[Swan-dev] tricky arithmetic

[Antony Antony]

On Wed, Jan 28, 2015 at 06:21:58PM -0500, D. Hugh Redelmeier wrote:
> tv_diff = (tv2.tv_sec - tv1.tv_sec) * 1000000 + (tv2.tv_usec - tv1.tv_usec);
> tv_diff = tv_diff != 0.0 ? tv_diff/1000 : 0;
> 
> I assumed that .tv_usec was an unsigned type.  So
> 	tv2.tv_usec - tv1.tv_usec
> would wrap around in a binary fashion if
> 	tv2.tv_usec < tv1.tv_usec
> This would produce the wrong result.
> 
> It turns out that .tv_usec is defined by POSIX to be of type
> suseconds_t and
> 	"The type suseconds_t shall be a signed integer type capable
> 	of storing values at least in the range [-1, 1000000]."
> This is a wierd type for C.  Any normal C signed type capable of
> representing 1000000 is also capable of representing -1000000.

In C tv_usec is signed int, then while printing why not cast the difference to int?

-               DBG_log("elapsed time in %s for hostname lookup %lu.%06lu",
+               DBG_log("elapsed time in %s for hostname lookup %lu.%06d",
                        __func__,
                        (unsigned long)(tv2.tv_sec - borrow - tv2.tv_sec),
-                       (unsigned long)(tv2.tv_usec + borrow * 1000000 - tv2.tv_
+                       (int)(tv2.tv_usec + borrow * 1000000 - tv2.tv_usec));  

Or should it %ld? to be safe. Some compilers may use long int for __suseconds_t?

Without cast there is a warning. I wonder why derived type should be casted:) I found this annoying side effects to too much typdef.  It is also easy to make mistake such as this you cast to wrong type. Or we we need function as we do with deltaseconds.

 warning: format ‘%d’ expects argument of type ‘int’, but argument 4 has type ‘__suseconds_t’ [-Wformat=] 

float warning to Hugh:) In this case using a double could make the code easier to read! 

-antony