# Using \`findlib.dynload\` inside a PPX

**URL:** <https://discuss.ocaml.org/t/using-findlib-dynload-inside-a-ppx/5222>\
**Category:** Learning\
**Tags:** ppx, findlib\
**Created:** [February 26, 2020, 4:22pm UTC](https://discuss.ocaml.org/t/using-findlib-dynload-inside-a-ppx/5222 "2020-02-26T16:22:29Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![thierry-martinez](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/thierry-martinez/32/1481_2.png) [@thierry-martinez](https://discuss.ocaml.org/u/thierry-martinez)\
**Post date:** [February 26, 2020, 4:22pm UTC](https://discuss.ocaml.org/t/using-findlib-dynload-inside-a-ppx/5222/1 "2020-02-26T16:22:29Z")

</div>

Inside a PPX transformer, I do not manage to use `Fl_dynload.load_packages` from `findlib.dynload` package to load dynamically packages: packages are found but no dynamic modules seem to be loaded. The problem only appears inside a PPX transformer: if the same loading is made in a simple executable, it works as expected.

I put a simple example to illustrate the problem in [https://github.com/thierry-martinez/test\_findlib](https://github.com/thierry-martinez/test_findlib).

A module `Plugin` is defined in the directory `plugin/`, which relies on the library `pcre`.  
The dynamic library `plugin/plugin.cmxs` can be built with `dune build plugin/plugin.cmxs`.

There is no problem when `plugin/plugin.cmxs` is loaded by a simple executable, such as the one defined in `test_direct/`. The code of `test_direct` is fairly simple.

```ocaml
let () =
  Findlib.init ();
  Fl_dynload.load_packages ~debug:true ["pcre"];
  Dynlink.loadfile "plugin/plugin.cmxs"

```

By building `dune build test_direct/test_direct.exe`, one can check that executing `test_direct/test_direct.exe` in the directory `_build/default` succeeds without error (it is important to execute the program with `_build/default` as current directory, in order to let the program find `plugin/plugin.cmxs`). The debugging log is reproduced below. In particular, we can notice that `pcre.cmxs` is loaded.

```auto
[DEBUG] Fl_dynload: about to load: unix
[DEBUG] Fl_dynload: files="unix.cmxs"
[DEBUG] Fl_dynload: loading "unix.cmxs"
[DEBUG] Fl_dynload: about to load: threads.posix
[DEBUG] Fl_dynload: files=""
[DEBUG] Fl_dynload: about to load: threads
[DEBUG] Fl_dynload: files=""
[DEBUG] Fl_dynload: about to load: pcre
[DEBUG] Fl_dynload: files="pcre.cmxs"
[DEBUG] Fl_dynload: loading "pcre.cmxs"

```

By contrast, if the same code as in `test_direct` is executed inside a PPX, `pcre.cmxs` is not loaded and, therefore, loading `plugin.cmxs` fails. One can check this by trying to compile `dune build test_ppx/test_ppx.exe`, which invokes the PPX transformer defined in `ppx/`. The latter just wraps the test above in an `Ast_mapper.mapper`. The log is reproduced below, where it appears that `pcre.cmxs` does not seem to be loaded.

```auto
[DEBUG] Fl_dynload: about to load: pcre
[DEBUG] Fl_dynload: files=""
Fatal error: exception Dynlink.Error (Dynlink.Cannot_open_dll "Dynlink.Error (Dynlink.Cannot_open_dll \"Failure(\\\"/home/tmartine/tmp/test_findlib/_build/default/plugin/plugin.cmxs: undefined symbol: camlPcre__exec_583\\\")\")")  

```

---

<div class="post-metadata">

**Author:** ![thierry-martinez](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/thierry-martinez/32/1481_2.png) [@thierry-martinez](https://discuss.ocaml.org/u/thierry-martinez)\
**Post date:** [February 27, 2020, 12:26pm UTC](https://discuss.ocaml.org/t/using-findlib-dynload-inside-a-ppx/5222/2 "2020-02-27T12:26:15Z")

</div>

It appears that to make `findlib.dynload` work, `dune` generates a file `findlib_initl.ml-gen` to initialize findlib predicates and record packages linked with the executable. But this file is generated for executable target only, and is not generated for the intermediate executable generated for ppx transformations. In the directory `ppx_fixed/` of [https://github.com/thierry-martinez/test\_findlib](https://github.com/thierry-martinez/test_findlib), the module `Findlib_for_ppx` provides a function `init_predicates` to initialize findlib predicates, and a function `load_packages` that does not fail if a module is already linked (since we do not know which packages have already been linked).

I filled an issue for dune: [https://github.com/ocaml/dune/issues/3214](https://github.com/ocaml/dune/issues/3214)