rt0

A minimal C runtime for Linux on i386 & x86_64

Features

• Implemented in just 195 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, and syscall0/1/2/3/4/5/6
• Small binary sizes vs. other libc's

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

Building

Try:

• make
• make librt0.a
• make test
• make runtest
• In bash, DEST=/ PREFIX=usr/local make install, or simply, make install
• You can also do make uninstall

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 -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 a slightly faster version of the syscalls, in the Makefile, comment line 30 and uncomment line 31, and add #define __RT0_WITH_FASTER_SYSCALL__ before you include rt0/syscall.h
• To enable -fdata-sections -ffunction-sections, in the Makefile, comment line 30 and uncomment line 32
• To enable a debug build, in the Makefile, comment line 30 & 36, then uncomment line 33 & 38

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

Other small libc's and runtimes

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