<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>3.3.2.2. Typecasting</title><link rel="stylesheet" type="text/css" href="Common_Content/css/default.css" /><link rel="stylesheet" media="print" href="Common_Content/css/print.css" type="text/css" /><meta name="generator" content="publican 2.8" /><meta name="package" content="Systemtap-SystemTap_Beginners_Guide-2.1-en-US-2.1-2" /><link rel="home" href="index.html" title="SystemTap Beginners Guide" /><link rel="up" href="targetvariables.html" title="3.3.2. Target Variables" /><link rel="prev" href="targetvariables.html" title="3.3.2. Target Variables" /><link rel="next" href="targetavailable.html" title="3.3.2.3. Checking Target Variable Availablility" /></head><body><p id="title"><a class="left" href="http://www.fedoraproject.org"><img src="Common_Content/images/image_left.png" alt="Product Site" /></a><a class="right" href="http://docs.fedoraproject.org"><img src="Common_Content/images/image_right.png" alt="Documentation Site" /></a></p><ul class="docnav"><li class="previous"><a accesskey="p" href="targetvariables.html"><strong>Prev</strong></a></li><li class="next"><a accesskey="n" href="targetavailable.html"><strong>Next</strong></a></li></ul><div class="section" id="typecasting"><div class="titlepage"><div><div keep-together.within-column="always"><h4 class="title" id="typecasting">3.3.2.2. Typecasting</h4></div></div></div><a id="idp11961848" class="indexterm"></a><div class="para">
In most cases SystemTap can determine a variable's type from the debug information. However, code may use void pointers for variables (for example memory allocation routines) and typing information is not available. Also the typing information available within a probe handler is not available within a function; SystemTap functions arguments use a long in place of a typed pointer. SystemTap's <code class="command">@cast</code> operator (first available in SystemTap 0.9) can be used to indicate the correct type of the object.
</div><div class="para">
The <a class="xref" href="typecasting.html#casting">Example 3.9, “Casting Example”</a> is from the <code class="command">task.stp</code> tapset. The function returns the value of the <code class="command">state</code> field from a <code class="command">task_struct</code> pointed to by the long <code class="command">task</code>. The first argument of the <code class="command">@cast</code> operator, <code class="command">task</code>, is the pointer to the object. The second argument is the type to cast the object to, <code class="command">task_struct</code>. The third arument lists what file that the type definition information comes from and is optional. With the <code class="command">@cast</code> operator the various fields of this particular <code class="command">task_struct</code> <code class="command">task</code> can be accessed; in this example the <code class="command">state</code> field is obtained.
</div><div class="example" id="casting"><h6>Example 3.9. Casting Example</h6><div class="example-contents"><pre class="programlisting">function task_state:long (task:long)
{
return @cast(task, "task_struct", "kernel<linux/sched.h>")->state
}
</pre></div></div><br class="example-break" /></div><ul class="docnav"><li class="previous"><a accesskey="p" href="targetvariables.html"><strong>Prev</strong>3.3.2. Target Variables</a></li><li class="up"><a accesskey="u" href="#"><strong>Up</strong></a></li><li class="home"><a accesskey="h" href="index.html"><strong>Home</strong></a></li><li class="next"><a accesskey="n" href="targetavailable.html"><strong>Next</strong>3.3.2.3. Checking Target Variable Availablility</a></li></ul></body></html>
