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ocaml-solo5 -- OCaml compiler with Solo5 backend

This package provides a OCaml compiler suitable for linking with a Solo5 base layer.

License and contributions

All original contributions to this package are licensed under the standard MIT license.

This package incorporates components derived or copied from musl libc, OpenBSD, OpenLibm and other third parties. For full details of the licenses of these third party components refer to the included LICENSE file.

The OCaml runtime ("OCaml Core System") built by this package is distributed under the terms of the GNU LGPL version 2.1 with a special exception for static or dynamic linking to produce an executable file. For details refer to the LICENSE file included in the version of the ocaml-src OPAM package installed on your system as a dependency when you build this package.


The following components are built and installed:

In PREFIX/solo5-sysroot/bin:

  • ocamlc: a bytecode OCaml compiler configured for the chosen target. Please note that the bytecode runtime is not supported.
  • ocamlopt: a native OCaml compiler configured for the chosen target.

In PREFIX/solo5-sysroot/lib/ocaml:

  • libasmrun.a, libotherlibs.a: OCaml native code runtime.
  • Compiler libraries.
  • In caml/: Header files for the OCaml runtime.

In PREFIX/solo5-sysroot/lib/nolibc:

  • libnolibc.a: libc interfaces required by the OCaml runtime.
  • libopenlibm.a: libm required by the OCaml runtime.

In PREFIX/solo5-sysroot/include/nolibc:

  • Header files for nolibc and openlibm.

In PREFIX/lib/findlib.conf.d:

  • solo5.conf: ocamlfind definition of the cross-compilation switch.


The installed compiler is able to build solo5 executables. The solo5 bindings (xen, hvt, spt, ...) is chosen at link time, using the solo5-specific -z solo5-abi=XXX compiler/linker option. Linking an executable with no bindings results in a dummy executable.

To build with the Solo5 compiler toolchain, it has to be selected using ocamlfind or dune:

  • ocamlfind: ocamlfind -toolchain solo5 ...
  • dune: dune build -x solo5, or add the toolchain in a build context in the dune workspace file.


The example describes the minimal structure needed to build an ocaml-solo5 executable with dune, linked with the hvt bindings. It requires an application manifest and a startup file to initialize the libc.

Build: dune build -x solo5 Run: solo5-hvt _build/default.solo5/main.exe

Supported compiler versions

Tested against OCaml 4.12.1 through 4.13.0. Other versions may require changing

Porting to a different (uni)kernel base layer

Assuming your unikernel base layer is packaged for OPAM in a similar fashion to Solo5 this should be as simple as:

  1. Adding the appropriate clauses to determine the OPAM packages required and MAKECONF_CFLAGS for compilation to
  2. Implementing a nolibc/sysdeps_yourkernel.c.

Note that the nolibc code is intentionally strict about namespacing of APIs and header files. If your base layer exports symbols or defines types which conflict with nolibc then the recommended course of action is to fix your base layer to not export anything defined by "POSIX" or "standard C".

Updating the vendored copy of OpenLibm

OpenLibm is "vendored" into this repository using git subtree:

git subtree add --prefix openlibm v0.5.4 --squash

To update the vendored copy of OpenLibm to the newer upstream version TAG, use the following command on a branch and then file a PR:

git subtree pull --prefix openlibm TAG --squash