From: Keith.S.Thompson+u@gmail.com
David Brown writes:
> On 14/01/2026 04:19, James Russell Kuyper Jr. wrote:
>> On 2026-01-12 13:08, Michael S wrote:
>>> On Mon, 12 Jan 2026 15:58:15 +0000
>>> bart wrote:
>> ...
>>>> struct bar1 {
>>>> union {
>>>> struct {
>>>> int table[4];
>>>> int other_table[4];
>>>> };
>>>> int xtable[8];
>>>> };
>>>> };
>> ...
>>>> I'm not even sure about there being no padding between .table and
>>>> .other_table.
>>>
>>> Considering that they both 'int' I don't think that it could happen,
>>> even in standard C.
>> "Each non-bit-field member of a structure or union object is aligned
>> in an implementation-defined manner appropriate to its type."
>> (6.7.3.2p16)
>> "... There can be unnamed padding within a structure object, but not
>> at its beginning." (6.7.3.2p17)
>
> Does this allow different alignment rules for a type when it is
> stand-alone, in an array, or in a struct? I don't think so - I have
> always interpreted this to mean that the alignment is tied to the
> type, not where the type is used.
Note that the alignof operator applies to a type, not to an expression
or object.
> Thus if "int" has 4-byte size and 4-byte alignment, and you have :
>
> struct X {
> char a;
> int b;
> int c;
> int ds[4];
> }
>
> then there will be 3 bytes of padding between "a" and "b", but cannot
> be any between "b" and "c" or between "c" and "ds".
There can be arbitrary padding between struct members, or after the last
member. Almost(?) all implementations add padding only to satisfy
alignment requirements, but the standard doesn't state any restrictions.
There can be no padding before the first member, and offsets of members
must be increasing.
If alignof (int) is 4, a compiler must place an int object at an address
that's a multiple of 4. It's free to place it at a multiple of 8, or
16, if it chooses.
> Even if you have a weird system that has, say, 3-byte "int" with
> 4-byte alignment, where you would have a byte of padding between "b"
> and "c", you would have the same padding there as between "ds[0]" and
> "ds[1]".
sizeof (int) == 3 and alignof (int) == 4 is not possible. Each type's
size is a multiple of its alignment. There is no padding between array
elements.
[...]
--
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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