MSVC + MSYS2 toolchain

TLDR: Some OCaml packages rely on C libraries or embed C code. These C-reliant packages may need to be patched to work with the MSVC + MSYS2 toolchain used by Diskuv OCaml.


Many OCaml packages that use the C language have assumed a GCC toolchain on a UNIX system. If you do encounter problems compiling these packages with the Diskuv OCaml distribution, they are likely due to

  1. The GCC toolchain vs the Microsoft Studio (MSVC) toolchain. GCC has non-standard extensions to the C language that MSVC’s CL.EXE compiler does not support.

    > This situation commonly presents itself as a syntax error

  2. UNIX vs Windows paths. UNIX paths have forward slashes while Windows paths have backslashes. Most Windows programs, and definitely the MSYS2 provided programs, support both Windows and Unix paths. However,

    • a) Often UNIX commands for OCaml builds are interpreted by the bash or dash shell; those shells will interpret a backslash as an escape character.

      > This situation commonly presents itself as garbled text, like > C:ProgramFilesMicrosoftNET

    • b) Windows paths commonly have spaces in them (ex. C:Program Files) while spaces in UNIX paths are fairly uncommon.

      > This situation commonly presents itself as an invalid argument or file not found. For > example someprog –option1 C:Program Files would behave as if someprog had “option1” > with the value C:Program and Files was an argument.

  3. Windows does not support the same APIs that Linux supports. These APIs include but are not limited to POSIX and GLIBC APIs.

    > This situation commonly presents itself as Cannot open include file: 'xxx.h': No such file or directory


Why use the Microsoft toolchain at all?

OCaml lives in a software ecosystem where there are few OCaml packages but many orders of magnitude more C packages. And although GCC-linked libraries are generally interchangable with MSVC-linked libraries, they are not 100% interchangable especially when it comes to shared libraries and C++ code. There is very little reason to have very difficult-to-diagnose problems with your programs in production or in your customer’s hands simply because we chose to use a lesser supported compiler on Windows. We’d think the choices should center on when to use OCaml versus when to use C (or Rust, etc.), rather than add more complexity with an unusual (for Windows) compiler toolchain. We’d say somewhat similar things for why we’d want to compile with clang on macOS/iOS rather than gcc, although the argument for clang is much weaker because clang is newer than Microsoft’s toolchain and Apple used to work well with gcc.

Real example of resolving toolchain problems

We picked this real package because it was the first package which gave us an opportunity to cover all three common toolchain problems while writing the documentation. We patched the package because we use it and like it!

Everything you see below came while using the Creating your own package patches steps.

Problem 1

First we replicated the problem we had during compilation by following the boilerplate opam source and git steps from Creating your own package patches steps.

PS Z:\source\diskuv-ocaml-starter> ./makeit build-dev
>> ... some errors in package core_kernel.v0.14.2 ...

PS Z:\source\diskuv-ocaml-starter> ./makeit shell-dev
>> [diskuv-ocaml-starter]$
 1[diskuv-ocaml-starter]$ cd /tmp
 2[tmp]$ opam source core_kernel.v0.14.2
 3[tmp]$ cd core_kernel.v0.14.2
 4[core_kernel.v0.14.2]$ [ -z "$USERPROFILE" ] || HOME=$(cygpath -au "$USERPROFILE")
 5[core_kernel.v0.14.2]$ git init && git config core.safecrlf false && git add -A
 6[core_kernel.v0.14.2]$ git commit -m "Baseline for patch"
 7[core_kernel.v0.14.2]$ git tag baseline-patch
 9[core_kernel.v0.14.2]$ opam install ./core_kernel.opam -v --debug-level 2
10> #=== ERROR while compiling core_kernel.v0.14.0 ================================#
11> # context     2.1.0 | win32/x86_64 | ocaml-variants.4.12.0+msvc64+msys2 | pinned(git+file://C:/Users/you/AppData/Local/Programs/DiskuvOCaml/1/tools/MSYS2/tmp/core_kernel.v0.14.2#master#6e50f367)
12> # path        Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\.opam-switch\build\core_kernel.v0.14.0
13> # command     Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\bin\dune.exe build -p core_kernel -j 11
14> # exit-code   1
15> # env-file    ~\.opam\log\core_kernel-44284-942604.env
16> # output-file ~\.opam\log\core_kernel-44284-942604.out
17> ### output ###
18> #         bash src/config.h,src/rt-flags (exit 1)
19> # (cd _build/default/src && C:\Users\you\AppData\Local\Programs\DiskuvOCaml\1\tools\MSYS2\usr\bin\bash.exe -e -u -o pipefail -c "cp Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\jst-config\config.h Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\jst-config\rt-flags .")
20> # cp: cannot stat 'Z:sourcediskuv-ocaml-starterbuilddevDebug_opamlibjst-configconfig.h': No such file or directory
21> # cp: cannot stat 'Z:sourcediskuv-ocaml-starterbuilddevDebug_opamlibjst-configrt-flags': No such file or directory

