Like C, C3 uses undefined behaviour. In contrast, C3 will trap - that is, print an error trace and abort – on undefined behaviour in debug builds. This is similar to using C with a UB sanitizer. It is only during release builds that actual undefined behaviour occurs.
In C3, undefined behaviour means that the compiler is free to interpret undefined behaviour as if behaviour cannot occur.
In the example below:
uint x = foo(); uint z = 255 / x; return x != 0;
The case of
x == 0 would invoke undefined behaviour for
255/x. For that reason,
the compiler may assume that
x != 0 and compile it into the following code:
foo(); return true;
As a contrast, the safe build will compile code equivalent to the following.
uint x = foo(); if (x == 0) trap("Division by zero") return true;
List of undefined behaviours
The following operations cause undefined behaviour in release builds of C3:
|operation||will trap in safe builds|
|int / 0||Yes|
|int % 0||Yes|
|reading explicitly uninitialized memory||Possible*|
|array index out of bounds||Yes|
|dereferencing memory not allocated||Possible*|
|dereferencing memory outside of its lifetime||Possible*|
|casting pointer to the incorrect array or vararray||Possible*|
|violating pre or post conditions||Yes|
* "Possible" indicates trapping is implementation dependent.
List of implementation dependent behaviours
Some behaviour is allowed to differ between implementations and platforms.
|operation||will trap in safe builds||permitted behaviour|
|comparing pointers of different provenance||Optional||Any result|
|subtracting pointers of different provenance||Optional||Any result|
|shifting by more or equal to the bit width||Yes||Any result|
|shifting by negative amount||Yes||Any result|
|conversion floating point <-> integer type is out of range||Optional||Any result|
|conversion between pointer types produces one with incorrect alignment||Optional||Any result / Error|
|calling a function through a function pointer that does not match the function||Optional||Any result / Error|
|attempt to modify a string literal||Optional||Partial modification / Error|
||Optional||Partial modification / Error|
List of undefined behaviour in C, which is defined in C3
Signed Integer Overflow
Signed integer is always wrapped using 2s complement.
Modifying the intermediate results of an expression
Behaves as if the intermediate result was stored in a variable on the stack.