1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
|
<?xml version='1.0'?>
<!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
]>
<section id="associativearrays">
<title>Associative Arrays</title>
<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>
<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 manipulating arrays (i.e. accessing elements in an associative array) is similar to that of <command>awk</command>, and is as follows:</para>
<screen>
<replaceable>array_name</replaceable>[<replaceable>index_expression</replaceable>] <replaceable>operation</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 (or set of unique keys) in the array, and the <command><replaceable>operation</replaceable></command> defines what to do with the <command><replaceable>index_expression</replaceable></command>. 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>
<example id="arraysimplestexample">
<title>Basic Array Statements</title>
<screen>
foo["tom"] = 23
foo["dick"] = 24
foo["harry"] = 25
</screen>
</example>
<important>
<title>Important</title>
<para>Arrays are normally used in multiple probes throughout a script: in most cases, one probe builds the arrays while another probe processes the information collected by the array (e.g. print its elements). Since the treatment of arrays is similar to that of variables, arrays must also be declared globally with the statement <command>global</command> when they are used by multiple probes.</para>
</important>
<section id="tuples">
<title>Tuples</title>
<para>Another important point to remember in arrays is that each element therein (i.e. the indexed expression) exists in a <emphasis>slot</emphasis>. A key pair's slot is defined by the order in which each pair's unique key is defined. In our sample array <command>foo</command> in <xref linkend="arraysimplestexample"/>, the key pair that uses the unique key <command>tom</command> is in the first slot, since <command>tom</command> was the first unique key to be defined. <command>dick</command> is in the second slot, and so on.</para>
<para>The sequence in which each key pair appears in an array (as defined by each pair's slot) is referred to as a <emphasis>tuple</emphasis>. Tuples allow us to refer to key pairs in an array by the order in which they appear in the sequence.</para>
<para>For example, the array statements in <xref linkend="arraysimplestexample"/> set 23 as the associated value of the unique key <command>tom</command>. Given the same array <command>foo</command>, we can increment the associated value of <command>tom</command> by 1 using the operator <command>++</command>, like so:</para>
<screen>
foo["tom"] ++
</screen>
<para>The above statement will increase the associated value of unique key <command>tom</command> to 24. Now, looking back at <xref linkend="arraysimplestexample"/>, we know that <command>dick</command> was the first uniqe key to be defined. As such, we can perform the same operation (i.e. incrementing associated value by 1) to <command>dick</command> using the following statement:</para>
<screen>
foo[2] ++
</screen>
<note>
<title>Note</title>
<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 perform the same operation to a set of key pairs. For example, to increase the associated value of all the key pairs defined by <xref linkend="arraysimplestexample"/>, you can use the following statement:</para>
<screen>
foo["tom",2,"harry"] ++
</screen>
</note>
</section>
<xi:include href="Array-Operations.xml" xmlns:xi="http://www.w3.org/2001/XInclude" />
<!--
<varlistentry>
<term></term>
<listitem>
<para></para>
</listitem>
</varlistentry>
-->
<!--
<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 kernel function <command>vfs_read</command>, 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 kernel.function("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>
|
