rt0

A minimal C runtime for Linux on i386 & x86_64

Features

• Implemented in just 202 SLOC of C code.
• Just 9 lines of GCC inline ASM for i386, or,
• Just 6 lines GCC inline ASM for x86_64
• Small runtime providing just argc, argv, envp, __environ, errno, _exit, _end, _edata, _etext, __executable_start and syscall0/1/2/3/4/5/6
• An example implementation of sys_brk, brk, and sbrk
• Can be used with gold (Google LD, part of GNU Binutils)
• Hello World in just 608 bytes (i386) or 792 bytes (x86_64)
• Small binary sizes vs. other libc's
• Tested with RHEL 7, Ubuntu 14.04, i386/i686, x86_64

See STATS.md to see how rt0 fares. See the musl libc comparison to see how other libc's fare.

You might be interested in Linux native/built-in and fairly portable nolibc header.

Building

Try:

• make
• make librt0.a
• make test
• make runtest
• make DESTDIR=/usr/local install, or simply, make install
• You can also do make uninstall
• It is highly recommended that you build using WITH_FAST_SYSCALL=1, e.g., make WITH_FAST_SYSCALL=1

Usage

• Include rt0/rt0.h for __environ, _exit
• Include rt0/syscall.h for SYS_*, syscall0/1/2/3/4/5/6
• Define main as int main( int, char**, char** )
• Compile your code with at least -nostdlib, e.g., cc -c prog.c -nostdlib -o prog.o
• On Linux, link with librt0, e.g., cc prog.o -nostdlib -lrt0 -o prog
• On FreeBSD, link with librt0 using, cc prog.o -Wl,-u_start -nostdlib -lrt0 -o prog
• To enable Google ld, type make LD=gold
• To enable GNU ld, type make LD=ld (default)
• To enable a slightly faster version of the syscalls, type make WITH_FAST_SYSCALL=1
• To enable -fdata-sections -ffunction-sections, type make WITH_SECTIONS=1
• To enable a debug build, type make WITH_DEBUG=1

References

• Hello from a libc free world!
• musl libc
• Minimum C prgram
• SmallerC
• GCC Inline Assembly HOWTO
• System V ABI/Calling Convention
• Linux syscall interface
• gold - Google ld
• brk(2) man page
• malloc implementation@Stack Overflow
• A Quick Tutorial on Implementing and Debugging Malloc, Free, Calloc, and Realloc
• Tips of malloc & free
• Writing C software without the standard library - Linux Edition

Other small libc's and runtimes

• nolibc
• musl libc
• newlib libc
• uclibc
• dietlibc
• Plan9 libc
• ELKS libc
• gcmurphy libc
• lib43
• libowfat
• libdjb
• x86_64 crt0 in a single file