Shared libraries [LWN.net]

Shared libraries

Posted Nov 25, 2025 14:59 UTC (Tue) by farnz (subscriber, #17727)
In reply to: Shared libraries by gspr
Parent article: APT Rust requirement raises questions

Returning a new error value that was previously impossible is an ABI break, in both C and Rust, unless it's clearly documented beforehand that other errors are possible.

For example, if I truncate the error value to 8 bits to make it fit an existing struct, because all known error values are under 255, and you introduce error value 256, I've got a problem in C. This gets worse in Rust, because enums aren't just a value, they can carry data, too, so the enum may get larger as a result of the change, and upstream won't care that the old enum compiled by Debian was 72 bytes, and the new one is 80 bytes - especially if compiled with a newer compiler, they're both 64 bytes.

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Shared libraries

Posted Nov 25, 2025 15:34 UTC (Tue) by gspr (subscriber, #91542) [Link] (1 responses)

Ok, sure, we have an ABI break for both C and Rust. I'm just saying: the situation doesn't get worse, does it?

Shared libraries

Posted Nov 25, 2025 15:42 UTC (Tue) by farnz (subscriber, #17727) [Link]

It's slightly worse, because years of habit mean that C programmers are used to thinking about whether a change will break distro ABIs; Rust programmers aren't, and because Rust makes it easier to express things that are hard to express in C (sum types, for example), they're more likely to make changes to fix a bug that make the situation worse.

Remember that the state we're in with C is in part because the language requires programmers to get it right, or risk UB, and as a result, C programmers doing security fixes tend to be thinking about all the ways they can accidentally break someone; Rust programmers tend not to be doing that, because the result of breaking someone is a compiler error, not UB.

That cultural difference matters, and is part of why the aim on the Rust side is to have a state where swapping in an incompatible dynamic library is a dynamic linker failure, not UB as it is in C.