implement libc in zig

[andrewrk]

If Zig ships with a libc implementation, it seals the deal for streamlined cross platform compilation. It improves the situation for native compilation as well. When a zig project depends on a C library such as GLFW, one could package it up with zig build system to build with zig (See #490). This accomplishes several things:

• A single zig build from the root project on any platform will fetch the project's dependencies and perform a deterministic build using only zig and no system dependencies.
• Zig can be intelligent about how it links dependencies together. It can link a bunch of .o files together instead of doing dynamic linking, where only 1 of the .o files has the libc implementation in it. So any code shared by zig std library and the libc implementation is actually shared in the binary, including stdio buffers and such. The default allocator can be shared between zig std lib and malloc/free.
• This build would have no native dependencies, so cross compiling works perfectly out-of-the-box, even though you have C library dependencies that depend on libc.

Our goal is for developers to reach for zig instead of gcc, clang, or msvc, even if they intend to use it only to compile C code.

For linux, we can port musl. A lot of our stdlib is ported from there anyway - a lot of this code would be shared between zig standard library and the libc implementation. We should also look at glibc, for example for the getpwnam function, which in musl reads /etc/passwd but in glibc has an extension for a more advanced user account system.

For windows it might be worth looking into newlib-cygwin, reactos libc, and wine libc.

For macos, we always link dynamically against libSystem which has libc in it because this is the stable syscall interface. When cross compiling we allow functions to be unresolved that come from libc. We still need to ship .h files with appropriate constant values, however. I think the .h files from apple might be BSD licensed and thus we could redistribute those.

[pluto439]

On windows, from what part of libc should people start?

[andrewrk]

Can you elaborate on the question?

[pluto439]

Do you need all libc, or only some parts of it? Which parts are more important right now?

[andrewrk]

The goal is to provide API compatibility for existing C projects. We can let that drive the use case. So to figure out what is more important, we would try to, for example, build ffmpeg against a zig-libc.

[pluto439]

I'd start with libjpeg, but I need exceptions for that. Or at least setjmp/longjmp.

[andrewrk]

You can implement setjmp/longjmp with inline assembly.

[tiehuis]

For anyone interested in this, I've started a simple repository which should give you some ideas here. Anyone reading, feel free to use this as a base or as inspiration.
https://github.com/tiehuis/zligc

I won't be doing much implementation here (too many things I already have to work on) but I may use this to figure out some zig -> c questions and some missing build system details needed in zig to accomodate the required goals.