On Thu, Mar 30, 2006 at 01:03:15AM +0200, Joerg Schilling wrote:
>Steve Bennett <[EMAIL PROTECTED]> wrote:
>
>> > After reading the C-99 Standard, it turned out that the last method
>> > is the "official" way to prevent tail padding of structures.
>> >
>> > Could you please run the last test again but use:
>> >
>> > #pragma pack(1)
>> >
>> > struct test1
>> > {
>> > char blerg[1];
>> > char type[4];
>> > char flibble[3];
>> > char more[2];
>> > char dummy[];
>> > };
>> >
>>
>> sizeof(test1) is 12
>>
>> And just in case you are wondering, I ran all those previous tests again
>> with -std=c99 with exactly the same results.
>
>Thank you for the test!
>
>So GCC is in clear violation with the C-99 standard.
Which part of the C-99 standard, exactly? Section 6.7.2.1, paragraph
#12,#14,#15 (see draft N869 from
http://cbfalconer.home.att.net/download/n869_txt.bz2) says the
following:
---------------------------------------
[#12] Within a structure object, the non-bit-field members
and the units in which bit-fields reside have addresses that
increase in the order in which they are declared. A pointer
to a structure object, suitably converted, points to its
initial member (or if that member is a bit-field, then to
the unit in which it resides), and vice versa. There may be
unnamed padding within a structure object, but not at its
beginning.
...
[#14] There may be unnamed padding at the end of a structure
or union.
[#15] As a special case, the last element of a structure
with more than one named member may have an incomplete array
type. This is called a flexible array member, and the size
of the structure shall be equal to the offset of the last
element of an otherwise identical structure that replaces
the flexible array member with an array of unspecified
length.97) When an lvalue whose type is a structure with a
flexible array member is used to access an object, it
behaves as if that member were replaced with the longest
array, with the same element type, that would not make the
structure larger than the object being accessed; the offset
of the array shall remain that of the flexible array member,
even if this would differ from that of the replacement
array. If this array would have no elements, then it
behaves as if it had one element, but the behavior is
undefined if any attempt is made to access that element or
to generate a pointer one past it.
____________________
97)The length is unspecified to allow for the fact that
implementations may give array members different
alignments according to their lengths.
---------------------------------------
i.e. there may be no padding before the first member of a structure,
but there may be elsewhere within the structure including right at the
end. As a special case, if you specify the last member of your
structure as a "flexible array member" then there will be no padding
_at the end of the structure_. It does not affect padding _within_ the
structure.
I'm sorry, I don't see anything here to support your assertion that
gcc is behaving incorrectly.
--
Steve McIntyre, Cambridge, UK. [EMAIL PROTECTED]
"Since phone messaging became popular, the young generation has lost the
ability to read or write anything that is longer than one hundred and sixty
characters." -- Ignatios Souvatzis
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