We aren’t big fans of exploring code from the command line, so we opened the package in Visual Studio Code:

[core_kernel.v0.14.2]$ code .

We searched the code (Ctrl-Shift-F in Visual Studio Code) for rt-flags (just search for uncommon text strings that you see in the ERROR). That gave the code in src/dune that what was invoking the highlighted lines above:

1(rule (targets config.h rt-flags) (deps)
2    (action (bash "cp %{lib:jst-config:config.h} %{lib:jst-config:rt-flags} .")))

From our toolchain Differences we can see the explanation of

This situation commonly presents itself as garbled text

matches the ERROR description, and the solution for that difference is to introduce quotes.

We changed the code to:

1(rule (targets config.h rt-flags) (deps)
2    (action (bash "cp '%{lib:jst-config:config.h}' '%{lib:jst-config:rt-flags}' .")))

and then committed the code:

[core_kernel.v0.14.2]$ git commit -m 'Put quotes around jst-config invocation in bash' src/

Problem 2

We check to see if Problem 1 is fixed, and discover a second problem:

1[core_kernel.v0.14.2]$ opam install ./core_kernel.opam -v --debug-level 2
2> - (cd _build/default/src && C:\DiskuvOCaml\BuildTools\VC\Tools\MSVC\14.29.30133\bin\HostX64\x64\cl.exe -nologo -O2 -Gy- -MD -D_CRT_SECURE_NO_DEPRECATE -nologo -O2 -Gy- -MD -D_LARGEFILE64_SOURCE -I Z:/source/diskuv-ocaml-starter/build/dev/Debug/_opam/lib/ocaml -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\base -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\base\base_internalhash_types -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\base\caml -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\base\md5 -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\base\shadow_stdlib -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\base_bigstring -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\base_quickcheck -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\bin_prot -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\bin_prot\shape -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\fieldslib -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\jane-street-headers -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\parsexp -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\ppx_assert\runtime-lib -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\ppx_bench\runtime-lib -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\ppx_compare\runtime-lib -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\ppx_enumerate\runtime-lib -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\ppx_expect\collector -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\ppx_expect\common -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\ppx_expect\config -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\ppx_expect\config_types -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\ppx_hash\runtime-lib -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\ppx_here\runtime-lib -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\ppx_inline_test\config -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\ppx_inline_test\runtime-lib -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\ppx_module_timer\runtime -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\ppx_sexp_conv\runtime-lib -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\sexplib -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\sexplib0 -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\splittable_random -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\stdio -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\time_now -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\typerep -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\variantslib -I ../base_for_tests/src /Fogc_stubs.obj -c gc_stubs.c)
3> - gc_stubs.c
4> - gc_stubs.c(16): error C2146: syntax error: missing ')' before identifier '__attribute__'
5> - gc_stubs.c(16): error C2061: syntax error: identifier '__attribute__'
6> - gc_stubs.c(16): error C2059: syntax error: ';'
7> - gc_stubs.c(16): error C2059: syntax error: ')'
8> - gc_stubs.c(17): error C2054: expected '(' to follow 'unused'

