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