I'm no expert on C, but I was under the impression that type aliases like off_t are introduced to have the possibility to change then later. This clearly doesn't work. Am I wrong?
Instead, dependencies are built and then installed, then dependents are built against the installed versions and then installed, etc. In the middle of this process, the system can technically be half-broken, because it could be attempting to run older dependents against newer dependencies. Usually this is not a real problem. Because of how pervasive time_t is, though, the half-broken state here is potentially very broken to the point that you may be unable to resume the update process if anything breaks for any reason.
So such upgrade needs care.
glibc specifically has support for the 32-bit and 64-bit time_t abi simultaneously.
From the post:
> What’s important here is that a single glibc build remains compatible with all three variants. However, libraries that use these types in their API are not.
Presumably if that had been done in glibc when 64-bit time_t support was added, we could have had multi-size-time ABI support in things like zlib by now. Seems like a mistake on glibc's part not to create that initially (years ago).
Though if other distros have already switched, I'd posit that perhaps Gentoo needs to rethink its design a bit so it doesn't run into this issue instead.
I like it, though it sounds like something that'd be unlikely to see adoption in the Linux world without the endorsement of multiple major distros.
Another option that's closer to how things work with LFS (large file support) (mostly in the past now) is to use create different interfaces to support 64-bit time_t, and pick defaults for time_t at compile time with a macro that picks the right impl.
Also possible could be having something like a version script (https://sourceware.org/binutils/docs/ld/VERSION.html) to tell the linker what symbols to use when 64-bit time_t was enabled. While this one might have some benefits, generally folks avoid using version scripts when possible, and would require changes in glibc, making it unlikely.
Both of those options (version script extention, and LFS-like pattern) could allow re-using the same binary (ie: smaller file size, no need to build code twice in general), and potentially enable mixing 32-bit time_t and 64-bit time_t code together in a single executable (not desirable, but does remove weird link issues).
Then all the time_t size-independent code would go into a member associated with both 32-bit and 64-bit time_t ABIs, and all the time_t size-dependent code would be separately compiled into single-ABI members, one for each ABI.
For both executables and libraries, name conflicts between members associated with the same ABI would be prohibited at compile/link time.
The effective symbol table for both compile time and runtime linking would then be the union of symbol tables defined by all members associated with the active ABI.
Language-level mechanisms that allow compilers to recognize and separately compile time_t-dependent symbols would be required, ideally implemented in ways that allow such dependence to be inferred in most if not all cases.
While compilers would be free to size-optimize code by splitting functions into dependent and independent subparts, I see no immediate reason why this needs to be explicitly supported at link level.
Finally, as I imagine it, mixing 32- and 64-bit time_t ABIs would be prohibited — implicitly, as 64-bit time_t symbols aren't ever visible to 32-bit time_t code and vice versa — with code that needs to support non-native time_t values (for I/O, IPC, etc.) left its own devices, just like code dealing with, e.g., non-native integer and floating-point formats today.
Admittedly this sounds like a lot of unnecessary work vs. similar alternatives built on existing multi-arch mechanisms, but it's still an interesting idea to contemplate.
It's pretty easy to imagine having both 32-bit time_t and 64-bit time_t in a single "executable" as long as the interfaces actually in use between 32-bit-time and 64-bit-time components don't use `time_t` (or derived types).
iow: if the fact that `time_t` is 32-bits is kept entirely internal to some library A used by some library B (by virtue of library A not having any exposed types with time_t that are used by library B, and library A not having any functions that accept time_t or derived types), there's nothing preventing mixing 32-bit-time_t code and 64-bit-time_t code in a single executable/process (in this theoretical case where we use a LFS (ala _FILE_OFFSET_BITS, etc) for time_t).
LFS had/has the same capability for mixing (with off_t being the type in question there).
The linked article is wrong to imply this isn't possible - and it really doesn't depend on "GLIBC provide a mechanism". All you have to do is:
* Compile the library itself with traditional time_t (32-bit or 64-bit depending on platform), but convert and call time64 APIs internally.
* Make a copy of every public structure that embeds a time_t (directly or indirectly), using time64_t (or whatever contains it) instead.
* Make a copy of every function that takes any time-using type, to use the time64 types.
* In the public headers, check if everything is compiled with 64-bit time_t, and if so make all the traditional types/functions aliases for the time64 versions.
* Disable most of this on platforms that already used 64-bit time_t. Instead (for convenience of external callers) make all the time64 names aliases for the traditional names.
It's just that this is a lot of work, and little benefit to any particular library. the GCC 5 std::string transition is probably a better story than LFS, in particular the compiler-supported `abi_tag` to help detect errors (but I think that's only for C++, ugh - language without room for automatic mangling suck).
(minor note: using typedefs rather than struct tags for your API makes this easier)
Note that for example the std::string transition did not do this.
I always understood the challenge as binary compatibility, when you can't just switch the entire world at once.
"treat it as a cross compilation scenario" is essentially what the post discusses when they mention "use a different CHOST". A CHOST is a unique name identifying a system configuration, like "x86_64-unknown-linux-gnu" (etc). Gentoo treats building for different CHOSTs as cross compiling.
NixOS sits between Debian and Gentoo, as it maintains an output that's capable of existing independently of the rest of the system (like Debian) but is designed to use the current host as a builder (like Gentoo). Gentoo doesn't have any way to keep individual builds separate from the system as a whole, as intimated in the article, so you need to work out how to keep the two worlds separate while you do the build.
I think what they're suggesting winds up being pretty similar to what you suggest, just with the right plumbing to make it work in a Gentoo system. NixOS would need different plumbing, I'm not sure whether they've done it yet or how but I can easily imagine it being more straightforward than what Gentoo is needing to do.
Where it will be hell is running Nix-built packages on a non-NixOS system with non-ABI-compatible glibc. That is something that desperately needs fixing on the glibc side, mostly from the design of nss and networking, that prevent linking against glibc statically.
This isn't a thing. Nix-built binaries all each use the hardcoded glibc they were built with. You can have any number of glibc's simultaneously in use.
And as someone who regularly uses traditional Linux distros like Debian and Fedora, the idea that a package management system would allow a set of packages known to be incompatible with one another to be installed without force, or that core package maintainers would knowingly specify incorrect requirements, is terrifying.
While I'm not familiar with Gentoo, my reading of this article suggests that its maintainers are well aware of these sorts of problems and that Gentoo does not, in fact, suffer from them (intentionally; inevitably mistakes happen just as they occasionally do on the bleeding edge of FreeBSD and other Linux distros).
With larger types that otherwise have similar semantics, you can still have breakage. A straightforward one would be padding in structs. Another one is that a lot of use cases convert pointers to integers and back, so if you change the underlying representation, that's guaranteed to break. Whether that's a good or not is another question, but it's certainly not uncommon.
(Edit: sibling comments make the same point much more succinctly: ABI compatibility!)
In order not to have this problem you'd need the API to use opaque struts, for which only pointers would be exchanged.