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Make adjustments to a clock object.


#include <zircon/syscalls.h>

zx_status_t zx_clock_update(zx_handle_t handle,
                            uint64_t options,
                            const void* args);


handle must be of type ZX_OBJ_TYPE_CLOCK and have ZX_RIGHT_WRITE.


Three different parameters may be dynamically controlled by a clock maintainer. They are

  • The clock's current value.
  • The clock's rate adjustment, expressed in PPM deviation from nominal.
  • The clock's current estimated error bounds.

When a clock maintainer wishes to change one or more of these parameters, they may do so using the zx_clock_update syscall. Updating a clock's parameters is an atomic operation from the perspective of all other users in the system.

The first update operation performed by a clock maintainer must include a valid value. This update is the update that starts the clock and defines its initial value. Before this update operation has succeeded, the ZX_CLOCK_STARTED signal will be de-asserted, and afterwards it will be asserted and remain so for the lifetime of the clock.

In order to update a clock, a user fills out the fields of a zx_clock_update_args_v2_t structure that they wish to adjust, then passes the structure to the update call, setting the bits in options which indicate both the explicit version of the structure (version 2), and which of these fields are valid and should be set. Defined options bits are


The version of the structure is passed using the ZX_CLOCK_ARGS_VERSION(...) macro, specifically ZX_CLOCK_ARGS_VERSION(2) for the version 2 structure.

For example

#include <zircon/syscalls.h>
#include <zircon/syscalls/clock.h>

void MaintainMyClock(zx_handle_t the_clock) {
  zx_clock_update_args_v2_t args;
  zx_status_t status;

  // Set the clock's value to 1500. Note that this also starts the clock.
  args.synthetic_value = 1500;
  status = zx_clock_update(the_clock,
  if (status != ZX_OK) {
    // Panic!

  // Make the clock run 23 PPM slower than nominal relative to clock monotonic.
  args.rate_adjust = -23;
  status = zx_clock_update(the_clock,
  if (status != ZX_OK) {
    // Halt and catch fire

  // Set the clock to 100,000, make it run 50 PPM faster than nominal, and specify an error bound of
  // +/- 400mSec, all at the same time.
  const uint64_t options = ZX_CLOCK_ARGS_VERSION(2) |
                           ZX_CLOCK_UPDATE_OPTION_RATE_ADJUST_VALID |
  args.synthetic_value = 100000;
  args.rate_adjust = 50;
  args.error_bound = ZX_MSEC(400);
  status = zx_clock_update(the_clock, options, &args);
  if (status != ZX_OK) {
    // Burn down, fall over, and then sink into the swamp.

Explicitly provided reference times.

With the addition of the V2 update structure, it is now possible (with some limitations) to explicitly control the reference time used for a clock update operation. Note that, upon success, the actual new reference <-> synthetic transformation specified by the user's update arguments will replace the old transformation during the call to zx_clock_update. Supplying an explicit reference time does not affect when the actual transformation is updated, it will always take effect during the call to zx_clock_update.

Diagrams provided in RFC-0077 may help to understand the effects of the operations described below.

Synthetic value updates with an explicitly provided reference time.

When users update the synthetic value (S) of a clock with an explicitly provided reference time (R), they are specifying a point ([R, S]) through which the new transformation will pass. In other words, the new transformation explicitly specifies that "when it is time R on the reference timeline, it will be time S on the synthetic timeline".

#include <zircon/syscalls.h>
#include <zircon/syscalls/clock.h>

// Set the syntheic value of the clock to be "synth" at the explicitly provided
// reference time "ref". In other words, upon success, this update operation will
// cause the clock's transformation from reference to synthetic time to
// specifically pass through the point (ref, synth)
zx_status_t SetSynthAtRef(zx_handle_t the_clock, zx_time_t ref, zx_time_t synth) {
  zx_clock_update_args_v2_t args;

  uint64_t options = ZX_CLOCK_ARGS_VERSION(2) |

  // Note that these options are equivalent, they just use a shorthand to
  // specify that both values are valid.

  args.reference_value = ref;
  args.synthetic_value = synth;
  return zx_clock_update(the_clock, options, args);

Rate adjustment updates with an explicitly provided reference time.

Let T(R) be the function which transforms a reference time R to a synthetic time for a clock before an update operation. When users adjust the rate of a clock with an explicitly provided reference time (R), they are specifying the slope of the new transformation T'(R) for the clock, such that T'(R) = T(R). In other words, at reference time R, the new transformation will pass through the same synthetic time that the old transformation did, but with a different slope.

#include <zircon/syscalls.h>
#include <zircon/syscalls/clock.h>

zx_status_t SetRateAtRef(zx_handle_t the_clock, zx_time_t ref, int32_t ppm_adj) {
  zx_clock_update_args_v2_t args;

  const uint64_t options = ZX_CLOCK_ARGS_VERSION(2) |
  args.reference_value = ref;
  args.rate_adjust = ppm_adj;

  return zx_clock_update(the_clock, options, args);

Notes, rules, and limitations.

  • Explicit reference values are not required. It is still possible to omit the reference value during an update operation. The update operation will simply use the current reference time when the operation is processed.
  • When providing an explicit reference value for a clock update operation, either a synthetic value, or a rate adjustment, or both, must also be provided. It is not legal to attempt to update only the error bounds at an explicit reference value.
  • Explicitly providing a reference value to an update operation for a continuous clock is never allowed as it would virtually always imply a discontinuity.
  • Explicitly providing a reference value to an update operation for a monotonic clock is allowed, but only if the behavior of the clock remains monotonic after the update.
  • An explicitly provided reference value during an update operation which would cause a read performed at a reference time of "now" to violate the configured backstop time of the clock will cause the operation to be rejected.
  • When updating a monotonic clock, it is not possible to effect both a synthetic value update and a rate adjustment simultaneously.

Details provided in RFC-0077 may help to understand the reasoning behind some of these rules and limitations.

Return value

On success, returns ZX_OK.


  • ZX_ERR_BAD_HANDLE : handle is either an invalid handle, or a handle to an object type that is not ZX_OBJ_TYPE_CLOCK.
  • ZX_ERR_ACCESS_DENIED : handle lacks the ZX_RIGHT_WRITE right.
  • ZX_ERR_INVALID_ARGS : The update request made is incompatible with the properties of the clock. See the DESCRIPTION section for details of permissible clock update operations. Otherwise, the version/pointer of the arguments structure is incorrect.

See also