1. Introduction

The GNU Hurd(1) is the GNU Project's replacement for the Unix kernel. The Hurd is a collection of servers that run on the Mach microkernel to implement file systems, network protocols, file access control, and other features that are normally implemented by the Unix kernel or similar kernels (such as Linux).

1.1 Audience

This manual is designed to be useful to everybody who is interested in using, administering, or programming the Hurd.

If you are an end-user and you are looking for help on running the Hurd, the first few chapters of this manual describe the essential parts of installing, starting up, and shutting down a Hurd workstation. If you need help with a specific program, the best way to use this manual is to find the program's name in the index and go directly to the appropriate section. You may also wish to try running program --help, which will display a brief usage message for program (see section Foundations).

The rest of this manual is a technical discussion of the Hurd servers and their implementation, and would not be helpful until you want to learn how to modify the Hurd.

This manual is organized according to the subsystems of the Hurd, and each chapter begins with descriptions of utilities and servers that are related to that subsystem. If you are a system administrator, and you want to learn more about, say, the Hurd networking subsystem, you can skip to the networking chapter (see section Networking), and read about the related utilities and servers.

Programmers who are interested in learning how to modify Hurd servers, or write new ones, should begin by learning about a microkernel to which the Hurd has been ported (currently only GNU Mach) and reading Foundations. You should then familiarize yourself with a subsystem that interests you by reading about existing servers and the libraries they use. At that point, you should be able to study the source code of existing Hurd servers and understand how they use the Hurd libraries.

The final level of mastery is learning the about the RPC interfaces which the Hurd libraries implement. The last section of each chapter describes any Hurd interfaces used in that subsystem. Those sections assume that you are perusing the referenced interface definitions as you read. After you have understood a given interface, you will be in a good position to improve the Hurd libraries, design your own interfaces, and implement new subsystems.

1.2 Features

The Hurd is not the most advanced operating system known to the planet (yet), but it does have a number of enticing features:

it's free software

Anybody can use, modify, and redistribute it under the terms of the GNU General Public License (see section GNU General Public License). The Hurd is part of the GNU system, which is a complete operating system licensed under the GPL.

it's compatible

The Hurd provides a familiar programming and user environment. For all intents and purposes, the Hurd is a modern Unix-like kernel. The Hurd uses the GNU C Library, whose development closely tracks standards such as ANSI/ISO, BSD, POSIX, Single Unix, SVID, and X/Open.

it's built to survive

Unlike other popular kernel software, the Hurd has an object-oriented structure that allows it to evolve without compromising its design. This structure will help the Hurd undergo major redesign and modifications without having to be entirely rewritten.

it's scalable

The Hurd implementation is aggressively multithreaded so that it runs efficiently on both single processors and symmetric multiprocessors. The Hurd interfaces are designed to allow transparent network clusters (collectives), although this feature has not yet been implemented.

it's extensible

The Hurd is an attractive platform for learning how to become a kernel hacker or for implementing new ideas in kernel technology. Every part of the system is designed to be modified and extended.

it's stable

It is possible to develop and test new Hurd kernel components without rebooting the machine (not even accidentally). Running your own kernel components doesn't interfere with other users, and so no special system privileges are required. The mechanism for kernel extensions is secure by design: it is impossible to impose your changes upon other users unless they authorize them or you are the system administrator.

it exists

The Hurd is real software that works Right Now. It is not a research project or a proposal. You don't have to wait at all before you can start using and developing it.

1.3 Overview

An operating system kernel provides a framework for programs to share a computer's hardware resources securely and efficiently. This framework includes mechanisms for programs to communicate safely, even if they do not trust one another (see section Ports Library).

The GNU Hurd divides up the work of the traditional kernel, and implements it in separate programs, or kernel servers. The Hurd formally defines the communication protocols that each of the servers understands, so that it is possible for different servers to implement the same interface.

The GNU C Library provides a POSIX environment on the Hurd, by translating standard POSIX system calls into interactions with the appropriate Hurd server.

1.4 History

Richard Stallman (RMS) started GNU in 1983, as a project to create a complete free operating system. In the text of the GNU Manifesto, he mentioned that there is a primitive kernel. In the first GNUsletter, Feb. 1986, he says that GNU's kernel is TRIX, which was developed at the Massachusetts Institute of Technology.

By December of 1986, the Free Software Foundation (FSF) had "started working on the changes needed to TRIX" [Gnusletter, Jan. 1987]. Shortly thereafter, the FSF began "negotiating with Professor Rashid of Carnegie-Mellon University about working with them on the development of the Mach kernel" [Gnusletter, June, 1987]. The text implies that the FSF wanted to use someone else's work, rather than have to fix TRIX.

In [Gnusletter, Feb. 1988], RMS was talking about taking Mach and putting the Berkeley Sprite filesystem on top of it, "after the parts of Berkeley Unix… have been replaced."

Six months later, the FSF is saying that "if we can't get Mach, we'll use TRIX or Berkeley's Sprite." Here, they present Sprite as a full-kernel option, rather than just a filesystem.

In January, 1990, they say "we aren't doing any kernel work. It does not make sense for us to start a kernel project now, when we still hope to use Mach" [Gnusletter, Jan. 1990]. Nothing significant occurs until 1991, when a more detailed plan is announced:

``We are still interested in a multi-process kernel running on top of
Mach. The CMU lawyers are currently deciding if they can release Mach
with distribution conditions that will enable us to distribute it. If
they decide to do so, then we will probably start work. CMU has
available under the same terms as Mach a single-server partial Unix
emulator named Poe; it is rather slow and provides minimal
functionality. We would probably begin by extending Poe to provide full
functionality. Later we hope to have a modular emulator divided into
multiple processes.'' [Gnusletter, Jan. 1991].

RMS explains the relationship between the Hurd and Linux in http://www.gnu.org/software/hurd/hurd-and-linux.html, where he mentions that the FSF started developing the Hurd in 1990. As of [Gnusletter, Nov. 1991], the Hurd (running on Mach) is GNU's official kernel.

1.5 GNU General Public License

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