Question

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;
    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.

Answer 1

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