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    SubjectRe: [00/02] add BUILD_BUG_DECL assertion (for 3.4??)
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    On Sun, 2012-04-08 at 17:59 -0600, Jim Cromie wrote:
    > On Sun, Apr 8, 2012 at 4:52 PM, Joe Perches <joe@perches.com> wrote:
    > > On Sun, 2012-04-08 at 16:38 -0600, Jim Cromie wrote:
    > >
    > >> this patch (0001) adds new bug.h macro, BUILD_BUG_DECL(name, cond),
    > >> which unlike other *BUG* assertions is usable at file scope. Its
    > >> primary purpose is to enforce identical sizes of 2 separate arrays,
    > >> which but for considerations of packing/padding/section, would be
    > >> together in a struct.
    > >>
    > >> const char const *names[] = { "bart", "lisa", "homer", "marge" };
    > >> int a[] = {1,2,3,4};
    > >> int b[] = {1,2,3,5};
    > >> long d[] = {1,2};
    > >>
    > >> BUILD_BUG_DECL(foo, ARRAY_SIZE(a) != ARRAY_SIZE(b));
    > >> BUILD_BUG_DECL(buz, sizeof(a) != sizeof(b)); // good
    > >> BUILD_BUG_DECL(a, sizeof(a) != sizeof(d)); // ok on x32, error x64
    > >> BUILD_BUG_DECL(b, ARRAY_SIZE(a) != ARRAY_SIZE(names)); // good
    > >>
    > >> macro expands as:
    > >> static __attribute__ ((__section__(".init.data"))) struct {
    > >> int BUILD_BUG_DECL_buz[1 - 2*!!(sizeof(a) != sizeof(b))];
    > >> } BUILD_BUG_DECL_buz[0] __attribute__((unused));
    > >>
    > >
    > > If possible, it might be better to wrap the
    > > declarations themselves in a macro that ensures
    > > the sizes are the same.
    > >
    > > Something like:
    > >
    > > declare_same_size_arrays(
    > > typeof array1[] = {...},
    > > typeof array2[] = {...}
    > > );
    > >
    > >
    >
    > Unless Im mis-reading you, this has a couple disadvantages:
    >
    > - bigger patches where its added.
    > granted these are mostly whitespace, but theyre less trivial to inspect.

    Not a problem in my view, it moves the
    related declarations closer together.

    > - not useful for array definitions which are not contiguous
    > granted thats a minority case, and the definitions could often be moved,
    > but not always.

    Discontiguous array definitions must be ugly.

    > Do you see advantages other than stylistic ones ?

    Not really.

    Contiguous declarations.
    No need for other markings.

    Seems useful enough.



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