Sorry for the delayed response.
I've had some experience in enterprise C++ a couple years ago (high
availability systems for a big corporation) and I've worked with some
brilliant C++ developers and I believe it. It's due to the fact that
such systems are usually packaged together and rebuilt from scratch.
Add to it a strong "not-invented-here" syndrome and you can basically
disregard any ABI issues.
That's exactly it.
> 2. Provide a C++ ABI, but implement the PImpl pattern such that it is
> possible to maintain ABI compatibility for the major version releases
> of the library (implementation details may be changed to implement bug
> fixes, etc, which is not possible without using the PImpl pattern).
> Even with this approach, there are limitations as to what should be
> passed over the shared library - application boundary.
>
Could you elaborate on the last sentence? Since my gut is telling me
PImpl would be the right choice, I'd like to know what these
limitations are.
Sure. The expansion of that comment is thus:
In order to reduce ABI compatibility problems in this scenario, do not
pass anything over the boundary unless you can be certain that the ABI
of those items are stable and compatible. This should be taken to mean
objects where ABI stability is guaranteed by the provider or you have
control of the ABI stability yourself. On some platforms and with some
C++ standard library implementations, the exposure of standard library
types in the public interface is out of the question, as ABI
compatibility is not retained between major releases of the standard
library implementation. These are the 'pain points' for the Qt
libraries on certain platforms and with certain compiler collections.
You also have to consider the binary compatibility of how certain
language features are implemented by the dependency and reverse
dependency. For example, the manner in which exceptions are handled may
differ between the dependency and reverse dependency. This issue in
particular is why Qt makes exclusive use of return codes (and not
exceptions) for error handling. Of course, the implementation of such
features is determined by the compiler collection being used to build
each binary module (as well as any settings applied during the build).
Fortunately, both libc++ and libstdc++ are relatively stable and
somewhat compatible from an ABI point of view. Both gcc and clang
attempt to implement ABI stability and compatibility, with both
implementing portions of the Itanium C++ ABI. There are still some
issues though, but with time these issues are being resolved in later
releases of the compiler collections.
The detailed status of compatibility between other compiler collections
for GNU / Linux is beyond me at current, though it would be safe to
assume that ABI compatibility is not completely guaranteed.
Thanks for taking the time to write this down, very helpful!
You're very welcome. Obviously this is a very high-level and general
overview.
As I mentioned above - I'm leaning towards PImpl. The reasons I see
for that are: we don't expose any templates to the user and we don't
need any polymorphism. We also seem to have a rather well defined
scope for the library - I can't imagine huge changes happening after
the v2.0 release.
I agree that the scope of the library is well-defined and relatively
limited. As you also point out, templates and polymorphism are not
utilised in the current form of the library and its public interface.
Of course, it is possible that future features might come along within
the gpio subsystem / libgpiod, where making use of these language
features within the C++ binding might be desired. That being said, it's
not like we're developing a library of highly generic components. If
required, we can make use of Qt as a reference for implementing
polymorphic inheritance hierarchies and limited [dynamic] strong data
type variation with the PImpl pattern. So PImpl is a very viable option
for the C++ binding, should you want to retain it being a shared library.
Header-only approach means every user includes everything and we still
need to recompile every user to update the library even with minor
changes. How do distros handle this anyway? Let's say boost gets a
bugfix - do all reverse dependencies get a bugfix release?
That's how it usually works. And that fact means that the reverse
dependency can be confident as to whether a library bug fix is available
at runtime, because it is compiled into the reverse dependency. This
differs from the shared library approach, as whether or not the bug fix
is available will depend on which version of the shared library is
available and which gets loaded and linked at runtime.
With a lot of C++ developers, there is a cultural element around
steering away from these types of issues if possible (and not for bad
reason either). So that is, in my opinion, one of the contributing
reasons as to why this approach is seen as acceptable / preferred by a
lot of this community. If you make a bug fix to the library, you have
to build the library and deploy it to the target anyway. Personally, I'd
rather build the reverse dependency with the bug fix 'baked in' and
deploy that.
Don't forget that if you make an ABI breaking change to the public
interface of a shared library, you have to do a re-build and
re-deployment of the library and reverse dependencies anyway. At least
if you go down the header-only library route, you remove the pain of
having to worry about ABI compatibility and stability entirely (while
being able to make full use of the features offered by the language).
I've been looking at what C++ shared libraries I have installed on my
regular Debian 10 system and then also browsed their code a bit. It
turns out that many of them also put the entire implementation in the
header (libjsoncpp, libmpeg2encpp and several others) and they're
still at relatively low ABI major versions of the shared object - so
I'm wondering if that's really such an issue? Or do so few people
realize this is a problem?
ABI compatibility for C++ libraries is certainly less of an issue on GNU
/ Linux (as compared to other platforms), but it is still an issue.
I've not had experience with either of the packages you've mentioned, so
it's possible they have not made ABI breaking changes or they just don't
care (maybe through ignorance).
I guess, like everything, it all comes down to what you're trying to
achieve and what you really care about. I've met individuals who didn't
give a hoot about this particular issue, as the carnage it caused
downstream "wasn't their problem" and they "still get paid at the end of
the month".
If you need me to expand on anything above, then let me know.
Personally, I'm still for a header-only approach, but if you still want
the C++ binding to be available as a shared library, the PImpl pattern
would also be well suited in this case. In either case, implementing
the required changes should not be too taxing.
On a somewhat related note, I did a build of the libgpiod master branch
yesterday. Could you just confirm what the version should be for the
resultant libgpiodcxx.so? The built library has a version of 1.1.1.
Jack
