On Tue, 2005-01-11 at 22:36 +0100, Thiemo Seufer wrote:
> Jim Gettys wrote:
> [snip]
> > > Strictly speaking, the ARM impementation of gcc is allowed to behave
> > > that way by the C standard. Not exercising this degree of freedom may
> > > be desireable to keep broken code working, but I'll leave it to the
> > > ARM people to weigh the tradeoff.
> >
> > Are you sure? Remember it is an array of such structures, not a
> > structure embedded in another structure or on its own....
> >
> > If you have a pointer to the language in the standard, it might
> > elucidate the situation a bit.
>
> >From a slightly outdated C99 draft, about the definition of arrays
> and structures:
>
> [#19] Any number of derived types can be constructed from
> the object, function, and incomplete types, as follows:
>
> -- An array type describes a contiguously allocated
> nonempty set of objects with a particular member object
> type, called the element type.31) Array types are
> characterized by their element type and by the number
> of elements in the array. An array type is said to be
> derived from its element type, and if its element type
> is T, the array type is sometimes called ``array of
> T''. The construction of an array type from an element
> type is called ``array type derivation''.
>
> -- A structure type describes a sequentially allocated
> nonempty set of member objects (and, in certain
> circumstances, an incomplete array), each of which has
> an optionally specified name and possibly distinct
> type.
>
Seems pretty silent on whether an array of structures are packed or not.
> About the sizeof operator:
>
> [#3] When applied to an operand that has type char, unsigned
> char, or signed char, (or a qualified version thereof) the
> result is 1. When applied to an operand that has array
> type, the result is the total number of bytes in the
> array.73) When applied to an operand that has structure or
> union type, the result is the total number of bytes in such
> an object, including internal and trailing padding.
>
> This means internal and trailing padding is part of the object, and
> each member object of an aggregation has it. (It would be plain insane
> to define sizeof(struct ...) in dependence of its context.) This, BTW,
> is the reason why the canonical way to compute the number of elements
> in an array is:
>
> sizeof array / sizeof array[0]
>
Note that this works consistently whether the object is padded in size
or or not, in this case, since the array would double in size even as
the individual structure size doubled.
- Jim
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