Configurator is a small library designed to query features available on the system, in order to generate configuration for dune builds. Such generated configuration is usually in the form of command line flags, generated headers, stubs, but there are no limitations on this.

Configurator allows you to query for the following features:

  • Variables defined in ocamlc - config,
  • pkg-config flags for packages,
  • Test features by compiling C code,
  • Extract compile time information such as #define variables.

Configurator is designed to be cross compilation friendly and avoids _running_ any compiled code to extract any of the information above.

Configurator started as an independent library, but now lives in dune. You do not need to install anything to use configurator.


We’ll describe configurator with a simple example. Everything else can be easily learned by studying configurator’s API.

To use configurator, we write an executable that will query the system using configurator’s API and output a set of targets reflecting the results. For example:

module C = Configurator.V1

let clock_gettime_code = {|
#include <time.h>

int main()
  struct timespec ts;
  clock_gettime(CLOCK_REALTIME, &ts);
  return 0;

let () =
  C.main ~name:"foo" (fun c ->
    let has_clock_gettime = C.c_test c clock_gettime_code ~link_flags:["-lrt"] in

    C.C_define.gen_header_file c ~fname:"config.h"
      [ "HAS_CKOCK_GETTIME", Switch has_ckock_gettime ]);

Usually, the module above would be named The next step is to invoke it as an executable and tell dune about the targets that it produces:

 ((name discover)
  (libraries (jbuilder.configurator))))

 ((targets (config.h))
  (action (run ./discover.exe))))

Another common pattern is to produce a flags file with configurator and then use this flag file using :include:

 ((name mylib)
  (c_names (foo))
  (c_library_flags (:include (flags.sexp)))))

For this, generate the list of flags for your library — for example using Configurator.V1.Pkg_config — and then write them to a file, in the above example flags.sexp, with Configurator.V1.write_flags "flags.sexp" flags.

Upgrading from the old Configurator

The old configurator is the independent configurator opam package. It is deprecated and users are encouraged to migrate to dune’s own configurator. The advantage of the transition include:

  • No extra dependencies,
  • No need to manually pass -ocamlc flag,
  • New configurator is cross compilation compatible.

The following steps must be taken to transition from the old configurator:

  • Mentions of the configurator opam package should be removed.
  • The library name configurator should be changed jbuilder.configurator.
  • The -ocamlc flag in rules that run configurator scripts should be removed. This information is now passed automatically by dune.
  • The new configurator API is versioned explicitly. The version that is compatible with old configurator is under the V1 module. Hence, to transition one’s code it’s enough to add this module alias:
module Configurator = Configurator.V1