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 works funny.
The problem with the following code is that the first if-statment does not happen ("hello") where as the second ("world") does. This I have interpreted as the calculation that is done inside the if-statment generates a negative number but the exact same calculation done with the result saved to a variables does not (even though the result is being saved to a signed variable).
The compiler used is gcc 4.4.
unsigned short u16_varHigh;
unsigned short u16_varLow;
unsigned short u16_Res1;
signed short s16_Res1;
u16_varHigh = 0xFFFF;
u16_varLow = 10;
u16_Res1 = u16_varLow - u16_varHigh; // response is 11 as expected
s16_Res1 = u16_varLow - u16_varHigh; // response is 11 as expected
// Does not enter
if( (u16_varLow - u16_varHigh) > (unsigned short)5 )
{
printf( "hello" );
}
// Does enter
if( (unsigned short)(u16_varLow - u16_varHigh) > 5 )
{
printf( "world" );
}
Can anyone explain this for me and perhaps come up with a solution for a fix so that the first if-statement works as well?
In the expression:
if( (u16_varLow - u16_varHigh) > (unsigned short)5 )
(u16_varLow - u16_varHigh) will be promoted to an int and evaluate to -65525. The fix for your problem is to cast to an unsigned type, like you do in the "Does enter"-code.
The reason s16_Res1 = u16_varLow - u16_varHigh; yields 11 is that the result of the subtraction, -65525, doesn't fit in a short.