🚀 go-pugleaf

rgp@seiko.mpd.tandem.com (Ramon Pantin) writes:

>Somewhere along the evolution of C this type of expressions where made illegal:
>	char	*p = ...;
>	p &= ~1;

>the workaround:
>	p = (char *) ((int)p & ~1);

>makes my code:
>	- harder to write
>	- less portable because it assumes that there is an integral type
>	  (if not "int", "long") that has enough precision (bits) so that
>	  the casts will be harmless.

You could use 'unsigned long' rather than 'int' to improve the chances of
the code working.

The workaround is not less portable, since bitwise operations on pointers
are not portable at all.  Any such messing with pointers has to be
confined to machine-specific modules to have any hope of porting your
program.  If it makes sense to bit-twiddle a pointer, nothing prevents
a compiler from providing an extension to do so -- the Standard only
requires the compiler to have a way to diagnose such code as an error.

The reason the C Standard makes such twiddling illegal is that there is
no way to specify semantics for it (in the context of a programming-
language Standard).  It doesn't make any sense to me to say that a
construct is completely legal but has no defined semantics.

Those things in the C Standard which have undefined behavior are
those which are in general infeasible to detect at compile time.
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com
Vice Chair, ANSI C++ Committee, X3J16