Perl Debugged

Description

• Copyright 2001
• Edition: 1st

• Book
• ISBN-10: 0-201-70054-9
• ISBN-13: 978-0-201-70054-1

"This book was a joy to read. It covered all sorts of techniques for debugging, including 'defensive' paradigms that will eliminate bugs in the first place. As coach of the USA Programming Team, I find the most difficult thing to teach is debugging. This is the first text I've even heard of that attacks the problem. It does a fine job. Please encourage these guys to write more."–Rob Kolstad

Perl Debugged provides the expertise and solutions developers require for coding better, faster, and more reliably in Perl. Focusing on debugging, the most vexing aspect of programming in Perl, this example-rich reference and how-to guide minimizes development, troubleshooting, and maintenance time resulting in the creation of elegant and error-free Perl code.

Designed for the novice to intermediate software developer, Perl Debugged will save the programmer time and frustration in debugging Perl programs. Based on the authors’ extensive experience with the language, this book guides developers through the entire programming process, tackling the benefits, plights, and pitfalls of Perl programming. Beginning with a guided tour of the Perl documentation, the book progresses to debugging, testing, and performance issues, and also devotes a chapter to CGI programming in Perl. Throughout the book, the authors espouse defensible paradigms for improving the accuracy and performance of Perl code. In addition, Perl Debugged includes Scott and Wright’s "Perls of Wisdom" which summarize key ideas from each of the chapters, and an appendix containing a comprehensive listing of Perl debugger commands.

In this exceptional reference and debugging guide, the authors cover every aspect of efficient Perl programming, including:

• CGI programming–special tips for debugging this type of Perl program
• How to develop the proper mindset for developing and programming effectively in Perl
• Perl "gotchas"–how to understand them, work around them, and avoid them
• "Antibugging"–the authors’ rules of thumb on how to code defensively
• The Perl debugger –the authors’ guide to using this Perl built-in
• Common syntax errors and how to track down their causes
• Semantical errors–why code may appear correct but the programs do not work
• How to improve the performance of resource-hungry programs
• Tips and advice for programmers moving to Perl from other language environments

Focusing on the process of Perl programming and guidelines for identifying and correcting mistakes, Perl Debugged helps the developer to write better Perl programs immediately and become better programmers in general.

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Table of Contents

Preface.


Perlness.


Who Are You?


What This Book Covers.


Getting Perl.


Typographical Conventions.


For Further Reference.


Perl Versions.


Acknowledgments.


 1. Introduction.


 2. The Zen of Perl Developing.


 3. Antibugging.


 4. Perl Pitfalls, Syntactical Sugaring.


 5. Tracing Code.


 6. Testing Perl Programs.


 7. The Perl Debugger.


 8. Syntax Errors.


 9. Run-Time Exceptions.


10. Semantical Errors.


11. Resource Failure.


12. Perl as a Second Language.


13. Debugging CGI Programs.


Appendix A: Perl Debugger Commands.


Appendix B: Perls of Wisdom.


Index.

Preface

 

Perlness

In the world of languages, the country of Perl is the great melting pot which welcomes all cultures, religions, and beliefs. "Give me your tired, your poorly-supported programmers, your huddled masses yearning to be free of artificial limitations," says Perl, and those who land on its shores find an environment where they are no longer hampered by a language designer's whimsical notions of elegant semantics and stifling syntactical purity.

Perl's universal availability and ease-of-use make it the most democratic programming language. Unlike many other languages, a relative beginner can write useful programs, whereas effective programmers in other languages normally need to spend a lot longer to learn syntax, operators, and functions. A Perl programmer may possess such expertise, or may be a newcomer who modified some example script to perform a new function.

But the newcomer has another problem: lack of debugging skills. Experience forces the canny to develop an innate knack for debugging due to years of accumulated pain. We want to minimize that pain, because we have suffered it. Perl's ease of use allows programmers with little knowledge to create usable, if fragile, code. The amount of time it takes to debug a Perl program can vary dramatically from person to person. Our goal is to help you minimize the development, debugging, and maintenance time you need for your own Perl programs.

Do not take the title of this book to imply we are debugging Perl itself in these pages. What few bugs exist in the Perl interpreter are a matter of minute exotica (or exotic minutiae), rapidly squashed by the fine volunteer crew supporting Perl. A more accurate title would have been Debugging Your Perl Programs, but that felt too pedestrian and loses the "unplugged" pun.

