Subscribe for signals on an object.


#include <zircon/syscalls.h>

zx_status_t zx_object_wait_async(zx_handle_t handle,
                                 zx_handle_t port,
                                 uint64_t key,
                                 zx_signals_t signals,
                                 uint32_t options);


zx_object_wait_async() is a non-blocking syscall which causes packets to be enqueued on port when the specified condition is met. Use zx_port_wait() to retrieve the packets.

handle points to the object that is to be watched for changes and must be a waitable object.

The options argument can be either ZX_WAIT_ASYNC_ONCE or ZX_WAIT_ASYNC_REPEATING. See notes below for ZX_WAIT_ASYNC_REPEATING.

In both cases, signals indicates which signals on the object specified by handle will cause a packet to be enqueued, and if any of those signals are asserted when zx_object_wait_async() is called, or become asserted afterwards, a packet will be enqueued on port containing all of the currently-asserted signals (not just the ones listed in the signals argument).

In the case of ZX_WAIT_ASYNC_ONCE, once a packet has been enqueued the asynchronous waiting ends. No further packets will be enqueued.

In the case of ZX_WAIT_ASYNC_REPEATING the asynchronous waiting continues until canceled. If any of signals are asserted and a packet is not currently in port‘s queue on behalf of this wait, a packet is enqueued. If a packet is already in the queue, the packet’s observed field is updated to include all of the currently-asserted signals (without removing the existing signals).

In either mode, zx_port_cancel() will terminate the operation and if a packet was in the queue on behalf of the operation, that packet will be removed from the queue.

If handle is closed, the operation will also be terminated, but packets already in the queue are not affected.

Packets generated via this syscall will have type set to either ZX_PKT_TYPE_SIGNAL_ONE or ZX_PKT_TYPE_SIGNAL_REP, and the union is of type zx_packet_signal_t:

typedef struct zx_packet_signal {
    zx_signals_t trigger;
    zx_signals_t observed;
    uint64_t count;
} zx_packet_signal_t;

trigger is the signals used in the call to zx_object_wait_async(), observed is the signals actually observed, and count is a per object defined count of pending operations. Use the zx_port_packet_t's key member to track what object this packet corresponds to and therefore match count with the operation.


handle must have ZX_RIGHT_WAIT.

port must be of type ZX_OBJ_TYPE_PORT and have ZX_RIGHT_WRITE.


zx_object_wait_async() returns ZX_OK if the subscription succeeded.



ZX_ERR_BAD_HANDLE handle is not a valid handle or port is not a valid handle.

ZX_ERR_WRONG_TYPE port is not a Port handle.

ZX_ERR_ACCESS_DENIED handle does not have ZX_RIGHT_WAIT or port does not have ZX_RIGHT_WRITE.

ZX_ERR_NOT_SUPPORTED handle is a handle that cannot be waited on.

ZX_ERR_NO_MEMORY Failure due to lack of memory. There is no good way for userspace to handle this (unlikely) error. In a future build this error will no longer occur.


See signals for more information about signals and their terminology.

ZX_WAIT_ASYNC_REPEATING is being deprecated and should not be used. After a packet has been retrieved, it can be re-armed with ZX_WAIT_ASYNC_ONCE to achieve similar behavior.