Question

My program calls function 183 (getcwd) of interrupt 80h which copies an absolute pathname of the current working directory to the memory location pointed to by buff, which is of length 4096. Returned absolute pathname length is usually less than 4096 bytes so i want to get its true length. How can i do that?

%define LF 0Ah      ; Line feed ASCII code.
%define STDOUT_FILENO 1 ; Standard output stream.
%define SYS_exit    1
%define SYS_write   4
%define SYS_getcwd  183
SECTION .bss
    buff resb 4096
SECTION .text
    global _start
_start:
    mov eax, SYS_getcwd ; getcwd
    mov ebx, buff
    mov ecx, 4096
    int 80h
    mov eax, SYS_write  ; print result to stdout
    mov ebx, STDOUT_FILENO
    mov ecx, buff
    mov edx, 4096
    int 80h
    mov eax, SYS_exit   ; exit
    mov ebx, 0
    int 80h

I add code to find length of null terminated string to my program as following and it works:

%define LF 0Ah      ; Line feed ASCII code.
%define STDOUT_FILENO 1 ; Standard output stream.
%define SYS_exit    1
%define SYS_write   4
%define SYS_getcwd  183
SECTION .data
    mesg1 db "Can't not find string length.",LF
    mesg1_l db $-mesg1
SECTION .bss
    buff resb 4096
SECTION .text
    global _start
_start:
    mov eax, SYS_getcwd ; getcwd
    mov ebx, buff
    mov ecx, 4096
    int 80h
    mov al, 0       ; find string length with scasb
    mov edi, buff
    cld
    repne scasb
    jne error1
    sub ecx, 4096
    neg ecx
    mov edx,ecx
print:  mov byte [buff + ecx],LF
    mov byte [buff + ecx + 1], 0
    inc edx
    mov eax, SYS_write  ; print result to stdout
    mov ebx, STDOUT_FILENO
    mov ecx, buff
    int 80h
    jmp exit
error1: mov eax, SYS_write
    mov ebx, STDOUT_FILENO
    mov ecx, error1 
    mov edx, mesg1_l
    int 80h
exit:   mov eax, SYS_exit   ; exit
    mov ebx, 0
    int 80h

Answer 1

Q: How do I find the length of a string (e.g. the string returned by “getcwd()”)?

A: The same way the standard library function “strlen()” would do it: parse the string until you find a ‘\0’ delimiter, then return that position as the string length.

PS:
I’d strongly urge you to consider using Gnu Assembler “gas” instead of “nasm”. As soon as you start playing with assemblers other than x86, the “bass-ackwards” Intel syntax gets really annoying.

IMHO …