On 31/07/14 19:00, Ricardo Telichevesky wrote:
> Hi, hope this is the right list.
>
> Here is my code and output, at the bottom of the e-mail. y is "correct",
> w and z obviously have problems - multiplying two 32-bit integers
> "hoping" the result would be correct assigning to 64-bit - I guess it is
> the same problem as double oneThird= 1/3; the result being zero, and
> not 0.3333.
>
> I was wondering if there is any strict warning that would flag the w and
> z assignments below, or the 1/3 above - the whole right hand side is
> evaluated as a 32-bit integer number, and assigned to a 64-bit integer
> or double. Not advocating this should be a default, but turning it on
> would help me detect some flaws in the code. Took me hours to catch a
> similar bug in my code, trying to solve a sparse system that has
> hundreds of millions of variables...
>
> Thanks!
> Ricardo
>
> laplace utils % cat ovr.c
> #include <stdio.h>
> int main()
> {
>
> unsigned int x = 1015625426;
> unsigned int t = sizeof(double);
>
> size_t y = x * sizeof(double);
> size_t w = x << 3;
> size_t z = x * t;
>
> printf("y= %zd w = %zd z = %zd\n", y, w, z);
> }
> laplace utils % gcc -Wall -o ovr ovr.c
> laplace utils % ovr
> y= 8125003408 w = 3830036112 z = 3830036112
>
>
Hi,
As others have said, it's not easy to warn about this sort of thing
since it is perfectly valid C - and many programs rely on the overflow
behaviour of unsigned integers.
But as a stylistic point, you should probably avoid using types like
"unsigned int" and "size_t" when you are concerned about integer sizes -
it is far safer, clearer, and more portable to use the size-specific
types in <stdint.h> such as "uint32_t" and "uint64_t". Of course, you
might want to use typedefs to make things even clearer, or to allow you
to easily change the sizes at a later date. But start from the
<stdint.h> types.
Another point is to remember to enable optimisation. It won't help in
this case, but some warnings work better when optimisation (at least
-O1) is enabled. And of course your code will run far faster.
David
