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Setting up an OCaml project with OASIS

OASIS allows to give a high level description of your project and will generate a build system and packaging information for you. It allows to set up a project in a quick and simple way. The goal of this tutorial is not to explain all the details of OASIS, but to provide the necessary information to setup a project.

Getting started

You first need to install OASIS. The preferred way is with opam:

opam install oasis

You can also install it under debian-based system.

sudo apt-get install oasis

Finally, a tarball is available on OASIS's main page.

What is your project ?

Your project description is going to be located in an _oasis file, at the root of your project. Let's start by a general description of your project!

Name: FooBar
Version: 0.1
Synopsis: A library about Foo and Bar.
Authors: Me <me@myhost.com>
License: LGPL with OCaml linking exception

Description: FooBar allows to combine foo and bar efficiently. It also allows to interface with baz.
Homepage: http://foobar.org

This description might seem unnecessary at first, but will be mandatory if you want to package and distribute your library later on. It's also a nice source of information for the interested user.

Only the first set of field is mandatory. You can find a list of all the available licenses here.

A little bit of OASIS boilerplate

OASIS needs some information to proceed. We will come back and explain the details later on. Just copy this part to your _oasis for now.

OASISFormat: 0.4
BuildTools: ocamlbuild
Plugins: META (0.4), DevFiles (0.4)

From now on, we will assume that the _oasis contains everything described previously.

The meat of your project

An executable

An _oasis file is organized in sections, each section defining a small part of your project. _oasis files are white-space sensitive, fields in a section must be properly indented.

We will start by defining an executable foo with sources located in an src directory.

Executable "foo"
  Path: src
  MainIs: foo.ml

Now that the _oasis file is defined, we can generate a build system. At the root of your project, run the following command:

oasis setup -setup-update dynamic

This will produce the following files : setup.ml, configure and a Makefile. These files are the only one necessary for OASIS to work, and you should put only those in a version control system. OASIS will produce some other files during building that you can safely ignore. More details about the various OASIS files are given here.

You can now build your project by running:


It will produce the foo.byte file than you can execute.

To produce native executables and use external libraries, we add some fields to the section:

Executable "foo"
  Path: src
  MainIs: foo.ml
  CompiledObject: best

CompiledObject can take the values byte, native or best. best is equal to native if it is available or byte otherwise. When the compilation is native, the produced binary is called foo.native. BuildDepends is a comma separated list of ocamlfind packages. ocamlfind handles packages locally in your system and is used by opam. You can list ocamlfind packages with ocamlfind list.

Defining libraries

To define a library, we use the Library section. Here we will define two libraries with some inter dependencies.

Library "foo"
  Path: foo_src
  Modules: Foo
  InternalModules: InternalFoo

Library "bar"
  Path: bar_src
  Modules: Bar
    InternalBar, InternalFooBar
  BuildDepends: foo, unix

As you can see, BuildDepends is also used for intra-project dependencies: the bar library is dependent on the foo library. Modules and InternalModules are comma separated list of modules. They should contain only capitalized name with no file extensions. They can include module provided by any type of file, being .ml, .mli, .mll or .mly (for ocamllex, ocamlyacc and menhir). InternalModules is used to list modules that are part of the library but should be kept hidden and not be part of the public interface of the library.

make builds the various libraries and make install installs them. OASIS handles the installation using ocamlfind.

Other optional sections

Executable and Library are enough to create a basic project. The following sections allow to provide more information and build extra things to distribute your project.


Documentation is a very important part of a library. OASIS allows to build an ocamldoc-powered HTML documentation for the complete project quite easily.

AlphaFeatures: ocamlbuild_more_args
Document "foobar_api"
  Type: ocamlbuild (0.4)
  BuildTools: ocamldoc

  Title: API reference for FooBar
  XOCamlbuildPath: .
    "-docflags '-colorize-code -short-functors -charset utf-8'"
  XOCamlbuildLibraries: foo, bar

The important part is the XOCamlbuildLibraries field, which is a list of ocamlfind libraries defined in the _oasis file. XOCamlbuildExtraArgs allows to give some ocamldoc command line flags. Those presented here are considered essential. Running make doc will create a folder foobar_api.docdir which contains the documentation as HTML files.

SourceRepository and .gitignore

As meta-data, OASIS also allows to define the address of the source repository. This can be used by packaging tools.

