Re: Compiler warnings, overflow

[David Brown <david@westcontrol.com>]

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