General Concepts


Any directory containing at least one <package>.opam file defines a scope. This scope is the subtree starting from this directory, excluding any other scopes rooted in subdirectories.

Typically, any given project will define a single scope. Libraries and executables that aren’t meant to be installed will be visible inside this scope only.

Because scopes are exclusive, if you wish to include your current project’s dependencies in your workspace, you can copy them in a vendor directory, or any name of your choice. Dune will look for them there rather than in the installed world, and there will be no overlap between the various scopes.

Ordered Set Language

A few fields take an ordered set as argument and can be specified using a small DSL.

This DSL is interpreted by Dune into an ordered set of strings using the following rules:

  • :standard denotes the standard value of the field when it’s absent

  • an atom not starting with a : is a singleton containing only this atom

  • a list of sets is the concatenation of its inner sets

  • (<sets1> \ <sets2>) is the set composed of elements of <sets1> that do not appear in <sets2>

In addition, some fields support the inclusion of an external file using the syntax (:include <filename>). For instance, this is useful when you need to run a script to figure out some compilation flags. <filename> is expected to contain a single S-expression and cannot contain (:include ...) forms.

Note that inside an ordered set, the first element of a list cannot be an atom except if it starts with - or :. The reason for this is that we’re planning to add simple programmatic features in the future so that one may write:

(flags (if (>= %{ocaml_version} 4.06) ...))

This restriction will allow you to add this feature without introducing breaking changes. If you want to write a list where the first element doesn’t start with -, you can simply quote it: ("x" y z).

Most fields using the ordered set language also support Variables. Variables are expanded after the set language is interpreted.

Boolean Language

The Boolean language allows the user to define simple Boolean expressions that Dune can evaluate. Here’s a semi-formal specification of the language:

op   ::=  '=' | '<' | '>' | '<>' | '>=' | '<='
expr ::=  (and <expr>+)
          (or <expr>+)
          (<op> <template> <template>)
          (not <expr>)

After an expression is evaluated, it must be exactly the string true or false to be considered as a Boolean. Any other value will be treated as an error.

Below is a simple example of a condition expressing that the build has a Flambda compiler, with the help of variable expansion, and is targeting OSX:

(and %{ocaml-config:flambda} (= %{ocaml-config:system} macosx))

Predicate Language

The predicate language allows the user to define simple predicates (Boolean-valued functions) that Dune can evaluate. Here is a semi-formal specification of the predicate language:

pred ::=  (and pred pred)
          (or pred pred)
          (not pred)

The exact meaning of :standard and the nature of element depends on the context. For example, in the case of the dirs, an element corresponds to file glob patterns. Another example is the user action (with-accepted-exit-codes …), where an element corresponds to a literal integer.


Some fields can contains variables that are expanded by Dune. The syntax of variables is as follows:


or, for more complex forms that take an argument:


In order to write a plain %{, you need to write \%{ in a string.

Dune supports the following variables:

  • project_root is the root of the current project. It is typically the root of your project, and as long as you have a dune-project file there, project_root is independent of the workspace configuration.

  • workspace_root is the root of the current workspace. Note that the value of workspace_root isn’t constant and depends on whether your project is vendored or not.

  • CC is the C compiler command line (list made of the compiler name followed by its flags) that will be used to compile foreign code. For more details about its content, please see this section.

  • CXX is the C++ compiler command line being used in the current build context.

  • ocaml_bin is the path where ocamlc lives.

  • ocaml is the ocaml binary.

  • ocamlc is the ocamlc binary.

  • ocamlopt is the ocamlopt binary.

  • ocaml_version is the version of the compiler used in the current build context.

  • ocaml_where is the output of ocamlc -where.

  • arch_sixtyfour is true if using a compiler that targets a 64-bit architecture and false otherwise.

  • null is /dev/null on Unix or nul on Windows.

  • ext_obj, ext_asm, ext_lib, ext_dll, and ext_exe are the file extensions used for various artifacts.

  • ext_plugin is .cmxs if natdynlink is supported and .cma otherwise.

