A quick thumbnail to cross debugging core files

This is just a quick thumbnail about how to take a core file from a FreeBSD/arm box and debug it on a FreeBSD/i386 box you cross compiled the FreeBSD/arm image on.

For normal gdb, one would just type:
  • gdb prog prog.core
and be done with it. However, without careful setup, this won't work when cross debugging. If you do the above, you'll get a number of warnings or errors from gdb when it tries to load the i386 libraries with the arm binary.

There is a simple trick, however, that gets around these problems. It can be used both when debugging cross architecture and when debugging different a core from one release on a newer/older release of FreeBSD. This example, I use 'gdb-arm' which is the debugger I use for cross debugging.
  • # gdb-arm
  • (gdb) set solib-absolute-prefix /path/to/built/image
  • (gdb) file prog
  • (gdb) core-file prog.core
  • (gdb)
By setting solib-absolute-prefix, you are instructing gdb to use that path as a prefix for all files it has to look up. So rather than /lib/libc.so.7, it will look in /path/to/built/image/lib/libc.so.7. Now that you have all the libraries that you need for debugging, you'll be able to track down the problems in no time.

This is just the barest of sketches for what to do. There are many other ways to make debugging easier in a cross debugging environment, and gdb can do more than just look at core files. But those topics will have to wait for another day. The curious are invited to read through the gdb info files. They are very complete, and well indexed by google or other search engines.