Foreword
In Federalist 51, James Madison wrote:“If men were angels, no government would
be necessary.” If he lived today, Madison might have written: “If software devel-
opers were angels, debugging would be unnecessary.” Most of us, however, make
mistakes, and many of us even make errors while designing and writing software.
Our mistakes need to be found and fixed, an activity called debugging that origi-
nated with the first computer programs. Today every computer program written
is also debugged, but debugging is not a widely studied or taught skill. Few books,
beyond this one, present a systematic approach to finding and fixing programming
errors.
Be honest: Does debugging seem as important, difficult, or worthy of study as
writing a program in the first place? Or, is it just one of those things that you need
to do to finish a project? Software developers though spend huge amounts of time
debugging—estimates range up to half or more of their workdays. Finding and fixing
bugs faster and more effectively directly increases productivity and can improve
program quality by eliminating more defects with available resources. Preventing
mistakes in the first place would be even better, but no one has yet found the
technique to prevent errors, so effective debugging will remain essential.
Improved programming languages and tools can supplant, but not eliminate,
debugging, by statically identifying errors and by dynamically detecting invariant
violations. For example, the type system in modern languages such as Java and
C# prevents many simple mistakes that slip by C programmers. Moreover, these
languages’ runtime bounds checks stop a program when it strays out of bounds,
which may be billions of instructions before the error manifests itself. Unfortunately,
there are countless ways in which a program can go wrong, almost all of which
languages and tools cannot detect or prevent. For example, in recent years there
has been considerable work in verifying sequences of operations in a program.Tools
can ensure that a file is opened before a program reads it, but they cannot check
that the correct file is accessed or that the program properly interprets its contents.
If either mistake occurs, someone still must debug the program to understand the
error and determine how to fix it.
In addition, debugging can be an enjoyable activity that shares the thrill of the
hunt and chase found in a good detective novel or video game. On the other hand,
a protracted, unsuccessful search for a bug in your code quickly loses its charm,
particularly when your boss is asking repeatedly about your (lack of ) progress.
Learning to debug well is essential to enjoying software development.
This book can teach you how to debug more effectively. It is a complete and
pragmatic overview of debugging, written by a talented researcher who has devel-
oped many clever ways to isolate bugs. It explains best practices for finding and
fixing errors in programs,ranging from systematically tracking error reports, repro-
ducing failures, observing symptoms, isolating the cause, and correcting defects.
Along with basic techniques and commonly used tools, the book also explores the
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