Cannot load required shared library dllunix on Cygwin

[mdesharnais]

The installation of the package yojson failed on Cygwin, claiming that dynamic loading [is] not supported on this platform.

$ opam install yojson
The following actions will be performed:
  ∗  install yojson 1.4.0

=-=- Gathering sources =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
[yojson] Archive in cache

=-=- Processing actions -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
[ERROR] The compilation of yojson failed at "jbuilder build -p yojson -j 4".

#=== ERROR while installing yojson.1.4.0 ======================================#
# opam-version 1.2.2
# os           cygwin
# command      jbuilder build -p yojson -j 4
# path         /home/MDS7/.opam/4.06.0/build/yojson.1.4.0
# compiler     4.06.0
# exit-code    1
# env-file     /home/MDS7/.opam/4.06.0/build/yojson.1.4.0/yojson-9824-5f1023.env
# stdout-file  /home/MDS7/.opam/4.06.0/build/yojson.1.4.0/yojson-9824-5f1023.out
# stderr-file  /home/MDS7/.opam/4.06.0/build/yojson.1.4.0/yojson-9824-5f1023.err
### stderr ###
#        ocaml (internal) (exit 2)
# (cd lib && /home/MDS7/.opam/4.06.0/bin/ocaml -I +compiler-libs -I /home/MDS7/.opam/4.06.0/lib/ocaml /home/MDS7/.opam/4.06.0/lib/ocaml/unix.cma /home/MDS7/.opam/4.06.0/build/yojson.1.4.0/_build/.jbuilds/default/lib/jbuild.ml)
# Cannot load required shared library dllunix.
# Reason: dllunix.so: dynamic loading not supported on this platform.

=-=- Error report -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
The following actions failed
  ∗  install yojson 1.4.0
No changes have been performed

The content of /home/MDS7/.opam/4.06.0/build/yojson.1.4.0/yojson-9824-5f1023.env is:

