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, but 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

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 it 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:

 (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 installs header files, they are made visible to users of the library via the include search path.

Stub Generation with Dune Ctypes

Beginning in Dune 3.0, it’s possible to use the ctypes field to generate bindings for C libraries without writing any C code.

Note that Dune support for this feature is experimental and is not subject to backward compatibility guarantees.

To use Dune ctypes stub generation, you must provide two OCaml modules: a “type description” module for describing the C library types and constants, and a “function description” module for describing the C library functions. Additionally, you must list any C headers and a method for resolving build and link flags.

If you’re binding a library distributed by your OS, you can use the pkg-config utility to resolve any build and link flags. Alternatively, if you’re using a locally installed library or a vendored library, you can provide the flags manually.

The “type description” module must define a functor named Types with signature Ctypes.TYPE. The “function description” module must define a functor named Functions with signature Ctypes.FOREIGN.

A Toy Example

To begin, you must declare the ctypes extension in your dune-project file:

(lang dune 3.16)
(using ctypes 0.3)

Next, here is a dune file you can use to define an OCaml program that binds a C system library called libfoo, which offers foo.h in a standard location.

 (name foo)
 (libraries core)
 ; ctypes backward compatibility shims warn sometimes; suppress them
 (flags (:standard -w -9-27))
  (external_library_name libfoo)
  (build_flags_resolver pkg_config)
  (headers (include "foo.h"))
   (instance Types)
   (functor Type_description))
   (concurrency unlocked)
   (instance Functions)
   (functor Function_description))
  (generated_types Types_generated)
  (generated_entry_point C)))

This field will introduce a module named C into your project, with the sub-modules Types and Functions that will have your fully-bound C types, constants, and functions.

Given libfoo with the C header file foo.h:

#define FOO_VERSION 1

int foo_init(void);

int foo_fnubar(char *);

void foo_exit(void);

Your example file is:

open Ctypes

module Types (F : Ctypes.TYPE) = struct
  open F

  let foo_version = constant "FOO_VERSION" int

Your example file is:

open Ctypes

(* This Types_generated module is an instantiation of the Types
   functor defined in the file. It's generated by
   a C program that Dune creates and runs behind the scenes. *)
module Types = Types_generated

module Functions (F : Ctypes.FOREIGN) = struct
  open F

  let foo_init = foreign "foo_init" (void @-> returning int)

  let foo_fnubar = foreign "foo_fnubar" (string_opt @-> returning int)

  let foo_exit = foreign "foo_exit" (void @-> returning void)

Finally, the entry point of your executable named above,, demonstrates how to access the bound C library functions and values:

let () =
  if (C.Types.foo_version <> 1) then
    failwith "foo only works with libfoo version 1";

  match C.Functions.foo_init () with
  | 0 ->
    C.Functions.foo_fnubar "fnubar!";
    C.Functions.foo_exit ()
  | err_code ->
    Printf.eprintf "foo_init failed: %d" err_code;

From here, one only needs to run dune build ./foo.exe to generate the stubs and build and link the example foo.exe program.

Complete information about the ctypes combinators used above is available at the ctypes project.

Ctypes Field Reference

The ctypes field can be used in any executable(s) or library stanza.

    (external_library_name <package-name>)
      (instance <module-name>)
      (functor <module-name>))
      (instance <module-name>)
      (functor <module-name>)
    (generated_entry_point <module-name>)
  • type_description: the functor module is a description of the C library types and constants written in the ctypes domain-specific language you wish to bind. The instance module is the name of the instantiated functor, inserted into the top-level of the generated_entry_point module.

  • function_description: the functor module is a description of the C library functions written in the ctypes domain-specific language you wish to bind. The instance module is the name of the instantiated functor, inserted into the top-level of the generated_entry_point module. The function_description field can be repeated. This is useful if you need to specify sets of functions with different concurrency policies (see below).

The instantiated types described above can be accessed from the function descriptions by referencing them as the module specified in optional generated_types field.

<optional-ctypes-fields> are:

  • (build_flags_resolver <pkg_config|vendored-field>) tells Dune how to compile and link your foreign library. Specifying pkg_config will use the pkg-config tool to query the compilation and link flags for external_library_name. For vendored libraries, provide the build and link flags using vendored field. If build_flags_resolver is not specified, the default of pkg_config will be used.

  • (generated_types <module-name>) is the name of an intermediate module. By default, it’s named Types_generated. You can use this module to access the types defined in Type_description from your Function_description module(s).

  • (generated_entry_point <module-name>) is the name of a generated module that your instantiated Types and Functions modules will instantiated under. We suggest calling it C.

  • Headers can be added to the generated C files:

    • (headers (include "include1" "include2" ...)) adds #include <include1>, #include <include2>. It uses the Ordered Set Language.

    • (headers (preamble <preamble>) adds directly the preamble. Variables can be used in <preamble> such as %{read: }.

  • Since the Dune’s ctypes feature is still experimental, it could be useful to add additional dependencies in order to make sure that local headers or libraries are available: (deps <deps-conf list>). See Dependency Specification for more details.

<optional-function-description-fields> are:

  • (concurrency <sequential|unlocked|lwt_jobs|lwt_preemptive>) tells ctypes stubgen whether to call your C functions with the runtime lock held or released. These correspond to the concurrency_policy type in the ctypes library. If concurrency is not specified, the default of sequential will be used.

  • (errno_policy <ignore_errno|return_errno>) specifies the errno_policy passed to the code generator. With ignore_errno, the errno variable is not accessed or returned by function calls. With return_errno, all functions will return the tuple (retval, errno).

<vendored-field> is:

  • (vendored (c_flags <flags>) (c_library_flags <flags>)) provide the build and link flags for binding your vendored code. You must also provide instructions in your dune file on how to build the vendored foreign library; see the foreign_library stanza. Usually the <flags> should contain :standard in order to add the default flags used by the OCaml compiler for C files use_standard_c_and_cxx_flags.

Foreign Build Sandboxing

When the build of a C library is too complicated to express in the Dune language, it’s 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

    • copies the generated files

    • the C archive .a must be built with -fpic

    • the must be copied as, and no should appear, otherwise the dynamic linking of the C library will be attempted. However, this usually fails because the isn’t available at the time of the execution.

  • 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:

 (deps (source_tree libfoo))
 (targets libfoo.a
   (chdir libfoo (run make))
   (copy libfoo/libfoo.a libfoo.a)
   (copy libfoo/

We copy the resulting archive files to the top directory where they can be declared as targets. The build is done in a no-infer action because libfoo/libfoo.a and libfoo/ are dependencies produced by an external build system.

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

 (name bar)
 (foreign_archives foo))

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


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’re 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.