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# zx_object_wait_one
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Wait for signals on an object.
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#include <zircon/syscalls.h>
zx_status_t zx_object_wait_one(zx_handle_t handle,
zx_signals_t signals,
zx_time_t deadline,
zx_signals_t* observed);
`zx_object_wait_one()` is a blocking syscall which causes the caller to wait
until either the *deadline* passes or the object to which *handle* refers
asserts at least one [signal][signals] specified by the bitmask *signals*. If
the object is already asserting at least one of the specified *signals*, the
wait ends immediately with **ZX_OK**.
Upon return, if non-NULL, *observed* is a bitmap of *all* of the signals
asserted on the object. If one of the specified *signals* was asserted,
*observed* will be the set of signals asserted at the moment the specified
signal was first asserted. Otherwise, if *deadline* passes or *handle* is
closed, *observed* will contain the state of the object's signals at the time
the `zx_object_wait_one` syscall completed.
The *deadline* parameter specifies a deadline with respect to
**ZX_CLOCK_MONOTONIC** and will be automatically adjusted according to the job's
[timer slack] policy. **ZX_TIME_INFINITE** is a special value meaning wait
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*handle* must have **ZX_RIGHT_WAIT**.
`zx_object_wait_one()` returns **ZX_OK** if any of *signals* were active when
the call was made, or observed on the object before *deadline* passes.
In the event of **ZX_ERR_TIMED_OUT**, *observed* may reflect state changes
that occurred after the deadline passed, but before the syscall returned.
In the event of **ZX_ERR_CANCELED**, *handle* has been closed,
and *observed* will have the **ZX_SIGNAL_HANDLE_CLOSED** bit set.
For any other return value, *observed* is undefined.
**ZX_ERR_INVALID_ARGS** *observed* is an invalid pointer.
**ZX_ERR_BAD_HANDLE** *handle* is not a valid handle.
**ZX_ERR_ACCESS_DENIED** *handle* does not have **ZX_RIGHT_WAIT** and may
not be waited upon.
**ZX_ERR_CANCELED** *handle* was invalidated (e.g., closed) during the wait.
**ZX_ERR_TIMED_OUT** The specified deadline passed before any of the specified
*signals* are observed on *handle*.
**ZX_ERR_NOT_SUPPORTED** *handle* is a handle that cannot be waited on
(for example, a Port handle).
See [signals] for more information about signals and their terminology.
- [timer slack](/docs/concepts/kernel/
- [`zx_object_wait_async()`]
- [`zx_object_wait_many()`]
[signals]: /docs/concepts/kernel/
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