1Devel::Cover::Tutorial(U3s)er Contributed Perl DocumentatDieovnel::Cover::Tutorial(3)
2
6 Here's part of a message I sent to perl-qa about code coverage metrics.
7 1.0 Introduction
9 It is wise to remember the following quote from Dijkstra, who said:
10 Testing never proves the absence of faults, it only shows their presence.
12 In particular, code coverage is just one weapon in the software
14 engineer's testing arsenal.
15 Any discussion of code coverage metrics is hampered by the fact that
17 many authors use different terms to describe the same kind of coverage.
18 Here, I shall provide only a brief introduction to some of the most
19 common metrics.
20 2.0 Metrics
22 2.1 Statement coverage
23 This is the most basic form of code coverage. A statement is covered
24 if it is executed. Note that statement != line of code. Multiple
25 statements on a single line can confuse issues - the reporting if
26 nothing else.
27 Where there are sequences of statements without branches it is not
29 necessary to count the execution of every statement, just one will
30 suffice, but people often like the count of every line to be reported,
31 especially in summary statistics. However it is not clear to me that
32 this is actually useful.
33 This type of coverage is fairly weak in that even with 100% statement
35 coverage there may still be serious problems in a program which could
36 be discovered through other types of metric.
37 It can be quite difficult to achieve 100% statement coverage. There
39 may be sections of code designed to deal with error conditions, or
40 rarely occurring events such as a signal received during a certain
41 section of code. There may also be code that should never be executed:
42 if ($param > 20)
44 {
45 die "This should never happen!";
46 }
47 It can be useful to mark such code in some way and flag an error if it
49 is executed.
50 Statement coverage, or something very similar, can be called statement
52 execution, line, block, basic block or segment coverage. I tend to
53 favour block coverage which does not attempt to extend its results to
54 each statement.
55 2.2 Branch coverage
57 The goal of branch coverage is to ensure that whenever a program can
58 jump, it jumps to all possible destinations. The most simple example
59 is a complete if statement:
60 if ($x)
62 {
63 print "a";
64 }
65 else
66 {
67 print "b";
68 }
69 In such a simple example statement coverage is as powerful, but branch
71 coverage should also allow for the case where the else part is missing:
72 if ($x)
74 {
75 print "a";
76 }
77 Full coverage is only achieved here if $x is true on one occasion and
79 false on another.
80 100% branch coverage implies 100% statement coverage.
82 Branch coverage is also called decision or all edges coverage.
84 2.3 Path coverage
86 There are classes of errors that branch coverage cannot detect, such
87 as:
88 $h = undef;
90 if ($x)
91 {
92 $h = { a => 1 };
93 }
94 if ($y)
95 {
96 print $h->{a};
97 }
98 100% branch coverage can be achieved by setting ($x, $y) to (1, 1) and
100 then to (0, 0). But if we have (0, 1) then things go bang.
101 The purpose of path coverage is to ensure that all paths through the
103 program are taken. In any reasonably sized program there will be an
104 enormous number of paths through the program and so in practice the
105 paths can be limited to a single subroutine, if the subroutine is not
106 too big, or simply to two consecutive branches.
107 In the above example there are four paths which correspond to the truth
109 table for $x and $y. To achieve 100% path coverage they must all be
110 taken. Note that missing elses count as paths.
111 In some cases it may be impossible to achieve 100% path coverage:
113 a if $x;
115 b;
116 c if $x;
117 50% path coverage is the best you can get here.
119 Loops also contribute to paths, and pose their own problems which I'll
121 ignore for now.
122 100% path coverage implies 100% branch coverage.
124 Path coverage and some of its close cousins, are also known as
126 predicate, basis path and LCSAJ (Linear Code Sequence and Jump)
127 coverage.
128 2.4 Expression coverage
130 When a boolean expression is evaluated it can be useful to ensure that
131 all the terms in the expression are exercised. For example:
132 a if $x || $y
134 The expression should be exercised with ($x, $y) set to (0, 0)
136 (required for branch coverage), (0, 1) and (1, 0) (to ensure that $x
137 and $y are independent) and possibly with (1, 1).
138 Expression coverage gets complicated, and difficult to achieve, as the
140 expression gets complicated.
141 Expressions which are not directly a part of a branching construct
143 should also be covered:
144 $z = $x || $y;
146 a if $z;
147 Expression coverage is also known as condition, condition-decision and
149 multiple decision coverage.
150 3.0 Other considerations
152 In order to get people to actually use code coverage it needs to be
153 simple to use. It should also be simple to understand the results and
154 to rectify any problems thrown up. Finally, if the overhead is too
155 great it won't get used either.
156 So there's a basic tutorial on code coverage, or at least my version of
158 it. Typing a few of these terms into google will probably provide a
159 basis for future research.
160
162 Version 0.66 - 12th April 2010
163
165 Copyright 2001-2010, Paul Johnson (pjcj@cpan.org)
166 This software is free. It is licensed under the same terms as Perl
168 itself.
169 The latest version of this software should be available from my
171 homepage: http://www.pjcj.net
172perl v5.12.3 2010-04-12 Devel::Cover::Tutorial(3)