dlvr.it
Static linking in OCaml
Most of the time, you don’t think about how your file is linked. We’ve come to love dynamically linked files with their small file sizes and reduced memory requirements, but there are times when the convenience of a single binary download from a GitHub release page is really what you need.
To do this in OCaml, we need to add -ccopt -static to the ocamlopt. I’m building with dune, so I can configure that in my dune file using a flags directive.
(flags (:standard -ccopt -static))
This can be extended for maximum compatibility by additionally adding -ccopt -march=x86-64, which ensures the generated code will run on any x86_64 processor and will not use newer instruction set extensions like SSE3, AVX, etc.
So what about Windows? The Mingw tool chain accepts -static. Including (flags (:standard -ccopt "-link -Wl,-static -v")) got my options applied to my dune build:
x86_64-w64-mingw32-gcc -mconsole -L. -I"C:/Users/Administrator/my-app/_opam/lib/ocaml" -I"C:\Users\Administrator\my-app\_opam\lib\mccs" -I"C:\Users\Administrator\my-app\_opam\lib\mccs\glpk/internal" -I"C:\Users\Administrator\my-app\_opam\lib\opam-core" -I"C:\Users\Administrator\my-app\_opam\lib\sha" -I"C:/Users/Administrator/my-app/_opam/lib/ocaml\flexdll" -L"C:/Users/Administrator/my-app/_opam/lib/ocaml" -L"C:\Users\Administrator\my-app\_opam\lib\mccs" -L"C:\Users\Administrator\my-app\_opam\lib\mccs\glpk/internal" -L"C:\Users\Administrator\my-app\_opam\lib\opam-core" -L"C:\Users\Administrator\my-app\_opam\lib\sha" -L"C:/Users/Administrator/my-app/_opam/lib/ocaml\flexdll" -o "bin/main.exe" "C:\Users\ADMINI~1\AppData\Local\Temp\2\build_d62d04_dune\dyndllb7e0e8.o" "@C:\Users\ADMINI~1\AppData\Local\Temp\2\build_d62d04_dune\camlrespec7816" "-municode" "-Wl,-static"
However, ldd showed that this wasn’t working:
$ ldd main.exe | grep mingw
libstdc++-6.dll => /mingw64/bin/libstdc++-6.dll (0x7ffabf3e0000)
libgcc_s_seh-1.dll => /mingw64/bin/libgcc_s_seh-1.dll (0x7ffac3130000)
libwinpthread-1.dll => /mingw64/bin/libwinpthread-1.dll (0x7ffac4b40000)
I tried a lot of different variations. I asked Claude… then I asked @dra27 who recalled @kit-ty-kate working on this for opam. PR#5680
The issue is the auto-response file, which precedes my static option. We can remove that by adding -noautolink, but now we must do all the work by hand and build a massive command line.
(executable
(public_name main)
(name main)
(flags (:standard -noautolink -cclib -lunixnat -cclib -lmccs_stubs -cclib -lmccs_glpk_stubs -cclib -lsha_stubs -cclib -lopam_core_stubs -cclib -l:libstdc++.a -cclib -l:libpthread.a -cclib -Wl,-static -cclib -ladvapi32 -cclib -lgdi32 -cclib -luser32 -cclib -lshell32 -cclib -lole32 -cclib -luuid -cclib -luserenv -cclib -lwindowsapp))
(libraries opam-client))
It works, but it’s not for the faint-hearted.
I additionally added (enabled_if (= %{os_type} Win32)) to my rule so it only runs on Windows.
