library version does not follow what the package (the overall project's versioning). Other times, you get some projects that decide to change versioning schemes later on (like going from being on version 12, then version being version 3). There is some cases, where some projects change a library but don't change the library versioning, so it really breaks anything that links to those libraries).

[erm67]

[jwdonal]