Created attachment 28835 [details] [review]
The patch for XSyncValueSubtract

Subtracting the same XSyncValue from XSyncValue, XSyncValueSubtract does not return 0.

XSyncIntToValue(&a, 100); // a.hi = 0, a.lo = 100

XSyncValueSubtract(&result, &a, &a, overflow); 
printf("a.hi = %d, a.lo = %d, overflow = %d", a.hi, a.lo, overflow); 

// a.hi = -1, a.lo = 0 overflow is set to true


In the _XSyncValueSubtract MACRO 

if (t>(presult)->lo) (presult)->hi--;\ 

must be replaced with 

if (t<(presult)->lo) (presult)->hi--;\

Sign '>' with '<'


Subtracting different values also work not in a correct form.
P.S. Copy Paste sometimes work against us.