Debugging with GDB (September 2007)

324 Debugging with GDB
Sometimes people give a few sketchy facts and ask, “Does this ring a bell?” Those bug
reports are useless, and we urge everyone to refuse to respond to them except to chide the
sender to report bugs properly.
To enable us to fix the bug, you should include all these things:
• The version of GDB. GDB announces it if you start with no arguments; you can also
print it at any time using show version.
Without this, we will not know whether there is any point in looking for the bug in the
current version of GDB.
• The type of m achine you are using, and the operating system name and version number.
• What compiler (and its version) was used to compile the program you are debugging—
e.g. “HP92453-01 A.10.32.03 HP C Compiler”. Use the what command with the
pathname of the compile command (‘what /opt/ansic/bin/cc’, for example) to obtain
this information.
• The command arguments you gave the compiler to compile your example and observe
the bug. For example, did you use ‘-O’? To guarantee you will not omit something
important, list them all. A copy of the Makefile (or the output from make) is sufficient.
If we were to try to guess the arguments, we would probably guess wrong and then we
might not encounter the bug.
• A complete input script, and all necessary source files, that will reproduce the bug.
• A description of what behavior you observe that you believe is incorrect. For example,
“It gets a fatal signal.”
Of course, if the bug is that GDB gets a fatal signal, then we will certainly notice it.
But if the bug is incorrect output, we might not notice unless it is glaringly wrong.
You might as well not give us a chance to make a mistake.
Even if the problem you experience is a fatal signal, you should still say so explicitly.
Suppose something strange is going on, such as, your copy of GDB is out of synch, or
you have encountered a bug in the C library on your system. (This has happened!)
Your copy might crash and ours would not. If you told us to expect a crash, then when
ours fails to crash, we would know that the bug was not happening for us. If you had
not told us to expect a crash, then we would not be able to draw any conclusion from
our observations.
Here are some things that are not necessary:
• A description of the envelope of the bug.
Often people who encounter a bug spend a lot of time investigating which changes to
the input file will make the bug go away and which changes will not affect it.
This is often time consuming and not very useful, because the way we will find the
bug is by running a single example under the debugger with breakpoints, not by pure
deduction from a series of examples. We recommend that you save your time for
something else.
Of course, if you can find a simpler example to report instead of the original one, that
is a convenience for us. Errors in the output will be easier to spot, running under the
debugger will take less time, and so on.
However, simplification is not vital; if you do not want to do this, report the bug
anyway and send us the entire test case you used.