How to Report Bugs Effectively 
Simon Tatham

Introduction

Anybody who has written software for public use will probably have
received at least one bad bug report. Reports that say nothing ("It
doesn't work!"); reports that make no sense; reports that don't give
enough information; reports that give wrong information. Reports of
problems that turn out to be user error; reports of problems that turn
out to be the fault of somebody else's program; reports of problems
that turn out to be network failures.

There's a reason why technical support is seen as a horrible job to be
in, and that reason is bad bug reports. However, not all bug reports
are unpleasant: I maintain free software, when I'm not earning my
living, and sometimes I receive wonderfully clear, helpful, informative
bug reports.

In this essay I'll try to state clearly what makes a good bug report.
Ideally, I would like everybody in the world to read this essay before
reporting any bugs to anybody. Certainly I would like everybody who
reports bugs to me to have read it.

In a nutshell, the aim of a bug report is to enable the programmer to
see the program failing in front of him. You can either show him in
person, or give him careful and detailed instructions on how to make it
fail. If he can make it fail, he will try to gather extra information
until he knows the cause. If he can't make it fail, he will have to ask
you to gather that information for him.

In bug reports, try to make very clear what are actual facts ("I was at
the computer and this happened") and what are speculations ("I think
the problem might be this"). Leave out speculations if you want to, but
don't leave out facts.

When you report a bug, you are doing so because you want the bug fixed.
There is no point in swearing at the programmer or being deliberately
unhelpful. It may be his fault and your problem, and you might be right
to be angry with him, but the bug will get fixed faster if you help by
supplying all the information he needs. Remember also that if the
program is free, the author is providing it out of kindness, so if too
many people are rude to him, he may stop feeling kind.

"It doesn't work."

Give the programmer some credit for basic intelligence. If the program
really didn't work at all, he would probably have noticed. Since he
hasn't noticed, it must be working for him. Therefore, either you are
doing something differently, or your environment is different from
his.  He needs information; providing this information is the purpose
of a bug report. More information is almost always better than less.

Many programs, particularly free ones, publish their list of known
bugs. If you can find a list of known bugs, it's worth reading it to
see if the bug you've just found is already known or not. If it's
already known, it probably isn't worth reporting again, but if you
think you have more information than the report in the bug list, you
might want to contact the programmer anyway. He might be able to fix
the bug more easily if you can give him information he didn't already
have.

This essay is full of guidelines. None of them is an absolute rule.
Particular programmers have particular ways they like bugs to be
reported. If the program comes with its own set of bug-reporting
guidelines, read them. If the guidelines that come with the program
contradict the guidelines in this essay, follow the ones that come with
the program!

If you are not reporting a bug but just asking for help using the
program, you should state where you have already looked for the answer
to your question. ("I looked in chapter 4 and section 5.2 but couldn't
find anything that told me if this is possible.") This will let the
programmer know where people will expect to find the answer, so they
can make the documentation easier to use.

"Show me."

One of the very best ways you can report a bug is by showing it to the
programmer. Stand him in front of your computer, fire up his software,
and demonstrate the thing that goes wrong. Let him watch you start the
machine, watch you run the software, watch how you interact with the
software, and watch what the software does in response to your inputs.

He knows that software like the back of his hand. He knows which parts
he trusts, and he knows which parts are likely to have faults. He knows
intuitively what to watch for. By the time the software does something
obviously wrong, he may well have already noticed something subtly
wrong earlier which might give him a clue. He can observe everything
the computer does during the test run, and he can pick out the
important bits for himself.

This may not be enough. He may decide he needs more information, and
ask you to show him the same thing again. He may ask you to talk him
through the procedure, so that he can reproduce the bug for himself as
many times as he wants. He might try varying the procedure a few times,
to see whether the problem occurs in only one case or in a family of
related cases. If you're unlucky, he may need to sit down for a couple
of hours with a set of development tools and really start
investigating. But the most important thing is to have the programmer
looking at the computer when it goes wrong. Once he can see the problem
happening, he can usually take it from there and start trying to fix
it.

"Show me how to show myself."

This is the era of the Internet. This is the era of worldwide
communication. This is the era in which I can send my software to
somebody in Russia at the touch of a button, and he can send me
comments about it just as easily. But if he has a problem with my
program, he can't have me standing in front of it while it fails. "Show
me" is good when you can, but often you can't.

If you have to report a bug to a programmer who can't be present in
person, the aim of the exercise is to enable him to reproduce the
problem. You want the programmer to run his own copy of the program, do
the same things to it, and make it fail in the same way. When he can
see the problem happening in front of his eyes, then he can deal with
it.

So tell him exactly what you did. If it's a graphical program, tell him
which buttons you pressed and in what order you pressed them. If it's a
program you run by typing a command, show him precisely what command
you typed. Wherever possible, you should provide a verbatim transcript
of the session, showing what commands you typed and what the computer
output in response.

Give the programmer all the input you can think of. If the program
reads from a file, you will probably need to send a copy of the file.
If the program talks to another computer over a network, you probably
can't send a copy of that computer, but you can at least say what kind
of computer it is, and (if you can) what software is running on it.

