Undefined Behaviour

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
dereference `null`	Yes
dereferencing memory not allocated	Possible*
dereferencing memory outside of its lifetime	Possible*
casting pointer to the incorrect array	Possible*
violating pre or post conditions	Yes
violating asserts	Yes
reaching `unreachable()` code	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
modifying a `const` variable	Optional	Partial modification / Error

Undefined Behaviour

List of undefined behaviours

List of implementation dependent behaviours

List of undefined behaviour in C, which is defined in C3

Signed Integer Overflow

Modifying the intermediate results of an expression