1Devel::Cover::Tutorial(U3s)er Contributed Perl DocumentatDieovnel::Cover::Tutorial(3)
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6 Devel::Cover::Tutorial - An introduction to code coverage
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9 version 1.36
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12 Here's part of a message I sent to perl-qa about code coverage metrics.
13 1.0 Introduction
15 It is wise to remember the following quote from Dijkstra, who said:
16 Testing never proves the absence of faults, it only shows their presence.
18 In particular, code coverage is just one weapon in the software
20 engineer's testing arsenal.
21 Any discussion of code coverage metrics is hampered by the fact that
23 many authors use different terms to describe the same kind of coverage.
24 Here, I shall provide only a brief introduction to some of the most
25 common metrics.
26 2.0 Metrics
28 2.1 Statement coverage
29 This is the most basic form of code coverage. A statement is covered
30 if it is executed. Note that statement != line of code. Multiple
31 statements on a single line can confuse issues - the reporting if
32 nothing else.
33 Where there are sequences of statements without branches it is not
35 necessary to count the execution of every statement, just one will
36 suffice, but people often like the count of every line to be reported,
37 especially in summary statistics. However it is not clear to me that
38 this is actually useful.
39 This type of coverage is fairly weak in that even with 100% statement
41 coverage there may still be serious problems in a program which could
42 be discovered through other types of metric.
43 It can be quite difficult to achieve 100% statement coverage. There
45 may be sections of code designed to deal with error conditions, or
46 rarely occurring events such as a signal received during a certain
47 section of code. There may also be code that should never be executed:
48 if ($param > 20) {
50 die "This should never happen!";
51 }
52 It can be useful to mark such code in some way and flag an error if it
54 is executed.
55 Statement coverage, or something very similar, can be called statement
57 execution, line, block, basic block or segment coverage. I tend to
58 favour block coverage which does not attempt to extend its results to
59 each statement.
60 2.2 Branch coverage
62 The goal of branch coverage is to ensure that whenever a program can
63 jump, it jumps to all possible destinations. The most simple example
64 is a complete if statement:
65 if ($x) {
67 print "a";
68 } else {
69 print "b";
70 }
71 In such a simple example statement coverage is as powerful, but branch
73 coverage should also allow for the case where the else part is missing:
74 if ($x) {
76 print "a";
77 }
78 Full coverage is only achieved here if $x is true on one occasion and
80 false on another.
81 100% branch coverage implies 100% statement coverage.
83 Branch coverage is also called decision or all edges coverage.
85 2.3 Path coverage
87 There are classes of errors that branch coverage cannot detect, such
88 as:
89 $h = undef;
91 if ($x) {
92 $h = { a => 1 };
93 } if ($y) {
94 print $h->{a};
95 }
96 100% branch coverage can be achieved by setting ($x, $y) to (1, 1) and
98 then to (0, 0). But if we have (0, 1) then things go bang.
99 The purpose of path coverage is to ensure that all paths through the
101 program are taken. In any reasonably sized program there will be an
102 enormous number of paths through the program and so in practice the
103 paths can be limited to a single subroutine, if the subroutine is not
104 too big, or simply to two consecutive branches.
105 In the above example there are four paths which correspond to the truth
107 table for $x and $y. To achieve 100% path coverage they must all be
108 taken. Note that missing elses count as paths.
109 In some cases it may be impossible to achieve 100% path coverage:
111 a if $x;
113 b;
114 c if $x;
115 50% path coverage is the best you can get here.
117 Loops also contribute to paths, and pose their own problems which I'll
119 ignore for now.
120 100% path coverage implies 100% branch coverage.
122 Path coverage and some of its close cousins, are also known as
124 predicate, basis path and LCSAJ (Linear Code Sequence and Jump)
125 coverage.
126 2.4 Expression coverage
128 When a boolean expression is evaluated it can be useful to ensure that
129 all the terms in the expression are exercised. For example:
130 a if $x || $y
132 The expression should be exercised with ($x, $y) set to (0, 0)
134 (required for branch coverage), (0, 1) and (1, 0) (to ensure that $x
135 and $y are independent) and possibly with (1, 1).
136 Expression coverage gets complicated, and difficult to achieve, as the
138 expression gets complicated.
139 Expressions which are not directly a part of a branching construct
141 should also be covered:
142 $z = $x || $y;
144 a if $z;
145 Expression coverage is also known as condition, condition-decision and
147 multiple decision coverage.
148 3.0 Other considerations
150 In order to get people to actually use code coverage it needs to be
151 simple to use. It should also be simple to understand the results and
152 to rectify any problems thrown up. Finally, if the overhead is too
153 great it won't get used either.
154 So there's a basic tutorial on code coverage, or at least my version of
156 it. Typing a few of these terms into google will probably provide a
157 basis for future research.
158
160 Copyright 2001-2019, Paul Johnson (paul@pjcj.net)
161 This software is free. It is licensed under the same terms as Perl
163 itself.
164 The latest version of this software should be available from my
166 homepage: http://www.pjcj.net
167perl v5.34.0 2021-07-22 Devel::Cover::Tutorial(3)