Opam Integration


Each Opam switch created by Diskuv OCaml uses the following repositories in order:

  1. diskuv-0.3.3

  2. fdopen-mingw-0.3.3 only for Windows

  3. default

The diskuv-0.3.3 repository has all the patches required for third-party OCaml packages to support the Microsoft compiler. Any switch that is created by Diskuv OCaml will have pinned versions for each package in this repository. That means you will always get the Diskuv OCaml patched versions and reproducible behavior.

The fdopen-mingw-0.3.3 repository has all the MinGW patches for _many_ third-party OCaml packages to work with MinGW (an alternative compiler popular on Linux). Unlike diskuv-0.3.3 the packages are not pinned, so it is possible that a newer package version is introduced into your switch that has no MinGW patches.


If you suspect a stale fdopen repository is causing you problems, run opam repository list to find where the repository is physically located and then look in its packages/ subdirectory to see what versions of your problematic package are supported by the fdopen repository. opam list will tell you which versions you are currently using. An issue can be filed at https://gitlab.com/diskuv/diskuv-ocaml/-/issues to move the package into an upcoming diskuv-* repository.

The default repository is the central Opam repository. Most of your packages are unpatched and will come directly from this repository.

Opam Root

Each Opam root created by Diskuv OCaml includes a plugin directory OPAMROOT/plugins/diskuvocaml/.

It contains:

  • vcpkg which has the C/C++ packages needed by some OCaml packages


diskuv-boot-DO-NOT-DELETE Global Switch

OCaml IDEs like Visual Studio Code with the OCaml Platform Extension work best when there is at least one global Opam switch installed and selected. The installation will install diskuv-boot-DO-NOT-DELETE with no OCaml packages.

This switch is reserved for future use so do not delete it.

(Expert Users) The OCaml Platform Extension, for example, will execute opam var root which will fail if no (global) Opam switch has been selected. But you won’t be able to pick any external (also known as ‘local’) Opam switches because opam var root failed. With a global switch that problem goes away.

diskuv-host-tools Local Switch

OCaml package directories, C header “include” directories and other critical locations are hardcoded into essential OCaml executables like ocamlc.exe during opam switch create and opam install. If we move the Opam switch that contains ocamlc many of your OCaml builds will break.

diskuv-host-tools is the Opam switch for ocamlc and other essentials that is built in its final resting place during installation and never moved after. If and when we need to deploy new versions of the essentials we use a new directory and rebuild all of the OCaml packages from scratch.

The system applications, along with Opam itself, are installed in your User Profile. Since they are also available on your PATH, you can just type dune, utop, ocamlformat, opam, etc.

Global Variables


Refer to the Opam Manual “Variables” documentation if you are not familiar with Opam variables.

The global variables that will be present in a Diskuv OCaml installation are:

<><> Global opam variables ><><><><><><><><><><><><><><><><><><><><><><><><><><>
arch              x86_64                                                     # Inferred from system
exe               .exe                                                       # Suffix needed for executable filenames (Windows)
jobs              11                                                         # The number of parallel jobs set up in opam configuration
make              make                                                       # The 'make' command to use
opam-version      2.1.0                                                      # The currently running opam version
os                win32                                                      # Inferred from system
os-distribution   win32                                                      # Inferred from system
os-family         windows                                                    # Inferred from system
os-version        10.0.22000                                                 # Inferred from system
root              C:\Users\you\.opam                                         # The current opam root directory
switch            C:\Users\you\AppData\Local\Programs\DiskuvOCaml\1\system   # The identifier of the current switch
sys-ocaml-arch    x86_64                                                     # Target architecture of the OCaml compiler present on your system
sys-ocaml-cc      msvc                                                       # Host C Compiler type of the OCaml compiler present on your system
sys-ocaml-libc    msvc                                                       # Host C Runtime Library type of the OCaml compiler present on your system
sys-ocaml-version 4.12.0                                                     # OCaml version present on your system independently of opam, if any

C Compiler

The Microsoft compiler and linker environment variables must be setup before use. Microsoft provides a vcvarsall.bat and vsdevcmd.bat scripts to set environment variables.

