Reference no: EM132400899

C programming language questions:

Consider the function below, which has two arguments but otherwise has no local variables.

void f(int *a, int *b)
{
    *a = *a ^ *b;
    *b = *a ^ *b;
    *a = *a ^ *b;
}

1. If, per this function's declaration, each of a and b is the address of an int, exactly what does this function do to *a and *b, the values at those addresses? (Consider, for instance, if *a is 1 and *b is 2.) Put another way, what would be a better name for this function than f?

Backups

XOR can also be used to back up your data! Suppose that your computer has just one drive on which to store files. If that drive were to fail, you could lose all of your data. Having a second drive with copies of your files as a backup would be better. But suppose that you have so many files that you need that second drive for more storage. Backing up both drives would seem to require two more drives for a total of four. But not so with XOR! Thanks to XOR, you can save a bit of space (and money!) by buying just one additional drive, for a total of three. But instead of storing files on that third drive, you can instead store the XOR of your other two drives. If any of those three drives fail, you can recover its bits by XORing the other two drives onto a new drive.

For the sake of discussion, suppose that these drives are not very large. They can each store, say, only 8 bits. (You should have shopped around.) Suppose that the first drive contains 11110000 and the second drive contains 00011000. You should thus store 11101000 on your third drive because:

  11110000
^ 00011000
  --------
  11101000

1. Suppose that one of your three drives fails and, before you have time to XOR your remaining two drives onto a new third drive, one of your remaining two drives fails as well. What might the effect be on your files?
2. Suppose that you now have not two but three hard drives with files that you'd like to back up, such that if any one of the three drives fails you'd like to be able to recover the data. How many additional drives do you minimally need? And what would you store on the additional drive(s)?