Why does this x86_64 assembly code does not work?

section .text
    global my_strlen

my_strlen:
    xor rax, rax
.LOOP:
    cmp BYTE[rdi+rax], 0
    jne .LOOP
    inc rax
    ret

I execute it with:

#include <stddef.h>
#include <stdio.h>

extern size_t my_strlen(const char *str);

int main(int argc, char *argv[]) {
    if(argc!=2) return 1;
   开发者_如何学运维 printf("%lu\n", (unsigned long)my_strlen(argv[1]));
    return 0;
}

But when I execute the program, it does not output anything and it does not close.

You need to increment rax inside the loop.

Increment rax in the loop. Otherwise, you always test the same byte.

my_strlen:
    xor rax, rax
    dec rax
.LOOP:
    inc rax
    cmp BYTE[rdi+rax], 0
    jne .LOOP

Increasing rax outside the loop