1<h2>device_read_async</h2>
2<hr>
3<p>
4<strong>System Trap</strong> - Read a sequence of bytes from a device object.
5<h3>SYNOPSIS</h3>
6<pre>
7<strong>kern_return_t   device_read_async</strong>
8                <strong>(mach_port_t</strong>                             <var>device</var>,
9                 <strong>mach_port_t</strong>                              <var>queue</var>,
10                 <strong>mach_port_t</strong>                         <var>request_id</var>,
11                 <strong>dev_mode_t</strong>                                <var>mode</var>,
12                 <strong>recnum_t</strong>                                <var>recnum</var>,
13                 <strong>io_buf_len_t</strong>                      <var>bytes_wanted</var><strong>);</strong>
14</pre>
15<h3>PARAMETERS</h3>
16<dl>
17<p>
18<dt> <var>device</var>
19<dd>
20[in device send right] A device port to the device to be read.
21<p>
22<dt> <var>queue</var>
23<dd>
24[in io_done queue send right] The port returned from 
25io_done_queue_create.
26<p>
27<dt> <var>request_id</var>
28<dd>
29[in send right] An unique request identifier that will be passed back as 
30part of the io_done_result structure.
31<p>
32<dt> <var>mode</var>
33<dd>
34[in scalar] I/O mode value. Meaningful options are:
35<p>
36     <dl>
37<dt> <strong>D_NOWAIT</strong>
38<dd>
39Do not wait if data is unavailable.
40     </dl>
41<p>
42<dt> <var>recnum</var>
43<dd>
44[in scalar] Record number to be read.
45<p>
46<dt> <var>bytes_wanted</var>
47<dd>
48[in scalar] Size of data transfer.
49</dl>
50<h3>DESCRIPTION</h3>
51<p>
52The <strong>device_read_async</strong> interface enqueues a read operation for a
53sequence of bytes from a device object. The meaning of recnum as well
54as the specific operation performed is device dependent.
55<h3>RETURN VALUES</h3>
56<dl>
57<strong>device_read_async</strong> returns only invalid parameter errors.
58<dd>
59</dl>
60<h3>RELATED INFORMATION</h3>
61<p>
62Functions:
63<a href="device_read_async_inband.html"><strong>device_read_async_inband</strong></a>,
64<a href="DR_overwrite_async.html"><strong>device_read_overwrite_async</strong></a>,
65<a href="device_write_async.html"><strong>device_write_async</strong></a>,
66<a href="device_write_async_inband.html"><strong>device_write_async_inband</strong></a>,
67<a href="io_done_queue_create.html"><strong>io_done_queue_create</strong></a>.
68