  • ocaml-config:v is for every variable v in the output of ocamlc -config. Note that Dune processes the output of ocamlc -config in order to make it a bit more stable across versions, so the exact set of variables accessible this way might not be exactly the same as what you can see in the output of ocamlc -config. In particular, variables added in new OCaml versions need to be registered in Dune before they can be used.

  • profile is the profile selected via --profile.

  • context_name is the name of the context (default, or defined in the workspace file)

  • os_type is the type of the OS the build is targeting. This is the same as ocaml-config:os_type.

  • architecture is the type of the architecture the build is targeting. This is the same as ocaml-config:architecture.

  • model is the type of the CPU the build is targeting. This is the same as ocaml-config:model.

  • system is the name of the OS the build is targeting. This is the same as ocaml-config:system.

  • ignoring_promoted_rule is true if --ignore-promoted-rules was passed on the command line and false otherwise.

  • <ext>:<path> where <ext> is one of cmo, cmi, cma, cmx, or cmxa. See Variables for Artifacts.

  • env:<var>=<default expands to the value of the environment variable <var>, or <default> if it does not exist. For example, %{env:BIN=/usr/bin}. Available since Dune 1.4.0.

  • There are some Coq-specific variables detailed in Coq-Specific Variables.

In addition, (action ...) fields support the following special variables:

  • target expands to the one target.

  • targets expands to the list of target.

  • deps expands to the list of dependencies.

  • ^ expands to the list of dependencies, separated by spaces.

  • dep:<path> expands to <path> (and adds <path> as a dependency of the action).

  • exe:<path> is the same as <path>, except when cross-compiling, in which case it will expand to <path> from the host build context.

  • bin:<program> expands <path> to program. If program is installed by a workspace package (see install stanzas), the locally built binary will be used, otherwise it will be searched in the <path> of the current build context. Note that (run %{bin:program} ...) and (run program ...) behave in the same way. %{bin:...} is only necessary when you are using (bash ...) or (system ...).

  • bin-available:<program> expands to true or false, depending on whether <program> is available or not.

  • lib:<public-library-name>:<file> expands to the file’s installation path <file> in the library <public-library-name>. If <public-library-name> is available in the current workspace, the local file will be used, otherwise the one from the installed world will be used.

  • lib-private:<library-name>:<file> expands to the file’s build path <file> in the library <library-name>. Both public and private library names are allowed as long as they refer to libraries within the same project.

  • libexec:<public-library-name>:<file> is the same as lib:..., except when cross-compiling, in which case it will expand to the file from the host build context.

  • libexec-private:<library-name>:<file> is the same as lib-private:... except when cross-compiling, in which case it will expand to the file from the host build context.

  • lib-available:<library-name> expands to true or false depending on whether the library is available or not. A library is available if at least one of the following conditions holds:

    • It’s part the installed world.

    • It’s available locally and is not optional.

    • It’s available locally, and all its library dependencies are available.

  • version:<package> expands to the version of the given package. Packages defined in the current scope have priority over the public packages. Public packages that don’t install any libraries will not be detected. How Dune determines the version of a package is described here.

  • read:<path> expands to the contents of the given file.

  • read-lines:<path> expands to the list of lines in the given file.

  • read-strings:<path> expands to the list of lines in the given file, unescaped using OCaml lexical convention.

The %{<kind>:...} forms are what allows you to write custom rules that work transparently, whether things are installed or not.

Note that aliases are ignored by %{deps}

The intent of this last form is to reliably read a list of strings generated by an OCaml program via:

List.iter (fun s -> print_string (String.escaped s)) l
  1. Dealing with circular dependencies introduced by variables

If you ever see Dune reporting a dependency cycle that involves a variable such as %{read:<path>}, try to move <path> to a different directory.

The reason you might see such dependency cycle is because Dune is trying to evaluate the %{read:<path>} too early. For instance, let’s consider the following example:

 (targets x)
 (enabled_if %{read:y})
 (action ...)

 (with-stdout-to y (...)))

When Dune loads and interprets this file, it decides whether the first rule is enabled by evaluating %{read:y}. To evaluate %{read:y}, it must build y. To build y, it must figure out the build rule that produces y, and in order to do that, it must first load and evaluate the above dune file. You can see how this creates a cycle.