"Works for me. So what goes wrong?"

If you give the programmer a long list of inputs and actions, and he
fires up his own copy of the program and nothing goes wrong, then you
haven't given him enough information. Possibly the fault doesn't show
up on every computer; your system and his may differ in some way.
Possibly you have misunderstood what the program is supposed to do, and
you are both looking at exactly the same display but you think it's
wrong and he knows it's right.

So also describe what happened. Tell him exactly what you saw. Tell him
why you think what you saw is wrong; better still, tell him exactly
what you expected to see. If you say "and then it went wrong", you have
left out some very important information.

If you saw error messages, tell the programmer, carefully and
precisely, what they were. They are important! At this stage, the
programmer is not trying to fix the problem; he's just trying to find
it. He needs to know what has gone wrong, and those error messages are
the computer's best effort to tell you that. Write the errors down if
you have no other easy way to remember them, but it's not worth
reporting that the program generated an error unless you can also
report what the error message was.

In particular, if the error message has numbers in it, do let the
programmer have those numbers. Just because you can't see any meaning
in them doesn't mean there isn't any. Numbers contain all kinds of
information that can be read by programmers, and they are likely to
contain vital clues. Numbers in error messages are there because the
computer is too confused to report the error in words, but is doing the
best it can to get the important information to you somehow.

At this stage, the programmer is effectively doing detective work. He
doesn't know what happened, and he can't get close enough to watch it
happening for himself, so he's searching for clues that might give it
away. Error messages, incomprehensible strings of numbers, and even
unexplained delays are all just as important as fingerprints at the
scene of a crime. Keep them!

If you are using Unix, the program may have produced a core dump. Core
dumps are a particularly good source of clues, so don't throw them
away. On the other hand, most programmers don't like to receive huge
core files by e-mail without warning, so ask before mailing one to
anybody. Also, be aware that the core file contains a record of the
complete state of the program: any "secrets" involved (maybe the
program was handling a personal message, or dealing with confidential
data) may be contained in the core file.

"So then I tried..."

There are a lot of things you might do when an error or bug comes up.
Many of them make the problem worse. A friend of mine at school deleted
all her Word documents by mistake, and before calling in any expert
help, she tried reinstalling Word, and then she tried running Defrag.
Neither of these helped recover her files, and between them they
scrambled her disk to the extent that no Undelete program in the world
would have been able to recover anything. If she'd only left it alone,
she might have had a chance.

Users like this are like a mongoose backed into a corner: with its back
to the wall and seeing certain death staring it in the face, it attacks
frantically, because doing something has to be better than doing
nothing. This is not well adapted to the type of problems computers
produce.

Instead of being a mongoose, be an antelope. When an antelope is
confronted with something unexpected or frightening, it freezes. It
stays absolutely still and tries not to attract any attention, while it
stops and thinks and works out the best thing to do. (If antelopes had
a technical support line, it would be telephoning it at this point.)
Then, once it has decided what the safest thing to do is, it does it.

When something goes wrong, immediately stop doing anything. Don't touch
any buttons at all. Look at the screen and notice everything out of the
ordinary, and remember it or write it down. Then perhaps start
cautiously pressing "OK" or "Cancel", whichever seems safest.  Try to
develop a reflex reaction - if a computer does anything unexpected,
freeze.

If you manage to get out of the problem, whether by closing down the
affected program or by rebooting the computer, a good thing to do is to
try to make it happen again. Programmers like problems that they can
reproduce more than once. Happy programmers fix bugs faster and more
efficiently.

"I think the tachyon modulation must be wrongly polarised."

It isn't only non-programmers who produce bad bug reports. Some of the
worst bug reports I've ever seen come from programmers, and even from
good programmers.

I worked with another programmer once who kept finding bugs in his own
code and trying to fix them. Every so often he'd hit a bug he couldn't
solve, and he'd call me over to help. "What's gone wrong?" I'd ask. He
would reply by telling me his current opinion of what had caused the
problem.

This worked fine when his current opinion was right. It meant he'd
already done half the work and we were able to finish the job
together.  It was efficient and useful.

But quite often he was wrong. We would work for some time trying to
figure out why some particular part of the program was producing
incorrect data, and eventually we would discover that it wasn't, that
we'd been investigating a perfectly good piece of code for half an
hour, and that the actual problem was somewhere else.

I'm sure he wouldn't do that to a doctor. "Doctor, I think I've got
Perkin's disease." People know not to say that to a doctor: you
describe the symptoms, the actual discomforts and aches and pains and
rashes and fevers, and you let the doctor do the diagnosis of what
caused the problem. Otherwise, the doctor dismisses you as a
hypochondriac or crackpot, and quite rightly so.

It's the same with programmers. Providing your own diagnosis might be
helpful sometimes, but always state the symptoms. The diagnosis is an
optional extra, and not an alternative to giving the symptoms. Equally,
sending a modification to the code to fix the problem is a useful
addition to a bug report but not an adequate substitute for one.

