how to identify the exact padding size of a structure considering all situations ?

rpittala · 04-17-2012, 10:18 AM

The below code is for your reference :

-->In the below code the size of structure is,I`m expecting it as 20bytes but it is 16 bytes.

-->How the word boundary is aligned for it ?

-->what is the secret behind padding ?

-->how to identify the exact padding size of a structure considering all situations ?
#include<stdio.h>
int main()
{
struct name{
int a;
char b;
short c;
char d;
short e
int e;
}p;
printf("%d \n",sizeof(p));
return 0;
}

manu-tm · 04-17-2012, 12:18 PM

I suppose it depends on your machine architecture and on your compiler.

suicidaleggroll · 04-17-2012, 12:23 PM

Where do you get 20? I get 14, 4 per int, 2 per short, 1 per char.

Anyway, about padding:
http://vcfaq.mvps.org/lang/11.htm

In particular:

Quote:

the compiler pads the end of the structure with dummy bytes until the structure size is a multiple of the largest alignment in the structure. That way the alignment for all consecutive structures is valid.

Your largest type is 4 bytes, so the struct is padded up to the nearest multiple of 4, which is 16. This behavior can be modified with the pragma described in the above link.

Each compiler will likely be different, but it sounds like yours is doing something similar to this.

cliffordw · 04-17-2012, 12:31 PM

This may depend on the word size of your machine, your compiler's default behaviour, and any options you may have passed to the compiler to change the default behaviour.

Assuming you are using gcc, it appears that the default is to "pack all structure members together without holes", but that you can change this with the "-fpack-struct[=n]" option. See http://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html.