pub struct SystemInstant(/* private fields */);
std
only.Expand description
std
A measurement of a monotonically nondecreasing clock.
Re-exported from std
::time::
Instant
→SystemInstant
.
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. As part of this non-guarantee it is also not specified whether system suspends count as elapsed time or not. The behavior varies across platforms and Rust versions.
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_seconds = u64::MAX / 1_000_000_000;
let duration = Duration::new(max_seconds, 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 | clock_gettime (Monotonic Clock) |
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 Instant
s 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§
Source§impl Instant
impl Instant
1.8.0 · Sourcepub fn now() -> Instant
pub fn now() -> Instant
Returns an instant corresponding to “now”.
§Examples
use std::time::Instant;
let now = Instant::now();
1.8.0 · Sourcepub fn duration_since(&self, earlier: Instant) -> Duration
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
1.39.0 · Sourcepub fn checked_duration_since(&self, earlier: Instant) -> Option<Duration> ⓘ
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 Instant
s,
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
1.39.0 · Sourcepub fn saturating_duration_since(&self, earlier: Instant) -> Duration
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
1.8.0 · Sourcepub fn elapsed(&self) -> Duration
pub fn elapsed(&self) -> Duration
Returns the amount of time elapsed since this instant.
§Panics
Previous Rust versions panicked when the current time was earlier than self. 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);
Trait Implementations§
1.9.0 · Source§impl AddAssign<Duration> for Instant
impl AddAssign<Duration> for Instant
Source§fn add_assign(&mut self, other: Duration)
fn add_assign(&mut self, other: Duration)
+=
operation. Read moreSource§impl BitSized<128> for SystemInstant
impl BitSized<128> for SystemInstant
Source§const BIT_SIZE: usize = _
const BIT_SIZE: usize = _
Source§const MIN_BYTE_SIZE: usize = _
const MIN_BYTE_SIZE: usize = _
1.8.0 · Source§impl Ord for Instant
impl Ord for Instant
1.8.0 · Source§impl PartialOrd for Instant
impl PartialOrd for Instant
1.8.0 · Source§impl Sub for Instant
impl Sub for Instant
Source§fn sub(self, other: Instant) -> Duration
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.
1.9.0 · Source§impl SubAssign<Duration> for Instant
impl SubAssign<Duration> for Instant
Source§fn sub_assign(&mut self, other: Duration)
fn sub_assign(&mut self, other: Duration)
-=
operation. Read moreimpl Copy for Instant
impl Eq for Instant
impl StructuralPartialEq for Instant
Auto Trait Implementations§
impl Freeze for Instant
impl RefUnwindSafe for Instant
impl Send for Instant
impl Sync for Instant
impl Unpin for Instant
impl UnwindSafe for Instant
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