rustlibc

A full libc implementation as a single Rust crate, exposing C bindings (static and dynamic) for use by ordinary C/C++ programs in place of glibc or musl.

The crate is freestanding (#![no_std]): it talks to the kernel directly through raw syscalls and provides its own C runtime (_start). The resulting librustlibc.a / librustlibc.so need no other libc underneath.

Status: early scaffold. The structure spans the C standard / POSIX headers broadly; pure-computation surfaces are implemented and tested, while I/O, allocation and math are a mix of real and stubbed. See Status.

Targets

Target	Build	Notes
x86_64-unknown-linux-gnu	✅	Primary; end-to-end example runs.
aarch64-unknown-linux-gnu	✅	Builds; cross-links via bundled rust-lld.

Toolchain is pinned to nightly (edition 2024) via rust-toolchain.toml — the c_variadic feature is required to implement the printf family.

Layout

src/
  arch/            per-architecture primitives (raw syscall, setjmp, _start asm)
    x86_64.rs
    aarch64.rs
  platform/linux/  syscall numbers + the errno-aware syscall wrapper
  crt.rs           _start / __libc_start_main (C runtime bootstrap)
  errno.rs types.rs
  ctype.rs string.rs strings.rs stdlib.rs malloc.rs stdio.rs
  unistd.rs fcntl.rs time.rs signal.rs math.rs wchar.rs setjmp.rs
  poll.rs sched.rs dirent.rs pthread.rs locale.rs
  sys/             mman.rs stat.rs uio.rs utsname.rs wait.rs
include/           the C headers programs #include (self-contained)
examples/          hello.c (strings/printf/malloc), sysprobe.c (uname/stat/dir)

Every user-visible libc function is exported with the C ABI under its canonical name via #[cfg_attr(not(test), unsafe(no_mangle))] extern "C". The not(test) gate drops the C symbols during cargo test (where the std harness already links the system libc); unit tests call the Rust paths directly.

Building & running

make lib                       # build static + shared libs (release)
make run                       # build & run the examples against the .a
make test                      # cargo test --lib
make hello ARCH=aarch64-unknown-linux-gnu   # cross-build an example

make run links the examples fully freestanding (-nostdinc -Iinclude -nostdlib -nostartfiles -static) and prints, e.g.:

=== hello ===
Hello from rustlibc!
argc=1, argv[0]=./hello
int=42 hex=0xff oct=100 char=!
float=3.142 str=trun pct=%
=== sysprobe ===
uname: Linux 6.12.41-gentoo-x86_64 (x86_64)
/etc/hostname: 7 bytes, mode 644
entries in ".": ./ ../ .git/ LICENSE src/ ...

Status

Implemented (real):

• string.h / strings.h: mem*, str* family, strerror, bcmp.
• ctype.h: full classification/conversion (C locale).
• stdio.h: FILE, standard streams, fwrite/fread/fputs/fputc/puts/ putchar/perror/fileno/fflush (unbuffered), and the full printf family — printf/fprintf/snprintf/sprintf + v* — via c_variadic (integers/strings/char/pointer complete; floats best-effort).
• stdlib.h: strtol/atoi family, abs/labs/llabs/imaxabs, qsort, bsearch, exit, abort, __assert_fail.
• malloc.h/stdlib.h: malloc/free/calloc/realloc/aligned_alloc (one mmap per allocation — correct, not yet optimized).
• unistd.h: read/write/close/lseek/getpid/_exit/sleep/getcwd.
• fcntl.h: open/openat/fcntl.
• sys/stat.h: per-arch struct stat, stat/lstat/fstat/fstatat, mkdir/chmod/fchmod.
• sys/mman.h: mmap/munmap/mprotect/madvise.
• sys/uio.h readv/writev; sys/utsname.h uname; sys/wait.h wait/waitpid (+ W* macros); poll.h poll; sched.h sched_yield.
• dirent.h: opendir/readdir/closedir/dirfd (over getdents64).
• time.h: clock_gettime, time. signal.h: numbers, kill, raise.
• math.h: fabs/copysign/fmax/fmin/fmod/sqrt.
• setjmp.h: setjmp/longjmp (naked asm, both arches).
• pthread.h: mutex/once as single-threaded no-ops (pthread_create stubbed).
• crt: _start → main, environ capture.
• Header-only: stdint/stddef/stdbool/stdarg/limits/float/ inttypes/iso646/stdalign/stdnoreturn.

Stubbed (marked // STUB, wired to link but not functional):

• scanf family, fopen, getenv, signal (handler install), pthread_create, gmtime_r, mbrtowc, setlocale (nominal "C"), math transcendentals (sin/cos/exp/log/pow/…).

Roadmap

1. Buffered stdio; fopen/fdopen; the scanf family.
2. A real segregated-free-list allocator.
3. Threads (pthread_create), TLS, per-thread errno.
4. Signal delivery (trampoline + sigaction).
5. libm port for the math transcendentals.
6. Sockets (sys/socket.h, netinet/, arpa/) and more sys/*.
7. Dynamic-linker / full static-PIE story.

License

MIT — see LICENSE.