Guido van Rossum wrote:
> I.e. a macro callable as
> SAFE_MULTIPLY(destination, src1, src2, on_overflow);
> meaning roughly
> destination = src1 * src2;
> if (<overflow detected>)
> on_overflow;
> There are also additions...These are easier to test: if you add a small
> positive constant to a size_t, and the result is smaller than the
> original size_t, an overflow occurred.
Why not use the same trick for multiplication? For src1,src2 > 0,
dest should always be >= MAX(src1,src2). SAFE_MULTIPLY could be
implemented something like this:
#define HALF_BITS (sizeof(size_t) * 4U)
#define LO_MASK ((((size_t)1) << (HALF_BITS))-1)
#define HI_MASK (~(LO_MASK))
#define MAX(a,b) (((a) >= (b)) ? (a) : (b))
#define SAFE_MULTIPLY(dest,src1,src2,on_error) \
{ \
size_t _x = src1; \
size_t _y = src2; \
size_t _dest = _x * _y; \
\
if (_x && _y && ((_x|_y) & HI_MASK)) \
{ \
if (_dest < MAX(_x,_y)) \
{ \
on_error; \
} \
} \
\
dest = _dest; \
}
-Jerry Williams
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