nulsl-libc

A very lightweight, from-scratch C17 libc for Linux, built for Null Linux and its 32 MB RAM target.

Status: skeleton. The structure is real, the code is honest stubs. The string core, the raw syscall layer, the entry point, and process teardown work end to end; everything else declares its intent and returns ENOSYS until it is implemented (see the roadmap).

Why another libc?

Null Linux guideline #6: "If it's small, if it's lean, you have ZERO reason to link with libC at ALL. Linux specific syscalls can do you well."

• Static-only, by design. Everything links with -nostdlib -static. No dynamic linker is loaded, relocated, or kept in memory for the lifetime of a process — that memory belongs to the GUI.
• The kernel is the API. One variadic syscall() is the only piece of code that touches the kernel ABI; read, write, _exit, and friends are thin wrappers around it.
• Freestanding C17. -ffreestanding -fno-builtin: the compiler never injects its own memcpy, so what you read is what runs.
• No dependencies. No libtool, no glibc, no kernel UAPI headers.

Requirements

• Linux on x86_64
• A C17 compiler (GCC ≥ 8 or Clang ≥ 6)
• autoconf ≥ 2.71, automake ≥ 1.16
• bear (optional, for LSP compile_commands.json)

Building

./autogen.sh          # or just run make — it regenerates when needed
make release          # -> bin/release/, CFLAGS='-O2'
make debug            # -> bin/debug/,   CFLAGS='-O0 -g'
make check            # smoke test (fully static, no glibc, no ld.so)
make bench            # benchmark the release build

Release is always -O2, per Null Linux guideline #4. make bench and make check run against the release build.

The repository root is never configured in-tree; configure.ac refuses it so the committed driver Makefile (which dispatches into bin/release and bin/debug) cannot be clobbered. make distclean removes both build directories.

What works today

Area	Real	Stub (ENOSYS)
string	strlen, strcmp, strncmp, strcpy,	—
	strncpy, memcpy, memmove, memset,
	memcmp
syscall	syscall() (x86_64)	other architectures
unistd	read, write, close, getpid, _exit	unlink
stdio	puts, putchar, fflush (trivially)	printf, fopen, fclose,
		fread, fwrite
stdlib	exit, abort, atoi	malloc, calloc, realloc,
		free, strtol
crt	crt0.o _start (x86_64)	other architectures

Stubs follow one convention: standard signature, documented error return, errno = ENOSYS, and a /* TODO */ comment naming what they need. See docs/architecture.md for the design and roadmap, docs/syscalls.md for the kernel interface.

Repository layout

Makefile              driver: make release / make debug / make bench / ...
configure.ac          autotools (C17 enforced, static-only)
autogen.sh            autoreconf -i
include/              public headers — declarations only
src/                  implementations; src/crt/crt0.S is the entry point
benchmarks/           make bench; must stay lean
tests/                make check; smoke test links -nostdlib -static
bin/release/          out-of-tree release build (-O2)
bin/debug/            out-of-tree debug build (-O0 -g)
docs/                 architecture + syscall documentation

Contributing

See CONTRIBUTING.md. Short version: C17, clang-format, benchmarks for anything performance-relevant, and per project policy — AI drafts, humans ship.