We wrote this book because we wanted you to see the development process at work. Most books on programming contain carefully crafted examples honed through sweaty practice to work perfectly and stand as mute testimonial to the elegant style of the author. They don't show you the ugly, irritating process it took to get the examples into shape; yet those examples did not in fact spring into existence fully formed from the forehead of their creator. Because you will experience this same process when developing your programs, we want to guide you through it and describe various ways around the embarrassment, humiliation, and surprising pitfalls that stand between you and Great Programming.

Within this book, we describe the most common and annoying mistakes a new Perl programmer might make, and then detail the procedures to identify and correct those bugs and any others. You should have some knowledge of Perl; several fine tutorials exist to free us from the onerous responsibility of explaining scalars and arrays and hashes and the like. This preface includes a few references to some of the most useful of these tutorials.

We will not attempt to define or describe a proper programming "style." Style is as unique as an individual--but a few general rules create a common reference so that we can easily read each other's programs.

Neither is this a "how to program" book. Although we will probe into the mechanics and underpinnings of the general principle of programming at times, it is not our intention to inculcate a complete newcomer with the mindset of the programmer's discipline.

Who Are You?

If you've been programming in Perl anywhere from a week to a year and want to speed up your development cycle, this book is for you. We'll also address some issues related to developing in a team. This book is intended to assist those who have started learning Perl by providing practical advice on development practices.

What This Book Covers

Here's what you'll find in the rest of this book:

• Chapter 1: Introduction and a guided tour of the Perl documentation
• Chapter 2: Developing the right mindset for programming and developing effectively
• Chapter 3: "Gotchas" in Perl: Working your way around some of the tricky things to understand or get right in Perl programming
• Chapter 4: Antibugging: How to code defensively
• Chapter 5: How to instrument your code
• Chapter 6: How to test your Perl programs
• Chapter 7: A tour of the perl debugger: our guide to using this built-in tool
• Chapter 8: Types of syntax error and how to track down their causes
• Chapter 9: Run-time errors
• Chapter 10: Semantical errors: When your program appears to work but doesn't do the right thing
• Chapter 11: How to improve the performance of a resource-hungry (memory, CPU cycles, and so on) program
• Chapter 12: Tips and pitfalls for people coming to Perl from other languages
• Chapter 13: Common Gateway Interface (CGI) programming: special tips for debugging this type of Perl program
• Chapter 14: Conclusion
• Appendix A: Reference for the Perl debugger commands
• Appendix B: List of our "Perls of Wisdom"

We will spend a lot of time going through examples of problems and how you might debug them.

Getting Perl

While this isn't a book about how to install or build perl,1 we owe you at least rudimentary instructions on how to get a perl of your own.

• For Windows machines, get the free ActivePerl distribution:

http://www.activeState.com/ActivePerl/

download.htm

• For Macintoshes:

http://www.cpan.org/ports/index.html#mac

• For binary distributions for all other machines:

http://www.cpan.org/ports/

• For the source of perl itself:

http://www.cpan.org/src/

Building perl from source on a supported Unix architecture requires just these commands after you download and unpack the right file:

./Configure

make

make test

make install # if the make test succeeds

The Configure step asks you zillions of questions, and most people won't have a clue what many of those questions are talking about; but the default answers Configure recommends are usually correct.

For educational purposes, you may want to build a perl that has debugging enabled. (Here we refer to a perl that lets you use the special -D flag to enable the output of information that tells you what perl is doing with your program. This has nothing to do with Perl's built-in interactive debugger--which we discuss in Chapter 7--all perls have that.) If you want to do that, build perl from the source, and when Configure asks, " Any additional cc flags? " paste in whatever it already shows between brackets as a default and add " -DDEBUGGING ". See the perlrun POD page (explained later) for more information.

We occasionally refer to modules that are not part of the core Perl distribution but that can be found on the Comprehensive Perl Archive Network (CPAN). For instructions on how to find, download, and install a module from CPAN, see http://www.cpan.org/misc/cpan-faq.html.

For Further Reference

Visit this book's Web site at http://www.perldebugged.com .

