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Advanced Programming in the UNIX® Environment: Second Edition By W. Richard Stevens, Stephen A. Rago

...

Publisher: Addison Wesley Professional Pub Date: June 17, 2005

ISBN: 0201433079 Pages: 960

Table of Contents | Index

"Stephen Rago's update is a long overdue benefit to the community of professionals using the versatile family of UNIX and UNIX-like operating environments. It removes obsolescence and includes newer developments. It also thoroughly updates the context of all topics, examples, and applications to recent releases of popular implementations of UNIX and UNIX-like environments. And yet, it does all this while retaining the style and taste of the original classic."--Mukesh Kacker, cofounder and former CTO of Pronto Networks, Inc."One of the essential classics of UNIX programming."--Eric S. Raymond, author of The Art of UNIX Programming"This is the definitive reference book for any serious or professional UNIX systems programmer. Rago has updated and extended the classic Stevens text while keeping true to the original. The APIs are illuminated by clear examples of their use. He also mentions many of the pitfalls to look out for when programming across different UNIX system implementations and points out how to avoid these pitfalls using relevant standards such as POSIX 1003.1, 2004 edition and the Single UNIX Specification, Version 3."--Andrew Josey, Director, Certification, The Open Group, and Chair of the POSIX 1003.1 Working Group"Advanced Programming in the UNIX® Environment, Second Edition, is an essential reference for anyone writing programs for a UNIX system. It's the first book I turn to when I want to understand or re-learn any of the various system interfaces. Stephen Rago has successfully revised this book to incorporate newer operating systems such as GNU/Linux and Apple's OS X while keeping true to the first edition in terms of both readability and usefulness. It will always have a place right next to my computer."--Dr. Benjamin Kuperman, Swarthmore CollegePraise for the First Edition"Advanced Programming in the UNIX® Environment is a must-have for any serious C programmer who works under UNIX. Its depth, thoroughness, and clarity of explana-tion are unmatched."--UniForum Monthly"Numerous readers recommended Advanced Programming in the UNIX® Environment by W. Richard Stevens (Addison-Wesley), and I'm glad they did; I hadn't even heard of this book, and it's been out since 1992. I just got my hands on a copy, and the first few chapters have been fascinating."--Open Systems Today"A much more readable and detailed treatment of UNIX internals can be found in Advanced Programming in the UNIX®

Environment by W. Richard Stevens (Addison-Wesley). This book includes lots of realistic examples, and I find it quite helpful when I have systems programming tasks to do."--RS/Magazine"This is the definitive reference book for any serious or professional UNIX systems programmer. Rago has updated and extended the original Stevens classic while keeping true to the original."--Andrew Josey, Director, Certification, The Open Group, and Chair of the POSIX 1003.1 Working GroupFor over a decade, serious C programmers have relied on one book for practical, in-depth knowledge of the programming interfaces that drive the UNIX and Linux kernels: W.

Richard Stevens' Advanced Programming in the UNIX® Environment. Now, Stevens' colleague Stephen Rago has thoroughly updated this classic to reflect the latest technical advances and add support for today's leading UNIX and Linux platforms.Rago carefully retains the spirit and approach that made this book a classic. Building on Stevens' work, he begins with basic topics such as files, directories, and processes, carefully laying the groundwork for understanding more advanced techniques, such as signal handling and terminal I/O.Substantial new material includes chapters on threads and multithreaded programming, using the socket interface to drive interprocess communication (IPC), and extensive coverage of the interfaces added to the latest version of the POSIX.1 standard.

Nearly all examples have been tested on four of today's most widely used UNIX/Linux platforms: FreeBSD 5.2.1; the Linux 2.4.22 kernel; Solaris 9; and Darwin 7.4.0, the FreeBSD/Mach hybrid underlying Apple's Mac OS X 10.3.As in the first edition, you'll learn through example, including more than 10,000 lines of downloadable, ANSI C source code. More than 400 system calls and functions are demonstrated with concise, complete programs that clearly illustrate their usage, arguments, and return values. To tie together what you've learned, the book presents several chapter-length case studies, each fully updated for contemporary

environments.Advanced Programming in the UNIX® Environment has helped a generation of programmers write code with exceptional power, performance, and reliability. Now updated for today's UNIX/Linux systems, this second edition will be even more indispensable.

