unsigned becomes signed in if-statement comparisons?

Question

I have searched this site for an answer and found many responses to unsigned/signed comparison but this problem is that only unsigned parameters are compared but still it wo

Answer 1

In the other answers we have seen that

u16_varLow - u16_varHigh

for you (with 16 bit short and 32 bit int) is equivalent to

(int)u16_varLow - (int)u16_varHigh

and thus its result is the int value -65525. Thus the assignment

s16_Res1 = u16_varLow - u16_varHigh; 

is equivalent to

s16_Res1 = -65525;

which in your case of 16 bit short yields "undefined behavior". You are just unlucky that your compiler decides to assign 11, instead. (Unlucky because I think that it is better to fail early.)

In contrast to that

u16_Res1 = -65525; 

is a valid assignment since u16_Res1 is of an unsigned type and arithmetic of unsigned types is modulo the appropriate power of two.