main/syscalls/object_wait_one.md

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# zx_object_wait_one

## Summary

Wait for signals on an object.

## Declaration

```c
#include <zircon/syscalls.h>

zx_status_t zx_object_wait_one(zx_handle_t handle,
                               zx_signals_t signals,
                               zx_instant_mono_t deadline,
                               zx_signals_t* observed);
```

## Description

`zx_object_wait_one()` is a blocking syscall that 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 forever.
  * `ZX_TIME_INFINITE_PAST` (or any value before the current time in
    `ZX_CLOCK_MONOTONIC`) will query the current value of the signal(s).

## Rights

*handle* must have `ZX_RIGHT_WAIT`.

## Return value

`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.

## Errors

`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).

## Notes

See [signals] for more information about signals and their terminology.

## See also

 - [signals]
 - [timer slack]
 - [`zx_object_wait_async()`]
 - [`zx_object_wait_many()`]

[signals]: /docs/concepts/kernel/signals.md
[timer slack]: /docs/concepts/kernel/timer_slack.md
[`zx_object_wait_async()`]: object_wait_async.md
[`zx_object_wait_many()`]: object_wait_many.md