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# zx_clock_update
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Make adjustments to a clock object.
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#include <zircon/syscalls.h>
zx_status_t zx_clock_update(zx_handle_t handle,
uint64_t options,
const void* args);
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*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_v1_t` structure that they wish to adjust, then passes the
structure to the update call, setting the bits in `options` which indicate which
of these fields are valid and should be set. Defined `options` bits are
In addition, maintainer **must** indicate that they are using the V1 version of
the struct using the ZX_CLOCK_ARGS_VERSION(...) macro.
For example
#include <zircon/syscalls.h>
#include <zircon/syscalls/clock.h>
void MaintainMyClock(zx_handle_t the_clock) {
zx_clock_update_args_v1_t args;
zx_handle_t the_clock;
zx_status_t status;
// Set the clock's value to 1500. Note that this also starts the clock.
args.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(1) |
args.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.
On success, returns **ZX_OK**.
- **ZX_ERR_BAD_HANDLE** : *handle* is either an invalid handle, or a handle to
an object type which 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.
- [clocks](/docs/reference/kernel_objects/
- [`zx_clock_create()`]
- [`zx_clock_get_details()`]
- [`zx_clock_read()`]
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