# MirageOS 4.0 Beta Release **URL:** \ **Category:** Community\ **Tags:** mirageos, announce\ **Created:** [February 10, 2022, 6:27pm UTC](https://discuss.ocaml.org/t/mirageos-4-0-beta-release/9302 "2022-02-10T18:27:47Z")\ **Posts on this page:** 1\ **Showing post:** 1
**Author:** ![samoht](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/samoht/32/81_2.png) [@samoht](https://discuss.ocaml.org/u/samoht)\ **Post date:** [February 10, 2022, 6:27pm UTC](https://discuss.ocaml.org/t/mirageos-4-0-beta-release/9302/1 "2022-02-10T18:27:47Z")
**On behalf of the Mirage team, I am delighted to announce the beta release of MirageOS 4.0!** [MirageOS](https://mirage.io) is a library operating system that constructs unikernels for secure, high-performance network applications across a variety of hypervisor and embedded platforms. For example, OCaml code can be developed on a standard OS, such as Linux or macOS, and then compiled into a fully standalone, specialised unikernel that runs under a Xen or KVM hypervisor. The MirageOS project also supplies several protocol and storage implementations written in pure OCaml, ranging from TCP/IP to TLS to a full Git-like storage stack. The beta of the MirageOS 4.00 release contains: - `mirage.4.0.0~beta`: the CLI tool; - `ocaml-freestanding.0.7.0`: a libc-free OCaml runtime; - and `solo5.0.7.0`: a cross-compiler for OCaml. They are all available in `opam` by using: ```auto opam install 'mirage>=4.0' ``` _Note_: you need to explicitly add the `mirage>=4.0` version here, otherwise `opam` will select the latest `3.*` stable release. For a good experience, check that at least version `4.0.0~beta3` is installed. ## New Features This new release of MirageOS adds systematic support for cross-compilation to all supported unikernel targets. This means that libraries that use C stubs (like Base, for example) can now seamlessly have those stubs cross-compiled to the desired target. Previous releases of MirageOS required specific support to accomplish this by adding the stubs to a central package. MirageOS implements cross-compilation using _Dune Workspaces_, which can take a whole collection of OCaml code (including all transitive dependencies) and compile it with a given set of C and OCaml compiler flags. This workflow also unlocks support for familiar IDE tools (such as `ocaml-lsp-server` and Merlin) while developing unikernels in OCaml. It makes day-to-day coding much faster because builds are decoupled from configuration and package updates. This means that live-builds, such as Dune’s watch mode, now work fine even for exotic build targets! A complete list of features can be found on the [MirageOS 4 release page](https://mirage.io/docs/mirage-4). ## Cross-Compilation and Dune Overlays This release introduces a significant change in the way MirageOS projects are compiled based on Dune Workspaces. This required implementing a new developer experience for Opam users in order to simplify cross-compilation of large OCaml projects. That new tool, called [opam-monorepo](https://github.com/ocamllabs/opam-monorepo) (née duniverse), separates package management from building the resulting source code. It is an Opam plugin that: - creates a lock file for the project dependencies - downloads and extracts the dependency sources locally - sets up a Dune Workspace so that `dune build` builds everything in one go. [![asciicast](https://asciinema.org/a/rRf6s8cNyHUbBsDDfZkBjkf7X.svg)](https://asciinema.org/a/rRf6s8cNyHUbBsDDfZkBjkf7X?speed=2) `opam-monorepo` is already available in Opam and can be used on many projects which use `dune` as a build system. However, as we don’t expect the complete set of OCaml dependencies to use `dune`, we MirageOS maintainers are committed to maintaining patches to build the most common dependencies with `dune`. These packages are hosted in a separate [dune-universe/mirage-opam-overlays](https://github.com/mirage/opam-overlays) repository, which can be used by `opam-monorepo` and is enabled by default when using the Mirage CLI tool. ## Next Steps Your feedback on this beta release is very much appreciated. You can follow the tutorials on [Mirage 4 | MirageOS](https://mirage.io/docs/mirage-4). Issues are very welcome on [GitHub · Where software is built](https://github.com/mirage/mirage/issues), or come find us on Matrix in the MirageOS channel: [#mirageos:matrix.org](https://matrix.to/#/#mirageos:matrix.org). The **final release** will happen in about a month. This release will incorporate your early feedback. It will also ensure the existing MirageOS ecosystem is compatible with MirageOS 4 by reducing the overlay packages to the bare minimum. We also plan to write more on `opam-monorepo` and all the new things MirageOS 4.0 will bring. --- _[View the full topic](https://discuss.ocaml.org/t/mirageos-4-0-beta-release/9302)._