Foreign Sources, Archives, and Objects

Dune provides basic support for including foreign source files as well as archives of foreign object files into OCaml projects via the foreign_stubs and foreign_archives fields. Individual object files can also be included via the extra_objects field.

Foreign Stubs

You can specify foreign sources using the foreign_stubs field of the library and executable stanzas. For example:

 (name lib)
 (foreign_stubs (language c) (names src1 src2))
 (foreign_stubs (language cxx) (names src3) (flags -O2)))

Here we declare an OCaml library lib, which contains two C sources src1 and src2, and one C++ source, src3, which need to be compiled with -O2. These source files will be compiled and packaged with the library, along with the link-time flags to be used when linking the final executables. When matching names to source files, Dune treats *.c files as C sources, and *.cpp, *.cc, and *.cxx files as C++ sources.

Here is a complete list of supported subfields:

  • language specifies the source language, where c means C and cxx means C++. In the future, more languages may be supported.

  • names specifies the names of source files. When specifying a source file, omit the extension and any relative parts of the path; Dune will scan all library directories to find all matching files and raise an error if multiple source files map to the same object name. If you need to have multiple object files with the same name, you can package them into different Foreign Archives via the foreign_archives field. This field uses the Ordered Set Language where the :standard value corresponds to the set of names of all source files whose extensions match the specified language.

  • flags are passed when compiling source files. This field is specified using the Ordered Set Language, where the :standard value comes from the environment settings c_flags and cxx_flags, respectively. Note that, for C stubs, Dune unconditionally adds the flags present in the OCaml config fields ocamlc_cflags and ocamlc_cppflags to the compiler command line. This behavior can be disabled since Dune 2.8 via the dune-project option use_standard_c_and_cxx_flags.

  • include_dirs are tracked as dependencies and passed to the compiler via the -I flag. You can use Variables in this field and refer to a library source directory using the (lib library-name) syntax. Additionally, the syntax (include filename) can be used to specify a file containing additional arguments to (include_dirs ...). The named file can either contain a single path to be added to this list of include directories, or an S-expression listing additional (include_dirs ...) arguments (the (lib ...) and (include ...) syntax is also supported in files included in this way). For example, (include_dirs dir1 (lib lib1) (lib lib2) (include inc1) dir2) specifies the directory dir1, the source directories of lib1, and lib2, the list of directories contained in the file inc1, and the directory dir2, in this order. Some examples of possible contents of the file inc1 are:

    • dir3 which would add dir3 to the list of include directories

    • ((lib lib3) dir4 (include inc2)) which would add the source directory of the library lib3, the directory dir4, and the result of recursively including the contents of the file inc2. The contents of included directories are tracked recursively, e.g., if you use (include_dir dir) and have headers dir/base.h and dir/lib/lib.h, they both will be tracked as dependencies.

    • extra_deps specifies any other dependencies that should be tracked. This is useful when dealing with #include statements that escape into a parent directory like #include "../a.h".

Mode-Dependent Stubs

Since Dune 3.5, it is possible to use different foreign stubs when building in native or byte mode. This feature needs to be activated by adding (using mode_specific_stubs 0.1) in the dune-project file.

Then it is allowed to use the mode field when describing foreign_stubs. If the same stub is defined twice, Dune will automatically chose the correct one. This allows the use of different sets of flags or even different source files from which the stubs are built.

 (name main)
 (modes native byte_complete)
   (language c)
   (mode byte)
   (names c_stubs))
   (language c)
   (mode native)
   (flags :standard -DNATIVE_CODE) ; A flag specific to native builds
   (names c_stubs)))  ; This could be the name of an implementation
                      ; specific to native builds

Note that, as of version 0.1 of this extension, this mechanism does not work for foreign_archives.

Foreign Archives

You can also specify archives of separately compiled foreign object files that need to be packaged with an OCaml library or linked into an OCaml executable. To do that, use the foreign_archives field of the corresponding library or executable stanza. For example:

 (name lib)
 (foreign_stubs (language c) (names src1 src2))
 (foreign_stubs (language cxx) (names src3) (flags -O2))
 (foreign_archives arch1 some/dir/arch2))

Here, in addition to Foreign Stubs, we also specify foreign archives arch1 and arch2, where the latter is stored in a subdirectory some/dir.

You can build a foreign archive manually, e.g., using a custom rule as described in Foreign Build Sandboxing, or ask Dune to build it via the foreign_library stanza:

 (archive_name arch1)
 (language c)
 (enabled_if true)
 (names src4 src5)
 (include_dir headers))

This asks Dune to compile C source files src4 and src5 with headers tracked in the headers directory and put the resulting object files into an archive arch1, whose full name is typically libarch1.a for static linking and for dynamic linking.

The foreign_library stanza supports all Foreign Stubs fields. The archive_name field specifies the archive’s name. You can refer to the same archive name from multiple OCaml libraries and executables, so a foreign archive is a bit like a foreign library, hence the name of the stanza. The enabled_if field has the same meaning as in the library stanza.

Foreign archives are particularly useful when embedding a library written in a foreign language and/or built with another build system. See Foreign Build Sandboxing for more details.

Extra Objects

It’s possible to specify native object files to be packaged with OCaml libraries or linked into OCaml executables. Do this by using the extra_objects field of the library or executable stanzas. For example:

 (public_name main)
 (extra_objects foo bar))

 (targets foo.o bar.o)
 (deps foo.c bar.c)
 (action (run ocamlopt %{deps})))

This example builds an executable which is linked against a pair of native object files, foo.o and bar.o. The extra_objects field takes a list of object names, which correspond to the object file names with their path and extension omitted.

In this example, the sources corresponding to the objects (foo.c and bar.c) are assumed to be present in the same directory as the OCaml source code, and a custom rule is used to compile the C source code into object files using ocamlopt. This is not necessary; one can instead compile foreign object files manually and place them next to the OCaml source code.


Depending on the use_standard_c_and_cxx_flags option, the base :standard set of flags for C will contain only ocamlc_cflags or both ocamlc_cflags and ocamlc_cppflags.

There are multiple levels where one can declare custom flags (using the Ordered Set Language), and each level inherits the flags of the previous one in its :standard set:

  • In the global env definition of a dune-workspace file

  • In the per-context env definitions in a dune-workspace file

  • In the env definition of a dune file

  • In a foreign_ field of an executable or a library

The %{cc} variable will contain the flags from the first three levels only.