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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>A single call back for many files</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="ch03s02.html" title="Writing data" /><link rel="next" href="ch04.html" title="Chapter 4. Tips and tricks" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">A single call back for many files</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch03s02.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Communicating with userland</th><td width="20%" align="right"> <a accesskey="n" href="ch04.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="A single call back for many files"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="usingdata"></a>A single call back for many files</h2></div></div></div><p>
When a large number of almost identical files is used, it's
quite inconvenient to use a separate call back function for
each file. A better approach is to have a single call back
function that distinguishes between the files by using the
<em class="structfield"><code>data</code></em> field in <span class="structname">struct
proc_dir_entry</span>. First of all, the
<em class="structfield"><code>data</code></em> field has to be initialised:
</p><pre class="programlisting">
struct proc_dir_entry* entry;
struct my_file_data *file_data;
file_data = kmalloc(sizeof(struct my_file_data), GFP_KERNEL);
entry->data = file_data;
</pre><p>
The <em class="structfield"><code>data</code></em> field is a <span class="type">void
*</span>, so it can be initialised with anything.
</p><p>
Now that the <em class="structfield"><code>data</code></em> field is set, the
<code class="function">read_proc</code> and
<code class="function">write_proc</code> can use it to distinguish
between files because they get it passed into their
<em class="parameter"><code>data</code></em> parameter:
</p><pre class="programlisting">
int foo_read_func(char *page, char **start, off_t off,
int count, int *eof, void *data)
{
int len;
if(data == file_data) {
/* special case for this file */
} else {
/* normal processing */
}
return len;
}
</pre><p>
Be sure to free the <em class="structfield"><code>data</code></em> data field
when removing the procfs entry.
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