[Python-Dev] Memory size overflows (original) (raw)

Gerald S. Williams gsw@agere.com
Wed, 16 Oct 2002 11:12:05 -0400

Guido van Rossum wrote:

I.e. a macro callable as SAFEMULTIPLY(destination, src1, src2, onoverflow); meaning roughly destination = src1 * src2; if () onoverflow;

There are also additions...These are easier to test: if you add a small positive constant to a sizet, and the result is smaller than the original sizet, 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