Skip to content


Folders and files

Last commit message
Last commit date

Latest commit


Repository files navigation

NOTE: This project is unmaintained

CloudABI is no longer being maintained. It was an awesome experiment, but it never got enough traction to be sustainable. If you like the idea behind CloudABI, please consider looking into the WebAssembly System Interface (WASI). WASI's design has been inspired by CloudABI.

The author of CloudABI (Ed Schouten) would like to thank all of the people who contributed to this project. Let's hope CloudABI lives on in spirit and influences the way software is designed in the future!

Introducing Nuxi CloudABI

Nuxi CloudABI is an application binary interface for UNIX-like operating systems built around the concept of capability-based security. It offers an environment in which processes can only interact with locally created resources (pipes, socket pairs, shared memory, subprocesses and threads), or global resources (files, sockets) when the process possesses a token granting the process access to the resource (a UNIX file descriptor). In a nutshell, it means that you can run processes directly on top of a UNIX kernel while keeping complete control over the actions the process is allowed to perform.

For example, consider the case where one wants to run a web service. In CloudABI this can be modeled by starting a web server process with the following file descriptors:

  • A listening network socket used to receive incoming HTTP requests,
  • One or more file descriptors to directories containing resources accessible through the web,
  • One or more network sockets connected to backend services used by the web server (e.g., database servers),
  • A file descriptor pointing to a log file.

During its lifetime this web server will never be capable of creating new tokens out of thin air. It may accept incoming network connections on the socket provided on startup, but not create an additional listening socket. It may open files stored within the directories provided on startup, but not access arbitrary paths on the system. This makes CloudABI a lot more secure than the runtime offered by most other UNIX-like systems. A security flaw in this web server will only allow an attacker to compromise the process; not the entire system.

As CloudABI is mostly legacy-free and only needs to implement the features that make sense in an environment that uses capability-based security, it is very small in comparison to other UNIX ABIs. At the time of writing, CloudABI only has 49 system calls. The number of types, datastructures and constants shared between kernelspace and userspace is very low when compared to existing operating systems. This makes it easy to add support for CloudABI executables to existing operating systems. The idea being that software only needs to be compiled once and can be run everywhere.

Nuxi CloudABI aims to become an ideal runtime environment for cluster/cloud computing. As processes become more isolated and less dependent on the host environment, the need for traditional system administration and configuration management will decrease. A cluster can be treated as a single abstract computation service, running processes that are not tied to individual logical operating system instances.

The design of CloudABI is based on the Capsicum lightweight OS capability and sandbox framework developed at the University of Cambridge Computer Laboratory. Nuxi would like to thank its authors for their work.

cloudlibc: Standard C library for CloudABI

cloudlibc is a standard C library built on top of CloudABI. It implements almost all features of the C11 standard, but also a large part of POSIX.1-2008. There are interfaces, however, that have been omitted for one of the following reasons:

  • Safety first.
    cloudlibc aims to make it easier for people to design and implement robust services. C applications may easily be prone to buffer overflows. Functions whose only purpose is to cause such bugs (e.g., gets(), strcpy()) are not implemented. The same holds for functions that are inherently thread unsafe (strtok()), degrade security (srand()), introduce unneeded global state (setlocale()) or offer bad abstraction (signal handlers).

  • Pure capability-based security.
    As access to global namespaces is prohibited, functions such as open(), fopen(), stat(), mkdir() and wait() have been omitted. For filesystem access it is possible to use the POSIX.1-2008 *at() functions instead. In some other cases alternatives have been developed. Fear not: most of these interfaces are either available on systems such as FreeBSD or have been designed in such a way that they can easily be added to existing operating systems.

  • Computing in the cloud.
    Though it is perfectly fine to run CloudABI processes from a command-line on your own workstation, the primary use case remains running networked services in the cloud or on a cluster. Certain concepts (e.g., UNIX user credentials, TTY handling, accounting) make little sense in such an environment.

The source tree is structured as follows:

Building cloudlibc

cloudlibc uses the Bazel build system. A copy of cloudlibc can be built by installing Bazel and running the following command:

bazel build //...

Testing cloudlibc

Unit tests can be run through Bazel as well. Test execution currently assumes the system has native (non-emulated) support for running CloudABI executables (i.e., you're using FreeBSD). These tests can be executed by running the following command:

bazel test //...

Installing cloudlibc

The Bazel build rules provide no support for explicitly installing cloudlibc. Instead of that, it is possible to use Bazel's support for external repositories to use cloudlibc as part of your C/C++ project. Instructions for that can be found on the Bazel Toolchains for CloudABI project page.


Public support

The mailing list is used to discuss the development of CloudABI. It is possible to sign up for this mailing list and browse its archive on Google Groups.

An IRC channel called #cloudabi exists on EFnet.

Bugs and pull requests can be submitted through GitHub.

Commercial support

CloudABI has been developed by Nuxi, an IT company based in the lovely city of 's-Hertogenbosch, the Netherlands. Are you interested in receiving commercial support on CloudABI or any of its components? Be sure to get in touch with us at