The highlighted lines are clear about which file and which line numbers are the cause, so we opened gc_stubs.c in Visual Studio Code:

 2#include <caml/memory.h>
 3#include <caml/gc_ctrl.h>
 5static intnat minor_words(void)
 7    return (intnat) (caml_stat_minor_words +
 8                (double) (caml_young_end - caml_young_ptr));
11static intnat promoted_words(void)
13    return ((intnat) caml_stat_promoted_words);
16CAMLprim value core_kernel_gc_minor_words(value unit __attribute__((unused)))
18    return Val_long(minor_words());

From our toolchain Differences we can see the explanation of

This situation commonly presents itself as a syntax error

matches the ERROR description, and the solution for that difference is to remove the GCC non-standard extension __attribute__((unused)).

We changed all the code that __attribute__((unused)) to look like:

 2#include <caml/memory.h>
 3#include <caml/gc_ctrl.h>
 5#if defined(_MSC_VER) && _MSC_VER >= 1500
 6# define __unused(x) __pragma( warning (push) ) \
 7    __pragma( warning (disable:4189 ) ) \
 8    x \
 9    __pragma( warning (pop))
11# define __unused(x) x __attribute__((unused))
14static intnat minor_words(void)
16    return (intnat) (caml_stat_minor_words +
17                (double) (caml_young_end - caml_young_ptr));
20static intnat promoted_words(void)
22    return ((intnat) caml_stat_promoted_words);
25CAMLprim value core_kernel_gc_minor_words(__unused(value unit))
27    return Val_long(minor_words());

We actually got the if defined macros from other OCaml code, but the top top Google search for msvc __attribute__((unused)) as of 2021-08-23 turns up something similar.

As always we commit the code:

[core_kernel.v0.14.2]$ git commit -a -m 'Do not assume the use of GCC'

Problem 3

We check to see if Problem 2 is fixed, and discover a third problem:

 1[core_kernel.v0.14.2]$ opam install ./core_kernel.opam -v --debug-level 2
 2> #=== ERROR while compiling core_kernel.v0.14.0 ================================#
 3> # context     2.1.0 | win32/x86_64 | ocaml-variants.4.12.0+msvc64+msys2 | pinned(git+file://C:/Users/you/AppData/Local/Programs/DiskuvOCaml/1/tools/MSYS2/tmp/core_kernel.v0.14.2#master#a5cf803a)
 4> # path        Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\.opam-switch\build\core_kernel.v0.14.0
 5> # command     Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\bin\dune.exe build -p core_kernel -j 11
 6> # exit-code   1
 7> # env-file    ~\.opam\log\core_kernel-1900-c83ce4.env
 8> # output-file ~\.opam\log\core_kernel-1900-c83ce4.out
 9> source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\base -I Z:\source\diskuv-ocaml-starter\build\dev\Debug\_opam\lib\base\base_in[...]
10> # md5_stubs.c
11> # md5_stubs.c(1): fatal error C1083: Cannot open include file: 'unistd.h': No such file or directory

We are experts now, so we open md5_stubs.c in Visual Studio Code:

1#include <unistd.h>
2#include <errno.h>
3#include <caml/alloc.h>
4#include <caml/memory.h>
5#include <caml/mlvalues.h>
6#include <caml/signals.h>
7#include <caml/bigarray.h>
8#include <core_params.h>

From our toolchain Differences we can see the explanation of

This situation commonly presents itself as Cannot open include file: 'xxx.h': No such file or directory

matches the ERROR description, and the solution for that difference is to place the non-Windows header with a Windows header.

