wlibc

Portable C standard library for Linux on x86_64, written from scratch. Takes design cues from [mmusl, mlibc and glibc.

wlibc is a small, freestanding-first libc: it does not depend on the host libc at all (the whole library builds with -nostdinc), ships its own crt0 and links everything statically. The goal is a codebase you can actually read end-to-end and port to a new OS or architecture by adding a single sysdeps directory.

Status

Area	State
crt0 (_start) + __libc_start_main	done
Linux/x86_64 sysdep layer (inline-asm syscalls, errno mapping)	done
string.h: mem*, str*, strdup, strnlen	done
stdio.h: printf family — d/i/u/o/x/X/c/s/p/f, flags, width, precision, length modifiers; puts, putchar, getchar	done
stdlib.h: malloc/calloc/realloc/free (mmap-backed), strtol/atoi/atol, qsort/bsearch, rand/srand	done
unistd.h / fcntl.h / sys/mman.h: read, write, open, close, lseek, unlink, mmap, mprotect, munmap, ...	done
errno, strerror, assert, ctype	done
TLS: __thread via local-exec model, per-thread errno / strerror buf / rand state	done
pthreads: create/join/detach/exit on clone(2), futex mutexes / condvars / pthread_once	done
buffered FILE* stdio: fopen/fdopen/fclose, fread/fwrite, fgetc/fputc, fgets/fputs, fseek/ftell/rewind, ungetc, setvbuf, fflush	done
signals: sigaction/signal, sigprocmask/sigpending/sigsuspend, sigset ops, kill/raise, sigreturn trampoline	done
process: fork + pthread_atfork, execve/execl*/execv* (PATH search), waitpid/wait, system, getenv	done
time: clock_gettime/clock_getres/gettimeofday/time/clock/nanosleep with vDSO fast path, syscall fallback	done
math: double/float from fdlibm — exp/log/pow/sin... (40 double + 42 float functions), float.h	done
calendar: gmtime/gmtime_r/localtime(_r), mktime/timegm, asctime/ctime, strftime, difftime	done
pthread_sigmask, pthread_kill	done
dynamic linking	not planned for now
more architectures (aarch64, riscv64)	planned

Known simplifications: printf has no %n (silently ignored) and %f is a fixed-point approximation without exponent form; pthreads has no cancellation, rwlocks, timed waits or per-thread TS yet; condvars use a sequence-counter protocol (spurious wakeups are allowed, which while (!pred) wait loops tolerate by design); signals have no SA_SIGINFO/siginfo_t, no sigaltstack and no sigaltstack yet; fork in a multithreaded process only resets stdio and heap locks — memory mapped by other threads' stacks stays mapped in the child; calendar functions treat local time as UTC (TZ is not parsed yet) and strftime has no ISO week specifiers (%G/%g/%V); math is ported from fdlibm (via musl) and follows the musl conventions: transcendentals do not set errno, there is no fenv.h control (nearbyint behaves like rint), and fma/erf/lgamma/Bessel are still on the roadmap; no timers or posix_spawn yet.

Design

include/          public headers (self-contained, no host libc)
src/              arch-independent core: never issues a raw syscall
  internal/       internal headers shared by the core
  string/ stdio/ stdlib/ malloc/ unistd/ fcntl/ mman/ signal/ time/ errno/ exit/ ctype/ assert/
sysdeps/
  linux/x86_64/   syscall numbers, inline-asm entry points, crt0, sigreturn

The core talks to the outside world only through the sysdeps interface, so porting to another OS or architecture means adding one directory under sysdeps/ — the same idea mlibc uses. The allocator reserves a large virtual region with PROT_NONE and commits pages on demand via mprotect, which keeps the heap contiguous and makes block coalescing trivial.

Signals use the kernel rt_sigaction layout with a mandatory SA_RESTORER trampoline (__restore_rt in sysdeps/linux/x86_64/sigreturn.s), since the x86_64 kernel provides no default restorer. On fork() the child reclaims stdio and heap locks and re-arms set_tid_address, so a single-threaded copy of a multithreaded parent stays usable.

Math is ported from fdlibm (the musl 1.1.15 lineage of the Sun sources, copyright notices preserved): bit-exact scalar algorithms with union-based word access, no floating-point environment dependencies. All math lives in libc.a — there is no separate libm.

clock_gettime and friends first probe the vDSO (AT_SYSINFO_EHDR is parsed in __libc_start_main, symbols resolved via DT_GNU_HASH/DT_HASH with a load bias, mirroring musl) and fall back to the raw syscall when the vDSO declines an unknown clock id.

Static linking only, by design. No dynamic loader yet.

Building

make            # builds libc.a and obj/crt1.o
make test       # builds and runs the test suite

Requires gcc or clang on Linux/x86_64. Nothing else.

Using wlibc

gcc -nostdinc -ffreestanding -I/path/to/wlibc/include -c prog.c -o prog.o
gcc -static -nostdlib -o prog prog.o /path/to/wlibc/obj/crt1.o /path/to/wlibc/libc.a

or with the helper script (after make):

./scripts/wlibc-gcc -o prog prog.c

hello world

#include <stdio.h>

int main(void)
{
    printf("hello from wlibc\n");
    return 0;
}

Roadmap

• math completion: fma, erf/erfc, lgamma/tgamma, Bessel, long double
• pthreads completion: timed waits, rwlocks, cancellation
• SA_SIGINFO, sigaltstack, posix_spawn, poll/select, opendir/readdir
• TZ parsing for localtime/mktime, ISO week specifiers in strftime
• aarch64 and riscv64 sysdeps
• locale skeleton (C + UTF-8)

Acknowledgements

• musl — the cleanest libc source to learn from
• mlibc — the sysdeps architecture
• glibc — the reference behavior

Contributing

Issues and PRs are welcome. Keep the core syscall-free, keep new OS/arch support inside sysdeps/, and add a test for anything user-visible.

License

MIT — see LICENSE.