練習：低レベルなHelloWorld

Hello Worldを生で書く

linux 64bitを想定

初級

#include "stdio.h"
int main(void){
    printf("Hello, world!\n");
    return 0;
}

中級

#include "unistd.h"
int main(void){
        write(1,"Hello, world!\n",14);
        return 0;
}

上級

int main(void)
{
        const char mes[] = "hello, world\n";
        asm("movq %0, %%rdx" :: "r"(sizeof(mes)) );
        asm("movq %0, %%rsi" :: "r"(mes) );
        asm("movq $1, %%rdi" ::);
        asm("movl $1, %%eax" ::);
        asm("syscall");
}

超上級

gcc -O3 -static -fomit-frame-pointer -nostartfiles -fno-stack-protector hello.c

void _start(void){
        const char mes[] = "hello, world\n";
        asm("movq %0, %%rdx" :: "r"(sizeof(mes)) );
        asm("movq %0, %%rsi" :: "r"(mes) );
        asm("movq $1, %%rdi" ::); /* stdout */
        asm("movl $1, %%eax" ::); /* write */
        asm("syscall");
        asm("movq $0, %%rdi" ::);
        asm("movl $0x3c, %%eax" ::); /* _exit */
        asm("syscall");
}