#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
int main(int argc, char * argv[])
{
    int num;
    int rightd;
    int sum = 0;

    printf ("Please enter a positive integer value:");
    scanf("%i", &num);

    if (num <= 1)
    {
         printf ("Invalid integer type, please reenter\n");
         return EXIT_FAILURE;
    }
    else
    {
         while (num != 0)
         {
               rightd = num % 10;
               sum += rightd;
               num /= 10;
          }
     }
      printf ("the sum of the digits is %i\n", sum);
      return EXIT_SUCCESS;
}

#include <stdio.h>
#include <ctype.h>

// Returns negative on error, zero or positive on success
int sum_of_digits()
{
  int sum=0;
  int n;
  char buf[512];

  // Prompt
  printf("Enter digits:  ");
  // Read in entire line into buf
  if(fgets(buf,512,stdin) == NULL)
  {
    // If couldn't read, return error
    fprintf(stderr,"Couldn't read from stdin\n);
    return(-1);
  }

  // Loop through entire string
  for(n=0; buf[n]; n++)
  {
    // If it's a newline or carriage return, stop scanning
    if((buf[n] == '\n') || (buf[n] == '\r'))
      break;

    // If it's not a digit, return error
    if(! isdigit(buf[n]))
    {
      fprintf(stderr,"Input string is not digits\n");
      return(-1);
    }

    // Add value of digit to sum
    sum += (buf[n]-'0');
  }

  // Return sum
  return(sum);
}

int main()
{
  while(1)
  {
    int val=sum_of_digits();
    if(val < 0)
      break;

    printf("Sum of digits is %d\n",val);
  }

  return(0);
}