vonbrand@sleipnir.valparaiso.cl (Horst von Brand) wrote on 24.06.00 in <200006250221.e5P2LPG04091@sleipnir.valparaiso.cl>:
> Urban Widmark <urban@svenskatest.se> said:
> > On Sat, 24 Jun 2000, Tigran Aivazian wrote:
> > > K&R says (page 213, A8.3):
> > >
> > > Adjacent field members of structures are packed into
> > > implementation-dependent storage units in an implementation-dependent
> > > direction. ... The members of a structure have addresses increasing
> > > in the order of their declaration.
> > >
> > > I think Alan Cox is right - there is no guarantee that field members of
> > > a structure can be found one after another starting from the first
> > > field.
>
> AFAIU, the "implementation dependent direction" here is big vs
> little enedian, at most. See that that _addresses_ are guaranteed to be
> increasing. Modulo padding (which in the example below isn't applicable
> anywhere I know), they are the same anywhere.
Bull. Language like that would *never* be used for endianness. And
structure layout isn't something endianness has ever come into anyway.
> > So all drivers (I'm sure there are a few) that use something like
> >
> > struct foo {
> > u32 a;
> > u32 b;
> > u32 c;
> > u32 d;
> > }
> >
> > to communicate with some hardware (4 32-bit values with addresses in
> > sequence) should be fixed not to make assumptions about the layout of a
> > struct?
>
> This would break one of the important uses of C (hardware fiddling) big
> way.
Well, there is no portable support for hardware fiddling in the C
standard, and there never has been any. And lots of hardware (like
anything on x86 I/O ports) can't be accessed from C anyway.
MfG Kai
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