Jim Gettys wrote:
[snip]
> > >From a slightly outdated C99 draft, about the definition of arrays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.
The size is an intrinsic property of the object type. Objects can't
legally overlap (outside of unions). A compiler is free to choose a
different actual memory layout, but then it has to hide this fact,
including pointer alignment etc., which means it is normally not done.
("Packed", btw, means "not in ABI layout" in gcc. The standard has no
notion of "packed", except for bitfield storage layout.)
> > 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.
Exactly. xlib seems to use the sum of the size of the primitives in an
element instead of the size of the first element.
Thiemo
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