Learn to Debug
Posted by admin on July 12 2010 10:02:02
Debugging is the cornerstone of being a programmer. The first meaning of the verb to debug is to
remove errors, but the meaning that really matters is to see into the execution of a program by examining
it. A programmer that cannot debug effectively is blind.

Idealists that think design, or analysis, or complexity theory, or whatnot, are more fundamental are
not working programmers. The working programmer does not live in an ideal world. Even if you
are perfect, your are surrounded by and must interact with code written by major software companies,
organizations like GNU, and your colleagues. Most of this code is imperfect and imperfectly
documented. Without the ability to gain visibility into the execution of this code the slightest
bump will throw you permanently. Often this visibility can only be gained by experimentation,
that is, debugging.

Debugging is about the running of programs, not programs themselves. If you buy something from
a major software company, you usually don't get to see the program. But there will still arise
places where the code does not conform to the documentation (crashing your entire machine is a
common and spectacular example), or where the documentation is mute. More commonly, you
create an error, examine the code you wrote and have no clue how the error can be occurring. Inevitably,
this means some assumption you are making is not quite correct, or some condition arises
that you did not anticipate. Sometimes the magic trick of staring into the source code works. When
it doesn't, you must debug.

To get visibility into the execution of a program you must be able to execute the code and observe
something about it. Sometimes this is visible, like what is being displayed on a screen, or the delay
between two events. In many other cases, it involves things that are not meant to be visible, like
the state of some variables inside the code, which lines of code are actually being executed, or
whether certain assertions hold across a complicated data structure. These hidden things must be

The common ways of looking into the ‘innards’ of an executing program can be categorized as:
• Using a debugging tool,
• Printlining --- Making a temporary modification to the program, typically adding lines that
print information out, and

• Logging --- Creating a permanent window into the programs execution in the form of a log.
Debugging tools are wonderful when they are stable and available, but the printlining and logging
are even more important. Debugging tools often lag behind language development, so at any point
in time they may not be available. In addition, because the debugging tool may subtly change the
way the program executes it may not always be practical. Finally, there are some kinds of debugging,
such as checking an assertion against a large data structure, that require writing code and
changing the execution of the program. It is good to know how to use debugging tools when they
are stable, but it is critical to be able to employ the other two methods.

Some beginners fear debugging when it requires modifying code. This is understandable---it is a
little like exploratory surgery. But you have to learn to poke at the code and make it jump; you
have to learn to experiment on it, and understand that nothing that you temporarily do to it will
make it worse. If you feel this fear, seek out a mentor---we lose a lot of good programmers at the
delicate onset of their learning to this fear.

by Robert L. Read