Some cycles might be more complex. In any case, when you see such an error, the easiest thing to do is move the file that’s being read to a different directory, preferably a standalone one. You can use the subdir stanza to keep the logic self-contained in the same dune file:

 (targets x)
 (enabled_if %{read:dir-for-y/y})
 (action ...)

  (with-stdout-to y (...))))

Expansion of Lists

Forms that expand to a list of items, such as %{cc}, %{deps}, %{targets}, or %{read-lines:...}, are suitable to be used in (run <prog> <arguments>). For instance in:

(run foo %{deps})

If there are two dependencies, a and b, the produced command will be equivalent to the shell command:

$ foo "a" "b"

If you want both dependencies to be passed as a single argument, you must quote the variable:

(run foo "%{deps}")

which is equivalent to the following shell command:

$ foo "a b"

(The items of the list are concatenated with space.) Please note: since %{deps} is a list of items, the first one may be used as a program name. For instance:

 (targets result.txt)
 (deps    foo.exe (glob_files *.txt))
 (action  (run %{deps})))

Here is another example:

 (target foo.exe)
 (deps   foo.c)
 (action (run %{cc} -o %{target} %{deps} -lfoolib)))

Library Dependencies

Library dependencies are specified using (libraries ...) fields in library and executables stanzas.

For libraries defined in the current scope, you can either use the real name or the public name. For libraries that are part of the installed world, or for libraries that are part of the current workspace but in another scope, you need to use the public name. For instance: (libraries base re).

When resolving libraries, ones that are part of the workspace are always preferred to ones that are part of the installed world.

Alternative Dependencies

Sometimes, one doesn’t want to depend on a specific library but rather on whatever is already installed, e.g., to use a different backend, depending on the target.

Dune allows this by using a (select ... from ...) form inside the list of library dependencies.

Select forms are specified as follows:

(select <target-filename> from
 (<literals> -> <filename>)
 (<literals> -> <filename>)

<literals> are lists of literals, where each literal is one of:

  • <library-name>, which will evaluate to true if <library-name> is available, either in the workspace or in the installed world

  • !<library-name>, which will evaluate to true if <library-name> is not available in the workspace or in the installed world

When evaluating a select form, Dune will create <target-filename> by copying the file given by the first (<literals> -> <filename>) case where all the literals evaluate to true. It is an error if none of the clauses are selectable. You can add a fallback by adding a clause of the form (-> <file>) at the end of the list.

Re-exported dependencies

A dependency foo may be marked as always re-exported using the following syntax:

(re_export foo)

For instance:

 (name bar)
 (libraries (re_export foo)))

This states that this library explicitly re-exports the interface of foo. Concretely, when something depends on bar, it will also be able to see foo independently of whether implicit transitive dependencies are allowed or not. When they are allowed, which is the default, all transitive dependencies are visible, whether they are marked as re-exported or not.

Preprocessing Specification

Dune accepts three kinds of preprocessing:

  • no_preprocessing means that files are given as-is to the compiler, which is the default.

  • (action <action>) is used to preprocess files using the given action.

  • (pps <ppx-rewriters-and-flags>) used to preprocess files using the given list of PPX rewriters.

  • (staged_pps <ppx-rewriters-and-flags>) is similar to (pps ...) but behave slightly differently. It’s needed for certain PPX rewriters (see below for details).

  • future_syntax is a special value that brings some of the newer OCaml syntaxes to older compilers. See Future syntax for more details.

Dune normally assumes that the compilation pipeline is sequenced as follows:

  • code generation (including preprocessing)

  • dependency analysis

  • compilation

Dune uses this fact to optimize the pipeline and, in particular, share the result of code generation and preprocessing between the dependency analysis and compilation phases. However, some specific code generators or preprocessors require feedback from the compilation phase. As a result, they must be applied in stages as follows:

  • first stage of code generation

  • dependency analysis

  • second step of code generation in parallel with compilation

This is the case for PPX rewriters using the OCaml typer, for instance. When using such PPX rewriters, you must use staged_pps instead of pps in order to force Dune to use the second pipeline, which is slower but necessary in this case.

Preprocessing with Actions

<action> uses the same DSL as described in the User actions section, and for the same reason given in that section, it will be executed from the root of the current build context. It’s expected to be an action that reads the file given as a dependency named input-file and outputs the preprocessed file on its standard output.

