//
// An application that updates file "grades" so that
// after a crash the file will have either the new
// or old contents (but not be e.g. empty).
// In real life you should check all syscall returns.
//

main()
{
  int fd, cc;
  char buf[512];

  // read existing file into memory
  fd = open("grades", 0);
  cc = read(fd, buf, sizeof(buf));
  close(fd);

  // change a grade in memory
  buf[0] = 'A';

  // write to a temporary file
  fd = creat("temp1234", 0600);
  write(fd, buf, cc);
  fsync(fd); // wait until everything on disk
  close(fd);

  // atomically replace old file with new
  rename("temp1234", "grades");
}