If a programmer asks you for extra information, don't make it up!
Somebody reported a bug to me once, and I asked him to try a command
that I knew wouldn't work. The reason I asked him to try it was that I
wanted to know which of two different error messages it would give.
Knowing which error message came back would give a vital clue. But he
didn't actually try it; he just mailed me back and said "No, that won't
work". It took me some time to persuade him to try it for real.

Using your intelligence to help the programmer is fine. Even if your
deductions are wrong, the programmer should be grateful that you at
least tried to make his life easier. But report the symptoms as well,
or you may well make his life much more difficult instead.

"That's funny, it did it a moment ago."

Say "intermittent fault" to any programmer and watch his face fall. The
easy problems are the ones in which failure is caused by a simple
sequence of actions. The programmer can then repeat those actions under
closely observed test conditions and watch what happens in great
detail. Too many problems simply don't work that way: there will be
programs that fail once a week, or fail once in a blue moon, or never
fail when you try them in front of the programmer but always fail when
you have a deadline coming up.

Most intermittent faults are not truly intermittent. Most of them have
some logic somewhere. Some might occur when the machine is running out
of memory, some might occur when another program tries to modify a
critical file at the wrong moment, and some might occur only in the
first half of every hour! (I've actually seen one of these.)

Also, if you can reproduce the bug but the programmer can't, it could
very well be that his computer and your computer are different in some
way and this difference is causing the problem. I had a program once
whose window curled up into a little ball in the top left corner of the
screen, and sat there and sulked. But it only did it on 800x600
screens; it was fine on my 1024x768 monitor.

The programmer will want to know anything you can find out about the
problem. Try it on another machine, perhaps. Try it twice or three
times and see how often it fails. If it goes wrong when you're doing
serious work but not when you're trying to demonstrate it, it might be
long running times or large files that make it fall over. Try to
remember as much detail as you can about what you were doing to it when
it did fall over, and if you see any patterns, mention them. Anything
you can provide has to be some help. Even if it's only probabilistic
(such as "it tends to crash more often when Emacs is running"), it
might not provide direct clues to the cause of the problem, but it
might help the programmer reproduce it.

Most importantly, the programmer will want to be sure of whether he's
dealing with a true intermittent fault or a machine-specific fault.  He
will want to know lots of details about your computer, so he can work
out how it differs from his. A lot of these details will depend on the
particular program, but one thing you should definitely be ready to
provide is version numbers. The version number of the program itself,
and the version number of the operating system, and probably the
version numbers of any other programs that are involved in the
problem.

"So I loaded the disk on to my Windows..."

Writing clearly is essential in a bug report. If the programmer can't
tell what you meant, you might as well have not said anything.

I get bug reports from all around the world. Many of them are from
non-native English speakers, and a lot of those apologize for their
poor English. In general, the bug reports with apologies for their poor
English are actually very clear and useful. All the most unclear
reports come from native English speakers who assume that I will
understand them even if they don't make any effort to be clear or
precise.

Be specific.
      If you can do the same thing two different ways, state which one
      you used. "I selected Load" might mean "I clicked on Load" or "I
      pressed Alt-L". Say which you did. Sometimes it matters.  

Be verbose.
      Give more information rather than less. If you say too much, the
      programmer can ignore some of it. If you say too little, he has
      to come back and ask more questions. One bug report I received
      was a single sentence. Every time I asked for more information,
      the reporter would reply with another single sentence. It took me
      several weeks to get a useful amount of information, because it
      turned up one short sentence at a time.  

Be careful of pronouns.
      Don't use words like "it", or references like "the window", when
      it's unclear what they mean. Consider this: "I started FooApp. It
      put up a warning window. I tried to close it and it crashed." It
      isn't clear what the user tried to close. Did he try to close the
      warning window, or the whole of FooApp? It makes a difference.
      Instead, you could say "I started FooApp, which put up a warning
      window. I tried to close the warning window, and FooApp crashed."
      This is longer and more repetitive, but also clearer and easier
      to understand.  

Read what you wrote.
      Read the report back to yourself, and see if you think it's
      clear. If you have listed a sequence of actions which should
      produce the failure, try following them yourself, to see if you
      missed a step.

Summary

 - The first aim of a bug report is to let the programmer see the failure
   with his own eyes. If you can't be with him to make it fail in front of
   him, give him detailed instructions so that he can make it fail for
   himself.  

 - In case the first aim doesn't succeed, and the programmer can't see it
   failing himself, the second aim of a bug report is to describe what went
   wrong.  Describe everything in detail. State what you saw, and also
   state what you expected to see. Write down the error messages,
   especially if they have numbers in them.  

 - When your computer does something unexpected, freeze. Do nothing until
   you're calm, and don't do anything that you think might be dangerous.  

 - By all means, try to diagnose the fault yourself if you think you can,
   but if you do, you should still report the symptoms as well.  

 - Be ready to provide extra information if the programmer needs it. If he
   didn't need it, he wouldn't be asking for it. He isn't being
   deliberately awkward. Have version numbers at your fingertips, because
   they will probably be needed.  

 - Write clearly. Say what you mean, and make sure it can't be
   misinterpreted.  

 - Above all, be precise. Programmers like precision.

Disclaimer: I've never actually seen a mongoose or an antelope. My
zoology may be inaccurate.