More precisely, (preprocess (action <action>)) acts as if you had set up a rule for every file of the form:

 (action (with-stdout-to %{target}
          (chdir %{workspace_root} <action>))))

The equivalent of a -pp <command> option passed to the OCaml compiler is (system "<command> %{input-file}").

Preprocessing with PPX Rewriters

<ppx-rewriters-and-flags> is expected to be a sequence where each element is either a command line flag if starting with a - or the name of a library. If you want to pass command line flags that don’t start with a -, you can separate library names from flags using --. So for instance from the following preprocess field:

(preprocess (pps ppx1 -foo ppx2 -- -bar 42))

The list of libraries will be ppx1 and ppx2, and the command line arguments will be: -foo -bar 42.

Libraries listed here should be ones implementing an OCaml AST rewriter and registering themselves using the ocaml-migrate-parsetree.driver API.

Dune will build a single executable by linking all these libraries and their dependencies together. Note that it is important that all these libraries are linked with -linkall. Dune automatically uses -linkall when the (kind ...) field is set to ppx_rewriter or ppx_deriver.

Per Module Preprocessing Specification

By default, a preprocessing specification applies to all modules in the library/set of executables. It’s possible to select the preprocessing on a module-by-module basis by using the following syntax:

(preprocess (per_module
             (<spec1> <module-list1>)
             (<spec2> <module-list2>)

Where <spec1>, <spec2>, etc. are preprocessing specifications and <module-list1>, <module-list2>, etc., are list of module names.

For instance:

(preprocess (per_module
             (((action (run ./ X=1 %{input-file})) foo bar))
             (((action (run ./ X=2 %{input-file})) baz))))

Future Syntax

The future_syntax preprocessing specification is equivalent to no_preprocessing when using one of the most recent versions of the compiler. When using an older one, it is a shim preprocessor that backports some of the newer syntax elements. This allows you to use some of the new OCaml features while keeping compatibility with older compilers.

One example of supported syntax is the custom let-syntax that was introduced in 4.08, allowing the user to define custom let operators.

Note that this feature is implemented by the third-party ocaml-syntax-shims project, so if you use this feature, you must also declare a dependency on this package.

Preprocessor Dependencies

If your preprocessor needs extra dependencies you should use the preprocessor_deps field available in the library, executable, and executables stanzas.

Dependency Specification

Dependencies in dune files can be specified using one of the following:

  • (:name <dependencies>) will bind the list of dependencies to the name variable. This variable will be available as %{name} in actions.

  • (file <filename>), or simply <filename>, depend on this file.

  • (alias <alias-name>) depends on the construction of this alias. For instance: (alias src/runtest).

  • (alias_rec <alias-name>) depends on the construction of this alias recursively in all children directories wherever it is defined. For instance: (alias_rec src/runtest) might depend on (alias src/runtest), (alias src/foo/bar/runtest), etc.

  • (glob_files <glob>) depends on all files matched by <glob>. See the glob for details.

  • (glob_files_rec <glob>) is the recursive version of (glob_files <glob>). See the glob for details.

  • (source_tree <dir>) depends on all source files in the subtree with root <dir>.

  • (universe) depends on everything in the universe. This is for cases where dependencies are too hard to specify. Note that Dune will not be able to cache the result of actions that depend on the universe. In any case, this is only for dependencies in the installed world. You must still specify all dependencies that come from the workspace.

  • (package <pkg>) depends on all files installed by <package>, as well as on the transitive package dependencies of <package>. This can be used to test a command against the files that will be installed.

  • (env_var <var>) depends on the value of the environment variable <var>. If this variable becomes set, becomes unset, or changes value, the target will be rebuilt.

  • (sandbox <config>) requires a particular sandboxing configuration. <config> can be one (or many) of:

    • always: the action requires a clean environment

    • none: the action must run in the build directory

    • preserve_file_kind: the action needs the files it reads to look like normal files (so Dune won’t use symlinks for sandboxing)

  • (include <file>) read the s-expression in <file> and interpret it as additional dependencies. The s-expression is expected to be a list of the same constructs enumerated here.