PATH=/home/MDS7/.opam/4.06.0/bin:/cygdrive/c/Program Files (x86)/Microsoft Visual Studio 14.0/VC/BIN/amd64:/usr/local/bin:/usr/bin:/cygdrive/c/PROGRA~1/ds_shell/code/bin/ds_shell:/cygdrive/c/Python27:/cygdrive/c/Python27/Scripts:/cygdrive/c/ProgramData/Oracle/Java/javapath:/cygdrive/c/Oracle11gR2_64bCli/client_1/bin:/cygdrive/c/Oracle11gR2_32bCli/client_1/bin:/cygdrive/c/Program Files/Microsoft MPI/Bin:/cygdrive/c/Windows/system32:/cygdrive/c/Windows:/cygdrive/c/Windows/System32/Wbem:/cygdrive/c/Windows/System32/WindowsPowerShell/v1.0:/cygdrive/c/Program Files (x86)/Microsoft SQL Server/90/Tools/binn:/cygdrive/c/Program Files/Microsoft/Web Platform Installer:/cygdrive/c/Program Files (x86)/Microsoft ASP.NET/ASP.NET Web Pages/v1.0:/cygdrive/c/Program Files/Microsoft SQL Server/110/Tools/Binn:/cygdrive/c/Windows/system32/config/systemprofile/.dnx/bin:/cygdrive/c/Program Files/Microsoft DNX/Dnvm:/cygdrive/c/Program Files (x86)/Windows Kits/8.1/Windows Performance Toolkit:/cygdrive/c/Program Files/Microsoft SQL Server/130/Tools/Binn:/cygdrive/c/Program Files (x86)/Windows Kits/10/Windows Performance Toolkit:/cygdrive/c/Program Files/Git/cmd:/cygdrive/c/Program Files/Git LFS:/cygdrive/c/Program Files (x86)/Pandoc:/cygdrive/c/Program Files/doxygen/bin:/cygdrive/c/Program Files/CMake/bin
OCAML_TOPLEVEL_PATH=/home/MDS7/.opam/4.06.0/lib/toplevel
PERL5LIB=/home/MDS7/.opam/4.06.0/lib/perl5:
MANPATH=:/home/MDS7/.opam/4.06.0/man
OPAMSWITCH=4.06.0
CAML_LD_LIBRARY_PATH=/home/MDS7/.opam/4.06.0/lib/stublibs
USERDOMAIN=DSONE
OS=Windows_NT
VS90COMNTOOLS=c:\Program Files (x86)\Microsoft Visual Studio 9.0\Common7\Tools\
COMMONPROGRAMFILES=C:\Program Files\Common Files
MSMPI_INC=C:\Program Files (x86)\Microsoft SDKs\MPI\Include\
PROCESSOR_LEVEL=6
PSModulePath=C:\windows\system32\WindowsPowerShell\v1.0\Modules\
ZTI_System32==
CommonProgramW6432=C:\Program Files\Common Files
INCLUDE=C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE;C:\Program Files (x86)\Windows Kits\10\include\10.0.10240.0\ucrt;C:\Program Files (x86)\Windows Kits\8.1\include\shared;C:\Program Files (x86)\Windows Kits\8.1\include\um;
CommonProgramFiles(x86)=C:\Program Files (x86)\Common Files
FP_NO_HOST_CHECK=NO
LANG=C.UTF-8
TZ=Europe/Berlin
HISTCONTROL=ignoredups,ignoredups
HOSTNAME=MUCWS128
PUBLIC=C:\Users\Public
OLDPWD=/cygdrive/c/Users/MDS7/Desktop
UATDATA=C:\windows\CCM\UATData\D9F8C395-CAB8-491d-B8AC-179A1FE1BE77
USERNAME=MDS7
ZTI_ProgramFiles32==
LOGONSERVER=\\EU-MUN-DCS04
PROCESSOR_ARCHITECTURE=AMD64
LOCALAPPDATA=C:\Users\MDS7\AppData\Local
COMPUTERNAME=MUCWS128
USER=MDS7
SYSTEMDRIVE=C:
USERPROFILE=C:\Users\MDS7
PATHEXT=.COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;.MSC
SYSTEMROOT=C:\windows
HOMESHARE=\\vs003.dsone.3ds.com\MDS7
PROCESSOR_IDENTIFIER=Intel64 Family 6 Model 45 Stepping 7, GenuineIntel
PWD=/cygdrive/e/stellar/mlcc
MSMPI_BIN=C:\Program Files\Microsoft MPI\Bin\
HOME=/home/MDS7
TMP=/tmp
windows_tracing_logfile=C:\BVTBin\Tests\installpackage\csilogfile.log
TMUX=/tmp/tmux-1907358/default,8328,0
VS100COMNTOOLS=c:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\Tools\
PROCESSOR_REVISION=2d07
PROFILEREAD=true
LIB=C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\LIB\amd64;C:\Program Files (x86)\Windows Kits\10\lib\10.0.10240.0\ucrt\x64;C:\Program Files (x86)\Windows Kits\8.1\lib\winv6.3\um\x64;
MSMPI_LIB32=C:\Program Files (x86)\Microsoft SDKs\MPI\Lib\x86\
USERDNSDOMAIN=DSONE.3DS.COM
NUMBER_OF_PROCESSORS=16
ProgramW6432=C:\Program Files
windows_tracing_flags=3
GIT_LFS_PATH=C:\Program Files\Git LFS
MSMPI_LIB64=C:\Program Files (x86)\Microsoft SDKs\MPI\Lib\x64\
COMSPEC=C:\windows\system32\cmd.exe
APPDATA=C:\Users\MDS7\AppData\Roaming
TERM=screen
SHELL=/bin/bash
WINDIR=C:\windows
_NT_SYMBOLS_PATH=SRV*C:\Windows\Symbols*http://msdl.microsoft.com/download/symbols
TMUX_PANE=%2
ProgramData=C:\ProgramData
SHLVL=2
PRINTER=\\EU-MUN-PSS01.dsone.3ds.com\EU-MUN-PRD13 (HPCLJCP3505)
PROGRAMFILES=C:\Program Files
ZTI_UserID==
ALLUSERSPROFILE=C:\ProgramData
TEMP=/tmp
SESSIONNAME=Console
ZTI_Password==
ProgramFiles(x86)=C:\Program Files (x86)
ZTI_CommonProgramFiles32==
HOMEDRIVE=Z:
INFOPATH=/usr/local/info:/usr/share/info:/usr/info
HOMEPATH=\
ORIGINAL_PATH=/cygdrive/c/PROGRA~1/ds_shell/code/bin/ds_shell:/cygdrive/c/Python27:/cygdrive/c/Python27/Scripts:/cygdrive/c/ProgramData/Oracle/Java/javapath:/cygdrive/c/Oracle11gR2_64bCli/client_1/bin:/cygdrive/c/Oracle11gR2_32bCli/client_1/bin:/cygdrive/c/Program Files/Microsoft MPI/Bin:/cygdrive/c/Windows/system32:/cygdrive/c/Windows:/cygdrive/c/Windows/System32/Wbem:/cygdrive/c/Windows/System32/WindowsPowerShell/v1.0:/cygdrive/c/Program Files (x86)/Microsoft SQL Server/90/Tools/binn:/cygdrive/c/Program Files/Microsoft/Web Platform Installer:/cygdrive/c/Program Files (x86)/Microsoft ASP.NET/ASP.NET Web Pages/v1.0:/cygdrive/c/Program Files/Microsoft SQL Server/110/Tools/Binn:/cygdrive/c/Windows/system32/config/systemprofile/.dnx/bin:/cygdrive/c/Program Files/Microsoft DNX/Dnvm:/cygdrive/c/Program Files (x86)/Windows Kits/8.1/Windows Performance Toolkit:/cygdrive/c/Program Files/Microsoft SQL Server/130/Tools/Binn:/cygdrive/c/Program Files (x86)/Windows Kits/10/Windows Performance Toolkit:/cygdrive/c/Program Files/Git/cmd:/cygdrive/c/Program Files/Git LFS:/cygdrive/c/Program Files (x86)/Pandoc:/cygdrive/c/Program Files/doxygen/bin:/cygdrive/c/Program Files/CMake/bin
EXECIGNORE=*.dll
VS140COMNTOOLS=C:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\Tools\
VS110COMNTOOLS=C:\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\Tools\
_=/usr/bin/opam
OPAM_PACKAGE_VERSION=1.4.0
OPAM_PACKAGE_NAME=yojson
MAKELEVEL=
MAKEFLAGS=

[dra27]

This is an ocaml or opam-repository issue, rather than jbuilder.

You need to install the flexdll Cygwin package and recompile your switch. If you run ocaml and then try #load "unix.cma";; you'll get the same error.

[ghost]

BTW, looking at yojson it's easy to avoid this problem, I submitted a PR