Using break and continue

#include <stdlib.h> /* for EXIT_xxx macros */
#include <stdio.h>  /* for printf() and getchar() */
#include <ctype.h> /* for isdigit() */

void flush_input_stream(FILE * fp);

int main(void)
{
  int sum = 0;
  printf("Enter digits to be summed up or 0 to exit:\n");

  do
  {
    int c = getchar();
    if (EOF == c)
    {
      printf("Read 'end-of-file', exiting!\n");

      break;
    }

    if ('\n' != c)
    {
      flush_input_stream(stdin);
    }

    if (!isdigit(c))
    {
      printf("%c is not a digit! Start over!\n", c);

      continue;
    }

    if ('0' == c)
    {
      printf("Exit requested.\n");

      break;
    }

    sum += c - '0';

    printf("The current sum is %d.\n", sum);
  } while (1);

  return EXIT_SUCCESS;
}

void flush_input_stream(FILE * fp)
{
  size_t i = 0;
  int c;
  while ((c = fgetc(fp)) != '\n' && c != EOF) /* Pull all until and including the next new-line. */
  {
    ++i;
  }

  if (0 != i)
  {
    fprintf(stderr, "Flushed %zu characters from input.\n", i);
  }
}