Alignment

[fishpond]

How to declare a variable guaranteed to have the strictest possible
alignment?

--
The defense attorney was hammering away at the plaintiff:
"You claim," he jeered, "that my client came at you with a broken bottle
in his hand. But is it not true, that you had something in YOUR hand?"
"Yes," the man admitted, "his wife. Very charming, of course,
but not much good in a fight."

[Eric Sosman]

Ark Khasin wrote:

fi******@invali d.com wrote:

>How to declare a variable guaranteed to have the strictest possible
alignment?

I guess the first question is, what's alignment? C99 3.2 says
"requiremen t that objects of a particular type be located on storage
boundaries with addresses that are particular multiples of a byte address."

That's a motivational explanation (like a math explanation of a set) but
not a definition: addresses are not numbers, and a multiple of a byte
address is... what?

Is it fair to say that all we can say (axiomatically) that
- For a type T there is an alignment good for storing a T object, and
- if T *p is an address good for storing a T object, then so is p+1?
[And then whatever alignment hierarchy the standard defines.]

I cannot imagine why one needs a strictest-aligned variable of static
duration, but the following would work
typedef union {
char c;
short s;
int i;
long l;
float f;
double d;
long double ld;
long long ll;
//and whatever native types I missed

"Aye: There's the rub." -- H, P of D

} strict_align_t;
strict_align_t myvar;

--
Eric Sosman
es*****@ieee-dot-org.invalid

[Ark Khasin]

Eric Sosman wrote:

Ark Khasin wrote:

>>
I cannot imagine why one needs a strictest-aligned variable of static
duration, but the following would work
typedef union {
char c;
short s;
int i;
long l;
float f;
double d;
long double ld;
long long ll;
//and whatever native types I missed

"Aye: There's the rub." -- H, P of D

>} strict_align_t;
strict_align _t myvar;

Of course. Will the addition of "void *v; const void *cv;" do it?

And, taking off my assembler motley, may I ask again what is alignment?
Is there some sort of conspiracy and addresses in fact /are/ some sort
of numbers?

[Ian Collins]

Ark Khasin wrote:

Eric Sosman wrote:

>Ark Khasin wrote:

>>>
I cannot imagine why one needs a strictest-aligned variable of static
duration, but the following would work
typedef union {
char c;
short s;
int i;
long l;
float f;
double d;
long double ld;
long long ll;
//and whatever native types I missed

"Aye: There's the rub." -- H, P of D

>>} strict_align_t;
strict_align_ t myvar;

Of course. Will the addition of "void *v; const void *cv;" do it?

Don't forget function pointers.

--
Ian Collins.

[Ark Khasin]

Ian Collins wrote:

Ark Khasin wrote:

>Eric Sosman wrote:

>>Ark Khasin wrote:
I cannot imagine why one needs a strictest-aligned variable of static
duration, but the following would work
typedef union {
char c;
short s;
int i;
long l;
float f;
double d;
long double ld;
long long ll;
//and whatever native types I missed
"Aye: There's the rub." -- H, P of D

} strict_align_t;
strict_align _t myvar;

Of course. Will the addition of "void *v; const void *cv;" do it?

Don't forget function pointers.

Thanks! I've missed that C99: J.5.7 is informative, not normative.

[Keith Thompson]

Ark Khasin <ak*****@macroe xpressions.comw rites:
[...]

I cannot imagine why one needs a strictest-aligned variable of static
duration, but the following would work
typedef union {
char c;
short s;
int i;
long l;
float f;
double d;
long double ld;
long long ll;
//and whatever native types I missed
} strict_align_t;
strict_align_t myvar;

Unsigned and complex stuff can be inferred to fit this alignment.

It's very likely to work, but it's certainly not guaranteed to work.
There are arbitrarily many types, and any of them could theoretically
have stricter alignment requirements than any of the types you've
listed.

If you're going to use that approach, you should include at least a
few pointer types (void*, a struct pointer, and a function pointer).

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Peter J. Holzer]

On 2007-08-20 03:47, Ark Khasin <ak*****@macroe xpressions.comw rote:

And, taking off my assembler motley, may I ask again what is alignment?
Is there some sort of conspiracy and addresses in fact /are/ some sort
of numbers?

Addresses aren't numbers, but each address includes at least one number
(if it didn't, pointer arithmetic would not be possible). In a flat
address space (the usual case today), the address consists only of a
single number. In a segmented address space, it contains (at least) two
numbers: A segment number and a segment offset, where arithmetic is
often only meaningful on the offset. An address can contain other
information (type information, flags, etc.).

