[swift-evolution] pitch: Unified libc import (again)

[Daniel Dunbar]

> On Aug 17, 2017, at 9:26 AM, Taylor Swift via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I know this has come up before without any action, but having the standard C library be packaged under `Darwin` on OSX and `Glibc` under Linux is something that’s really becoming an issue as the Swift package ecosystem matures. Right now a lot of packages are a lot less portable than they could be because somewhere along the dependency line, there is a package that only imports the C library for one platform. Unifying it under one import would allow people to write packages that are portable by default.

What we (SwiftPM) have done for now is use a `libc` target to start by normalizing the name:
  https://github.com/apple/swift-package-manager/tree/master/Sources/libc
(and in the past, when we find missing things in Glibc getting them added to the compiler). Also, this name is technically a misnomer, but we couldn’t think of a better one (“os” might have been a good one).

Unfortunately, I think this change alone is really only the tip of the iceberg. It would be nice to not have it the difference, but we found we very quickly needed several other layers on top to get to having a relatively stable cross-platform base:
  https://github.com/apple/swift-package-manager/tree/master/Sources/POSIX
  https://github.com/apple/swift-package-manager/tree/master/Sources/Basic

My hope is that one minimal improvement we can get soon is multi-package repo support in SwiftPM, which will at least allow us to share those targets & functionality with other packages.

> Since I think this got hung up in the past over “what constitutes” a universal libc, I propose a unified package should just consist of the functionality that is common between Darwin and Glibc right now, since those are the only two supported platforms anyway.

What would the concrete proposal be? It isn’t trivial to determine that subset and make it well-defined what the exact API is. Is the proposal to just to pick a standard name, and reexport Darwin and Glibc from it?

> Alternatively, Swift could make it a priority to get basic functionality like file IO and math functions shipped with the compiler, which would probably cover 98% of cases where people currently import Darwin/Glibc. A large portion of the standard C libraries are redundant to the Swift standard library anyway.

I’m not sure I agree with these statements about the percentages. For some clients (I’m biased, the areas I work in tend to be in this boat), what we largely need is good platform-agnostic access to the POSIX APIs. This is a hard problem to make a good cross-platform API for (esp. if Windows support is in your head), and which many projects struggle with (Netscape :: NSPR, LLVM :: libSupport, many many more).

The sticking point I see is this: if the proposal is just to unify the name & that doesn’t actually buy us all that much (still need standard layers on top), then have we really solved enough of a problem to be worth standardizing on?

+1 in general agreement with the meta-issue being an important one to discuss.

 - Daniel

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