ABI compatibility in C++

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I am creating Debian packages for a SNMP library in C++ which unfortunately does not properly set soname or versioning information in the makefile. It is unacceptable to ship a library in those conditions under Debian guidelines.

If you are a C programmer you don’t need to worry as binary-compatibility is usually not an issue in C as it lacks polymorphism.

If you are a C++ programmer and you don’t know what an ABI is and why binary compatibility matters, you must first read and understand what an API, an ABI and name mangling are.

Even if you already knew about ABI and name mangling, you probably don’t know about binary compatibility in shared objects (libraries). Don’t worry: ABI-compatibility is non-obvius and most C++ programmers don’t know a word about that.

The idea is quite simple: if you uncarefully change the API of your library you have also changed the ABI of your library and, most probably, due to name-mangling and virtual tables (needed to support polymorphism), the new version of the library is ABI-incompatible with the older version.

There are two kind of changes you can make: extend the API/ABI but leave it compatible with former versions, or extend the API/ABI and break binary-compatibility. There is very good information about what changes render your library binary-compatible and which don’t in the KDE wiki and in the Qt FAQ.

Once ABI compatibility is clear, the next question arises: how does my library state its binary compatibiliy? There are essentially two mechanisms for that: the Sun Solaris-specific mechanism (which is great, by the way) and sonames (used by most other Unices and Linux). There is plenty of information about sonames at the libtool manual and the Debian Libraries Packaging Guide and a good example at LinuxQuestions. For more information about the Solaris mechanism, read carefully the DSO howto by Ulrich Drepper (page 35, section 3.3: “ABI Versioning”).

Two important things you have to remember:

  • If your library version is the same as your soname version, you have not understood a thing of what I said above
  • If your library is stable, follow these guidelines.

In case you really want to keep ABI-compatibility for whatever reason (you are developing a very spreaded C++ library and the user might have a newer/older version already installed, you don’t want to update versions unless totally necessary, etc), take a look at the opaque pointer technique (AKA D-Pointer) invented by Trolltech.

Arisnova, IPMS, Flash, XEmbed

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For almost 4 years I worked for a small company, Venue Network, as a Systems Administrator. At the beginning my job meant dealing with Windows 2000 Server and Windows 2003 Server systems. As time went by I was able to introduce Linux and FreeBSD servers in some clients, saving them money and us hassle. The last 18 months there I barely touched Windows systems: the increasing demand for Linux and the storage-hungry users led me to focus on SANs and NASes and Linux. I still did some very specific (read: complex) work on webservers, but that was the exception as I was already overloaded with work.

One day at the end of October 2006 I received an e-mail from another company saying they read about me in the aKademy 2006 site (I gave a conference last year) and would like to know more about me. I sent them my phone number and the next day we talked on the phone for about 20 minutes: they wanted me to work as a C++/Linux/Qt developer. I told Jesús (the CTO and one of the founders of the company) I had never had a developer job. The most ressembling job I had held was a summer internship in 2002 as a multimedia script writer but I didn’t think that qualified. I was not the person they were looking for. He insisted and we arranged a meeting for next week at their offices. Truth is I thought Jesús was crazy and I would be wasting his time and mine, but I agreed. How could I possibly have a job as a C++ developer? It had been years since I programmed in C/C++ and I only developed in Ruby and as a hobby (Ruby, QtRuby, Rails, etc). My visit to Arisnova went very well: Jesús was full of confidence I would be able to do the job and he was so convicing even I started to believe it (actually he was so confident I tried to hand him my resume and he declined the offer :-O)

Would it work? Venue Network was a tiny company where I held a very comfortable position and I already had earned my medals, I did not need to demonstrate anything anymore. At Arisnova I was going to start from scratch!

Fast forward to May 2007.

Turns out I accepted the offer and I have been working for Arisnova for 4 months now. My main job is porting our Integrated Platform Management System from Windows to Linux (auxiliary libraries, middleware, applications, everything). This software manages ships (frigates, corvettes, etc) and has been in use on Windows for several years now, ships have been sold for several countries and they all are very impressed with the software.

We use a lot of open source for the IPMS: Qt, Boost, ACE, ZeroC ICE, OpenSceneGraph, Lua and the list goes on. As the building blocks were already cross-platform, the port is being easier than everybody expected (including me).

The main innovation coming with the Linux version is the movement to KDE: the Windows version depends on several ActiveX components for video, documentation, videoconferencing and some other features. Obviously ActiveX do not work on Linux, so the first thing you think is we would need two different branches of code or a hell of a lot of #ifdef‘s. Not! (sorry, I couldn’t resist). Thankfully, being a KDE bigot is going to benefit our IPMS: KDE4 is multiplatform (Linux/Unix, Mac and Windows), therefore we will be making extensive use of KParts and almost every new technology KDE4 features: Phonon, Decibel, Strigi, etc (by the way, GNOME is not even close to this). We will also be using CMake.

As the port has progressed at a faster pace than we expected and we’d like the KDE4 to be quite stable when we invest our time, I have some time to fiddle with other things. Something I am looking at for the third version of our IPMS, which is currently in its inception, is Flash. Is it possible to integrate Flash in a desktop application (our GUI) and make it feel natural for the user? Will we need to embed a WebKit/Konqueror/whatever component as a "proxy" between the application and Flash? I don’t know yet, but I am currently investigating every lead: dlopen, libflashsupport, XEmbed (which has pretty easy to use since Qt 4.1).

Summarizing, I am very happy I moved to Arisnova: the job is interesting, I am learning a lot, people are nice, I am performing way better than I (and everybody) expected and I see exciting challenges coming. Thank you guys!