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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968" /><title>Access Flags</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="Writing an ALSA Driver" /><link rel="up" href="ch06.html" title="Chapter 6. Control Interface" /><link rel="prev" href="ch06s03.html" title="Control Names" /><link rel="next" href="ch06s05.html" title="Callbacks" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Access Flags</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch06s03.html">Prev</a> </td><th width="60%" align="center">Chapter 6. Control Interface</th><td width="20%" align="right"> <a accesskey="n" href="ch06s05.html">Next</a></td></tr></table><hr /></div><div class="section" title="Access Flags"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="control-interface-access-flags"></a>Access Flags</h2></div></div></div><p>
The access flag is the bitmask which specifies the access type
of the given control. The default access type is
<code class="constant">SNDRV_CTL_ELEM_ACCESS_READWRITE</code>,
which means both read and write are allowed to this control.
When the access flag is omitted (i.e. = 0), it is
considered as <code class="constant">READWRITE</code> access as default.
</p><p>
When the control is read-only, pass
<code class="constant">SNDRV_CTL_ELEM_ACCESS_READ</code> instead.
In this case, you don't have to define
the <em class="structfield"><code>put</code></em> callback.
Similarly, when the control is write-only (although it's a rare
case), you can use the <code class="constant">WRITE</code> flag instead, and
you don't need the <em class="structfield"><code>get</code></em> callback.
</p><p>
If the control value changes frequently (e.g. the VU meter),
<code class="constant">VOLATILE</code> flag should be given. This means
that the control may be changed without
<a class="link" href="ch06s07.html" title="Change Notification"><em class="citetitle">
notification</em></a>. Applications should poll such
a control constantly.
</p><p>
When the control is inactive, set
the <code class="constant">INACTIVE</code> flag, too.
There are <code class="constant">LOCK</code> and
<code class="constant">OWNER</code> flags to change the write
permissions.
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch06s03.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch06.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ch06s05.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Control Names </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Callbacks</td></tr></table></div></body></html>
