strlen in assembly

I made my own implementation of strlen in assembly, but it doesn't return the correct value. It returns the string length + 4. Consequently. I don't see why.. and I hope any of you do...

Assembly source:

section .text
    [GLOBAL stringlen:] ; C function

stringlen:  
    push ebp
    mov ebp, esp        ; setup the stack frame

    mov ecx, [ebp+8]

    xor eax, eax        ; loop counter


startLoop:
    xor edx, edx
    mov edx, [ecx+eax]
    inc eax

    cmp edx, 0x0 ; null byte    
    开发者_运维知识库jne startLoop
end:    
    pop ebp

    ret

And the main routine:

#include <stdio.h>

extern int stringlen(char *);

int main(void)
{
  printf("%d", stringlen("h"));

  return 0;
}

Thanks

You are not accessing bytes (characters), but doublewords. So your code is not looking for a single terminating zero, it is looking for 4 consecutive zeroes. Note that won't always return correct value +4, it depends on what the memory after your string contains.

To fix, you should use byte accesses, for example by changing edx to dl.

Thanks for your answers. Under here working code for anyone who has the same problem as me.

section .text
    [GLOBAL stringlen:]

stringlen:  
    push ebp
    mov ebp, esp

    mov edx, [ebp+8]    ; the string
    xor eax, eax        ; loop counter

    jmp if

then:
    inc eax

if:
    mov cl, [edx+eax]
    cmp cl, 0x0
    jne then

end:
    pop ebp
    ret

Not sure about the four, but it seems obvious it will always return the proper length + 1, since eax is always increased, even if the first byte read from the string is zero.

Change the line

mov edx, [ecx+eax]

to

mov dl, byte [ecx+eax]

and

  cmp edx, 0x0 ; null byte

to

  cmp dl, 0x0 ; null byte

Because you have to compare only byte at a time. Following is the code. Your original code got off-by-one error. For "h" it will return two h + null character.

section .text
    [GLOBAL stringlen:] ; C function

stringlen:
    push ebp
    mov ebp, esp        ; setup the stack frame

    mov ecx, [ebp+8]

    xor eax, eax        ; loop counter


startLoop:
    xor dx, dx
    mov dl, byte [ecx+eax]
    inc eax

    cmp dl, 0x0 ; null byte
    jne startLoop
end:
    pop ebp

    ret

More easy way here(ASCII zero terminated string only):

REPE SCAS m8

http://pdos.csail.mit.edu/6.828/2006/readings/i386/REP.htm

I think your inc should be after the jne. I'm not familiar with this assembly, so I don't really know.