Thread: Packing a struct

Quick question. If I have the following struct:

Code:

struct Wombat
{
   unsigned int Foo;
   char         Bar;
}

Can I be certain (assuming the int datatype is 4 bytes) that sizeof(Wombat) will always equal 5, or is there a possibility that the compiler may pad it out to 8 bytes to align it to word boundaries?

I'm pretty sure it'll pad based on what I think I know about structures

EDIT: And why of all things did you call it Wombat?
EDIT2: And I know that it's just a test struct. Right?

Hmm, in that case, if I had a struct that was 12 bytes, I guess I'd have to worry about 64-bit compilers padding it out to 16 bytes to align it on dword boundaries as well. Bah.

To answer your questions:

1) I like wombats.
2) Yep, just a test struct, as is demonstrated by the fact that I left out the trailing semicolon after the closing brace.

I should've noticed that. Meh.
I'm wiki'ing Wombat since I have no idea what it is. Well I do, sort of, I'm just not sure.

EDIT: Awesome. Someone thought it'd be funny to cross-breed a cat and a pig. Ouch.

I think you can use #pragma to align structures to x bytes, at least for most compilers. (I was looking at the SDL header begin_code.h earlier today.) Try googling it.

Or better yet don't rely on five byte structures.

Yes, they right compiler will probably pad structures. You may want to try this:

Code:

#include <cstdlib>
#include <iostream>
#pragma pack(1)
using namespace std;

struct test
{
    int x;
    char y;
};

int main ( ) 
{
    cout << sizeof(test);
    system("pause");
    return 0;
}

I test it on Win XP machine, Dev-Cpp.

Try google it for details

> Can I be certain (assuming the int datatype is 4 bytes) that sizeof(Wombat) will always equal 5
You can't be certain of anything about the size of a struct, except that it is always at least the size of the sum of it's members. Compilers are free to add internal and trailing padding to preserve alignment (though they can't add anything before the first member).

Compiler extensions like #pragma pack() are not universally portable, it may be ignored completely, or it may pack something else instead.

> 1) I like wombats.
I wouldn't play wom with anything else