Your symbol table is not your ABI

You’d say that for how much I have written on the topics related to shared libraries, ABIs, visibility and so on, I would be able to write a whole book about the subject. Unfortunately, between me not being a native speaker – thus requiring more editing – and the topic not being about any shiny web technology, there seem to be no publisher interested, again. At least this time it’s not that somebody else wrote it already. Heh.

When a wise man points at the moon, the fool looks at the finger.

Yesterday (well, today, but I’m going to post this in the morning or afternoon) I hit an interesting bug in the tinderbox: a game failing to build apparently because of a missing link to libesd (the client library of the now-obsolete ESounD. This seemed trivial to me, as it appeared that the new libgnome didn’t link to libesd any longer. I had to look a bit more to see how bad the situation was. Indeed, libgnome used to bring in libesd via pkg-config; then it started using Requires.private, so it was only brought in indirectly (transitively) through the NEEDED entries. Finally, with this release, it is not even in the NEEDED entries, as long as you’re using --as-needed.

What happened? Well, with the latest release, the old, long-time deprecated esd interfaces for sound support are finally gone, and all the gnome-sound API is based off Lennart’s libcanberra; this is the good part of the story. The other part of the story is that two functions that were tightly tied with the old ESounD interface (to load a sample into the ESounD session, and get a handler to the currently running esd session, respectively) are no longer functioning. Since GNOME is not supposed to break ABI (Application Binary Interface) between releases within version 2, the developers decided to keep the symbols around, and, quoting them, to keep linking to libesd to maintain binary compatibility.

Well, first off, the “binary compatibility” that they wish to keep by linking to libesd is rather compatibility with underlinked software that, in its own self, is a bad thing we shouldn’t be condoning at all. Software that uses esd should link against it, not rely on the fact that libgnome would bring it in.

On the other hand, they seem to assume that the ABI is little more than what the symbol tables provides. Of course there are the symbols exported that are part of the ABI, and their types; for the functions, also the type and order of their parameters. You also have to add the structures, the types of their content, their size and the order. All of this is part of the ABI, yet the only thing that the linker will enforce is the symbol table, which is why in my title I wanted to make it clear that the symbol table is not the only part of the ABI; or the fact that the ABI is not only what the linker can enforce — or the API what the compiler can enforce!

What is that I’m referring to? Well as I said there are two functions now that make no sense at all; they weren’t, though, removed, as that would have broken the part of the ABI visible by the linker. Instead, the functions now return the value -1 (i.e. there has been an error) without doing anything (almost; actually the sample loading is done through libcanberra, but it really doesn’t matter given that you’re trying to get the sample handle, that is not returned anyway). Even though this won’t cause a startup error, or a build-time error, it’s still breaking the ABI: software that was built relying on the two functions working will not work starting this version.

You’re not maintaining binary compatibility, you’re hiding your own ABI breakage under the rug!

This is not dissimilar to the infamous undefined symbol problem as it simply shuffles to runtime a mistake that could have been easily found at build-time, by making the build fail when the symbol was used, to run-time, by forcing an error condition where the code was unlikely to expect one.

In this particular case, only two packages directly express the need for esd to be enabled in libgnome2; they should be fixed or simply dropped out of Portage, as ESounD has been deprecated for many years now and thus make no sense to find a way to fix these packages, they are more than likely dead if they didn’t write a fallback already they have probably stopped developing it.

One thought on “Your symbol table is not your ABI

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

w

Connecting to %s