We also want to include vcpkg C headers and C libraries by default, so the following are adjusted to include vcpkg:

In Diskuv OCaml most targets (./makeit shell-dev, ./makeit build-dev, etc.) have their environment variables automatically set for Microsoft C compilation inside MSYS2 in a manner similar to the following:

source vendor/diskuv-ocaml/etc/contexts/linux-build/crossplatform-functions.sh
autodetect_compiler /tmp/launcher.sh "$LOCALAPPDATA/opam/plugins/diskuvocaml/vcpkg/0.3.3/installed/x86-windows" # if 64-bit
autodetect_compiler /tmp/launcher.sh "$LOCALAPPDATA/opam/plugins/diskuvocaml/vcpkg/0.3.3/installed/x64-windows" # if 32-bit

/tmp/launcher.sh bash

The choice of Microsoft compiler is configured during Diskuv OCaml installation and made available at $env:LOCALAPPDATA\Programs\DiskuvOCaml\vsstudio.dir.txt (full details at $env:LOCALAPPDATA\Programs\DiskuvOCaml\vsstudio.json).

There are two typical methods used to detect the C compiler during the installation of an OCaml package (ex. opam install):

  • Many packages use autoconf to generate a ./configure script that will automatically detect the presence of Microsoft environment variables. Those will have been set by autodetect_compiler.

  • Some packages, especially core OCaml packages like the OCaml compiler and Opam, will use msvs-tools. Recent versions of msvs-tools can detect an Diskuv OCaml auto-installed Visual Studio Build Tools but they will not add vcpkg installed packages to the INCLUDE and LIBPATH; msvs-tools may also select a more recent compiler. TODO: Fixme. In progress

OCaml Compiler

The compiler is built with Microsoft’s CL.EXE. Typically OCaml packages re-use the same C compiler flags as was used by the OCaml Compiler.

This comes from ocamlc -config (yours will vary slightly):

version: 4.12.0
standard_library_default: C:/Users/User/AppData/Local/Programs/DiskuvOCaml/0/host-tools/_opam/lib/ocaml
standard_library: C:/Users/User/AppData/Local/Programs/DiskuvOCaml/0/host-tools/_opam/lib/ocaml
ccomp_type: msvc
c_compiler: cl
ocamlc_cflags: -nologo -O2 -Gy- -MD
ocamlc_cppflags: -D_CRT_SECURE_NO_DEPRECATE
ocamlopt_cflags: -nologo -O2 -Gy- -MD
ocamlopt_cppflags: -D_CRT_SECURE_NO_DEPRECATE
bytecomp_c_compiler: cl -nologo -O2 -Gy- -MD -D_CRT_SECURE_NO_DEPRECATE
native_c_compiler: cl -nologo -O2 -Gy- -MD -D_CRT_SECURE_NO_DEPRECATE
bytecomp_c_libraries: advapi32.lib ws2_32.lib version.lib
native_c_libraries: advapi32.lib ws2_32.lib version.lib
native_pack_linker: link -lib -nologo -machine:AMD64  -out:
architecture: amd64
model: default
systhread_supported: true
host: x86_64-pc-windows
target: x86_64-pc-windows
flambda: false
safe_string: true
default_safe_string: true
flat_float_array: true
function_sections: false
afl_instrument: false
windows_unicode: true
supports_shared_libraries: true
exec_magic_number: Caml1999X029
cmi_magic_number: Caml1999I029
cmo_magic_number: Caml1999O029
cma_magic_number: Caml1999A029
cmx_magic_number: Caml1999Y029
cmxa_magic_number: Caml1999Z029
ast_impl_magic_number: Caml1999M029
ast_intf_magic_number: Caml1999N029
cmxs_magic_number: Caml1999D029
cmt_magic_number: Caml1999T029
linear_magic_number: Caml1999L029


voodoos@’s diagram at https://github.com/ocaml/dune/issues/3718 is one of the best pictures of how Dune built packages get their compiler flags:


Working with Native Windows


This section of the documentation is for OCaml package maintainers (anyone who creates an OCaml package for public consumption).

As an OCaml package maintainer you may want to customize the way your package builds if you are on native Windows. Native Windows installations differ from Cygwin Windows installations because Cygwin is a reasonably complete POSIX environment. You may need a few tweaks including but not limited to:

  • translating Windows paths into Unix paths (usually only a problem if you are using absolute paths)

  • use Windows libraries rather than Unix libraries

  • use LOCALAPPDATA rather than HOME to locate the user’s home directory

Typically you will customize your package build behavior with either Opam Filters (the topic of this section) or with Dune Configuration.

In this section we try to be distribution-agnostic. That means we will present techniques you can use even if your native Windows users are not using Diskuv OCaml.

Use the following Opam filter in your *.opam files to detect native Windows installations:

{ os-family = "win32" & sys-ocaml-cc = "msvc" }