When someone asks you to supply a backtrace (from a, for example), here is roughly what you have to do.
Run the faulting program like this:
$ gdb --args PROGRAM [ARGUMENTS] [...] (gdb) run [...] [Segmentation fault.] (gdb) bt [...]
This will only provide useful information if the executable (and additional libraries) contain debugging symbols, or those are available separartely.
If you're one a Debian GNU/Hurd system, you'll have to have the hurd-dbg package installed for debugging Hurd stuff, and perhaps libc0.3-dbg as well if it's inside the glibc.