> Only because people aren't putting in the effort to build their binaries properly.

Because Linux userland is an unmitigated clusterfuck of bad design that makes this really really really hard.

GCC/Clang and Glibc make it effectively impossible almost impossible to do this on their own. The only way you can actually do this is:

1. create a userland container from the past 2. use Zig which moved oceans and mountains to make it somewhat tractable

It's awful.

But to link against an old glibc version, you need to compile on an old distro, on a VM. And you'll have a rough time if some part of the build depends on a tool too new for your VM. It would be infinitely simpler if one could simply 'cross-compile' down to older symbol versions, but the tooling does not make this easy at all.

If it requires effort to be correct, that's a bad design.

Why doesn't the glibc use the version tag to do the appropriate mapping?

I don’t understand why this is the case, and would like to understand. If I want only functions f1 and f2 which were introduced in glibc versions v1 and v2, why do I have to build with v2 rather than v3? Shouldn’t the symbols be named something like glibc_v1_f1 and glibc_v2_f2 regardless of whether you’re compiling against glibc v2 or glibc v3? If it is instead something like “compiling against vN uses symbols glibc_vN_f1 and glibc_vN_f2” combined with glibc v3 providing glibc_v1_f1, glibc_v2_f1, glibc_v3_f1, glibc_v2_f2 and glbc_v3_f2… why would it be that way?

> You need to link against the oldest glibc version that has all the symbols you need

Or at least the oldest one made before glibc's latest backwards incompatible ABI break.

It's not at all straightforward, it should be the kind of thing that's just a compiler flag, as opposed to needing to restructure your build process to support it.

> There’re quite a few mechanics they use for that. The oldest one, call a special API function on startup [...]

Isn't the oldest one... to have the API/ABI version in the name of your DLL? Unlike on Linux which by default uses a flat namespace, on the Windows land imports are nearly always identified by a pair of the DLL name and the symbol name (or ordinal). You can even have multiple C runtimes (MSVCR71.DLL, MSVCR80.DLL, etc) linked together but working independently in the same executable.

I did forget to mention something important. Since about Vista, Microsoft tends to replace or supplement C WinAPI with IUnknown based object-oriented ones. Note IUnknown doesn’t necessarily imply COM; for example, Direct3D is not COM: no IDispatch, IPC, registration or type libraries.

IUnknown-based ABIs exposing methods of objects without any symbols exported from DLLs. Virtual method tables are internal implementation details, not public symbols. By testing SDK-defined magic numbers like SDKVersion argument of D3D11CreateDevice factory function, the DLL implementing the factory function may create very different objects for programs built against different versions of Windows SDK.

There’s also API Sets: where DLLs like api-win-blah-1.dll acts as a proxy for another DLL both literally, with forwarder exports, and figuratively, with a system-wide in-memory hashmap between api set and actual DLL.

Iirc this is both for versioning, but also so some software can target windows and Xbox OS’s whilst “importing” the same api-set DLL? Caused me a lot of grief writing a PE dynamic linker once.

https://bookkity.com/article/api-sets