Alignment

[fishpond]

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

--
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"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."

[Malcolm McLean]

<fi******@inval id.comwrote in message
news:sl******** ************@no spam.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.

In strictly correct terms, use compiler pragmas to achieve what you want.
double isn't guaranteed to be the most strictly aligned type and there is no
portable mechnaism for finding it.

--
Free games and programming goodies.
http://www.personal.leeds.ac.uk/~bgy1mm

[Harald van =?UTF-8?B?RMSzaw==?=]

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

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

Given that any object type can be contained in a structure or a union, and
all structure and union types have the same alignment requirements, any
structure or union type will do.

However, why do you need this? There's very little in standard C that you
can do with knowledge of alignment of non-dynamic objects. Even if you have
a structure containing an array of four chars, which is properly aligned
for an int, and sizeof(int)==4 on your platform, you're still not allowed
to access this structure as were it an int.

[Harald van =?UTF-8?B?RMSzaw==?=]

Harald van Dijk wrote:

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

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

Given that any object type can be contained in a structure or a union, and
all structure and union types have the same alignment requirements, any
structure or union type will do.

Sorry, this is of course incorrect. I was thinking of pointers to
structures. The below is still true, though you'd need to have another way
of making the structure properly aligned for it to be relevant.

However, why do you need this? There's very little in standard C that you
can do with knowledge of alignment of non-dynamic objects. Even if you
have a structure containing an array of four chars, which is properly
aligned for an int, and sizeof(int)==4 on your platform, you're still not
allowed to access this structure as were it an int.

[pete]

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

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

Allocated objects have strictest alignment.

Does it have to be a declared variable?

--
pete

[Thad Smith]

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

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

How to ask a question without a subject and verb?

I assume you mean something like "how can I declare a variable
guaranteed to have the strictest possible alignment?".

To get alignment suitable to any type, dynamically allocate it.
Otherwise, you can declare a union containing all the types that must be
aligned properly.

--
Thad

[Keith Thompson]

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

<fi******@inval id.comwrote in message
news:sl******** ************@no spam.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?

In strictly correct terms, use compiler pragmas to achieve what you
want. double isn't guaranteed to be the most strictly aligned type and
there is no portable mechnaism for finding it.

In strictly correct terms, (or, as I prefer to say, "correct terms")
there is no portable way to do this; the language doesn't define any
pragmas for this purpose.

--
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"

[Harald van =?UTF-8?B?RMSzaw==?=]

Keith Thompson wrote:

Harald van Dijk <tr*****@gmail. comwrites:

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

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

Given that any object type can be contained in a structure or a union,
and all structure and union types have the same alignment requirements,
any structure or union type will do.

[...]

Not true. All *pointers* to structure or union types have the same
alignment requirements, but that refers only to the alignment of
pointer objects, not to the alignment of what they point to. It's
entirely possible for 'struct { int n; }' to have stricter alignment
than 'struct { char c; }', for example.

To clarify beyond the message I sent earlier: I used to believe that all
structures must have the same alignment requirements. I have never read a
satisfactory explanation of how the representation requirements for
pointers to structures do not imply this, I accept that the intent was that
different structures may have different alignment requirements, and all
that has nothing to do with my first message in this thread, which was
nothing more a slip of the mind unrelated to that old argument.

Or, to shorten it somewhat: agreed.

[Malcolm McLean]

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

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

><fi******@inva lid.comwrote in message
news:sl******* *************@n ospam.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 ?

--
Free games and programming goodies.
http://www.personal.leeds.ac.uk/~bgy1mm

[Ark Khasin]

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
} strict_align_t;
strict_align_t myvar;

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

-- Ark