SourceRepository master
  Type: git
  Location: https://my_git_host.org/foobar.git
  Branch: master
  Browser: https://my_git_host.org/foobar

While we are on the subject of version control system, here is an example of .gitignore file that will ignore just the right file for your project using OASIS.



Going further

You now should be able to build simple projects with OASIS. You should also know enough to browse the The official OASIS documentation and look at some examples to find out the various fields you can put in an _oasis file.

However, OASIS has more features!

Sub libraries and OCamlfind manipulations

OCamlfind allows to organize libraries into hierarchies, for example let's look at some of lwt's ocamlfind libraries:


It is possible to define such hierarchy in OASIS using two additional fields: FindLibName and FindLibParent. Let say we want to isolate the core of foo in a library foo.core:

Library "foo_core"
  FindLibName: core
  FindLibParent: foo
  Path: foo
  Modules: Foo

Library "foo"
  Path: foo
  Modules: FooMore
  BuildDepends: foo.core

Flags and conditional compilation

OASIS allows to define a set of flags that can be activated by the user or by a package manager like opam.

Flag "with-baz"
  Description: Build baz support for bar
  Default: false

configure --enable-with-baz will turn this flag on. Be careful that _ will be turned into - in the command line option.

You can then condition the build of some library or executable by this flag.

Library "bar_with_baz"
  Build$: flag(baz)
  FindLibName: baz
  FindLibParent: bar
  Path: bar
  Modules: BarBaz
  BuildDepends: bar, baz

Notice the $: which is specific for boolean fields. Conditional values are explained in details here.


OASIS can also handle tests, be them unit tests or not. First we want to test our foo executable defined earlier.

Test "test_foo"
  TestTools: foo
  Command: $foo test/some_test_input

TestTools allows to give the dependency. To enable and run the tests, run configure --enable-tests and make test. Tests are disabled by default.

For unit testing, we can declare an executable only for tests, using the oUnit testing framework for example, and a test using it. For this, we use the predefined flag tests.

Executable "test_bar_and_baz"
  Path: test
  MainIs: test_bar.ml
  Build$: flag(tests) && flag(baz)
  CompiledObject: best
  Install: false
  BuildDepends: bar, baz, bar.baz, oUnit

Test "test_bar_and_baz"
  Run$: flag(tests) && flag(baz)
  TestTools: test_bar_and_baz
  Command: $test_bar_and_baz
  WorkingDirectory: test

Syntax extensions

Using a syntax extension

Camlp4 syntax extensions are almost always bundled as ocamlfind subpackages finishing in .syntax. When this is the case, we can just add the name of the package as a dependency:

Library "foo"
  Path: foo
  Modules: FooMore
  BuildDepends: foo.core, lwt, lwt.syntax

OASIS will deduce that camlp4 must be used.

If the package's name doesn't end by .syntax (which is pretty unlikely), you can add this to the _tags file, outside of the OASIS section (OASIS_START/OASIS_STOP):

true: syntax(camlp4o)

See this section for details about the _tags file.

Creating a syntax extension

To define a syntax extension, we must define the special properties XMETAType:

Library "foo_syntax"
  Path: foo_syntax
  FindLibName: syntax
  FindLibParent: foo
  Modules: Pa_foo
  BuildDepends: campl4
  XMETAType: syntax

This will ensure that this library is properly packaged by ocamlfind as a syntax extension.

Interfacing with C code

To build a joint library with OCaml and C code, we can use the CSources field. The CCLib allows to link against C libraries and the CCOpt field allows to pass options to the C compiler.

Library "foo-with-C"
  Path: lib
  Modules: FooC
  CSources: fooC_stubs.c, fooC_stubs.h
  BuildDepends: foo
  CCLib: -lfoo
  CCOpt: -O2 -Wall -Wextra -Wno-unused-parameter # Better safe than sorry.

You should be careful not to call your C files the same as your OCaml files, in order to avoid clashes between .so generated files. The usual idiom is to append _stubs to the name.

For details about C bindings, you can consult this tutorial.

Wrapping up a package

Earlier, we generated the build system with the command line oasis setup -setup-update dynamic. This dynamic feature means that OASIS will be called to regenerate the build system when the _oasis file changes. It also produces very few and small files. This means that it's very convenient for the developer. However, it makes the build system dependent on OASIS, which is not acceptable for releases. In order to generate an OASIS independent build system, you can simply run oasis setup. You can learn more about this here.

