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0 pointsgeofft5y ago0 comments

> Does any major software vendor support older versions of the ABI or machine code?

Yes, this is extraordinarily common. The ABI is an interface, a promise that new versions of the machine code for a library can both be used by binaries compiled against the old one. There's new machine code, but there's no "by definition" of whether they make this promise or not.

glibc (and the other common libraries) on basically all the GNU/Linux distros does this: that's why it's called "libc.so.6" after all these years. New functions can be introduced (and possibly new versions of functions, using symbol versioning), but old binaries compiled against a "libc.so.6" from 10 years ago will still run today. (This is how it's possible to distribute precompiled code for GNU/Linux, whether NumPy or Firefox or Steam, and have it run on more than a single version of a single distro.)

Apple does the same thing; code linked against an old libSystem will still run today. Android does the same thing; code written to an older SDK version will still run today, even though the runtime environment is different.

Oracle Java does the same thing: JARs built with an older version of the JDK can load in newer versions.

Microsoft does this at the OS level, but - notably - the Visual C++ runtime does not make this promise, and they follow a similar pattern to what Nvidia is suggesting. You need to include a copy of the "redistributable" runtime of whatever version (e.g. MSVCR71.DLL) along with your program; you can't necessarily use a newer version. However, old DLLs continue to work on new OSes, and they take great pains to ensure compatibility.

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aronpye5y ago

Excellent comment, I was wondering how glibc handled backwards compatibility.

Is symbol versioning an ELF object file thing, or is it more universal than that?

geofftOP5y ago

Almost all of the time, they do it via just adding new features and not breaking old ones.

But yeah, GNU/Linux and Solaris both have symbol versioning as part of ELF (I'm not sure if other executable formats have it; it doesn't actually require very much out of the format). The approach, roughly, is that each symbol in the file is named something like "memcpy@GLIBC_2.2.5", and if you see symbol versions in the library you're linking against, you include those references. The dynamic linker is also smart enough to resolve unqualified symbols against some default version the library specifies. This is important for backwards-compatibility, for the ability for distros to add symbol versions when upstream doesn't have them yet, and for things like dlsym("memcpy") keeping working. When they make a backwards-incompatible change (e.g., old memcpy supports overlapping ranges, new memcpy does not promise to do the right thing and you need to use memmove instead), they add a new version (e.g., "memcpy@GLIBC_2.14"). Anything compiled against the newer library will reference the new version, but an implementation of the old version still sticks around for older functions.

And yes, there were older versions before libc.so.6 - libc.so.5 was used, I think, in the early 2000s, but they've avoided changes since then. (The approach used there is that you can install both of them on a single system, but "libc.so" symlinks to one of them, and that name is used when you compile code. When you run gcc -lfoo, it looks libfoo.so, but if the library has a header saying its "real" name, called its "SONAME", is libfoo.so.1, the compiled program looks for libfoo.so.1 and not libfoo.so.) Now you only have to have a single glibc version and it works with many years of updates.

jcelerier5y ago

ELF: https://lists.debian.org/lsb-spec/1999/12/msg00017.html

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