clock - Kernel object used to track the progress of time.


A clock is a one dimensional affine transformation of the clock monotonic reference timeline, which may be atomically adjusted by a clock maintainer, and observed by clients.



The properties of a clock are established when the clock is created and cannot be changed afterwards. Currently, three clock properties are defined.


When set, the clock is guaranteed to have monotonic behavior. This is to say that any sequence of observations of the clock is guaranteed to produce a sequence of times that are always greater than or equal to the previous observations. A monotonic clock can never go backwards, although it can jump forwards. Formally:

Given a clock C, Let C(x) be the function that maps from the reference timeline C's timeline. C(x) is a piecewise linear function made up of all the affine transformation segments over all time as determined by C's maintainer. C is monotonic if and only if:

for all R1, R2 : R2 >= R1

C(R2) >= C(R1)


When set, the clock is guaranteed to have continuous behavior. This is to say that any update to the clock transformation is guaranteed to be first order continuous with the previous transformation segment. Formally:

Let Ci(x) be the ith affine transformation segment of C(x). Let Ri be the first point in time on the reference timeline for which Ci(x) is defined. A clock C is continuous if and only if: for all i

Ci(Ri + 1) = Ci + 1(Ri + 1)

Backstop Time

The backstop time of a clock represents the minimum value that a clock may ever be set to. Since clocks can only tick forwards, and never backwards, it is impossible for an observer of a clock to ever receive a value that is less than the backstop time configured by a clock's creator.

A backstop time may be provided via the zx_create_args_v1_t structure at creation time. Otherwise, it will default to 0.

During clock update operations, any attempt to set the clock's value to something less than the backstop time will fail with ZX_ERR_INVALID_ARGS. A clock that has not been initially set will always report the backstop time configured for the clock. Backtop times may never be less than the default value of zero.

Implied properties

  • The reference clock for all clock objects in the system is clock monotonic.
  • The nominal units of all clock objects are specified to be nanoseconds. This property is not configurable.
  • The units of frequency adjustment for all clock objects are specified to be parts per million, or PPM.
  • The maximum permissible range of frequency adjustment of a clock object is specified to be [-1000, +1000] PPM. This property is not configurable.

Additional creation options


When you use this option during clock creation, the clock begins in a started state instead of the default non-started state. See Starting a clock for details.

Reading the clock

Given a clock handle, users may query the current time given by that clock using the zx_clock_read() syscall. Clock reads ZX_RIGHT_READ permissions. Clock reads are guaranteed to be coherent for all observers. This is to say that, if two observers query the clock at exactly the same reference time R, that they will always see the same value C(R).

Reference timelines, zx_ticks_get(), and zx_clock_get_monotonic()

As noted earlier, zx_clock_get_monotonic() is the reference timeline for all user-created zircon clocks. This means that if a user knows a clock instance's current transformation, then given a value on the clock instance's timeline, the corresponding point on the clock monotonic timeline may be computed (and vice-versa). It also means that in the absence of a rate adjustment made to the kernel clock, clock monotonic and the kernel clock will tick at exactly the same rate.

In addition to the clock monotonic timeline, the zircon kernel also exposes the "ticks" timeline via zx_ticks_get() and zx_ticks_per_second(). Internally, ticks is actually the reference timeline for clock monotonic and is read directly from an architecture appropriate timer unit accessible to the kernel. Clock monotonic is actually a linear transformation of the ticks timeline normalized to nanosecond units. Both timelines start ticking from zero as the kernel starts.

Because clock monotonic is a static transformation based on ticks, and all kernel clocks are transformations based on clock monotonic, ticks may also serve as a reference clock for kernel clocks in addition to clock monotonic.

Fetching the clock's details

In addition to simply reading the current value of the clock, advanced users who possess ZX_RIGHT_READ permissions may also read the clock and get extended details in the process using zx_clock_get_details(). Upon a successful call, the details structure returned to callers will include:

  • The current clock monotonic to clock transformation.
  • The current ticks to clock transformation.
  • The current symmetric error bound estimate (if any) for the clock.
  • The last time the clock was updated as defined by the clock monotonic reference timeline.
  • An observation of the system tick counter, which was taken during the observation of the clock.
  • All of the static properties of the clock defined at creation time.
  • A generation nonce.

