mpi in toplevel

[pklehre-ocamlec]

Is it possible to use mpi via the toplevel (ocaml, utop, or jupyter)? I get the the following error message:

# #require "mpi";;
Error: The external function `caml_mpi_send_int' is not available

I am using the '4.14.0+domains+flambda' opam switch.

[xavierleroy]

I, too, find this error puzzling!

Looking at the output of ocamlobjinfo on the mpi.cma archive gives a clue:

File /home/xleroy/.opam/4.14.0/lib/mpi/mpi.cma
Force custom: YES
Extra C object files: -lcamlmpi -lmpi
Extra C options: -L/usr/lib/x86_64-linux-gnu
Extra dynamically-loaded libraries:

The library does not provide its OCaml-C stub code as a DLL, so that's why it cannot be loaded in a toplevel REPL. I would hope we could have the stub code as a DLL, if we used ocamlmklib to build mpi.cma. But there may be obscure MPI-specific problems.

TODO: try to build mpi.cma and mpi.cmxa using ocamlmklib.

Also, if you're using a distributed implementation of MPI (e.g. on a cluster), I'm not sure what happens if you use it interactively via the toplevel REPL on one node and non-interactively(?) on the other nodes.