Question about struct sizes

[CaptainPanda]

Hi all,

Basically been writing some code and I came across a funky error when reading a a file and was off by one with my struct definition.

This was the original code:

struct lumpinfo_t {
    int filepos;
    int size;
    char name[9]; // meant to be 8
};

So I decided to test the size of the struct and it turned out to be 20 bytes. When I noticed my error I changed 9 to 8 and the size jumped down to 16.

If I was to guess, modern computers don't used 1 byte chunks any more as most int's are at least 4 bytes so instead of creating a struct of size 17 bytes it pads the struct by 3 extra bytes.

If I'm wrong feel free to correct me but my basic question is why padding does this? Is it an optimisation or something?

[Peter87]

It's padding to make sure the types are align correctly. A primitive type that is 4 bytes is normally stored on a memory address that is divisible by 4. How important this is depends on the hardware. Some hardware can't handle unaligned data at all, some are just slower.

In your case 3 bytes of padding is added to make sure the ints are aligned properly if you were to store them in an array.

| int filepos   | int size      | char name[9]                      | padding   |
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 |11 |12 |13 |14 |15 |16 |17 |18 |19 |
---------------------------------------------------------------------------------
| int filepos   | int size      | char name[9]                      | padding   |
|20 |21 |22 |23 |24 |25 |26 |27 |28 |29 |30 |31 |32 |33 |34 |35 |36 |37 |38 |39 |
---------------------------------------------------------------------------------
| int filepos   | int size      | char name[9]                      | padding   |
|40 |41 |42 |43 |44 |45 |46 |47 |48 |49 |50 |51 |52 |53 |54 |55 |56 |57 |58 |59 |

If there was no padding the ints for some of the elements would not get aligned correctly.

| int filepos   | int size      | char name[9]                      |
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 |11 |12 |13 |14 |15 |16 |
---------------------------------------------------------------------
| int filepos   | int size      | char name[9]                      |
|17 |18 |19 |20 |21 |22 |23 |24 |25 |26 |27 |28 |29 |30 |31 |32 |33 |
---------------------------------------------------------------------
| int filepos   | int size      | char name[9]                      |
|34 |35 |36 |37 |38 |39 |40 |41 |42 |43 |44 |45 |46 |47 |48 |49 |50 |