While @SamGrondahl is on the right track, unfortunately, if you enter a negative number it gives unexpected results. This should work properly with negative numbers:
int sumdigits(int number) {
int sign = number < 0 ? -1 : 1;
number = sign * number; // abs the number
if (number < 10)
return sign * number;
else
return sign * (number % 10 + sumdigits(number / 10));
}
This will return the sum of the digits, and if the number is negative, negate that.
solved How to sum the digits of a number using Recursion [C]? [closed]