Debugging sasa ocaml Algorithms

This tutorial propose various ways to debug sasa Algorithms (others than from code review or printf debugging). rdbg is (currently) not able to inspect the ocaml runtime system, but the usual ocaml debugging tools can be used.

Set the seed

In order to debug, it is generally easier if the randomness is frozen. You can do that using the -seed option of sasa. This is good to know when you encounter a sporadic behavior that you want to replay from the debugger.

Each time you launch sasa with no seed, a random one is chosen and saved into the <graph-name>.seed file, as well as in the rif file. You can use this seed to replay a simulation. The --replay option do this automatically by reading the <graph-name>.seed file is it exists.

As far as custom daemons are concerned, Lutin also has a -seed option and a --replay option.

Using the ocaml toplevel #trace directive

Since rdbg is a wrapper around an ocaml top-level, you can use its directives, and in particular the #trace one.

For example, if your algorithm is implemented in a file named, in order to see the calls the function step_f, you can do the following:

(rdbg) #trace P.step_f

However, this trace mode is not always convenient to debug programs, and it is sometimes more handy to use ocamldebug.

Using ocamldebug

To use ocamldebug, you can use the --ocamldebug (-ocd) option of rdbg:

cd test/unison/
rdbg --sasa -ocd -sut "sasa -sd -seed 42"
(ocd) break @ unison 23
(ocd) run

nb: rdbg --sasa -ocd launches ocamldebug luretteSession/luretteSession<some integer>. This is good to know if you need to relaunch ocamldebug (or to launch it via emacs).

Printing 's Algo.neighbor values from ocamldebug

If you want to print values of variables of type 's Algo.neighbor, you can use the Algo.fmt_print_neighbor function. To be able to use it from ocamldebug, you first need to load and install the Algo.fmt_print_neighbor printer:

dir $OPAM_SWITCH_PREFIX/lib/sasacore
dir $OPAM_SWITCH_PREFIX/lib/sasa
dir $OPAM_SWITCH_PREFIX/lib/algo
dir $OPAM_SWITCH_PREFIX/lib/lutils
dir $OPAM_SWITCH_PREFIX/lib/ocamlgraph/
load_printer str.cma
load_printer nums.cma
load_printer graph.cma
load_printer lutils.cma
load_printer sasacore.cma
load_printer algo.cma
install_printer Algo.fmt_print_neighbor

nb: you can put the instructions above can be in the .ocamldebug resource file.

(ocd) print nl