[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [MULTIBOOT2 DOC PATCH 08/10] multiboot2: Add C structure alignment and padding consideration section
On 09/06/2016 21:30, Daniel Kiper wrote: > Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx> > --- > doc/multiboot.texi | 17 +++++++++++++++++ > 1 file changed, 17 insertions(+) > > diff --git a/doc/multiboot.texi b/doc/multiboot.texi > index c81b2ea..bf02a1b 100644 > --- a/doc/multiboot.texi > +++ b/doc/multiboot.texi > @@ -1384,6 +1384,7 @@ document, but are included for prospective operating > system and boot > loader writers. > > @menu > +* C structure alignment and padding consideration:: > * Notes on PC:: > * BIOS device mapping techniques:: > * Example OS code:: > @@ -1391,6 +1392,22 @@ loader writers. > @end menu > > > +@node C structure alignment and padding consideration > +@section C structure alignment and padding consideration > + > +Many C compilers try to optimize memory accesses aligning structure "by aligning" > +members properly. Usually they reach the goal by adding some padding. What does "properly" mean here? The default padding will be specified by the default ABI the compiler conforms to. > +This is very useful thing in general. However, if you try to mix assembler > +with C or use C to implement structure low level access this behavior > +may lead, at least, to quite surprising results. Hence, compiler should > +be instructed to not optimize such accesses. Usually it is done by special > +attribute added to structure definition, e.g. GCC compatible sources use > +@samp{__attribute__ ((__packed__))} for this purpose. However, this is not > +required if it is known that its members are properly aligned and compiler > +does not do any optimization. Very good example of this is shown below in > +@file{multiboot2.h} file. I am not sure what you are trying to say. ~Andrew _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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