Get introductions to Perl programming from the following (in rough order of usefulness):

• Learning Perl, 2nd ed., by Randal Schwartz and Tom Christiansen (O'Reilly & Associates, 1997)
• Programming Perl, 3rd ed., by Larry Wall, Tom Christiansen, and Jon Orwant (O'Reilly & Associates, 2000)
• Perl, the Programmer's Companion, by Nigel Chapman (John Wiley & Sons, 1998)
• Elements of Programming with Perl, by Andrew Johnson (Manning Publications, 1999)
• Effective Perl Programming, by Joseph Hall with Randal Schwartz (Addison-Wesley, 1998)

Perl Versions

In this book, we refer to the latest "stable" version of Perl, which is 5.6.0 as of this writing. The vast majority of what we say works unaltered on older versions of Perl 5, but not Perl 4. If you use any version of Perl older than 5.004_04, you should upgrade; 5.003 had issues such as security problems and memory leaks. You can find out the version number of your perl by passing it the -v flag:

% perl -v

This is perl, v5.6.0 built for i586-linux

Copyright 1987-2000, Larry Wall

...

Perl won't execute a script named on the command line if the -v flag is present. A more detailed description of your perl's configuration can be obtained with the -V flag; if you issue a bug report, the facility for doing that automatically includes this information with your report.

A separate development track exists for Perl; you will know if you have one of those versions because the release number either contains an underscore followed by a number of 50 or larger or contains an odd number between two dots. Nothing is guaranteed to work in such a distribution; it's intended for testing. If you find you have one and you didn't want it, the person who downloaded your perl probably visited the wrong FTP link.

It was announced at the fourth annual Perl Conference (Monterey, California, July 2000) that Perl 6 development was beginning in earnest, and backward compatibility need not stand in the way of doing good things. As of press time, discussion continues on new language features.

