How to debug GRASS modules and libraries with GNU gdb and DDD:

Re-compile the modules and libraries you want to debug with the -g switch to
the gcc compiler and without the -s switch to the linker: Edit
src/CMD/head/head and head.in for the above configuration. Remove the file
in src/CMD/next_step/. Then remove the OBJ.* and LIB.* directories and
recompile (with gmake5 or with make install).

Now you can start a grass5 session, start up the DDD debugger (ddd&) and
select the module to debug. The binary in $GISBASE/bin/ is not what you want
to look at, as this only switches between the interactive version and the
command line version of the module in many cases. The command line version
of the module is in $GISBASE/etc/bin/main/cmd, the interactive version in
$GISBASE/etc/bin/main/inter. For the "contributed" modules, the executable
is found in $GISBASE/etc/bin/contrib/inter or $GISBASE/etc/bin/contrib/cmd
respectively.

-- 
Andreas Lange, andreas.lange@rhein-main.de