In all these cases, the argument supports Variables.

Named Dependencies

Dune allows a user to organize dependency lists by naming them. The user is allowed to assign a group of dependencies a name that can later be referred to in actions (like the %{deps}, %{target}, and %{targets} built in variables).

One instance where this is useful is for naming globs. Here’s an example of an imaginary bundle command:

 (target archive.tar)
  (:css (glob_files *.css))
  (:js foo.js bar.js)
  (:img (glob_files *.png) (glob_files *.jpg)))
  (run %{bin:bundle} index.html -css %{css} -js %{js} -img %{img} -o %{target})))

Note that a named dependency list can also include unnamed dependencies (like index.html in the example above). Also, such user defined names will shadow build in variables, so (:workspace_root x) will shadow the built-in %{workspace_root} variable.


You can use globs to declare dependencies on a set of files. Note that globs will match files that exist in the source tree as well as buildable targets, so for instance you can depend on *.cmi.

Dune supports globbing files in a single directory via (glob_files ...) and, starting with Dune 3.0, in all sub-directories recursively via (glob_files_rec ...). The glob is interpreted as follows:

  • anything before the last / is taken as a literal path

  • anything after the last /, or everything if the glob contains no /, is interpreted using the glob syntax

Absolute paths are permitted in the (glob_files ...) term only. It’s an error to pass an absolute path (i.e., a path beginning with a /) to (glob_files_rec ...)`.

The glob syntax is interpreted as follows:

  • \<char> matches exactly <char>, even if it’s a special character (*, ?, …).

  • * matches any sequence of characters, except if it comes first, in which case it matches any character that is not . followed by anything.

  • ** matches any character that is not . followed by anything, except if it comes first, in which case it matches anything.

  • ? matches any single character.

  • [<set>] matches any character that is part of <set>.

  • [!<set>] matches any character that is not part of <set>.

  • {<glob1>,<glob2>,...,<globn>} matches any string that is matched by one of <glob1>, <glob2>, etc.

Glob syntax examples


Files matched

Files not matched








file1.txt, file2.txt








a/b, a.b



a1.txt, a2.txt

b1.txt, a10.txt


fx.txt, fy.txt, fz.txt

f2.txt, f.txt


f2.txt, fa.txt

fx.txt, f.txt

a.{ml,mli}, a.mli



../, ../a.mli

OCaml Flags

In library, executable, executables, and env stanzas, you can specify OCaml compilation flags using the following fields:

  • (flags <flags>) to specify flags passed to both ocamlc and ocamlopt

  • (ocamlc_flags <flags>) to specify flags passed to ocamlc only

  • (ocamlopt_flags <flags>) to specify flags passed to ocamlopt only

For all these fields, <flags> is specified in the Ordered set language. These fields all support (:include ...) forms.

The default value for (flags ...) is taken from the environment, as a result it’s recommended to write (flags ...) fields as follows:

(flags (:standard <my options>))

User Actions

(action ...) fields describe user actions.

User actions are always run from the same subdirectory of the current build context as the dune file they are defined in, so for instance, an action defined in src/foo/dune will be run from $build/<context>/src/foo.

The argument of (action ...) fields is a small DSL that’s interpreted by Dune directly and doesn’t require an external shell. All atoms in the DSL support Variables. Moreover, you don’t need to specify dependencies explicitly for the special %{<kind>:...} forms; these are recognized and automatically handled by Dune.

The DSL is currently quite limited, so if you want to do something complicated it’s recommended to write a small OCaml program and use the DSL to invoke it. You can use shexp to write portable scripts or Configurator for configuration related tasks. You can also use Sandboxing to express program dependencies directly in the source code.

