1<html><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968"><title>trace_printk</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"><link rel="home" href="index.html" title="Linux Device Drivers"><link rel="up" href="ch01s09.html" title="Kernel utility functions"><link rel="prev" href="API-kstrtol.html" title="kstrtol"><link rel="next" href="API-trace-puts.html" title="trace_puts"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center"><span class="phrase">trace_printk</span></th></tr><tr><td width="20%" align="left"><a accesskey="p" href="API-kstrtol.html">Prev</a> </td><th width="60%" align="center">Kernel utility functions</th><td width="20%" align="right"> <a accesskey="n" href="API-trace-puts.html">Next</a></td></tr></table><hr></div><div class="refentry"><a name="API-trace-printk"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>trace_printk — 2 printf formatting in the ftrace buffer 3 </p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><div class="funcsynopsis"><table border="0" class="funcprototype-table" summary="Function synopsis" style="cellspacing: 0; cellpadding: 0;"><tr><td><code class="funcdef"> <b class="fsfunc">trace_printk </b>(</code></td><td> <var class="pdparam">fmt</var>, </td></tr><tr><td> </td><td> <var class="pdparam">...</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div></div><div class="refsect1"><a name="idp1108352140"></a><h2>Arguments</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="parameter"><code>fmt</code></em></span></dt><dd><p> 4 the printf format for printing 5 </p></dd><dt><span class="term"><em class="parameter"><code>...</code></em></span></dt><dd><p> 6 variable arguments 7 </p></dd></dl></div></div><div class="refsect1"><a name="idp1108354756"></a><h2>Note</h2><p> 8 __trace_printk is an internal function for trace_printk and 9 the <em class="parameter"><code>ip</code></em> is passed in via the trace_printk macro. 10 </p><p> 11 12 This function allows a kernel developer to debug fast path sections 13 that printk is not appropriate for. By scattering in various 14 printk like tracing in the code, a developer can quickly see 15 where problems are occurring. 16 </p><p> 17 18 This is intended as a debugging tool for the developer only. 19 Please refrain from leaving trace_printks scattered around in 20 your code. (Extra memory is used for special buffers that are 21 allocated when <code class="function">trace_printk</code> is used) 22 </p><p> 23 24 A little optization trick is done here. If there's only one 25 argument, there's no need to scan the string for printf formats. 26 The <code class="function">trace_puts</code> will suffice. But how can we take advantage of 27 using <code class="function">trace_puts</code> when <code class="function">trace_printk</code> has only one argument? 28 By stringifying the args and checking the size we can tell 29 whether or not there are args. __stringify((__VA_ARGS__)) will 30 turn into <span class="quote">“<span class="quote">()\0</span>”</span> with a size of 3 when there are no args, anything 31 else will be bigger. All we need to do is define a string to this, 32 and then take its size and compare to 3. If it's bigger, use 33 <code class="function">do_trace_printk</code> otherwise, optimize it to <code class="function">trace_puts</code>. Then just 34 let gcc optimize the rest. 35</p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="API-kstrtol.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch01s09.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="API-trace-puts.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top"><span class="phrase">kstrtol</span> </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> <span class="phrase">trace_puts</span></td></tr></table></div></body></html> 36