path: root/doc/SystemTap_Beginners_Guide/en-US/Arrays.xml

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<section id="associativearrays">
	<title>Associative Arrays</title>
<indexterm>
<primary>arrays</primary>
<seealso>associative arrays</seealso>
</indexterm>

<indexterm>
<primary>associative arrays</primary>
<secondary>introduction</secondary>
</indexterm>	

<para>SystemTap also supports the use of associative arrays. While an ordinary variable represents a single value, associative arrays can represent a collection of values. Simply put, an associative array is a collection of unique keys; each key in the array has a value associated with it.</para>

<!--<para>SystemTap also supports the use of associative arrays. While an ordinary variable represents a single value, associative arrays can represent a list of values. Simply put, an associative array is a collection of unique keys; each key in the array has a value associated with it. Illustrating this visually would be similar to creating a two-column table: the first column would have the unique key, while the second column would have each key's associated value. Each unique key and its associated value is referred to as a <emphasis>key pair</emphasis>.</para>-->

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<indexterm>
<primary>arrays</primary>
<secondary>introduction</secondary>
<tertiary>key pairs</tertiary>
</indexterm>
-->

<indexterm>
<primary>associative arrays</primary>
<secondary>introduction</secondary>
<tertiary>key pairs</tertiary>
</indexterm>

<indexterm>
<primary>key pairs</primary>
<secondary>introduction</secondary>
<tertiary>arrays</tertiary>
</indexterm>

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<indexterm>
<primary>arrays</primary>
<secondary>introduction</secondary>
<tertiary>unique keys</tertiary>
</indexterm>
-->
<indexterm>
<primary>associative arrays</primary>
<secondary>introduction</secondary>
<tertiary>unique keys</tertiary>
</indexterm>

<indexterm>
<primary>unique keys</primary>
<secondary>introduction</secondary>
<tertiary>arrays</tertiary>
</indexterm>

<!-- next 2 indexterms for associated values 

<indexterm>
<primary>arrays</primary>
<secondary>introduction</secondary>
<tertiary>associated values</tertiary>
</indexterm>
-->
<indexterm>
<primary>associative arrays</primary>
<secondary>introduction</secondary>
<tertiary>associated values</tertiary>
</indexterm>

<indexterm>
<primary>associated values</primary>
<secondary>introduction</secondary>
<tertiary>arrays</tertiary>
</indexterm>

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<indexterm>
<primary>arrays</primary>
<secondary>introduction</secondary>
<tertiary>index expression</tertiary>
</indexterm>
-->
<indexterm>
<primary>associative arrays</primary>
<secondary>introduction</secondary>
<tertiary>index expression</tertiary>
</indexterm>

<indexterm>
<primary>index expression</primary>
<secondary>introduction</secondary>
<tertiary>arrays</tertiary>
</indexterm>

<para>Since associative arrays are normally processed in multiple probes (as we will demonstrate later), they are declared as <command>global</command> variables in the SystemTap script. The syntax for accessing an element in an associative array is similar to that of <command>awk</command>, and is as follows:</para>
<!-- next 3 indexterms for syntax 
<indexterm>
<primary>arrays</primary>
<secondary>introduction</secondary>
<tertiary>syntax</tertiary>
</indexterm>
-->
<indexterm>
<primary>associative arrays</primary>
<secondary>introduction</secondary>
<tertiary>syntax</tertiary>
</indexterm>

<indexterm>
<primary>syntax</primary>
<secondary>introduction</secondary>
<tertiary>arrays</tertiary>
</indexterm>

<indexterm>
<primary>format</primary>
<secondary>introduction</secondary>
<tertiary>arrays</tertiary>
</indexterm>


<screen>
<replaceable>array_name</replaceable>[<replaceable>index_expression</replaceable>]
</screen>

<para>Here, the <command><replaceable>array_name</replaceable></command> is any arbitrary name the array uses. The <command><replaceable>index_expression</replaceable></command> is used to refer to a specific unique key in the array. To illustrate, let us try to build an array named <command>foo</command> that specifies the ages of three people (i.e. the unique keys): <command>tom</command>, <command>dick</command>, and <command>harry</command>. To assign them the ages (i.e. associated values) of 23, 24, and 25 respectively, we'd use the following array statements:</para> 

