Return a value from assembly function to calling c++ function

I am trying to code the collatz problem in assembly (just for fun). I just finished the first part. I am calling the assembly code from a C++ function in a different file:

if (((n / 2) * 2) == n) n = n / 2;
else n = 3 * n + 1; 

.code
isCN proc n:dword
mov eax, n
xor edx, edx
mov ecx, 2
mov r8d, eax
div ecx
mov r9d, eax
mul ecx
cmp eax, r8d
je equal
;else
mov eax, 3
mul r8d
add eax, 1
ret


equal:
; if n is even
mov eax, r9d
ret



isCN ENDP
end

I am using the code like that:

extern "C" int isCN(int n);

while (n > 1) {
    c++;
    n = isCN(n);
}
validNumbers[i] = true;
counters[i] = c;

But when I try to use it, the value it returns is nonsense.

When I look at the disassembly, it seems to be correctly returned from eax.

Is the error in the way I use the code, or in the code itself?

Your code is inefficient but from a quick test it does seem to work. A more efficient version could be:

    mov eax, ecx
    shr eax, 1
    jnc done
    lea eax, [2*ecx + ecx + 1]
done:
    ret

(Assuming you are using windows calling convention.)