Wireshark-dev: Re: [Wireshark-dev] [Wireshark-commits] rev 40436: /trunk-1.6/ /trunk-1.6/debian
From: Guy Harris <[email protected]>
Date: Thu, 12 Jan 2012 15:09:28 -0800
On Jan 12, 2012, at 11:16 AM, Joerg Mayer wrote:

> Thanks for the pointer and the following explanations. Now that I feel I *may*
> have understood the numbering, I have a new proposal/question:
> As we strive to keep our libs downward compatible inside a stable release
> (i.e. the newer libs should be usable with lower versions of plugins and
> wireshark?) we may want to add -version as well?

Presumably you mean "add -release".

> How about naming the libs
> for Wireshark <major>.<minor>.<release> something like
> -release <major>.<minor> -version-info <release>:0:<release>?

I'm not sure they're supposed to be used together, at least in that fashion.  I tried changing epan/Makefile.am to pass

	-release 1.7.1 -version-info 0:1:0

and what I got was libwireshark-1.7.1.0.0.1.dylib and libwireshark-1.7.1.0.dylib, which have a lot more components in the name than is normally seen..  Without -release, I get libwireshark.0.0.1.dylib and libwireshark.0.dylib, which is more like what version numbers should be.

The libtool documentation says

	-release release
		Specify that the library was generated by release release of your package, so that users can easily tell which versions are newer than others. Be warned that no two releases of your package will be binary compatible if you use this flag. If you want binary compatibility, use the -version-info flag instead (see Versioning).

The "use the -version-info flag instead" seems to suggest that -version-info is intended to be an alternative to -release.  Section 7.4 "Managing release information" of the libtool documentation says

	Often, people want to encode the name of the package release into the shared library so that it is obvious to the user which package their programs are linked against. This convention is used especially on GNU/Linux:

	     trick$ ls /usr/lib/libbfd*

	     /usr/lib/libbfd.a           /usr/lib/libbfd.so.2.7.0.2
	     /usr/lib/libbfd.so
	     trick$

	On ‘trick’, /usr/lib/libbfd.so is a symbolic link to libbfd.so.2.7.0.2, which was distributed as a part of ‘binutils-2.7.0.2’.

	Unfortunately, this convention conflicts directly with libtool's idea of library interface versions, because the library interface rarely changes at the same time that the release number does, and the library suffix is never the same across all platforms.

	So, in order to accommodate both views, you can use the -release flag in order to set release information for libraries for which you do not want to use -version-info. ...

which further seems to suggest that -release and -version-info are not intended to be used together.

That section continues:

	... For the libbfdexample, the next release that uses libtool should be built with ‘-release 2.9.0’, which will produce the following files on GNU/Linux:

	     trick$ ls /usr/lib/libbfd*

	     /usr/lib/libbfd-2.9.0.so     /usr/lib/libbfd.a
	     /usr/lib/libbfd.so
	     trick$

		In this case, /usr/lib/libbfd.so is a symbolic link to libbfd-2.9.0.so. This makes it obvious that the user is dealing with ‘binutils-2.9.0’, without compromising libtool's idea of interface versions.

	Note that this option causes a modification of the library name, so do not use it unless you want to break binary compatibility with any past library releases. In general, you should only use -release for package-internal libraries or for ones whose interfaces change very frequently.

What they mean there is that, with that name, the library is "libbfd-2", version 9.0.

The shared library version number scheme used in ELF-based systems is based on the one in SunOS 4.x (because the ELF scheme originally came from System V Release 4, which was produced by a joint AT&T/Sun project that introduced a SunOS 4.x-derived shared library mechanism).  In the SunOS 4.x scheme, libraries had a major version number, which changed whenever the library changed in a non-backwards-binary-compatible fashion, and a minor version number, which changed whenever the library changed in a backwards-binary-compatible but not forwards-binary-compatible fashion (e.g., introducing a new routine).

That scheme isn't oriented towards matching project version numbers to library version numbers.

Given that the major version number of a shared library should change if and only if you make a backwards-binary-incompatible change to the library, which means that the library version number can track the project version number only if the project changes the major version number whenever it makes a backwards-binary-incompatible change to the library - which we don't do.