Re: [Tinycc-devel] Bug Report (offset+cast)

[kf]

On 5/11/07, Dave Dodge <address@hidden> wrote:
> On Fri, May 11, 2007 at 11:31:28AM -0400, David A. Wheeler wrote:
> > Gil Dabah:
> > > > It seems you can't cast a pointer from a specified offset:
> > > >
> > > > char* s = "\x34\x34\x12";
> > > > short word = *(short*)&s[1]; // <--buggy
> > > > printf("%x", word);
> > > >
> > > > will print 34 instead of 1234 (on LE machine, of course).
> >
> > Confirmed, it's a bug in tcc.
> >
> > kf:
> > > Happens here too. However,
> > >
> > > short word = *(short*)(s+1);
> > > works as expected.
> >
> > Confirmed, that one works correctly.
>
> Well, the term "correctly" is a bit suspect.  You can't convert an
> arbitrary char* to a short* and dereference it without risking
> undefined behavior, because among other things there's no guarantee
> that the pointed-to location will be aligned properly as a short.
>
> That code on a Sparc (with any compiler) will almost certainly core
> dump with a bus error.  On IA64 (with gcc) it traps and emits an
> "unaligned access..." message before continuing.  x86 only lets you
> get away with this sort of thing because of its loose alignment
> behavior; other real-world systems are much less forgiving.

This is true, of course. I sometimes use something like:

#define CAST(x) ({short __foo; memcpy(&__foo, &x, sizeof (short)); __foo; })

and then:

short word = CAST(s+1);

to be portable (no bus errors on Sparc), and efficient, as this
translates to a single mov instruction on x86, i.e. something like
movw 0x1234,(word) with gcc -O3. But tcc probably produces bad code
for this, emitting a call to memcpy...

kf

>                                                   -Dave Dodge
>
>
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