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Advanced Programming in the UNIX® Environment: Second Edition By W. Richard Stevens, Stephen A. Rago

...

Publisher: Addison Wesley Professional Pub Date: June 17, 2005

ISBN: 0201433079 Pages: 960

Table of Contents | Index

Copyright

Praise for Advanced Programming in the UNIX® Environment, Second Edition Praise for the First Edition

Addison-Wesley Professional Computing Series Foreword

Preface Introduction

Changes from the First Edition Acknowledgments

Preface to the First Edition Introduction

Unix Standards Organization of the Book Examples in the Text

Systems Used to Test the Examples Acknowledgments

Chapter 1. UNIX System Overview Section 1.1. Introduction

Section 1.2. UNIX Architecture Section 1.3. Logging In

Section 1.4. Files and Directories Section 1.5. Input and Output Section 1.6. Programs and Processes Section 1.7. Error Handling

Section 1.8. User Identification Section 1.9. Signals

Section 1.10. Time Values

Section 1.11. System Calls and Library Functions Section 1.12. Summary

Exercises

Chapter 2. UNIX Standardization and Implementations Section 2.1. Introduction

Section 2.2. UNIX Standardization Section 2.3. UNIX System Implementations

Section 2.4. Relationship of Standards and Implementations Section 2.5. Limits

Section 2.6. Options

Section 2.7. Feature Test Macros Section 2.8. Primitive System Data Types

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Section 2.9. Conflicts Between Standards Section 2.10. Summary

Exercises Chapter 3. File I/O Section 3.1. Introduction Section 3.2. File Descriptors Section 3.3. open Function Section 3.4. creat Function Section 3.5. close Function Section 3.6. lseek Function Section 3.7. read Function Section 3.8. write Function Section 3.9. I/O Efficiency Section 3.10. File Sharing Section 3.11. Atomic Operations Section 3.12. dup and dup2 Functions

Section 3.13. sync, fsync, and fdatasync Functions Section 3.14. fcntl Function

Section 3.15. ioctl Function Section 3.16. /dev/fd Section 3.17. Summary Exercises

Chapter 4. Files and Directories Section 4.1. Introduction

Section 4.2. stat, fstat, and lstat Functions Section 4.3. File Types

Section 4.4. Set-User-ID and Set-Group-ID Section 4.5. File Access Permissions

Section 4.6. Ownership of New Files and Directories Section 4.7. access Function

Section 4.8. umask Function

Section 4.9. chmod and fchmod Functions Section 4.10. Sticky Bit

Section 4.11. chown, fchown, and lchown Functions Section 4.12. File Size

Section 4.13. File Truncation Section 4.14. File Systems

Section 4.15. link, unlink, remove, and rename Functions Section 4.16. Symbolic Links

Section 4.17. symlink and readlink Functions Section 4.18. File Times

Section 4.19. utime Function

Section 4.20. mkdir and rmdir Functions Section 4.21. Reading Directories

Section 4.22. chdir, fchdir, and getcwd Functions Section 4.23. Device Special Files

Section 4.24. Summary of File Access Permission Bits Section 4.25. Summary

Exercises

Chapter 5. Standard I/O Library Section 5.1. Introduction

Section 5.2. Streams and FILE Objects

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Section 5.3. Standard Input, Standard Output, and Standard Error Section 5.4. Buffering

Section 5.5. Opening a Stream

Section 5.6. Reading and Writing a Stream Section 5.7. Line-at-a-Time I/O

Section 5.8. Standard I/O Efficiency Section 5.9. Binary I/O

Section 5.10. Positioning a Stream Section 5.11. Formatted I/O Section 5.12. Implementation Details Section 5.13. Temporary Files

Section 5.14. Alternatives to Standard I/O Section 5.15. Summary

Exercises

Chapter 6. System Data Files and Information Section 6.1. Introduction

Section 6.2. Password File Section 6.3. Shadow Passwords Section 6.4. Group File

Section 6.5. Supplementary Group IDs Section 6.6. Implementation Differences Section 6.7. Other Data Files

Section 6.8. Login Accounting Section 6.9. System Identification Section 6.10. Time and Date Routines Section 6.11. Summary

