Question

I have a problem about counting the length of a string.
I always get some number like 2432, thou I pass a string like “abc”.

I think the problem is in this line

mov bl, byte [esi]

but I would not know why.
Maybe it is something with character length in bits?

Could the problem be a 64bit operating system or dual core processor? (which I somehow doubt, because I think the first line “bits 32” should solve the problem).

PS.: This is an exercise, that is why I need to determine the length of the string like this.

The code:

bits 32
extern _printf
extern _scanf
global _main

section .data
number_frmt db 10,"%d",10,0
enter_str db "Enter some string: ", 10,0
string_frmt db "%s", 0

section .bss
entered_string resb 100

section .text

_main:
    pushad

    push dword enter_str
    call _printf
    add esp, 4

    push dword entered_string
    push dword string_frmt
    call _scanf
    add esp, 4  ;leave the entered string in the stack

    call count  ; count it and put the result to eax

    push dword eax
    push dword number_frmt
    call _printf
    add esp, 12

    popad
    ret

count:
    push esi    ;save it
    push ebx    ;save it
    mov eax, 0  ;init eax=0
    mov esi, [esp+12] ;put the entered string to esi

.loop:
    mov bl, byte [esi]  ;get the first char
    inc eax             ;eax++
    add esi,1           ;point to next char
    cmp bl,10           ;is it new line?
    jne .loop           ;if not loop
    dec eax             ;eax-- (because of newline at the end)
    pop ebx             ;retrieve ebx
    pop esi             ;retrieve esi
    ret

Answer 1

cmp bl,10           ;is it new line?

should be

cmp bl,0

because c/c++ strings are everytime ended/terminated by a 0, so you have in fact searched for an random position in the memory where the next 10 is.