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1 Introduction

The GNU Hurd1 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).


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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 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 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 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.


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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 Copying). 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.


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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 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.


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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.


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1.5 GNU GENERAL PUBLIC LICENSE

Version 2, June 1991
Copyright © 1989, 1991 Free Software Foundation, Inc.
59 Temple Place – Suite 330, Boston, MA 02111-1307, USA

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

Preamble

The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software—to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation’s software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too.

When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.

To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it.

For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.

We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software.

Also, for each author’s protection and ours, we want to make certain that everyone understands that there is no warranty for this free software. If the software is modified by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the original authors’ reputations.

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The precise terms and conditions for copying, distribution and modification follow.

  1. This License applies to any program or other work which contains a notice placed by the copyright holder saying it may be distributed under the terms of this General Public License. The “Program”, below, refers to any such program or work, and a “work based on the Program” means either the Program or any derivative work under copyright law: that is to say, a work containing the Program or a portion of it, either verbatim or with modifications and/or translated into another language. (Hereinafter, translation is included without limitation in the term “modification”.) Each licensee is addressed as “you”.

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How to Apply These Terms to Your New Programs

If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.

To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the “copyright” line and a pointer to where the full notice is found.

one line to give the program's name and an idea of what it does.
Copyright (C) 19yy  name of author

This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
as published by the Free Software Foundation; either version 2
of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License along
with this program; if not, write to the Free Software Foundation, Inc.,
59 Temple Place, Suite 330, Boston, MA 02111-1307, USA.

Also add information on how to contact you by electronic and paper mail.

If the program is interactive, make it output a short notice like this when it starts in an interactive mode:

Gnomovision version 69, Copyright (C) 19yy name of author
Gnomovision comes with ABSOLUTELY NO WARRANTY; for details
type `show w'.  This is free software, and you are welcome
to redistribute it under certain conditions; type `show c' 
for details.

The hypothetical commands ‘show w’ and ‘show c’ should show the appropriate parts of the General Public License. Of course, the commands you use may be called something other than ‘show w’ and ‘show c’; they could even be mouse-clicks or menu items—whatever suits your program.

You should also get your employer (if you work as a programmer) or your school, if any, to sign a “copyright disclaimer” for the program, if necessary. Here is a sample; alter the names:

Yoyodyne, Inc., hereby disclaims all copyright
interest in the program `Gnomovision'
(which makes passes at compilers) written 
by James Hacker.

signature of Ty Coon, 1 April 1989
Ty Coon, President of Vice

This General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Library General Public License instead of this License.


Footnotes

(1)

The name Hurd stands for “Hird of Unix-Replacing Daemons.” The name Hird stands for “Hurd of Interfaces Representing Depth.”


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