>>109684549
Undefined behavior means it's undefined by the C standard, however in practice some UBs are defined by compilers, but this is compiler specific.
>>109685165
Not true
Some UBs will always be undefined, even at runtime, like reading an uninitialized variable.
consider for example:
int main() {
int a; //uninitialized value
return a;
}
When compiled and then run multiple times, this program will give different exit values, there is no reliable way of predicting its return value and hence its behavior is undefined even at runtime
