Struct std::time::Instant

pub struct Instant(_);

A measurement of a monotonically nondecreasing clock. Opaque and useful only with Duration.

Instants are always guaranteed, barring platform bugs, to be no less than any previously measured instant when created, and are often useful for tasks such as measuring benchmarks or timing how long an operation takes.

Note, however, that instants are not guaranteed to be steady. In other words, each tick of the underlying clock might not be the same length (e.g. some seconds may be longer than others). An instant may jump forwards or experience time dilation (slow down or speed up), but it will never go backwards.

Instants are opaque types that can only be compared to one another. There is no method to get “the number of seconds” from an instant. Instead, it only allows measuring the duration between two instants (or comparing two instants).

The size of an Instant struct may vary depending on the target operating system.

Example:

use std::time::{Duration, Instant};
use std::thread::sleep;

fn main() {
   let now = Instant::now();

   // we sleep for 2 seconds
   sleep(Duration::new(2, 0));
   // it prints '2'
   println!("{}", now.elapsed().as_secs());
}

OS-specific behaviors

An Instant is a wrapper around system-specific types and it may behave differently depending on the underlying operating system. For example, the following snippet is fine on Linux but panics on macOS:

use std::time::{Instant, Duration};

let now = Instant::now();
let max_nanoseconds = u64::MAX / 1_000_000_000;
let duration = Duration::new(max_nanoseconds, 0);
println!("{:?}", now + duration);

Underlying System calls

The following system calls are currently being used by now() to find out the current time:

Platform	System call
SGX	insecure_time usercall. More information on timekeeping in SGX
UNIX	clock_gettime (Monotonic Clock)
Darwin	mach_absolute_time
VXWorks	clock_gettime (Monotonic Clock)
SOLID	get_tim
WASI	__wasi_clock_time_get (Monotonic Clock)
Windows	QueryPerformanceCounter

Disclaimer: These system calls might change over time.

Note: mathematical operations like add may panic if the underlying structure cannot represent the new point in time.

Monotonicity

On all platforms Instant will try to use an OS API that guarantees monotonic behavior if available, which is the case for all tier 1 platforms. In practice such guarantees are – under rare circumstances – broken by hardware, virtualization or operating system bugs. To work around these bugs and platforms not offering monotonic clocks duration_since, elapsed and sub saturate to zero. In older Rust versions this lead to a panic instead. checked_duration_since can be used to detect and handle situations where monotonicity is violated, or Instants are subtracted in the wrong order.

This workaround obscures programming errors where earlier and later instants are accidentally swapped. For this reason future rust versions may reintroduce panics.

Implementations

impl Instant

pub fn now() -> Instant

Returns an instant corresponding to “now”.

Examples

use std::time::Instant;

let now = Instant::now();

pub fn duration_since(&self, earlier: Instant) -> Duration

Returns the amount of time elapsed from another instant to this one, or zero duration if that instant is later than this one.

Panics

Previous rust versions panicked when earlier was later than self. Currently this method saturates. Future versions may reintroduce the panic in some circumstances. See Monotonicity.

Examples

use std::time::{Duration, Instant};
use std::thread::sleep;

let now = Instant::now();
sleep(Duration::new(1, 0));
let new_now = Instant::now();
println!("{:?}", new_now.duration_since(now));
println!("{:?}", now.duration_since(new_now)); // 0ns

pub fn checked_duration_since(&self, earlier: Instant) -> Option<Duration>

Returns the amount of time elapsed from another instant to this one, or None if that instant is later than this one.

Due to monotonicity bugs, even under correct logical ordering of the passed Instants, this method can return None.

Examples

use std::time::{Duration, Instant};
use std::thread::sleep;

let now = Instant::now();
sleep(Duration::new(1, 0));
let new_now = Instant::now();
println!("{:?}", new_now.checked_duration_since(now));
println!("{:?}", now.checked_duration_since(new_now)); // None

pub fn saturating_duration_since(&self, earlier: Instant) -> Duration

Returns the amount of time elapsed from another instant to this one, or zero duration if that instant is later than this one.

Examples

use std::time::{Duration, Instant};
use std::thread::sleep;

let now = Instant::now();
sleep(Duration::new(1, 0));
let new_now = Instant::now();
println!("{:?}", new_now.saturating_duration_since(now));
println!("{:?}", now.saturating_duration_since(new_now)); // 0ns

pub fn elapsed(&self) -> Duration

Returns the amount of time elapsed since this instant was created.

Panics

Previous rust versions panicked when self was earlier than the current time. Currently this method returns a Duration of zero in that case. Future versions may reintroduce the panic. See Monotonicity.

Examples

use std::thread::sleep;
use std::time::{Duration, Instant};

let instant = Instant::now();
let three_secs = Duration::from_secs(3);
sleep(three_secs);
assert!(instant.elapsed() >= three_secs);

pub fn checked_add(&self, duration: Duration) -> Option<Instant>

Returns Some(t) where t is the time self + duration if t can be represented as Instant (which means it’s inside the bounds of the underlying data structure), None otherwise.

pub fn checked_sub(&self, duration: Duration) -> Option<Instant>

Returns Some(t) where t is the time self - duration if t can be represented as Instant (which means it’s inside the bounds of the underlying data structure), None otherwise.

Trait Implementations

impl Add<Duration> for Instant

fn add(self, other: Duration) -> Instant

Panics

This function may panic if the resulting point in time cannot be represented by the underlying data structure. See Instant::checked_add for a version without panic.

type Output = Instant

The resulting type after applying the + operator.

impl AddAssign<Duration> for Instant

fn add_assign(&mut self, other: Duration)

Performs the += operation.

impl Clone for Instant

fn clone(&self) -> Instant

Returns a copy of the value.

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

Performs copy-assignment from source.

impl Debug for Instant

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

Formats the value using the given formatter.

impl Hash for Instant

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H) where
    H: Hasher, 

Feeds a slice of this type into the given Hasher.

impl Ord for Instant

fn cmp(&self, other: &Instant) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

Restrict a value to a certain interval.

impl PartialEq<Instant> for Instant

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

This method tests for self and other values to be equal, and is used by ==.

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

This method tests for !=.

impl PartialOrd<Instant> for Instant

fn partial_cmp(&self, other: &Instant) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

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

This method tests less than (for self and other) and is used by the < operator.

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

This method tests less than or equal to (for self and other) and is used by the <= operator.

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

This method tests greater than (for self and other) and is used by the > operator.

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

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl Sub<Duration> for Instant

type Output = Instant

The resulting type after applying the - operator.

fn sub(self, other: Duration) -> Instant

Performs the - operation.

impl Sub<Instant> for Instant

fn sub(self, other: Instant) -> Duration

Returns the amount of time elapsed from another instant to this one, or zero duration if that instant is later than this one.

Panics

Previous rust versions panicked when other was later than self. Currently this method saturates. Future versions may reintroduce the panic in some circumstances. See Monotonicity.

type Output = Duration

The resulting type after applying the - operator.

impl SubAssign<Duration> for Instant

fn sub_assign(&mut self, other: Duration)

Performs the -= operation.

impl Copy for Instant

impl Eq for Instant

impl StructuralEq for Instant

impl StructuralPartialEq for Instant