print without libc

main.c

// Just an example.
#include<unistd.h>
int main() {
  write(1, "hello\n", 6);
}

main.s

  .section  .rodata
.LC0:
  .string "hello\n"
  .text
  .globl  main
  .type main, @function
main:
.LFB0:
  .cfi_startproc
  pushq %rbp
  .cfi_def_cfa_offset 16
  .cfi_offset 6, -16
  movq  %rsp, %rbp
  .cfi_def_cfa_register 6 
  movl  $6, %edx
  movl  $.LC0, %esi 
  movl  $1, %edi
  movl  $1, %eax
  syscall
  movl  $0, %eax
  popq  %rbp
  .cfi_def_cfa 7, 8 
  ret
  .cfi_endproc

as -> ld -> a.out
https://kaworu.jpn.org/kaworu/2011-11-10-1.php#2011-11-10-1-ea19956fd16d4bbfd5c71de263cd2ace

$ as main.s -o main.o && ld main.o && ./a.out
ld: warning: cannot find entry symbol _start; defaulting to 0000000000401000
hello
fish: './a.out' terminated by signal SIGSEGV (Address boundary error)