The following constructions are available:

  • (run <prog> <args>) to execute a program. <prog> is resolved locally if it is available in the current workspace, otherwise it is resolved using the PATH

  • (dynamic-run <prog> <args>) to execute a program that was linked against dune-action-plugin library. <prog> is resolved in the same way as in run

  • (chdir <dir> <DSL>) to change the current directory

  • (setenv <var> <value> <DSL>) to set an environment variable

  • (with-<outputs>-to <file> <DSL>) to redirect the output to a file, where <outputs> is one of: stdout, stderr or outputs (for both stdout and stderr)

  • (ignore-<outputs> <DSL>) to ignore the output, where <outputs> is one of: stdout, stderr or outputs

  • (with-stdin-from <file> <DSL>) to redirect the input from a file

  • (with-accepted-exit-codes <pred> <DSL>) specifies the list of expected exit codes for the programs executed in <DSL>. <pred> is a predicate on integer values, and is specified using the Predicate Language. <DSL> can only contain nested occurrences of run, bash, system, chdir, setenv, ignore-<outputs>, with-stdin-from and with-<outputs>-to. This action is available since Dune 2.0.

  • (progn <DSL>...) to execute several commands in sequence

  • (echo <string>) to output a string on stdout

  • (write-file <file> <string>) writes <string> to <file>

  • (cat <file> ...) to sequentially print the contents of files to stdout

  • (copy <src> <dst>) to copy a file. If these files are OCaml sources you should follow the naming convention to preserve Merlin’s functionality.

  • (copy# <src> <dst>) to copy a file and add a line directive at the beginning

  • (system <cmd>) to execute a command using the system shell: sh on Unix and cmd on Windows

  • (bash <cmd>) to execute a command using /bin/bash. This is obviously not very portable.

  • (diff <file1> <file2>) is similar to (run diff <file1> <file2>) but is better and allows promotion. See Diffing and promotion for more details.

  • (diff? <file1> <file2>) is similar to (diff <file1> <file2>) except that <file2> should be produced by a part of the same action rather than be a dependency, is optional and will be consumed by diff?.

  • (cmp <file1> <file2>) is similar to (run cmp <file1> <file2>) but allows promotion. See Diffing and promotion for more details.

  • (no-infer <DSL>) to perform an action without inference of dependencies and targets. This is useful if you are generating dependencies in a way that Dune doesn’t know about, for instance by calling an external build system.

  • (pipe-<outputs> <DSL> <DSL> <DSL>...) to execute several actions (at least two) in sequence, filtering the <outputs> of the first command through the other command, piping the standard output of each one into the input of the next. This action is available since Dune 2.7.

As mentioned, copy# inserts a line directive at the beginning of the destination file. More precisely, it inserts the following line:

# 1 "<source file name>"

Most languages recognize such lines and update their current location to report errors in the original file rather than the copy. This is important because the copy exists only under the _build directory, and in order for editors to jump to errors when parsing the output of the build system, errors must point to files that exist in the source tree. In the beta versions of Dune, copy# was called copy-and-add-line-directive. However, most of time, one wants this behavior rather than a bare copy, so it was renamed to something shorter.

Note: expansion of the special %{<kind>:...} is done relative to the current working directory of the DSL being executed. So for instance, if you have this action in a src/foo/dune:

(action (chdir ../../.. (echo %{dep:dune})))

Then %{dep:dune} will expand to src/foo/dune. When you run various tools, they often use the filename given on the command line in error messages. As a result, if you execute the command from the original directory, it will only see the basename.

To understand why this is important, let’s consider this Dune file living in src/foo:

 (deps   blah.mll)
 (action (run ocamllex -o %{target} %{deps})))

Here the command that will be executed is:

ocamllex -o blah.mll

And it will be executed in _build/<context>/src/foo. As a result, if there is an error in the generated file it will be reported as:

File "", line 42, characters 5-10:
Error: ...

Which can be a problem, as you editor might think that is at the root of your project. Instead, this is a better way to write it:

 (deps   blah.mll)
 (action (chdir %{workspace_root} (run ocamllex -o %{target} %{deps}))))


The user actions that run external commands (run, bash, system) are opaque to Dune, so Dune has to rely on manual specification of dependencies and targets. One problem with manual specification is that it’s error-prone. It’s often hard to know in advance what files the command will read, and knowing a correct set of dependencies is very important for build reproducibility and incremental build correctness.

To help with this problem Dune supports sandboxing. An idealized view of sandboxing is that it runs the action in an environment where it can’t access anything except for its declared dependencies.

In practice, we have to make compromises and have some trade-offs between simplicity, information leakage, performance, and portability.