Of course you can interpret any collection of bits as a number ...

hp
--
_ | Peter J. Holzer | I know I'd be respectful of a pirate
|_|_) | Sysadmin WSR | with an emu on his shoulder.
| | | hj*@hjp.at |
__/ | http://www.hjp.at/ | -- Sam in "Freefall"

[Peter J. Holzer]

On 2007-08-19 22:07, Malcolm McLean <re*******@btin ternet.comwrote :

"Keith Thompson" <ks***@mib.orgw rote in message
news:ln******** ****@nuthaus.mi b.org...

>"Malcolm McLean" <re*******@btin ternet.comwrite s:

>><fi******@inv alid.comwrote in message
news:sl****** **************@ nospam.invalid. ..
How to declare a variable guaranteed to have the strictest possible
alignment?
In practise, align to a double.

[...]

>What if long double has stricter alignment requirements than double?

Computer like things in units of powers of two.
doubles are almost always 64 bits, in fact have to be if IEEE. long doubles
are 80.

No. long doubles are 80 on Intel x86. I've seen 96 bit and 128 bit long
doubles. It is entirely possible that a 128 bit long double needs to be
aligned on a 128 bit boundary.

hp
--
_ | Peter J. Holzer | I know I'd be respectful of a pirate
|_|_) | Sysadmin WSR | with an emu on his shoulder.
| | | hj*@hjp.at |
__/ | http://www.hjp.at/ | -- Sam in "Freefall"

[Ivar Rosquist]

On Sun, 19 Aug 2007 23:07:43 +0100, Malcolm McLean wrote:

"Keith Thompson" <ks***@mib.orgw rote in message
news:ln******** ****@nuthaus.mi b.org...

>"Malcolm McLean" <re*******@btin ternet.comwrite s:

>><fi******@inv alid.comwrote in message
news:sl****** **************@ nospam.invalid. ..
How to declare a variable guaranteed to have the strictest possible
alignment?

In practise, align to a double.
eg

union alignedint
{
double dummy;
int x;
}

x will now have double alignement.

What if long double has stricter alignment requirements than double?

Computer like things in units of powers of two. doubles are almost
always 64 bits, in fact have to be if IEEE. long doubles are 80. So the
alignment requirements for double are likely to be stricter. However you
are right, in practise isn't the same as "by the standard". It might
even be storing up trouble for the future to advise such a thing. I
haven't made my mind up about this one. How about a new type. align_t ?

When are you going to stop giving misleading advice? Please, do
go back to your specialty - C is NOT it.

[Eric Sosman]

Ark Khasin wrote:

Eric Sosman wrote:

>Ark Khasin wrote:

>>>
I cannot imagine why one needs a strictest-aligned variable of static
duration, but the following would work
typedef union {
char c;
short s;
int i;
long l;
float f;
double d;
long double ld;
long long ll;
//and whatever native types I missed

"Aye: There's the rub." -- H, P of D

>>} strict_align_t;
strict_align_ t myvar;

Of course. Will the addition of "void *v; const void *cv;" do it?

It can't hurt (but it's not necessary to add both; the
qualifiers don't affect the alignment requirement). Include
a function pointer while you're about it, and some struct
types (a struct might require stricter alignment than its
members), and size_t, and intmax_t, and ... The "rub" is
that the list of possible types is potentially infinite.

<off-topic>

There's another "rub," too, which is that some alignment
requirements of the platform may not correspond to any C type
at all. "Page alignment," for example, is likely to be hard
to achieve with this approach. On systems that support multiple
page sizes, testing for page alignment uses a divisor specific to
the "arena" containing the address. A static, location-independent
scheme like the above cannot work on such systems, even if you can
find a page-sized C data type.

</off-topic>

And, taking off my assembler motley, may I ask again what is alignment?
Is there some sort of conspiracy and addresses in fact /are/ some sort
of numbers?

Peter J. Holzer's explanation elsethread is good.

--
Eric Sosman
es*****@ieee-dot-org.invalid

[CBFalconer]

fi******@invali d.com wrote:

>

.... snip ...

>
But some type punning can be done in completely Standard C. For
example, deciding whether a float is positive or negative by
treating it as an int and extracting the sign bit uses only
bit-shifting which is a Standard C operator.

But the number of bits in an int isn't standard. What's wrong in:

if (value < 0) return 1;
else return 0;

or the equivalent in other terms?

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

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