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Message-ID: <20140428101156.GF12324@port70.net> Date: Mon, 28 Apr 2014 12:11:56 +0200 From: Szabolcs Nagy <nsz@...t70.net> To: musl@...ts.openwall.com Cc: Rich Felker <dalias@...c.org> Subject: Re: [PATCH 3/3] stddef: Define max_align_t * Khem Raj <raj.khem@...il.com> [2014-04-27 22:51:34 -0700]: > On Sun, Apr 27, 2014 at 7:03 PM, Rich Felker <dalias@...c.org> wrote: > >> +typedef struct { > >> + long long __max_align_ll __attribute__((__aligned__(__alignof__(long long)))); > >> + long double __max_align_ld __attribute__((__aligned__(__alignof__(long double)))); > >> +} max_align_t; > >> + > > > > As far as I can tell, there's no reason to use the attribute here. > > What's it there for? Also a union would probably be nicer than a > > struct, but perhaps it doesn't matter. > > union does not return correct alignment where as struct did. I just > tried to match > what clang also has > > http://reviews.llvm.org/rL201729 > nice.. i think i386 abi is non-conforming to the c11 alignment requirements now: long long has 8 byte alignment, but in a struct/union it has only 4 (this is why the attrs are needed above) long long x; // _Alignof(x) == 8 struct {long long x;} y; // _Alignof(y.x) == 4 i think the standard requires that all (addressable) long long objects should have the same alignment (or stricter) than _Alignof(x) max_align_t is defined to be the "greatest alignment supported in all contexts", i don't know why it is not just typedef char max_align_t __attribute__((aligned(__BIGGEST_ALIGNMENT__))); which gives 16 byte alignment on i386 gcc, i thought it was supported in all contexts if gcc and clang went with the same definition we should follow, but this makes the type less meaningful
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