The way sandboxing is currently implemented is that for each sandboxed action we build a separate directory tree (sandbox directory) that mirrors the build directory, filtering it to only contain the files that were declared as dependencies. We run the action in that directory, and then we copy the targets back to the build directory.

You can configure Dune to use sandboxing modes symlink, hardlink or copy, which determines how the individual files are populated (they will be symlinked, hardlinked, or copied into the sandbox directory).

This approach is very simple and portable, but that comes with certain limitations:

  • The actions in the sandbox can use absolute paths to refer to anywhere outside the sandbox. This means that only dependencies on relative paths in the build tree can be enforced/detected by sandboxing.

  • The sandboxed actions still run with full permissions of Dune itself so sandboxing is not a security feature. It won’t prevent network access either.

  • We don’t erase the environment variables of the sandboxed commands. This is something we want to change.

  • Performance impact is usually small, but it can get noticeable for fast actions with very large sets of dependencies.

Per-action Sandboxing Configuration

Some actions may rely on sandboxing to work correctly. For example, an action may need the input directory to contain nothing except the input files, or the action might create temporary files that break other build actions.

Some other actions may refuse to work with Sandboxing. Cor example, if they rely on absolute path to the build directory staying fixed, or if they deliberately use some files without declaring dependencies (this is usually a very bad idea, by the way).

Generally it’s better to improve the action so it works with or without sandboxing (especially with), but sometimes you just can’t do that.

Things like this can be described using the “sandbox” field in the dependency specification language (see Dependency Specification).

Global Sandboxing Configuration

Dune always respects per-action sandboxing specification. You can configure it globally to prefer a certain sandboxing mode if the action allows it.

This is controlled by:

  • dune --sandbox <...> cli flag (see man dune-build)

  • DUNE_SANDBOX environment (see man dune-build)

  • (sandboxing_preference ..) field in the dune config (see man dune-config)


Given two rules that are independent, Dune will assume that their associated actions can be run concurrently. Two rules are considered independent if neither of them depend on the other, either directly or through a chain of dependencies. This basic assumption allows Dune to parallelize the build.

However, it is sometimes the case that two independent rules cannot be executed concurrently. For instance this can happen for more complicated tests. In order to prevent Dune from running the actions at the same time, you can specify that both actions take the same lock:

 (alias  runtest)
 (deps   foo)
 (locks  m)
 (action (run test.exe %{deps})))

 (rule   runtest)
 (deps   bar)
 (locks  m)
 (action (run test.exe %{deps})))

Dune will make sure that the executions of test.exe foo and test.exe bar are serialized.

Although they don’t live in the filesystem, lock names are interpreted as file names. So for instance (with-lock m ...) in src/dune and (with-lock ../src/m) in test/dune refer to the same lock.

Note also that locks are per build context. So if your workspace has two build contexts setup, the same rule might still be executed concurrently between the two build contexts. If you want a lock that is global to all build contexts, simply use an absolute filename:

 (alias   runtest)
 (deps   foo)
 (locks  /tcp-port/1042)
 (action (run test.exe %{deps})))

Diffing and Promotion

(diff <file1> <file2>) is very similar to (run diff <file1> <file2>). In particular it behaves in the same way:

  • When <file1> and <file2> are equal, it does nothing.

  • When they are not, the differences are shown and the action fails.

However, it is different for the following reason:

  • The exact command used for diff files can be configured via the --diff-command command line argument. Note that it’s only called when the files are not byte equals

  • By default, it will use patdiff if it is installed. patdiff is a better diffing program. You can install it via opam with:

    $ opam install patdiff
  • On Windows, both (diff a b) and (diff? a b) normalize end-of-line characters before comparing the files.

  • Since (diff a b) is a built-in action, Dune knows that a and b are needed, so you don’t need to specify them explicitly as dependencies.

  • You can use (diff? a b) after a command that might or might not produce b, for cases where commands optionally produce a corrected file

  • If <file1> doesn’t exist, it will compare with the empty file.

  • It allows promotion. See below.

Note that (cmp a b) does no end-of-line normalization and doesn’t print a diff when the files differ. cmp is meant to be used with binary files.


