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

void die(char *operation,char *why)
{
  fprintf(stderr,"fatal: unable to %s file17.number: %s\n",operation,why);
  exit(111);
}

int readnumber(void)
{
  FILE *f;
  int result;
  char throwaway;

  f = fopen("file17.number","r");
  if (!f) {
    if (errno == ENOENT) return 0;
    die("read",strerror(errno));
  }

  switch (fscanf(f,"%d",&result)) {
    case -1:
      if (ferror(f)) die("read",strerror(errno));
      /* end of file; fall through to next case */
    case 0: /* something non-numeric */
      die("read","does not start with a number");
  }
  switch (fscanf(f,"%c",&throwaway)) {
    case 1:
      die("read","has something after the number");
    case -1:
      if (ferror(f)) die("read",strerror(errno));
  }

  fclose(f);
  return result;
}

void writenumber(int result)
{
  FILE *f = fopen("file17.number","w");
  if (!f) die("write",strerror(errno));
  if (fprintf(f,"%d",result) == -1) die("write",strerror(errno));
  if (fflush(f) == -1) die("write",strerror(errno));
  fclose(f);
}

int main(void)
{
  int result = readnumber();
  printf("Number of times I have been run: %d\n",++result);
  writenumber(result);
  return 0;
}