Alignment (of structs and packed structs // controlling of alignment...)

[anon...@gmail.com]

Hello!

__u32 (linux/types.h) is 4-byte aligned (__u32-aligned) - that's
certain!

But: Is a struct aligned according to its size, or according to the
largest component it contains:

Example:
What about a struct that has a sizeof 4 bytes, but contains at most a
__u16:
Will it be __u16 aligned? Or rather: will it be __u32 aligned since
the whole structure has 4 bytes?

But then also: What if a struct contains a __u8-array of say 4
elements?

What if sizeof(struct abc) is 6??
etc.

There are code-snippets below (that will hopefully make it easier to
discuss):

#include <linux/types.h>

/***************** (1a) ******************/
struct mystruct1a {
__u8 a;
__u16 b;
__u8 c;
};

struct mystruct1a ms1a;
/*
### will the whole struct be __u8-aligned, or __u16-aligned, or __u32-
aligned ??
*/

/***************** (1b-packed) ***********/
struct mystruct1b {
__u8 a;
__u16 b;
__u8 c;
} __attribute__ ((packed));

struct mystruct1b ms1b;
/*
### whole struct: __u8-aligned, or __u16-aligned, or __u32-aligned ??
*/

/***************** (2a) ******************/
struct mystruct2a {
__u8 arr[3];
};

struct mystruct2a ms2a;
/*
### whole struct: __u8-aligned, or __u16-aligned, or __u32-aligned ??
would a union behave differently?
*/

/***************** (2b-packed) ***********/
struct mystruct2b {
__u8 arr[3];
} __attribute__ ((packed));

struct mystruct2b ms2b;
/*
### whole struct: __u8-aligned, or __u16-aligned, or __u32-aligned ?
would a union behave differently?
*/

/***************** (3a) ******************/
struct mystruct3a {
__u8 arr[4];
};

struct mystruct3a ms3a;
/*
### whole struct: __u8-aligned, or __u16-aligned, or __u32-aligned ??
would a union behave differently?
*/

/***************** (3b-packed) ***********/
struct mystruct3b {
__u8 arr[4];
} __attribute__ ((packed));

struct mystruct3b ms3b;
/*### whole struct: __u8-aligned, or __u16-aligned, or __u32-
aligned ??
would a union behave differently?
*/

Also: Does gcc have a way of allowing the programmer to flexibly
control such alignments?

Thanks
-Albert

[anon...@gmail.com]

On Aug 29, 11:13 am, anon.a...@gmail.com wrote:
> Hello!
>
> __u32 (linux/types.h) is 4-byte aligned (__u32-aligned) - that's
> certain!
>
> But: Is a struct aligned according to its size, or according to the
> largest component it contains:

According to a multiple of " the least common multiple of its
elements' alignment requirements":

An answer (from Eric Sosman) to the questions concerning non-packed
structs can be seen here:
http://groups.google.com/group/comp.lang.c/msg/9e69e689975bc07e?

> Example:
> What about a struct that has a sizeof 4 bytes, but contains at most a
> __u16:
> Will it be __u16 aligned? Or rather: will it be __u32 aligned since
> the whole structure has 4 bytes?

Consider it __u16-aligned, to go save! could also be a multiple of
__u16, e.g. __u32

> But then also: What if a struct contains a __u8-array of say 4
> elements?

__u8 aligned to go save!

> /***************** (1a) ******************/
> struct mystruct1a {
> __u8 a;
> __u16 b;
> __u8 c;
>
> };
>
> struct mystruct1a ms1a;
> /*
> ### will the whole struct be __u8-aligned, or __u16-aligned, or __u32-
> aligned ??
> */

__u16 to go save!

>
> /***************** (1b-packed) ***********/
> struct mystruct1b {
> __u8 a;
> __u16 b;
> __u8 c;
>
> } __attribute__ ((packed));
>
> struct mystruct1b ms1b;
> /*
> ### whole struct: __u8-aligned, or __u16-aligned, or __u32-aligned ??
> */
>

???

> /***************** (2a) ******************/
> struct mystruct2a {
> __u8 arr[3];
>
> };
>
> struct mystruct2a ms2a;
> /*
> ### whole struct: __u8-aligned, or __u16-aligned, or __u32-aligned ??
> would a union behave differently?
> */
>

__u8 to go save

> /***************** (2b-packed) ***********/
> struct mystruct2b {
> __u8 arr[3];
>
> } __attribute__ ((packed));
>
> struct mystruct2b ms2b;
> /*
> ### whole struct: __u8-aligned, or __u16-aligned, or __u32-aligned ?
> would a union behave differently?
> */
>

???

> /***************** (3a) ******************/
> struct mystruct3a {
> __u8 arr[4];
>
> };
>
> struct mystruct3a ms3a;
> /*
> ### whole struct: __u8-aligned, or __u16-aligned, or __u32-aligned ??
> would a union behave differently?
> */
>

__u8 to go save!

> /***************** (3b-packed) ***********/
> struct mystruct3b {
> __u8 arr[4];
>
> } __attribute__ ((packed));
>
> struct mystruct3b ms3b;
> /*### whole struct: __u8-aligned, or __u16-aligned, or __u32-
> aligned ??
> would a union behave differently?
> */
>

???

> Also: Does gcc have a way of allowing the programmer to flexibly
> control such alignments?

???

-----------------------------------
Here's something interesting:

Consider a packed struct:

struct structi {
short a; //2 bytes
int b; //4bytes
short c; //2bytes
} __attribute__ ((packed));

struct structi run_this_by_me_again;

Depending on how this is accessed
e.g.

*((int *)(((short*)&run_this_by_me_again)+1))
!!

it might be useful to have this struct aligned at addresses x for
which:
x%4 = 2
x mod 4 = 2

Can this be achieved with gcc ??

-Albert