[Xcb] [ANNOUNCE] xcb-util 0.3.9

Jeremy Huddleston jeremyhu at freedesktop.org
Tue Jun 5 10:37:45 PDT 2012

On Jun 4, 2012, at 11:22 PM, Josh Triplett <josh at joshtriplett.org> wrote:

> On Mon, Jun 04, 2012 at 04:24:10PM -0700, Jeremy Huddleston wrote:
>> On Jun 4, 2012, at 3:25 PM, Josh Triplett <josh at joshtriplett.org> wrote:
>>> On Mon, Jun 04, 2012 at 02:52:51PM -0700, Jeremy Huddleston wrote:
>>>> On Jun 4, 2012, at 2:19 PM, Adam Jackson <ajax at redhat.com> wrote:
>>>>> On Mon, 2012-06-04 at 14:03 -0700, Jeremy Huddleston wrote:
>>>>>> On Jun 4, 2012, at 1:34 PM, Julien Cristau <jcristau at debian.org> wrote:
>>>>>> Think about this from the libc perspective.  libc *may have* strlcat
>>>>>> or not, but they're named the same because all functions in libc have
>>>>>> consistent signatures.
>>>>> A libc that had strlcat once, and then removed it, would no longer have
>>>>> the same ABI.  An application that had successfully linked against the
>>>>> old libc's strlcat would reasonably expect it to be present at runtime
>>>>> too.
>>>> That argument breaks down when you reverse it.  The "rules" state that the SONAME should not change when adding APIs.  If all you're basing this on is SONAME, then there is absolutely no difference between the adding and removing case.  If I link against a "newer" libc which has strlcat, then by your argument, I'd expect strlcat to be present on any libc matching that SONAME.  When I run my application with the older libc without strlcat, it will fail to find it.
>>> That represents the difference between major and minor version changes.
>>> When you add a new function (or otherwise extend the ABI, such as by
>>> adding new flags to a flags parameter), you increase the minor version,
>>> so that applications built against the new library won't run with the
>>> old one, but applications built against the old one (and thus not
>>> expecting the new function) will still work with the new library.
>>> However, when you *remove* a function, applications built against the
>>> old library will not work with the new one, so you have to bump the
>>> major version.
>> I guess this is where the "OS X" paradigm and the GNU paradigm just
>> break down.  Is there actually annotation done to specify that a
>> specific function was added for a given minor version bump of a
>> library?  Does the loader just require that the runtime version be >=
>> the linktime version (that seems particularly dangerous to me)?  How
>> is this actually enforced in practice?  My understanding was that the
>> minor version was nothing more than extra bits as a guide to the user
>> or packager and that there wasn't actually any "real" mechanism in
>> place to deal with this properly (ie weak linking the new symbols).
> The dynamic linker has a couple of independent mechanisms to handle
> this: the library major/minor version, and symbol versioning.
> The major/minor version have a simple rule: a program or library linked
> against version $linkmajor.$linkminor of libfoo can run with any libfoo
> with $runtimemajor == $linkmajor and $runtimeminor >= $linkminor.  With
> ELF on Linux, the minor number requirement just exists as a convention,
> not a dynamic linker requirement, though some libraries explicitly
> ask the dynamic linker to enforce it using symbol versioning.
> The dynamic linker *does* enforce the availability of symbols, though.
> If your program or library links against libfoo and expects to find
> foo_blah, and a newer libfoo removes foo_blah, the linker will complain
> and not load your program or library.

> If you want to do something more complicated, you can use symbol
> versioning, which lets you say things like "a program expecting the
> foo_blah symbol from libfoo.so.1.2 should use old_foo_blah instead; a
> program expecting foo_blah from libfoo.so.1.3 should use foo_blah".
> glibc does a ton of that, to give it much more fine-grained versioning
> so that programs built against one version of glibc will run against the
> widest possible selection of glibc versions.  More simple examples
> include enforcing the minor version rule, which almost every library
> using symbol versioning enforces: programs linking against libfoo.so.1.3
> will want various foo_* symbols from libfoo.so.1.3, and only
> libfoo.so.1.3 and newer minor versions will have mappings for those
> symbols; older minor versions won't have those mappings, so the linker
> knows to fail early.

Is this actually done in practice (outside of glibc)?  As you describe it, it's essentially exactly the same mechanism we use in OS X, but I haven't actually seen this done in X.org (or many other autotools/glibtool) projects.

Take libXi for example, since it's probably fresh in most of our minds.  With Xi2, libXi added a ton of new functionality and remained backwards compatible.  Thus, the minor version of the library was bumped (http://cgit.freedesktop.org/xorg/lib/libXi/commit/?id=0d19a3ec942aedf5432a9bda1e80f29f7186ce5b) from 6.0 to 6.1.

As far as I can tell, though, symbols are not annotated with "available in 6.1 and later" so the only difference is the minor version bump, and as you said, Linux/ELF essentially ignores this ... so in this case, there is no *practical* difference between going forward with an added symbol and going backwards with a removed symbol.  To make this a bit more clear (words fail me, sorry), consider:

case 1:
libABC.0.0 contains symbols a, b, c
libABC.1.0 contains symbols a, b
libABC.1.1 contains symbols a, b, c (c was added back in but not annotated as new in 1.1)

case 2:
libABC.0.0 contains symbols a, b, c
libABC.0.1 contains symbols a, b

In case 1, libABC.1.1 and libABC.0.0 are identical.  Since Linux/ELF ignores minor version, there is no effective difference between linking against libABC.1.1 and running with libABC.1.0 in case 1 compared to linking against libABC.0.0 and running with libABC.0.1 in case 2.

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