But at the moment we don’t know why the file needs unistd.h at all, so we comment out that line completely from all the files which include it, like so:

1// #include <unistd.h>
2#include <errno.h>
3#include <caml/alloc.h>
4#include <caml/memory.h>
5#include <caml/mlvalues.h>
6#include <caml/signals.h>
7#include <caml/bigarray.h>
8#include <core_params.h>

And then commit and rebuild:

1[core_kernel.v0.14.2]$ git commit -a -m 'Remove unistd.h'
2[core_kernel.v0.14.2]$ opam install ./core_kernel.opam -v --debug-level 2
3> # bigstring_stubs.c(39): fatal error C1083: Cannot open include file: 'endian.h': No such file or directory

We’ve got another non-Windows header, and we comment that out and rebuild:

1[core_kernel.v0.14.2]$ git commit -a -m 'Remove endian.h'
2[core_kernel.v0.14.2]$ opam install ./core_kernel.opam -v --debug-level 2
3> ✶ installed core_kernel.v0.14.0
4> ...
5> Done.
6> 01:49.074  ENV                    Environment is up-to-date
7> ...

Somewhat surprisingly the opam install completes successfully! Sometimes Windows header files include more C declarations than the equivalent Linux header, but other times we would have had to do a Google search for the corresponding Windows header.

We’ll clean up the last two commits to look like:

:emphasize-lines: 1

#ifndef _MSC_VER
# include <unistd.h>
#include <errno.h>
#include <caml/alloc.h>
#include <caml/memory.h>
#include <caml/mlvalues.h>
#include <caml/signals.h>
#include <caml/bigarray.h>
#include <core_params.h>

And then we create the Opam file and patch file from the Creating your own package patches instructions:

[core_kernel.v0.14.2]$ git commit -a -m 'Skip unistd.h and endian.h if MSVC toolchain'
[core_kernel.v0.14.2]$ opam install ./core_kernel.opam -v --debug-level 2
[core_kernel.v0.14.2]$ opam remove core_kernel
> <><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
> λ removed   core_kernel.v0.14.0
> Done.
[core_kernel.v0.14.2]$ opam pin remove core_kernel
> Ok, core_kernel is no longer pinned to git+file://C:/Users/you/AppData/Local/Programs/DiskuvOCaml/1/tools/MSYS2/tmp/core_kernel.v0.14.2#master (version v0.14.0)
[core_kernel.v0.14.2]$ git diff baseline-patch > /tmp/custom.patch
[core_kernel.v0.14.2]$ opam show core_kernel -f opam-file > /tmp/opam
[core_kernel.v0.14.2]$ echo 'patches: ["custom.patch"]' >> /tmp/opam

Creating your own package patches shows where to place these /tmp/opam and /tmp/custom.patch files, and you can see the final results in

The last step is to update the Opam package repository with our newly copied patches:

[core_kernel.v0.14.2]$ opam update diskuv-1.2.0

Creating your own package patches

This procedure will not work if the package already has a patch. For example you can’t use this procedure if the package is present in vendor/diskuv-opam-repository and has patches: [...] in its opam file. You may be able to remove the patches clause and then do a opam update diskuv-1.2.0 && opam upgrade before doing any of these instructions, but that procedure has not been tested.

The repository has a vendor/diskuv-opam-repository folder containing all of the patches necessary for the Microsoft Visual Studio toolchain on MSYS2 to work. Your local project vendors that repository as a git submodule at vendor/diskuv-ocaml (run git submodule status to see it). You can fork the repository (assuming you meet the license requirements of course), edit it, and update your git submodule with git submodule set-url vendor/diskuv-ocaml https://YOUR_GIT_FORK.

Follow these instructions to add patches to your own fork.

FIRST, find which OCaml package is failing and check out that package with:

opam source PACKAGE_NAME

The version of the package is important. Using the above command will check out the version of the package that Opam thinks should be installed, which is almost always what you should be patching.

