Reading 4 bytes from the end of a char array

Question

If I have a char* array:

char* c = getBytesFromSomewhere();

I'd like to read the last 4 bytes to a uint and then the previous 4 bytes etc. I tried this:

char* end = &c[size-5];

to get a pointer to the last 4 bytes in the array and then:

unsigned int n = *(reinterpret_cast<int *>(end));

but it doesn't seem to be working.... what am I doing wrong?

Answer 1

Lets say the returned array is of size 8, it would look something like this in memory:

+---+
| c |
+---+
  |
  v
  +---+---+---+---+---+---+---+---+
  | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
  +---+---+---+---+---+---+---+---+

(The numbers inside is the indexes.)

Now if you make a new variable e to point at c + size it will point point to one beyond the end of the data:

+---+                           +---+
| c |                           | e |
+---+                           +---+
  |                               |
  v                               v
  +---+---+---+---+---+---+---+---+
  | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
  +---+---+---+---+---+---+---+---+

If you subtract 1 from e it now points to index 7:

+---+                       +---+
| c |                       | e |
+---+                       +---+
  |                           |
  v                           v
  +---+---+---+---+---+---+---+---+
  | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
  +---+---+---+---+---+---+---+---+

If you subtract two (in total) e would point to index 6, subtract 3 and e would be pointing at index 5 and subtract 4 and the index pointed to would be 4. If you subtract 5 the pointer e would point to index 3:

+---+       +---+
| c |       | e |
+---+       +---+
  |           |
  v           v
  +---+---+---+---+---+---+---+---+
  | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
  +---+---+---+---+---+---+---+---+

And that's not four bytes from the end, that's five bytes from the end.

So you should be doing e.g.

char* end = c + size - 4;  /* Subtract by 4 and not 5 */

---

You should also be careful of the endianness, if the data comes from other systems e.g. over the Internet.