Compile (with gcc) and link (with ld) to create your executable (without C library).

x.h
===
int add(int a, int b);


a.c
===
#include "x.h"
extern int extra;
int main()
{
int a = 9;
int b = 8;
int c;
c = add(9, 8) + extra;

return 0;
}


b.c
===
#include "x.h"
int extra = 9;
int add(int a, int b)
{
return a+b;
}


start.S
=======
.globl _start

_start:
call main
movl $1, %eax
xorl %ebx, %ebx
int $0x80


Now do
~/# gcc -c -nostdlib a.c
~/# gcc -c -nostdlib b.c
~/# gcc -c -nostdlib start.S
~/# ld -o test a.o b.o start.o

Your executable test is ready!!


Source: https://blogs.oracle.com/ksplice/entry/hello_from_a_libc_free