The Luhn Mod-10 Method (ISO 2894/ANSI 4.13) involves a check digit in the
one's position.  The check digit is calculated starting at the right with
the digit immediately preceding the check digit (ten's digit) and moving
toward the left, doubling every other digit.  If a doubled digit is greater
than nine, the two digits are added to together to obtain a single-digit
result.  The sum of all the resulting digits (including those skipped) is then
taken modulo with 10, to obtain the check digit.

Below, is untested code.

/************************************************************************
*  LuhnMod10 - self-checking scheme for validating card account numbers
*  according to ISO 2894/ANSI 4.13.
* 
*    Ex:  cardNumber = "795102879015546"
*         strlen(cardNumber) == 15
*         LuhnMod10(cardNumber, 14) == 6 == cardNumber[14]
* 
************************************************************************/
int LuhnMod10(char* cardNumber, int size)
{
  static int table[2][10] = { {0,1,2,3,4,5,6,7,8,9}, {0,2,4,6,8,1,3,5,7,9} };
 
  for (int i=size-1, odd=0, sum=0; i>=0; i--)
    if (isdigit(cardNumber[i]))
      sum += table[(odd=1-odd)][cardNumber[i]-'0'];
  sum %= 10;
  return (sum ? 10-sum : 0);               /* return the check digit */
}