sunfishcode / c-ward

An implementation of libc written in Rust

c-ward

An implementation of libc written in Rust

c-ward is an implementation of the libc ABI written in Rust.

It consists of two crates:

• c-scape, which is no_std, and
• c-gull, which pulls in c-scape and additionally provides features using std.

It is a goal of c-ward to be a C ABI layer on top of Rust-idomatic libraries, rather than to have significant implementation code of its own.

In theory c-ward could be extended to be ABI-compatible with different platforms, however currently it is only known to be ABI-compatible with *-unknown-linux-gnu* platforms.

The primary way this is used is through Mustang and Eyra, as their libc implementations. It can also be used as a regular library in "coexist-with-libc" mode.

Runtime requirements

Resolving users and DNS records requires the execution of getent which prints the entries on stdout. On a regular glibc system the getent binary is provided by it and uses the NSS setup as usual. Similar, a musl system also provides getent (but does not use NSS).

Similar crates

Another libc implementation is relibc. tinyrlibc is a very minimal set of libc functions for bare-metal embedded platforms.

Where's the #![no_builtins]?

Normally, a libc implementation would use #[no_builtins] to prevent compilers from noticing the bodies of libc functions implement the semantics of libc functions and replacing them with calls, which effectively makes them uselessly recursive calls to themselves.

However, #[no_builtins] is too pessimistic, because we don't need to disable all pattern matching, just these specific cases.

So instead, c-scape and c-gull are just careful to avoid open-coding functions which are known to get pattern-matched into builtins, by just calling the compiler_builtins implementations directly themselves. This way, we can avoid using #![no_builtins].