Advanced users may use these details to not only compute a recent now value for the clock (by transforming the reported ticks-now observation using the ticks-to-clock transformation, both reported by the get details operation), but to also:

  • Know whether the clock transformation has been changed since the last zx_clock_get_details() operation (using the generation nonce).
  • Compose the clock transformation with other clocks' transformations to reason about the relationship between two clocks.
  • Know the clock maintainer's best estimate of error bound.
  • Reason about the range of possible future values of the clock relative to the reference clock based on the last correction time, the current transformation, and the maximum permissible correction factor for the clock (see the maximum permissive range of frequency adjustment described in the |Implied properties| section above.

Starting a clock and clock signals

Immediately after creation, a clock has not yet been started. All attempts to read the clock will return the clock's configured backstop time, which defaults to 0 if unspecified during creation.

A clock begins running after the very first update operation performed by a clock's maintainer, which must include a set-value operation. The clock will begin running at that point with a rate equal to the reference clock plus the deviation from nominal specified by the maintainer.

Clocks also have a ZX_CLOCK_STARTED signal, which may be used by users to know when the clock has actually been started. Initially, this signal is not set, but it becomes set after the first successful update operation. Once started, a clock will never stop and the ZX_CLOCK_STARTED signal will always be asserted.

Initially, the clock is a clone of clock monotonic, which makes the transformation between the clock monotonic timeline and synthetic timeline the identity function. This clock may still be maintained after creation, subject to the limitations imposed by rights, the ZX_CLOCK_OPT_MONOTONIC and ZX_CLOCK_OPT_CONTINUOUS properties, and the configured backstop time.

If a clock is created with the ZX_CLOCK_OPT_AUTO_START options, it cannot be configured with a backstop time that is greater than the current clock monotonic time. If this was allowed, this would result in a state where a clock's current time is set to a time before its backstop time.

Maintaining a clock

Users who possess ZX_RIGHT_WRITE permissions for a clock object may act as a maintainer of the clock using the zx_clock_update() syscall. Three parameters of the clock may be adjusted during each call to zx_clock_update(), but not all three need to be adjusted each time. These values are:

  • The clock's absolute value.
  • The frequency adjustment of the clock (deviation from nominal expressed in ppm)
  • The absolute error bound estimate of the clock (expressed in nanoseconds)

Changes to a clocks transformation occur during the syscall itself. The specific reference time of the adjustment may not be specified by the user.

Any change to the absolute value of a clock with the ZX_CLOCK_OPT_MONOTONIC property set on it which would result in non-monotonic behavior will fail with a return code of ZX_ERR_INVALID_ARGS.

The first update operation is what starts a clock ticking and must include a set-value operation.

Aside from the very first set-value operation, all attempts to set the absolute value of a clock with the ZX_CLOCK_OPT_CONTINUOUS property set on it will fail with a return code of ZX_ERR_INVALID_ARGS

Notes on the clock error bound estimate

The zx_clock_get_details() syscall provides the user with a number of fine grained details about a clock, including the "error bound estimate". This value, expressed in nanoseconds, represents the clock maintainer's best current estimate of how wrong the clock currently might be relative to the reference(s) being used by the maintainer. For example, if a user fetched a time X with an error bound estimate of E, then the clock maintainer is attempting to say that it believes that the clock's actual value is somewhere in the range [ X-E, X+E ].

The level of confidence in this estimate is not specified by the kernel APIs. It is possible that some clock maintainers are using a strict bound, while others are using a bound that is not provable but provides "high confidence", while others still might have little to no confidence in their estimates.

In the case that a user needs to understand the objective quality of the error estimates they are accessing (for example, to enforce certificate validity dates, or DRM license expiration), they should understand which component in the system is maintaining their clock, and what guarantees that maintainer provides when it comes to the confidence levels of its published error bound estimates.