#include <stdio.h>

void die_write(void)
{
  fprintf(stderr,"file6: fatal: unable to write data.txt\n");
  exit(111);
}

int main(void)
{
  FILE *f;
  int i;

  f = fopen("data.txt","w");
  if (!f) die_write();

  for (i = 0;i < 10;++i) fprintf(f,"%d\n",i);
  fflush(f);

  if (ferror(f)) die_write();

  return 0;
}