<!-- next 2 indexterms for example 

<indexterm>
<primary>arrays</primary>
<secondary>introduction</secondary>
<tertiary>example</tertiary>
</indexterm>
-->
<indexterm>
<primary>associative arrays</primary>
<secondary>introduction</secondary>
<tertiary>example</tertiary>
</indexterm>

<indexterm>
<primary>example</primary>
<secondary>introduction</secondary>
<tertiary>arrays</tertiary>
</indexterm>
<example id="arraysimplestexample">
	<title>Basic Array Statements</title>
<screen>
foo["tom"] = 23
foo["dick"] = 24
foo["harry"] = 25
</screen>
</example>

<para>You can specify up to 5 index expressons in an array statement, each one delimited by a comma (<command>,</command>). This is useful if you wish to have a key that contains multiple pieces of information. The following line from <xref linkend="scriptdisktop"/> uses 5 elements for the key: process ID, executable name, user ID, parent process ID, and string "W". It associates the value of <command>devname</command> with that key.</para>

<screen>
device[pid(),execname(),uid(),ppid(),"W"] = devname
</screen>

<important>
	<title>Important</title>
	<para>All associate arrays must be declared as <command>global</command>, regardless of whether the associate array is used in one or multiple probes. </para>
</important>



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<!-- 
	
<para>SystemTap supports numerous ways to manipulate associative arrays (i.e. <command><replaceable>operation</replaceable></command> in the syntax format). For the purposes of this book, we will only cover the most common examples of manipulating associative arrays, all of which appear in <xref linkend="useful-systemtap-scripts"/>.</para>

<formalpara id="aaexamplesimple">
	<title>Incrementing Associated Values of Unique Keys</title>
<para>The simplest form of data manipulation in associative arrays is incrementing the associated value of a unique key in the array. The syntax for this operation is as follows:</para>
</formalpara>
<screen>
<replaceable>array_name</replaceable>[<replaceable>index_expression</replaceable>"] ++
</screen>

<para>Here, the <command>++</command> operation instructs SystemTap to increment the associated value of <command><replaceable>unique_key</replaceable></command> by <command><replaceable>value</replaceable></command>. For example, to increase the associated value of unique key <command>hello</command> in array <command>foo</command> by 4, use:</para>

<formalpara id="aaexamplesimple-example">
	<title>explicit-incrementing-in-arrays.stp</title>
<para>
<programlisting>
probe begin {	foo[4,"hello"] ++ }
</programlisting>
</para>
</formalpara>

<formalpara id="aaexampleupdatecounting">
	<title>Incrementing Associated Values of Unique Keys (By Variable)</title>
	
<para>One of the most common uses of associative arrays is to tally events per unique key. To do this, use the operator <command>+= $count</command>, as in:</para>
</formalpara>

<screen>
<replaceable>array_name</replaceable>[<replaceable>unique_key</replaceable>] += $count	
</screen>

<para>You can also use a handler function in as the <command><replaceable>unique_key</replaceable></command>. Doing so creates an associate array that uses the values returned by the handler function as the unique keys. The first time that a probe using this array returns a string value, that value is set as a unique key with an initial value of 0. The next time that the probe returns the same string value, it increments the associated value of the unique key by 1.</para>

<para>For example, let's say you need to tally how many times each process performs a read to the virtual file system (VFS). To do this, probe the VFS read opeartion, use the handler <command>execname()</command> to identify which processes performed the VFS read, and tally the reads of each process using the associative array named <command>reads</command>, as in</para>

<formalpara id="aaexamplesimplevfsreads">
	<title>tallying-in-arrays.stp</title>
<para>
<programlisting>
probe vfs.read
{ reads[execname()] += $count }
</programlisting>
</para>
</formalpara>

<para>In <xref linkend="aaexamplesimplevfsreads"/>, the first time that the probe returns the process name <command>gnome-terminal</command> (i.e. the first time <command>gnome-terminal</command> performs a VFS read), that process name is set as a unique key. The next time that the probe returns the process name <command>gnome-terminal</command>, SystemTap increments the associated value of <command>gnome-terminal</command> by 1. SystemTap performs this operation for <emphasis>all</emphasis> process names as the probe returns them.</para>
-->
</section>