Executing the following code always gives -511 as a result, I performed many tests, and it seems that results are correct from 0 * 0 to 181 * 181, beyond that, results are abnormal, I tried many types for the z variable (int, long, float) without success, any idea? I'm using the Arduino Uno.
long z = 0;
void setup() {
Serial.begin(9600);
}
void loop() {
z = 255 * 255;
Serial.println(z);
delay(200);
}
No, this is not a bug. You are using a compile time constant expression which is signed int unless told otherwise. Therefore you can only represent numbers between -32768 and 32767. Your computation of 255 * 255 = 65025 exceeds the range. Thus you see an overflow. In the C/C++ standard the overflow of signed types is actually undefined behavior. That means that the compiler is allowed to do anything from showing the correct answer to halt and catch fire. Only unsigned types with a known size like uint16_t have a well-defined overflow behaviour. You should promote your right hand side computation to unsigned type like this:
z = 255U * 255U;
This way you tell the compiler your intention and don't land in "Undefined-Behaviour-Land".
int. But the latter is still true. Compile time constants are signed int, which is 16 bit. So the right side of the assignement is the problem.
wrap around doesn't sound right - but I know what you're saying
Commented
Feb 5, 2019 at 11:37
Assuming long is 4 bytes, it should work.
However, if somehow a long type is only 2 bytes, or (more likely) you used another type or there is some wrong define/typedef, and it is a signed long (default if you do not used the unsigned keyword), the value range is -32,768 to 32,767 and 255 * 255 = 65,025 which is out of range.
So you can first try to use unsigned long.
float z = 1 / 5result will be0because integer division results with integer that is later casted to float.