The OASIS description file can be used to generate packages. Two tools are available for that: oasis2opam will generate an opam descriptions and oasis2debian for .deb packages. Of course, don't forget to use the non dynamic setup before packaging, as describe earlier, or the package will need OASIS to be build.

In order to ease the deployment, small scripts can be used. For example, tyxml uses a Makefile.dist and lwt a dist.sh.


OASIS is better learned by examples _oasis files you can copy and tweak. OASIS sources contain a gallery of very instructive examples.

However, nothing is better than real world examples:

  • ocaml-inotify contains a very simple yet feature-full _oasis file that will showcase all the various elements explained in this tutorial.
  • lwt's _oasis is very complete and will probably cover all your needs.
  • cohttp shows how to avoid dependencies to syntax packages with XMETARequires.
  • batteries uses OASIS for meta-data only and handles the build system separately using the XCustom fields.
  • Some other examples.

Under the hood

This section aims to explain in slightly greater depth some of the technical details behind OASIS.


OASIS uses ocamlbuild as underlying build system. This has several consequences.

  • Compilation is done in the _build directory, not in the sources itself. binaries and documentations target are symlinked to the root directory of the project, in order to be accessible directly.
  • Some options and customization of the build system are only available by modifying the myocamlbuild.ml and _tags files. Those files are partially generated by OASIS so, if you modify them, you should keep the OASIS section. If the setup was dynamic, this files are not present at all, you should then add the section yourself.

For myocamlbuild:


For _tags:


The OASIS files

OASIS generates all the file needed by an OCaml installation, which is quite an important amount. In this section, we will give a detailed description of each files.

All these files will contain an OASIS section that should not be modified.

    • setup.ml contains the logic of the build system.
    • Makefile and configure are convenient files produced by the plugin DevFiles. They call setup.ml.
    • setup.log is produced by the configure step.
    • setup.data is produced during the build process.
  • OCamlbuild
    • _tags allows to give specific options to the various file in your project.
    • myocamlbuild.ml allows to customize the build system by using Ocamlbuild's library.
    • *.mllib, *.mldylib and *.clib are targets that indicate how to build the various sub libraries.
  • OCamlfind
    • META is the library specification for ocamlfind.

Setup options

oasis setup accepts an option -setup-update defined in the help as:

  -setup-update {none|weak|dynamic}  Define the way `setup.ml` should update
                                     when `_oasis` change.

This option allows to adapt the way OASIS will generate the build system according to your needs.

  • none, the normal mode, will generate all the files presented in the previous section and their content. The build system generated will be independent of OASIS and will only need Ocamlbuild.

  • dynamic will only generate three files : setup.ml, configure and Makefile. Other files are dynamically created when building. These three files are quite small (especially setup.ml) and are not going to be modified. Hence they can be safely pushed into a repository. If you want to include some code in _tags for example, just put an empty OASIS section, as described here, and OASIS will dynamically populate it at runtime. This allows to push only the minimum amount of generated file in the repository and is very convenient for developing. The downside is that this setup need OASIS to be installed.

Plugins and Features

We used during this tutorial some plugins without explaining exactly what they do. We will give here a brief description of each of these plugins, plus some other potentially useful ones.

  • Plugins (the Plugins field):
    • META makes OASIS generates the META file used by ocamlfind.
    • DevFiles makes OASIS generates configure and Makefile.
    • StdFiles makes OASIS generates README.txt, INSTALL.txt and AUTHORS.txt using the metadatas contained in _oasis.
  • Features (AlphaFeatures and BetaFeatures fields) are various recent additions to OASIS that are not completely battle-tested. We give here only those that are stable enough to be used. Note that they will be integrated into OASIS properly in a short to mid term.
    • ocamlbuild_more_args (alpha) enables the XOCamlbuildExtraArgs field that allows to provide arguments to Ocamlbuild directly. It's a way to internalize in _oasis some information that would be in _tags otherwise.
    • compiled_setup_ml (alpha) will make OASIS compiles the setup.ml file instead of interpreting it, giving a significant speed up.
    • pure_interface (alpha) is necessary to be able to compile standalone .mli files (without any .ml). This feature is used in tyxml for example.
    • stdfiles_markdown (alpha) makes OASIS generate StdFiles in markdown format.