A signal is a single bit information that magenta kernel objects expose to applications. Each object can expose one or more signals; some are generic and some are specific to the object.
Only waitable objects have signals. Below are the signals that apply to every waitable object.
To know if an object is waitable, call object_get_info with MX_INFO_HANDLE_BASIC topic and test for MX_OBJ_PROP_WAITABLE.
The handle associated with a pending object_wait_one or a object_wait_many call has been closed causing the wait operation to be aborted.
This signal can only be obtained as a result of the above two wait calls when the wait itself returns with MX_ERR_CANCELED.
The object handle count reached 1. The process observing this event was, at the moment of observation, the last handle holder. This signal can be used to reap local resources associated with an object whose handle was duplicated and sent over a channel.
A newly created object first handle has this signal asserted. If this handle is duplicated then the signal de-asserts.
All waitable objects have 8 user-avaliable signals which can be asserted and de-asserted via object_signal and for objects that have a peer such as channels, sockets and event pairs, the peer object user signals can be mutated via object_signal_peer.