nasm: jump when input is NULL

Question

I am recoding puts using nasm (64bit), and when puts receives NULL as argument it prints (null). I'm trying to recreate that behaviour, except I can't get the code to jump to the part where it prints (null). Instead it just prints nothing

here's my code:

    global _my_puts

    section .text


    %define WRITE 0x2000004
    %define STDOUT 1


_my_puts:
    cmp rdi, 0
    je is_null
    mov r8, rdi
    cmp byte [r8], 0
    jne print_and_add

print_and_add:
    mov rax, WRITE
    mov rdi, STDOUT
    mov rsi, r8
    mov rdx, 1
    syscall
    inc r8
    cmp byte [r8], 0
    jne print_and_add
    mov rax, WRITE
    mov rdi, STDOUT
    mov rdx, newline.len
    lea rsi, [rel newline]
    syscall
    mov rax, 1
    ret

is_null:
    mov rax, WRITE
    mov rdi, STDOUT
    lea rsi, [rel nullmsg]
    mov rdx, nullmsg.len
    syscall
    mov rax, 1
    ret

    section .data

nullmsg:    db "(null)", 10
.len:       equ $ - nullmsg
newline:    db 10
.len:       equ $ - newline

I also tried

test rdi, rdi
je is_null

with no change. Any ideas?

thanks for the help :)

Answer 1

Your first instruction is the problem: cmp rdi, 0. You're comparing the string pointer, passed to my_puts, to the value 0 to determine if you should print "(null)" rather than comparing the first byte of the string to 0. I'm supposing if you pass a 0 pointer, that would be an error.

Try:

_my_puts:
    mov r8, rdi
    cmp [r8], 0
    je is_null

print_and_add:
    ...

If you want the defensive check against a 0 pointer, you could include that as well, but you wouldn't be checking against any other bad pointers:

_my_puts:
    cmp rdi, 0
    je bad_pointer
    mov r8, rdi
    cmp [r8], 0
    je is_null

print_and_add:
    ...