Dealing with foreign libraries — dune documentation

# Dealing with foreign libraries

The OCaml programming language can interface with libraries written in foreign languages such as C. This section explains how to do this with Dune. Note that it does not cover how to write the C stubs themselves, this is covered by the OCaml manual

More precisely, this section covers: - how to add C/C++ stubs to an OCaml library - how to pass specific compilation flags for compiling the stubs - how to build a library with a foreign build system

Note that in general Dune has limited support for building source files written in foreign languages. This support is suitable for most OCaml projects containing C stubs, but is too limited for building complex libraries written in C or other languages. For such cases, Dune can integrate a foreign build system into a normal Dune build.

## Adding C/C++ stubs to an OCaml library

To add C stubs to an OCaml library, simply list the C files without the `.c` extension in the Foreign stubs field. For instance:

    (library
     (name mylib)
     (foreign_stubs (language c) (names file1 file2)))

You can also add C++ stubs to an OCaml library by specifying `(language cxx)` instead.

Dune is currently not flexible regarding the extension of the C/C++ source files. They have to be `.c` for C files and `.cpp`, `.cc` or `.cxx` for C++ files. If you have source files with other extensions and you want to build them with Dune, you need to rename them first. Alternatively, you can use the foreign build sandboxing method described below.

### Header files

C/C++ source files may include header files in the same directory as the C/C++ source files or in the same directory group when using include_subdirs.

The header files must have the `.h` extension.

### Installing header files

It is sometimes desirable to install header files with the library. For that you have two choices: install them explicitly with an install stanza or use the `install_c_headers` field of the library stanza. This field takes a list of header files names without the `.h` extension. When a library install header files, these are made visible to users of the library via the include search path.

## Foreign build sandboxing

When the build of a C library is too complicated to express in the Dune language, it is possible to simply sandbox a foreign build. Note that this method can be used to build other things, not just C libraries.

To do that, follow the following procedure:

- put all the foreign code in a sub-directory
- tell Dune not to interpret configuration files in this directory via an data_only_dirs stanza
- write a custom rule that:
  - depends on this directory recursively via source_tree
  - invokes the external build system
- attach the C archive files to an OCaml library via Foreign archives.

For instance, let’s assume that you want to build a C library `libfoo` using `libfoo`’s own build system and attach it to an OCaml library called `foo`.

The first step is to put the sources of `libfoo` in your project, for instance in `src/libfoo`. Then tell dune to consider `src/libfoo` as raw data by writing the following in `src/dune`:

    (data_only_dirs libfoo)

The next step is to setup the rule to build `libfoo`. For this, writing the following code `src/dune`:

    (rule
     (deps (source_tree libfoo))
     (targets libfoo.a dllfoo.so)
     (action (progn
              (chdir libfoo (run make))
              (copy libfoo/libfoo.a libfoo.a)
              (copy libfoo/libfoo.so dllfoo.so))))

We copy the resulting archive files to the top directory where they can be declared as `targets`.

The last step is to attach these archives to an OCaml library as follows:

    (library
     (name bar)
     (foreign_archives foo))

Then, whenever you use the `bar` library, you will also be able to use C functions from `libfoo`.

### Limitations

When using the sandboxing method, the following limitations apply:

- the build of the foreign code will be sequential
- the build of the foreign code won’t be incremental

both these points could be improved. If you are interested in helping make this happen, please let the Dune team know and someone will guide you.

### Real example

The re2 project uses this method to build the re2 C library. You can look at the file `re2/src/re2_c/dune` in this project to see a full working example.