Find the sum of all the digits of a string without using library function

Given a string, which consists of numbers, find the sum of the numbers.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

/* Convert the character digit to a number */
#define toindex(x) ((int)x-(int)'0')

int main(void) {

   char *s;

   int len=0,sum=0;

   /* Allocate memory for the input string */
  s=(char*)malloc(sizeof(char)*256);

    /* Get the input string from the user */
  scanf("%s",s);

    /* Until the pointer reaches the end of the string */
  while(*s)
   {
        /* Sum the integer values of the string */
    sum+=toindex(*s);

        /* Increment the pointer of the string to point next character in the string */
    s++;

        /* Calculate the length of the string */
    len++;
   }

    /* Finally print the sum of the numbers in the string */
  printf("the sum of the digits of %s is %d",(s-len),sum);

   return 0;
}