Debugging
This page is mainly about how to gather more information in connection with bug reports. Even though the word "debug" is used, it is not intended as a guide for how to debug programs while developing.
When an application fails
Run it from the commandline
If an application suddenly crashes, try running it from a terminal emulator. Type in the name of the application in lowercase letters. If you do not know the name of the executable, only the name of the package, the following command can find the name of the executable. Replace packagename with the name of the package:
$ pacman -Ql packagename | grep ' /usr/bin/.'
Check availability of a core dump
A core dump is a file containing a process's address space (memory) when the process terminates unexpectedly. If the application is compiled in a debug-friendly way, the "core" file can be used to find out where things went wrong.
The location of core dumps may vary depending on the operating system configuration. See core dump to find whether generation of core dump files is enabled on your system and where do they go.
Segmentation faults
There are several techniques that can be used to figure out what went wrong. Put your detective hat on.
Gdb
gdb is an ancient and well tested application for debugging applications. See Debugging/Getting traces#Getting the trace for more instructions how to use it to obtain a trace. While running from gdb you might have to wait for the segfault. Afterwards, post the trace to a pastebin service and include the URL in your bug report.
If you have a "core" file, it can be used together with gdb to get a backtrace:
$ gdb appname core bt full
Valgrind
Assuming you have an unstripped binary without inlined functions, it is usually a good idea to also run that program through valgrind. valgrind is a tool that emulates a CPU and usually shows where things go wrong or provide additional info in addition to gdb.
$ valgrind appname
it will provide a lot of helpful debug output if there is a crash. Consider -v and --leak-check=full to get even more info.
Alternatively, use:
$ valgrind --tool=callgrind appname
and run the output through kcachegrind to graphically explore the functions the program uses. If a program hangs, this makes it easier to pinpoint the location of the error.
Missing files or libraries
Strace
strace finds out in detail what an application is actually doing. If an application tries to open a file that is not there, it can be discovered by strace.
For finding which files a program named appname tries to open:
$ strace -eopen appname
Save the output, post it to a pastebin service and keep the URL in handy.
strace -o /dev/stdout appname | grep string.LD_DEBUG
Setting LD_DEBUG=files gives another overview of what files an application is looking for. For an application named appname:
$ LD_DEBUG=files appname > appname.log 2>&1
The output will end up in appname.log.
For more information, see ld-linux(8).
Readelf
If you get no such file or directory when running an application, try the following command:
$ readelf -a /usr/bin/appname | grep interp
(replace /usr/bin/appname with the location of your executable)
Make sure the interpreter in question (like /lib/ld-linux-x86-64.so.2) actually exists. Install ld-lsb if need be.
If it is not written in C or C++, but perhaps in Python
Use file on the executable to get more information:
$ file /usr/bin/appname
If it says ELF it is a binary executable and is usually written in C or C++. If it says Python script you know you are dealing with an application written in Python.
If it is a shell script, open up the shell script in a text editor and see (usually at the bottom of the file) if you can find the name of the real application (ELF file). You can then temporarily put "gdb" right in the shellscript, before the name of the executable, for debugging purposes. See the sections about gdb further up. Prefix the command with gdb --args if the executable in question needs arguments as well.
For pure shell scripts, you can also use bash -x script_name or bash -xv script_name.
For Python applications, the output will often say which file and line number the crash occurred at. If you are proficient with Python, you can try to fix this and include the fix in the bug report.
Report the bug
Please report a bug at https://bugs.archlinux.org and possibly also directly to the developers of the application in question, then include a link in the Arch Linux bug report. This helps us all.
However, if you think there is something wrong with the application itself, and not with how it is packaged, report the bug directly to upstream (which means the developers of the application). Normally, software streams from developers, through packagers/maintainers and down to users. Upstream means the other way, so for this case: directly to the developers of an application.