Unable to find length of String in Assembly(nasm Linux)

Question

I have made a program that takes a string as a input from user and then calculates its length and display it. But I am getting an error Segmentation fault (core dumped) when I run the program. My code is shown below

str_len:                    ;Procedure to calculate length of String
    xor rdx,rdx
    xor rcx,rcx             ;I want to store length in rcx
    mov rdx,[string]        ;string contains the input taken from the user
    mov rsi,rdx
    mov rcx,'0'             ;By default rcx will contain '0' decimal

up: cmp byte[rsi],0         ;Compare if end of string
    je str_o                ;If yes then jump to str_o

    inc rcx                 ;Increment rcx i.e., length
    inc rsi                 ;Point to next location of rdx i.e.,string
    jmp up                  ;Repeat till end of string

str_o:  mov rax,1           ;Print final length
        mov rdi,1
        mov rsi,rcx
        mov rdx,1
        syscall
ret

I can guarantee that rest of my program is correct. The error will be in the above part of the code. What is probably the error ?

Answer 1

The error is here:

mov rdx,[string] ; string contains the input taken from the user

You are loading RDX with the first 8 bytes of the string content and not the address of the string. So better use

lea rdx, string  ; LEA = Load Effective Address

Another problem is that your output routine tries to print a number that is not converted to ASCII with

mov rax,1      ; SYS_WRITE - Print final length
mov rdi,1      ; STDOUT handle
mov rsi,rcx    ; RSI should point to buffer
mov rdx,1      ; length of buffer - a value of one prints only one char
syscall

This doesn't work that way. You'd have to convert the qword number in RCX to an ASCII string before and then pass the address of this string in RSI. Search Stack Overflow for questions like this.