Exercises

Chapter 7. Process Environment Section 7.1. Introduction Section 7.2. main Function Section 7.3. Process Termination Section 7.4. Command-Line Arguments Section 7.5. Environment List

Section 7.6. Memory Layout of a C Program Section 7.7. Shared Libraries

Section 7.8. Memory Allocation Section 7.9. Environment Variables Section 7.10. setjmp and longjmp Functions Section 7.11. getrlimit and setrlimit Functions Section 7.12. Summary

Exercises

Chapter 8. Process Control Section 8.1. Introduction Section 8.2. Process Identifiers Section 8.3. fork Function Section 8.4. vfork Function Section 8.5. exit Functions

Section 8.6. wait and waitpid Functions Section 8.7. waitid Function

Section 8.8. wait3 and wait4 Functions Section 8.9. Race Conditions

Section 8.10. exec Functions

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Section 8.11. Changing User IDs and Group IDs Section 8.12. Interpreter Files

Section 8.13. system Function Section 8.14. Process Accounting Section 8.15. User Identification Section 8.16. Process Times Section 8.17. Summary Exercises

Chapter 9. Process Relationships Section 9.1. Introduction Section 9.2. Terminal Logins Section 9.3. Network Logins Section 9.4. Process Groups Section 9.5. Sessions

Section 9.6. Controlling Terminal

Section 9.7. tcgetpgrp, tcsetpgrp, and tcgetsid Functions Section 9.8. Job Control

Section 9.9. Shell Execution of Programs Section 9.10. Orphaned Process Groups Section 9.11. FreeBSD Implementation Section 9.12. Summary

Exercises

Chapter 10. Signals Section 10.1. Introduction Section 10.2. Signal Concepts Section 10.3. signal Function Section 10.4. Unreliable Signals Section 10.5. Interrupted System Calls Section 10.6. Reentrant Functions Section 10.7. SIGCLD Semantics

Section 10.8. Reliable-Signal Terminology and Semantics Section 10.9. kill and raise Functions

Section 10.10. alarm and pause Functions Section 10.11. Signal Sets

Section 10.12. sigprocmask Function Section 10.13. sigpending Function Section 10.14. sigaction Function

Section 10.15. sigsetjmp and siglongjmp Functions Section 10.16. sigsuspend Function

Section 10.17. abort Function Section 10.18. system Function Section 10.19. sleep Function Section 10.20. Job-Control Signals Section 10.21. Additional Features Section 10.22. Summary

Exercises

Chapter 11. Threads Section 11.1. Introduction Section 11.2. Thread Concepts Section 11.3. Thread Identification Section 11.4. Thread Creation Section 11.5. Thread Termination

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Section 11.6. Thread Synchronization Section 11.7. Summary

Exercises

Chapter 12. Thread Control Section 12.1. Introduction Section 12.2. Thread Limits Section 12.3. Thread Attributes Section 12.4. Synchronization Attributes Section 12.5. Reentrancy

Section 12.6. Thread-Specific Data Section 12.7. Cancel Options Section 12.8. Threads and Signals Section 12.9. Threads and fork Section 12.10. Threads and I/O Section 12.11. Summary Exercises

Chapter 13. Daemon Processes Section 13.1. Introduction

Section 13.2. Daemon Characteristics Section 13.3. Coding Rules

Section 13.4. Error Logging

Section 13.5. Single-Instance Daemons Section 13.6. Daemon Conventions Section 13.7. ClientServer Model Section 13.8. Summary Exercises

Chapter 14. Advanced I/O Section 14.1. Introduction Section 14.2. Nonblocking I/O Section 14.3. Record Locking Section 14.4. STREAMS Section 14.5. I/O Multiplexing Section 14.6. Asynchronous I/O Section 14.7. readv and writev Functions Section 14.8. readn and writen Functions Section 14.9. Memory-Mapped I/O Section 14.10. Summary

Exercises

Chapter 15. Interprocess Communication Section 15.1. Introduction

Section 15.2. Pipes

Section 15.3. popen and pclose Functions Section 15.4. Coprocesses

Section 15.5. FIFOs Section 15.6. XSI IPC

Section 15.7. Message Queues Section 15.8. Semaphores Section 15.9. Shared Memory Section 15.10. ClientServer Properties Section 15.11. Summary