SECOND, create a local git repository; we’ll be using git to create our patch:

[ -z "$USERPROFILE" ] || HOME=$(cygpath -au "$USERPROFILE")
git init && git config core.safecrlf false && git add -A
git commit -m "Baseline for patch"
git tag baseline-patch

If ``git init`` fails because Git is already there, skip to ``git tag baseline-patch`` and then proceed to the THIRD step

THIRD, verify you can recreate the error:

ls *.opam
basename $PWD
opam install ./PACKAGE_NAME.opam -v --debug-level 2

There may be many ``.opam`` files. Use the one that matches the ``basename`` without the version number


Fix the error in the source code with your favorite editor, do a git commit and test it with:

opam install ./PACKAGE_NAME.opam

Doing a ``git commit`` is required. Do not get concerned if you end up with a long string of bad git commits; they will be squashed in the SIXTH step.

Repeat until you get a successful install.

FIFTH, remove your edits so they do not hide the diskuv-opam-repository repository:

opam remove PACKAGE_NAME
opam pin remove PACKAGE_NAME

SIXTH, create a patch:

git diff baseline-patch > /tmp/custom.patch

SEVENTH, create a self-contained opam file:

opam show PACKAGE_NAME -f opam-file > /tmp/opam
echo 'patches: ["custom.patch"]' >> /tmp/opam

cat /tmp/opam

There should be a url { src: "..." checksum: "" } in your file. If not, make sure you ran opam pin remove PACKAGE_NAME

EIGHTH, create/modify the diskuv-opam-repository directory (on Windows PowerShell look in $env:DiskuvOCamlHome\vendor\drd\repos; in general look wherever opam repo list --all | awk '$1=="diskuv"{print $2}' tells you):

└── repos
    └── diskuv-opam-repository
        ├── packages
        │   └── <PACKAGE_NAME>
        │       └── <PACKAGE_NAME.PACKAGE_VERSION>
        │           ├── files
        │           │   └── custom.patch           <==  Copy /tmp/custom.patch
        │           └── opam                       <==  Copy /tmp/opam
        └── repo

For more details visit

NINTH, update your Opam switch with your new diskuv-opam-repository patch:

opam update diskuv-1.2.0

See `Troubleshooting: opam update diskuv <#opam-update-diskuv>`__ if this fails

TENTH, add your new package to the “PINNED_PACKAGES” variable in vendor/drd/src/unix/ if it is not there already.

Done! Go ahead and continue with your normal build. If your patches are useful to the open source community, please consider sending a Pull Request.


opam update diskuv

If after opam update diskuv-1.2.0 you get:

[diskuv] synchronised from file://Z:/somewhere/vendor/diskuv-opam-repository
[ERROR] Could not update repository "diskuv": "Z:\\somewhere\\build\\_tools\\common\\MSYS2\\usr\\bin\\patch.exe -p1 -i C:\\Users\\user\\.opam\\log\\patch-28544-5495c0" exited with code 1

then rerun the command as opam update diskuv-1.2.0 -vv. That will give you something like:

- patching file packages/dune-configurator/dune-configurator.2.9.0/files/custom.patch
- patching file packages/dune-configurator/dune-configurator.2.9.0/files/v1.patch
- Reversed (or previously applied) patch detected!  Assume -R? [n]
- Apply anyway? [n]
- Skipping patch.
- 1 out of 1 hunk ignored -- saving rejects to file packages/dune-configurator/dune-configurator.2.9.0/files/v1.patch.rej
- patching file packages/dune-configurator/dune-configurator.2.9.0/opam
- Hunk #1 FAILED at 47 (different line endings).
- 1 out of 1 hunk FAILED -- saving rejects to file packages/dune-configurator/dune-configurator.2.9.0/opam.rej

Anything with a saving rejects to file packages/SOME_PACKAGE_NAME/.../*.rej is showing a broken package. Just remove the broken package with opam remove SOME_PACKAGE_NAME and opam pin remove SOME_PACKAGE_NAME, like:

opam remove dune-configurator
opam pin remove dune-configurator

opam update diskuv-1.2.0

in the example above.

If that still doesn’t work just do:

opam repository remove diskuv-1.2.0 --all

# On Windows do: ./makeit init-dev
make init-dev

opam repository priority diskuv-1.2.0 1 --all
opam update diskuv-1.2.0

which will rebuild your repository.

Then you can do make prepare-dev to rebuild your switch.