How to generate only a statically linked clib?

[hannesm]

I want to build only a statically linked c library, as done by passing custom or failsafe to ocamlmklib, but I fail to find a way how to pass such an option via _tags (and/or myocamlbuild.ml) to the build system.

repository in question is mirage-xen on the topkg branch. Any hints?

[hannesm]

In the latest commit on the branch mentioned above, the target is only libmirage-xen_bindings.a, I don't understand why ocamlbuild (or ocamlmklib) nevertheless tries to build the shared object.

[gasche]

I actually know little about static vs. dynamic linking in the non-OCaml world, so I am not sure how to test whether my experiments work or not. Please be patient :-) Also, I haven't had the time to build all of mirage-xen's dependecies, so I decided to experiment with a small repro case.

I have a file lib.ml and lib.mli with dummy content. The only rule in ocamlbuild -documentation that mentions ocamlmklib is the .clib one, so I created a libfoo.clib file with the following content:

lib.o

then ocamlbuild -verbose 1 -tag custom libfoo.a (or listing true: custom in _tags) runs the following commands:

ocamldep.opt -modules lib.ml > lib.ml.depends
ocamldep.opt -modules lib.mli > lib.mli.depends
ocamlc.opt -c -o lib.cmi lib.mli
ocamlopt.opt -c -o lib.cmx lib.ml
ocamlmklib.opt -o foo lib.o

If I understand correctly, the lack of a -custom flag passed to ocamlmklib in the final call is the issue for you. This should clearly be fixed upstream, but in the meantime I wrote the following myocamlbuild.ml script:

open Ocamlbuild_plugin

let _ = dispatch begin function
  | After_rules ->
    flag ["ocamlmklib"; "custom"] & A"-custom";
  | _ -> ()    
end

With this plugin file, I get the following behavior for the same command:

ocamldep.opt -modules lib.ml > lib.ml.depends
ocamldep.opt -modules lib.mli > lib.mli.depends
ocamlc.opt -c -o lib.cmi lib.mli
ocamlopt.opt -c -o lib.cmx lib.ml
ocamlmklib.opt -o foo -custom lib.o

(the -custom flag is indeed passed.)

Does that suffice to fix your issue?

Note: if you wish to integrate the plugin within an OASIS project like mirage-xen, you should locate the

...
(*OASIS_STOP*)
Ocamlbuild_plugin.dispatch dispatch_default;;

line after the (OASIS_STOP) comment, and replace it with

...
(*OASIS_STOP*)
open Ocamlbuild_plugin

let _ = dispatch begin fun hook ->
  dispatch_default hook; (* this calls the OASIS stuff *)
  match hook with
  | After_rules ->
    flag ["ocamlmklib"; "custom"] & A"-custom";
  | _ -> ()    
end

[hannesm]

thanks @gasche, I used your snippets in mirage/mirage-xen#1