Exercises

Chapter 16. Network IPC: Sockets

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Section 16.1. Introduction Section 16.2. Socket Descriptors Section 16.3. Addressing

Section 16.4. Connection Establishment Section 16.5. Data Transfer

Section 16.6. Socket Options Section 16.7. Out-of-Band Data

Section 16.8. Nonblocking and Asynchronous I/O Section 16.9. Summary

Exercises

Chapter 17. Advanced IPC Section 17.1. Introduction

Section 17.2. STREAMS-Based Pipes Section 17.3. UNIX Domain Sockets Section 17.4. Passing File Descriptors Section 17.5. An Open Server, Version 1 Section 17.6. An Open Server, Version 2 Section 17.7. Summary

Exercises

Chapter 18. Terminal I/O Section 18.1. Introduction Section 18.2. Overview

Section 18.3. Special Input Characters

Section 18.4. Getting and Setting Terminal Attributes Section 18.5. Terminal Option Flags

Section 18.6. stty Command Section 18.7. Baud Rate Functions Section 18.8. Line Control Functions Section 18.9. Terminal Identification Section 18.10. Canonical Mode Section 18.11. Noncanonical Mode Section 18.12. Terminal Window Size Section 18.13. termcap, terminfo, and curses Section 18.14. Summary

Exercises

Chapter 19. Pseudo Terminals Section 19.1. Introduction Section 19.2. Overview

Section 19.3. Opening Pseudo-Terminal Devices Section 19.4. pty_fork Function

Section 19.5. pty Program

Section 19.6. Using the pty Program Section 19.7. Advanced Features Section 19.8. Summary

Exercises

Chapter 20. A Database Library Section 20.1. Introduction Section 20.2. History Section 20.3. The Library

Section 20.4. Implementation Overview Section 20.5. Centralized or Decentralized?

Section 20.6. Concurrency

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Section 20.7. Building the Library Section 20.8. Source Code Section 20.9. Performance Section 20.10. Summary Exercises

Chapter 21. Communicating with a Network Printer Section 21.1. Introduction

Section 21.2. The Internet Printing Protocol Section 21.3. The Hypertext Transfer Protocol Section 21.4. Printer Spooling

Section 21.5. Source Code Section 21.6. Summary Exercises

Appendix A. Function Prototypes Appendix B. Miscellaneous Source Code Section B.1. Our Header File

B.2 Standard Error Routines

Appendix C. Solutions to Selected Exercises Chapter 1

Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapter 10 Chapter 11 Chapter 12 Chapter 13 Chapter 14 Chapter 15 Chapter 16 Chapter 17 Chapter 18 Chapter 19 Chapter 20 Chapter 21 Bibliography Index

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Copyright

Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in this book, and the publisher was aware of a trademark claim, the designations have been printed with initial capital letters or in all capitals.

The authors and publisher have taken care in the preparation of this book, but make no expressed or implied warranty of any kind and assume no responsibility for errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of the use of the information or programs contained herein.

The publisher offers excellent discounts on this book when ordered in quantity for bulk purchases or special sales, which may include electronic versions and/or custom covers and content particular to your business, training goals, marketing focus, and branding interests.

For more information, please contact:

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(800) 382-3419

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Visit us on the Web: www.awprofessional.com

Library of Congress Cataloging-in-Publication Data:

Stevens, W. Richard.

Advanced programming in the Unix environment / W. Richard Stevens, Stephen A. Rago.2nd ed.

p. cm.

Includes bibliographical references and index.

ISBN 0-201-43307-9 (hardcover : alk. paper)

1. Operating systems (Computers) 2. UNIX (Computer file) I. Rago, Stephen A. II. Title.

QA76.76.O63S754 2005 005.4'32dc22

2005007943

Copyright © 2005 Pearson Education, Inc.

All rights reserved. Printed in the United States of America. This publication is protected by copyright, and permission must be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permissions, write to:

Pearson Education, Inc.

