//
// 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");
}
  $
  $ echo > x
  bwrite 4 ialloc inum 18
  bwrite 4 iupdate inum 18
  bwrite 29 writei inum 1 *
  $ echo z > x
  bwrite 28 balloc bn 337
  bwrite 337 writei inum 18
  bwrite 4 iupdate inum 18
  bwrite 337 writei inum 18
  bwrite 4 iupdate inum 18
  $ rm x
  bwrite 29 writei inum 1  **
  bwrite 4 iupdate inum 18
  bwrite 337 bzero
  bwrite 28 bfree bn 337
  bwrite 4 iupdate inum 18
  bwrite 4 iupdate inum 18
  $ echo > y
  bwrite 4 ialloc inum 18
  bwrite 4 iupdate inum 18
  bwrite 29 writei inum 1 **
  $ 
  (file /a already exists w/ i-num 19)
  $
  $ echo > b
  bwrite 4 ialloc inum 20
  bwrite 4 iupdate inum 20
  bwrite 29 writei inum 1
  $ rm a
  bwrite 29 writei inum 1
  bwrite 4 iupdate inum 19
  bwrite 4 iupdate inum 19
  bwrite 4 iupdate inum 19
  $