[PATCH] D144889: [C2x] Support <string.h> in freestanding

Mon Feb 27 13:56:52 PST 2023

efriedma added a comment.

In D144889#4156120 <https://reviews.llvm.org/D144889#4156120>, @bcraig wrote:

> A freestanding implementation doesn't necessarily mean that everything is header-only.  It's fine to require linking against a (freestanding) C runtime library.  All header-only is fine too though, if you want to make that work.
>
> Architecturally, I don't feel it is required that Clang be the location of all the freestanding headers for C.  I think that's fine for the C library in the final toolchain to own those headers.  I'm not super familiar with what interface contracts you have between the clang-provided C headers and the C-library provided C headers though.

In practice, there are basically three ways people can use clang:

1. A normal operating system target with a "hosted" libc.
2. A baremetal target with an "embedded" libc; basically a stripped down libc which only includes stuff that doesn't require an operating system.  What exactly this includes varies; may include some form of "malloc", some "POSIX" APIs, semihosting, etc.
3. A baremetal target with no libc, using custom implementations for everything.  This is what we conventionally referred to as "freestanding", and what -ffreestanding implements.  The user provides memcpy/memmove/memset, and uses the compiler-provided "builtins" lib if neceessary, but nothing we would traditionally call "libc".

We could just refuse to provide any APIs outside of our conventional notion of "freestanding", I guess, and say users that want a C23 freestanding environment need to find their own mini-libc. Or just provide the header, and say users are responsible for providing implementations, like they currently are for memcpy etc.  That doesn't really align with what the committee was hoping for, though...

Maybe eventually we can come up with some way to use implementations provided by LLVM libc, but I'm not sure what exactly that looks like.

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https://reviews.llvm.org/D144889