Hello world without using libraries

This was an onsite interview question and I was baffled.

I was asked to write a Hello world program for linux.. That too without using any libraries in the system. I think I have to use system calls or something.. The code should run using -nostdlib and -nostartfiles option..

It would be gr开发者_如何学运维eat if someone could help..

$ cat > hwa.S
write = 0x04
exit  = 0xfc
.text
_start:
        movl    $1, %ebx
        lea     str, %ecx
        movl    $len, %edx
        movl    $write, %eax
        int     $0x80
        xorl    %ebx, %ebx
        movl    $exit, %eax
        int     $0x80
.data
str:    .ascii "Hello, world!\n"
len = . -str
.globl  _start
$ as -o hwa.o hwa.S
$ ld hwa.o
$ ./a.out
Hello, world!

Have a look at example 4 (won't win a prize for portability):

#include <syscall.h>

void syscall1(int num, int arg1)
{
  asm("int\t$0x80\n\t":
      /* output */    :
      /* input  */    "a"(num), "b"(arg1)
      /* clobbered */ );
}

void syscall3(int num, int arg1, int arg2, int arg3)
{
  asm("int\t$0x80\n\t" :
      /* output */     :
      /* input  */    "a"(num), "b"(arg1), "c"(arg2), "d"(arg3) 
      /* clobbered */ );
}

char str[] = "Hello, world!\n";

int _start()
{
  syscall3(SYS_write, 0, (int) str, sizeof(str)-1);
  syscall1(SYS_exit,  0);
}

Edit: as pointed out by Zan Lynx below, the first argument to sys_write is the file descriptor. Thus this code does the uncommon thing of writing "Hello, world!\n" to stdin (fd 0) instead of stdout (fd 1).

How about writing it in pure assembly as in the example presented in the following link?

http://blog.var.cc/blog/archive/2004/11/10/hello_world_in_x86_assembly__programming_workshop.html

    .global _start

    .text

_start:
    mov     $1, %rax               
    mov     $1, %rdi                
    mov     $yourText, %rsi          
    mov     $13, %rdx              
    syscall                         

    mov     $60, %rax               
    xor     %rdi, %rdi              
    syscall                         

yourText:
    .ascii  "Hello, World\n"

You can assemble and run this using gcc:

$ vim hello.s
$ gcc -c hello.s && ld hello.o -o hello.out && ./hello.out

or using as:

$as hello.s -o hello.o && ld hello.o -o hello.out && ./hello.out

You'd have to talk to the OS directly. You could write to file descriptor 1, (stdout), by doing:

#include <unistd.h>

int main()
{
    write(1, "Hello World\n", 12);
}

What about a shell script? I didn't see any programming language requirement in the question.

echo "Hello World!"