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Index

!, 238, 239

$, 75

to begin variables, 196

$" variable, 78

$$, 177

$_, 54-56, 252

&, 73, 252

;, 75

:: (colons), 31

; (semicolon) as statement separator, 130

?: (trinary) operator, 198

{ } (braces), 61-62

( ) (parentheses), 57-60, 66-67

need for, in function arguments, 197, 198

precedence and, 66, 252

putting empty, after function, 61, 252

' (apostrophe), 31, 136

. debugger command, 112, 243

(\) (backslashes ), 31

debugger line ending in, 238

< debugger command, 244

<< debugger command, 244

&& operator, 198

|| operators, 198

@ to begin variables, 196

| (vertical bar), 108

| operator, 162

A debugger command, 115-16

a debugger command, 115-16

Acceptance testing, 91, 103

action, 242

Action/breakpoint display, 242

alias, 239

Alteration typo, 129, 135-36

Antibugging, 27-51

Apple, Big, 68

Appleton, Brad, 158

Arguments, memorizing function calls with common,187-89

assert, 96

Associativity, 64

AtExit module, 158

AutoTrace debugger option, 248

Autovivification, 67-69

b debugger command, 114-15

B language, 64

Barr, Graham, 157

BBEdit editor, 32

Benchmark, 176

Benchmarkmodule, 179, 181-82, 253

Binding operator, 65-66

Bowling, 235

Braces { }, 61-62

Breakpointing, 241

Built-in functions, 76, 252

Bunce, Tim, 179

Burke, Sean M., 193

bus error, 5

c debugger command, 113-14

-c flag, 95

C programmer, tips for, 197-200

C++ programmer, tips for, 204-5, 206-7

-c switch, 144

Carp::Assert module, 96

Cat, Cheshire, 209-210, 231

catch statement, 157-158

CGI::Carp module, 227

CGI.pm, 224-25

Cgi-test sample program, 220

Christiansen, Tom, 40

Class::Struct, 204

Closure block, 121

Code walkthroughs, 90

Colons ( :: ), 31

Command aliasing, 239

Command history, 238-239

Command line testing, 225-27

Common Gateway Interface (CGI), 210-11, 254

debugging, 209-31

compile form, 242

comp.infosystems.www.authoring.cgi, 211

comp.lang.perl.misc, 210

Complexity, 47

condition, 242

Conditional my, 169-70

confess function, 159

Consistent style, 30, 252

Constant folding, 119-20

constant.pm, 50-51

Content-type, 218

Continuation lines, 238

Conway, Damian, 96, 197n, 199, 205

Coverage analysis, 100

cperl, Emacs mode, 30-32

CPU usage, measuring, 178-81

croak, 159

D, 242

d, 242

-d command line option, 107

-D flag, 88

-d flag, 231

Data::Dumper module, 80, 253

DB_File module, 192

ddd, 122-23

Debugger interaction, 228-30

Debuggers, as teaching tool, 126

Debugging Common Gateway Interface (CGI) programs, 209-31

Defensive programming. 28. See also Antibugging

defined operator, 45n

Deletion typo, 129-34

Delimiters, mismatched, 32

Deparse module, 166-68, 253

DESTROY, 206

Design by Contract, 96

Devel::Coverage module, 100-102

Devel::DProf module, 182-85, 253

Diagnostics, 46

Directories, reading, 163-64

do BLOCK while EXPRESSION construct, 166

Documentation, 36-39, 252

online, 7-15

Dominus, Mark-Jason, 187

"(double quote), 136

Dynamic variables, 197

-e code, 93

-e flag, 94

Emacs

debugging with, 123-25

editing with, 30-31

EQUIVALENCE, avoiding, 201

Errors. See also Syntax errors

distinction between exception and, 145

heading off, 218-20

semantical, 161-72

eval operator, 156-57

Exceptions

distinction between error and, 145

options affecting handling, 247

trapping, 157, 253

Execution speed, improving, 185-89

Extreme Programming (XP), 28, 90

fatalsToBrowser, 228

ferrets, 167

Feynmann, Richard, 92n

@files, 203

Filter, 78

500 Server Error, 214, 227

for loop, 76

foreach loop, 57

FORTRAN programmer, tips for, 200-201

403 Forbidden error, 214

Fun with Perl mailing list, 235

Function arguments, 197-98

Function calls, memorizing with common arguments, 187-89

gethostbyname, 188

Getopt::Long, 87

Global variables, 241

Golf, 235

Hall, Joseph N., 42n

Hash key, 69, 252

Help, 239-240

Hierarchical data structures, 199

Hietaniemi, Jarkko, 100, 179

Indenting, 29

insecure dependency in path, 217

Insertion typo, 129, 134-35

Inspection testing, 90, 92-93

Integration testing, 91

Internal Server Error, 212-14

Java programmer, tips for, 204-6

keys function, 66-67

Keywords

local, 44

my, 133

this, 204

König, Andreas, 96

L debgger command, 115

-l flag, 95

Lexical data, examining, 241

Liljegren, Jonas, 228

load, 242

Load testing, 91

local keyword, 44

local statement, 197

llocaltime function, 7

Logical XOR operator, 162

Loop variables, 76, 252

MacPerl, Perl documentation in, 14-15

man command, 240

maxTraceLen, 248

Memoize function, 187, 253

Memory usage

improving, 189-91

measuring, 176-78

Mismatched delimiters, 32

MLDBM (MultiLevel Database) module, 192-93

Moore's Law, 175

my keyword, 133

my statement, 170, 253

n debugger command, 108-9

Named variable, 55, 252

Natural language, 28, 252

.nrecognized switch error, 144-45

O debugger command, 245

Obfuscation, 235

$obj, 206

$obj-<can ('meth'), 206

Object-oriented features, 204

Observation, 34

One-liners, 93-96

One-time breakpoint, 114

Online documentation, 7-15

Options

control by another program, 247

exception handling, 247

setting, 245

V, X, and x command, 246

or logical operator, 6

ornaments, 248

our pragma, 44n

p debugger command, 110

Package data, examining, 240-241

Paging, 238

Parentheses ( ), 57-60, 66-67

need for, in function arguments, 197, 198

precedence and, 66, 252

putting empty, after function, 61, 252

Partition Magic, 24

Perl debugger, 105-26, 237-249

commands, 239-245

environmental variables, 248-249

general syntax, 238-239

options, 245-248

Perl (Practical Extraction and Report Language)