Whenever an action (diff <file1> <file2>) or (diff?  <file1> <file2>) fails because the two files are different, Dune allows you to promote <file2> as <file1> if <file1> is a source file and <file2> is a generated file.

More precisely, let’s consider the following Dune file:

 (with-stdout-to data.out (run ./test.exe)))

 (alias   runtest)
 (action (diff data.expected data.out)))

Where data.expected is a file committed in the source repository. You can use the following workflow to update your test:

  • Update the code of your test.

  • Run dune runtest. The diff action will fail and a diff will be printed.

  • Check the diff to make sure it’s what you expect. This diff can be displayed again by running dune promotion diff.

  • Run dune promote. This will copy the generated data.out file to data.expected directly in the source tree.

You can also use dune runtest --auto-promote, which will automatically do the promotion.

Package Specification

Installation is the process of copying freshly built libraries, binaries, and other files from the build directory to the system. Dune offers two ways of doing this: via opam or directly via the install command. In particular, the installation model implemented by Dune was copied from opam. Opam is the standard OCaml package manager.

In both cases, Dune only know how to install whole packages. A package being a collection of executables, libraries, and other files. In this section, we’ll describe how to define a package, how to “attach” various elements to it, and how to proceed with installing it on the system.

Declaring a Package

To declare a package, simply add a package stanza to your dune-project file:

 (name mypackage)
 (synopsis "My first Dune package!")
 (description "\| This is my first attempt at creating
              "\| a project with Dune.

Once you have done this, Dune will know about the package named mypackage and you will be able to attach various elements to it. The package stanza accepts more fields, such as dependencies.

Note that package names are in a global namespace, so the name you choose must be universally unique. In particular, package managers never allow to release two packages with the same name.

In older projects using Dune, packages were defined by manually writing a file called <package-name>.opam at the root of the project. However, it’s not recommended to use this method in new projects, as we expect to deprecate it in the future. The right way to define a package is with a package stanza in the dune-project file.

See Generating opam Files for instructions on configuring Dune to automatically generate .opam files based on the package stanzas.

Attaching Elements to a Package

Attaching an element to a package means declaring to Dune that this element is part of the said package. The method to attach an element to a package depends on the kind of the element. In this sub-section, we will go through the various kinds of elements and describe how to attach each of them to a package.

In the rest of this section, <prefix> refers to the directory in which the user chooses to install packages. When installing via opam, it’s opam that sets this directory. When calling dune install, the installation directory is either guessed or can be manually specified by the user. Defaults directories which replace guessing can be set during the compilation of dune.

Sites of a Package

When packages need additional resources outside their binary, their location could be hard to find. Moreover some packages could add resources to another package, for example in the case of plugins. These location are called sites in Dune. One package can define them. During execution, one site corresponds to a list of directories. They are like layers, and the first directories have a higher priority. Examples and precisions are available at How to Load Additional Files at Runtime.


In order to attach a library to a package, merely add a public_name field to your library. This is the name that external users of your libraries must use in order to refer to it. Dune requires that the public name of a library is either the name of the package it is part of or start with the package name followed by a dot character.

For instance:

 (name mylib)
 (public_name mypackage.mylib))

After you have added a public name to a library, Dune will know to install it as part of the package it is attached to. Dune installs the library files in a directory <prefix>/lib/<package-name>.

If the library name contains dots, the full directory in which the library files are installed is lib/<comp1>/<comp2/.../<compn>, where <comp1>, <comp2>, … <compn> are the dot separated component of the public library name. By definition, <comp1> is always the package name.


Similarly to libraries, to attach an executable to a package simply add a public_name field to your executable stanza or a public_names field for executables stanzas. Designate this name to match the available executables through the installed PATH (i.e., the name users must type in their shell to execute the program), because Dune cannot guess an executable’s relevant package from its public name. It’s also necessary to add a package field unless the project contains a single package, in which case the executable will be attached to this package.

For instance:

 (name main)
 (public_name myprog)
 (package mypackage))

Once mypackage is installed on the system, the user will be able to type the following in their shell:

$ myprog

to execute the program.

Other Files

For all other kinds of elements, you must attach them manually via an install stanza.

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 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)
 (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 plus the archive_name field, which 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.

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.