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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>Writing data</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="Linux Kernel Procfs Guide" /><link rel="up" href="ch03.html" title="Chapter 3. Communicating with userland" /><link rel="prev" href="ch03.html" title="Chapter 3. Communicating with userland" /><link rel="next" href="ch03s03.html" title="A single call back for many files" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Writing data</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch03.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Communicating with userland</th><td width="20%" align="right"> <a accesskey="n" href="ch03s03.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="Writing data"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="Writing_data"></a>Writing data</h2></div></div></div><p>
The write call back function allows a userland process to write
data to the kernel, so it has some kind of control over the
kernel. The write function should have the following format:
</p><div class="funcsynopsis"><table border="0" summary="Function synopsis" cellspacing="0" cellpadding="0" class="funcprototype-table"><tr><td><code class="funcdef">int <b class="fsfunc">write_func</b>(</code></td><td>struct file* <var class="pdparam">file</var>, </td></tr><tr><td> </td><td>const char* <var class="pdparam">buffer</var>, </td></tr><tr><td> </td><td>unsigned long <var class="pdparam">count</var>, </td></tr><tr><td> </td><td>void* <var class="pdparam">data</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div><p>
The write function should read <em class="parameter"><code>count</code></em>
bytes at maximum from the <em class="parameter"><code>buffer</code></em>. Note
that the <em class="parameter"><code>buffer</code></em> doesn't live in the
kernel's memory space, so it should first be copied to kernel
space with <code class="function">copy_from_user</code>. The
<em class="parameter"><code>file</code></em> parameter is usually
ignored. <a class="xref" href="ch03s03.html" title="A single call back for many files">the section called “A single call back for many files”</a> shows how to use the
<em class="parameter"><code>data</code></em> parameter.
</p><p>
Again, <a class="xref" href="ch05.html" title="Chapter 5. Example">Chapter 5, <i>Example</i></a> shows how to use this call back
function.
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch03.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch03.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ch03s03.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 3. Communicating with userland </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> A single call back for many files</td></tr></table></div></body></html>