ease-of-use of, xii

pitfalls, 53-76

reasons for using, 3-4

as second language, 195-207

sources for, xv-xvi

testing programs, 89-103

universal availability, xii

Perl (Practical Extraction and Report Language) documentation, 20-21

in MacPerl, 14-15

on Windows, 8-9

Perl Mongers, 235

PERLDB_NOTTY, 249

PERLDB_RESTART, 249

perldoc command, 9-14

overload, 206-7

perl, 10

perldata, 11

perllexwarn, 46

perllol, 11

perlop, 11

perlre, 11

perlref, 11

perlsub, 12

perlsyn, 11

perltoot, 204

POD (plain old documentation), 38-39, 252

Polymorphism, 197

postpone form, 242

Precedence ordering, 62-67

Prechelt, Lutz, 4

printf, 74, 169

PrintRet, debugger option, 248

Pritikin, Joshua Nathaniel, 97

Prompt-time actions, 244

Prototypes, secret, 73-76

proxylog tool, 220-24

ps command, 178

ps -lp pid, 176

ps u pid, 176

Pseudo-hashes, 149-50

ptkdb debugger, 123-24, 230-31

PTKDB_DISPLAY, 231

purl, 236

q debugger command, 240

-q perldoc option, 12

qq quoting operator, 203

Quoting rules, 202

r debugger command, 109-10

readdir ( ), 163-64, 253

readline operator (<> ), 55

recallCommand debugger option, 238

Regression testing, 91

REMOTE_USER environment, 226

Reserved words, 71-73

Resource failure, 173-94

Restarting, 240

Return code, checking, 152-59

RFC 822 standard, 216

Run-time exceptions, 134, 143-59, 162, 205

return code, 152-59

symbolic references, 146-52

s debugger command, 108, 109

Sarathy, Gurusamy, 192

Saturation testing, 91, 102

Scalar variables, 197

Schwern, Michael, 96

Scope, 49-50

Security, 214-17

segmentation fault, 5

Semantical errors, 161-72

Semantics, 128

Semicolons as statement separators, 130

Shell interaction, 239

Shell programmer, tips for, 202-3

shellBang debugger option, 239

shuck, 15

Short-circuiting, 6, 162

Sigils, 196

Simpsons, The, 226

' (single quote), 31, 136

Software Engineering Institute's Capability Maturity Model for software development (SW-CMM), 25

sort argument, 58

Source listing, 243-44

splain program, 46

split function, 69-71

Stack display, 243

STATEMENT while EXPRESSION construct, 166

STDERR, 218

STDOUT, 219

Stepping, 240

Stopping, 240

String delimiters, 136

Style, 29

Subroutine prototypes, 207

Subroutines, 73, 252

substr function, 198-99

switch statement, 199

Symbolic references, 146-52

Syntax, 128

Syntax errors, 127-41, 253

alteration, 129, 135-36

deletion, 129, 130-34

insertion, 129, 134-35

transposition, 129, 136-38

System testing, 91, 96-102

t command, 118

-T flag, 215, 217, 230, 231

Too late for, 217

Taint mode, 215-17

debugging, 217

Temporary files, avoiding, 194

Test module, 97, 99

Test::Harness module, 96

Term::ReadLine module, 229

this keyword, 204

throw statement, 157-158

top command, 176, 178

Torkington, Nathan, 40

Tracing, code, 77-88

Tracing, debugger, 244

Transposition typo, 129, 136-38

Trinary (?:) operator, 198

try statement, 157-158

Turing, Alan, 174

Turing machine, 174

Typo, 128

union statement, 199

Unit testing, 90, 93-96

unpack, 177

use constant, 51

use diagnostics, 46, 252

use strict, 42-45, 132, 146, 252

use vars pragma, 44n

V, X, and x command, options affecting, 246

V debugger command, 111

$var, 241

Variables, 196

declaring, as lexical, 44, 252

dynamic, 197

scalar, 197

Version information, 241

Vertical bar ( | ), 108

vim editor, 32

-w,, 46, 61-62, 134, 218, 252

W debugger command, 117

w debugger command, 112

-w flag, 59, 94, 132, 145, 176

-w option set, 107

Wall, Larry, 4, 233, 236

Watchpoints, 117, 244

WEB, 39

Web servers, 211-12

while loop, 55, 56, 57, 252

Windows, Perl documentation on, 89

Writing code, 29

clarity, 32-34

style, 29-30

x debugger command, 110-11

YAPC, 235

Zakharevich, Ilya, 30

Zeller, Andreas, 122

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