ejchet/rt0

A minimal C runtime for Linux i386 & x86_64

rt0

A minimal C runtime for Linux i386 & x86_64

Features

• Implemented in just 87 SLOC of C code.
• Just 2 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, _exit, and syscall0/1/2/3/4/5/6
• Small binary sizes vs. other libc's

Binary	Arch	Size	Purpose	Compile Args
librt0.a	i386	4390	Startup code+syscall	-Os -nostdlib -fomit-frame-pointer
librt0.a	x86_64	6086	Startup code+syscall	-Os -nostdlib -fomit-frame-pointer
t/hello.exe	i386	1272	Hello World\n	-Os -s -nostdlib -fomit-frame-pointer
t/hello.exe	x86_64	1520	Hello World\n	-Os -s -nostdlib -fomit-frame-pointer
hello.exe	i386	5532	Hello World\n with glibc	-Os -s -fomit-frame-pointer
hello.exe	x86_64	6240	Hello World\n with glibc	-Os -s -fomit-frame-pointer

Compiler: GCC 4.8.2, OS: Ubuntu 14.04.1, Source: Author

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

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
• Link with librt0, e.g., cc prog.o -nostdlib -lrt0 -o prog

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
• lib43
• libowfat
• libdjb