1<h2>evc_wait</h2>
2<hr>
3<p>
4<strong>System Trap</strong> - Wait for a kernel (device) signalled event.
5<h3>SYNOPSIS</h3>
6<pre>
7<strong>kern_return_t	evc_wait</strong>
8		<strong>(unsigned int</strong>	<var>event</var><strong>);</strong>
9</pre>
10<h3>PARAMETERS</h3>
11<dl>
12<dt> <var>event</var>
13<dd>
14[in scalar] The task local event ID of the kernel event object.
15</dl>
16<h3>DESCRIPTION</h3>
17<p>
18The <strong>evc_wait</strong> function causes the invoking thread to wait until the
19specified kernel (device) generated <var>event</var> occurs. Device drivers
20(typically mapped devices intended to be supported by user space
21drivers) may supply an <var>event</var> service.  The <var>event</var> service defines one
22or more <var>event</var> objects, named by task local <var>event</var> IDs. Each of these
23<var>event</var> objects has an associated <var>event</var> count, initially zero. Whenever
24the associated <var>event</var> occurs (typically a device interrupt), the <var>event</var>
25count is incremented. If this count is zero when <strong>evc_wait</strong> is called,
26the calling thread waits for the next <var>event</var> to occur. Only one thread
27may be waiting for the <var>event</var> to occur. If the count is non-zero when
28<strong>evc_wait</strong> is called, the count is simply decremented without causing
29the thread to wait. The <var>event</var> count guarantees that no <var>event</var>s are
30lost.
31<h3>NOTES</h3>
32<p>
33The typical use of this service is within user space device
34drivers. When a device interrupt occurs, the (in this case, simple)
35kernel device driver would place device status in a shared (with the
36user device driver) memory window (established by <strong>device_map</strong>) and
37signal the associated <var>event</var>. The user space device driver would
38normally be waiting with <strong>evc_wait</strong>. The user thread then wakes,
39processes the device status, typically interacting with the device via
40its shared memory window, then waits for the next interrupt.
41<h3>RETURN VALUES</h3>
42<dl>
43<dt> <strong>KERN_NO_SPACE</strong>
44<dd>
45There is already a thread waiting for this event.
46</dl>
47<h3>RELATED INFORMATION</h3>
48<p>
49Functions:
50<a href="device_map.html"><strong>device_map</strong></a>.
51