Improve coding size

1. Attribute packed

packed is a variable attribute that is used with structures and unions in order to minimize the memory requirements.

#include <stdio.h>
struct foo {
    int a;
    char c;
};

struct __attribute__((__packed__))foo_packed {
    int a;
    char c;
};

int main()
{
    printf("Size of foo: %d\n", sizeof(struct foo));
    printf("Size of packed foo: %d\n", sizeof(struct foo_packed));
    return 0;
}

On my 64 bit Linux,

• Size of struct foo = 8 bytes
• Size of struct foo_packed = 5 bytes

struct t with no attributes and default alignment on 8-byte boundary:

+-+-------+--+-------+
|a|       |bb|       |
+-+-------+--+-------+

struct t when a and b are aligned on 16-byte boundaries:

+-+---------------+--+---------------+
|a|    padding    |bb|    padding    |
+-+---------------+--+---------------+

struct t when a and b have no alignment restrictions and t is packed:

+-+--+
|a|bb|
+-+--+