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The OMake plugin

Author: Gerd Stolpmann,

The OMake plugin is meant as a powerful alternative to the ocamlbuild plugin. Its special feature is that the user can modify the generated OMakefiles, and add further build rules that are beyond the scope of OASIS.

The OMake plugin uses a scheme that is largely compatible with the way ocamlbuild works. However, there are a few differences you should be aware of (see below in this doc). That means it is normally sufficient to change the plugin type in your _oasis file to "omake". At this point, generate the OMakefiles with oasis setup, modify them by your needs, and enjoy the new scripting capabilities.

What is included:

  • A new build plugin
  • A new doc plugin
  • A new install plugin

There is no new configuration plugin. The standard one coming with OASIS is good enough, and if you need to run some custom configuration script, you can still do with a PostConfCommand.

The new install plugin is optional. You can also use the standard install plugin of OASIS. The new install plugin is easier to extend, though, because it also runs OMake.

Switching to OMake

Just change:

  • In all sections for libraries and executables (or just globally):

    BuildType: OMake
  • If you want to use the install plugin:

    InstallType: OMake
  • In documents, change:

    Type: OMake

    Also, use XOMakePath and XOMakeLibraries instead of XOCamlbuildPath and XOCamlbuildLibraries

After that, don't forget to run oasis setup. This generates a bunch of files. After that, you are ready to go, and e.g.

ocaml -build

will invoke OMake. Note that you can also run OMake directly:

omake build
omake doc
omake install

When distributing your project, you should also pack up the generated files (OMakeroot, OMakefile, _oasis_*.om except

Generated files

After oasis setup, the following files are generated:

  • OMakeroot: This file marks the root of the directory hierarchy, and contains global OMake configurations. This file is constant, and never changed by oasis setup again once put into place.

  • OMakefile: This file exists in every directory of the hierarchy, and is starting point for the definition of rules for the directory. This file is constant, and never changed by oasis setup again once put into place. The idea is that this file can be freely modified by your needs.

  • This is the library with additional OMake functionality. It is overwritten with every oasis setup.

  • This file defines a variable with the subdirectories. It is overwritten with every oasis setup.

  • This file defines the build rules derived from _oasis. It is overwritten with every oasis setup.

  • This file defines the install rules derived from _oasis. It is overwritten with every oasis setup.

Note that OMake stores a binary version of the *.om files with suffix .omc.


OMake is able to read (the file where the result of ocaml -configure is written to). There is a converted file

  • All OASIS variables are prefixed with "oasis_". E.g. "bindir" is available as "oasis_bindir".

  • A couple of extra configurations are appended to, e.g. the C compiler to use (CC) is extracted from OASIS variables.

The file is included into the OMakefile in the root directory. If you need to adjust variables, you can do that after the "include" line.

Note that is automatically rebuilt once is changed.


OMake doesn't use a separate build directory. Object files are put into the source hierarchy side by side with the source.

(Note: there is a vmount() function, but it is still marked as experimental feature. See the OMake manual.)

There is one thing that is fundamentally different. First, there is no _tags file. OMake only gets the build flags from the OMakefile. Second, OMake takes the build flags from the OMakefile in the same directory as the module/library/executable. The build flags are derived from a number of variables defined in the OMakefiles. The exact set is documented in the OMake manual, but the most important variables are:

Variable Meaning
OCAMLPACKS: the findlib packages
OCAMLINCLUDES: other project directories to include
OCAMLFLAGS: flags for both ocamlc and ocamlopt
OCAMLCFLAGS: flags for ocamlc
OCAMLOPTFLAGS: flags for ocamlopt
OCAMLFINDFLAGS: flags for ocamlfind

The generated files already initialize these variables in a meaningful way. Nevertheless, you should know:

  • For compiling modules, the value of of these variable at the end of the OMakefile in the same directory counts.

  • If you want good control over the flags for a certain library or executable, build the library or executable in a directory of its own. This way, you can be sure that it is isolated from the settings of any other library or executable, as these are built in different directories.

  • It is a bad idea to put modules into subdirectories unless you want to achieve a special effect. For example, you could have in _oasis:


    The point here is now that R and S are now built in the subdirectory, and the flags are taken from the OMakefile in this subdirectory. The generator does not take care of anything in this situation, and probably does not generate the right flags.

    So, don't do this even if you only have something innocent in mind like grouping your files logically.

  • It is possible to set deviating flags for a module. There are a number of helper functions to do so. E.g. if a module X needs camlp4 syntax, the way to enable this only for this module is:

    OASIS_modify_OCAMLFINDFLAGS(X, -syntax camlp4)

    Put that into the OMakefile of the directory where X exists.

There is some documentation in the top-most OMakefile explaining further ways to set flags.


The doc plugin understands a couple of additional options in _oasis:

  • OMakePath: where to build the document
  • OMakeLibraries: libraries to include into the document
  • OMakeModules: modules to include into the document
  • OMakeTexts: texts to include (the files must have suffix .txt but omit the extension, e.g. "OMakeTexts: foo" would take foo.txt)
  • OMakeIntro: introductory text (also without extension)
  • OMakeFlags: additional flags for ocamldoc

You can set Format to either HTML, PDF, PostScript, or DVI.


The new install plugin should just do the same as the internal plugin, only that the installation is done via OMake rules.

If you e.g. want to run some additional commands, you can modify the install-here rule in the OMakefile in the directory where you want to install something. It normally reads:

install-here: $(INSTALL_TARGETS)

without commands, and you can simply add some commands, e.g.

install-here: $(INSTALL_TARGETS)
    cp myfile /dest/dir/

In order to comply with the configuration, you should copy the files into the right directories, e.g.

install-here: $(INSTALL_TARGETS)
    cp myconfig $(OASIS_destdir $(oasis_sysconfdir))

Here, the variable oasis_sysconfdir is the configured directory for configurations, and the function OASIS_destdir prepends, if configured, the destination prefix (-destdir). (NB. OASIS_destdir is defined in


This works also the same as for the internal plugin. Note that OMake keeps track of the installed files in