Struct Wrapping 

pub struct Wrapping {
    pub signed: bool,
    pub pointer: bool,
    pub trap: bool,
}

What overflows rather than being undefined, from -fwrapv and its relatives.

C says a signed addition that overflows and a pointer that walks off the end of the object it points into are both undefined, and an optimizer that believes it reads a great deal into every loop: that a counter going up one at a time never turns round, that an index widened to an address may be widened before the arithmetic rather than after, that a bound is reached. These flags withdraw exactly that. They do not make the program mean something else, they make it mean less, and the code that asks for them is code that overflows on purpose and wants the answer the machine gives rather than the answer the standard declines to give.

Two of them because gcc has two, and a build that wants one usually wants the other. Signed arithmetic and pointer arithmetic are separate assumptions and a kernel turns both off.

-ftrapv is the third answer to the first question and is here for that reason. Undefined, wrapping and stopping are the three things a signed overflow can be, and a command line picks one of them: the last of -fwrapv and -ftrapv wins, which is gcc’s behaviour and what makes them one field rather than two that can both be set.

Fields

signed: bool

Whether signed arithmetic wraps, from -fwrapv.

pointer: bool

Whether pointer arithmetic wraps, from -fwrapv-pointer.

trap: bool

Whether a signed overflow stops the program instead, from -ftrapv.

Never set at the same time as Wrapping::signed, since a program cannot both wrap and stop, and the driver is what keeps that true by clearing each when the other is asked for.

Implementations

impl Wrapping

pub const ALL: Self

Both of them, which is what -fno-strict-overflow asks for.

gcc says so itself: its help text for -fstrict-overflow reads “negated as -fwrapv -fwrapv-pointer”, so the older flag is a name for the pair rather than a third knob. And asking for wrapping is asking for not stopping, so this is the whole answer and not two thirds of one.

pub const NONE: Self

Neither, which is the default and what a command line that says nothing about any of this gets.

Trait Implementations

impl Clone for Wrapping

fn clone(&self) -> Wrapping

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for Wrapping

impl Debug for Wrapping

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for Wrapping

fn default() -> Wrapping

Returns the “default value” for a type.

impl Eq for Wrapping

impl PartialEq for Wrapping

fn eq(&self, other: &Wrapping) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for Wrapping