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Rights and Contracts Department One Lake Street

Upper Saddle River, NJ 07458

0-201-43307-9

Text printed in the United States on recycled paper at Courier in Westford, Massachusetts. First printing, June 2005

Dedication

To Jeanne

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Praise for Advanced Programming in the UNIX ® Environment, Second Edition

"Stephen Rago's update is a long overdue benefit to the community of professionals using the versatile family of UNIX and UNIX-like operating environments. It removes obsolescence and includes newer developments. It also thoroughly updates the context of all topics, examples, and applications to recent releases of popular

implementations of UNIX and UNIX-like environments. And yet, it does all this while retaining the style and taste of the original classic."

Mukesh Kacker, cofounder and former CTO of Pronto Networks, Inc.

"One of the essential classics of UNIX programming."

Eric S. Raymond, author of The Art of UNIX Programming

"This is the definitive reference book for any serious or professional UNIX systems programmer. Rago has updated and extended the classic Stevens text while keeping true to the original. The APIs are illuminated by clear examples of their use. He also mentions many of the pitfalls to look out for when programming across different UNIX system implementations and points out how to avoid these pitfalls using relevant standards such as POSIX 1003.1, 2004 edition and the Single UNIX Specification, Version 3."

Andrew Josey, Director, Certification, The Open Group, and Chair of the POSIX 1003.1 Working Group

"Advanced Programming in the UNIX®

Environment, Second Edition, is an essential reference for anyone writing programs for a UNIX system. It's the first book I turn to when I want to understand or re-learn any of the various system interfaces. Stephen Rago has successfully revised this book to incorporate newer operating systems such as GNU/Linux and Apple's OS X while keeping true to the first edition in terms of both readability and usefulness.

It will always have a place right next to my computer."

Dr. Benjamin Kuperman, Swarthmore College

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Praise for the First Edition

"Advanced Programming in the UNIX®

Environment is a must-have for any serious C programmer who works under UNIX. Its depth, thoroughness, and clarity of explanation are unmatched."

UniForum Monthly

"Numerous readers recommended Advanced Programming in the UNIX®

Environment by W. Richard Stevens (Addison-Wesley), and I'm glad they did; I hadn't even heard of this book, and it's been out since 1992. I just got my hands on a copy, and the first few chapters have been fascinating."

Open Systems Today

"A much more readable and detailed treatment of [UNIX internals] can be found in Advanced Programming in the UNIX®

Environment by W. Richard Stevens (Addison-Wesley). This book includes lots of realistic examples, and I find it quite helpful when I have systems programming tasks to do."

RS/Magazine

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.

(13)

Addison-Wesley Professional Computing Series

Brian W. Kernighan, Consulting Editor

Matthew H. Austern, Generic Programming and the STL: Using and Extending the C++ Standard Template Library

David R. Butenhof, Programming with POSIX® Threads

Brent Callaghan, NFS Illustrated

Tom Cargill, C++ Programming Style

William R. Cheswick/Steven M. Bellovin/Aviel D. Rubin, Firewalls and Internet Security, Second Edition: Repelling the Wily Hacker

David A. Curry, UNIX®

System Security: A Guide for Users and System Administrators

Stephen C. Dewhurst, C++ Gotchas: Avoiding Common Problems in Coding and Design

Dan Farmer/Wietse Venema, Forensic Discovery

Erich Gamma/Richard Helm/Ralph Johnson/John Vlissides, Design Patterns: Elements of Reusable Object-Oriented Software

Erich Gamma/Richard Helm/Ralph Johnson/John Vlissides, Design Patterns CD: Elements of Reusable Object-Oriented Software

Peter Haggar, Practical Java™

Programming Language Guide

David R. Hanson, C Interfaces and Implementations: Techniques for Creating Reusable Software

Mark Harrison/Michael McLennan, Effective Tcl/Tk Programming: Writing Better Programs with Tcl and Tk

Michi Henning/Steve Vinoski, Advanced CORBA®

Programming with C++

Brian W. Kernighan/Rob Pike, The Practice of Programming

S. Keshav, An Engineering Approach to Computer Networking: ATM Networks, the Internet, and the Telephone Network

John Lakos, Large-Scale C++ Software Design

Scott Meyers, Effective C++ CD: 85 Specific Ways to Improve Your Programs and Designs

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Scott Meyers, Effective C++, Third Edition: 55 Specific Ways to Improve Your Programs and Designs

Scott Meyers, More Effective C++: 35 New Ways to Improve Your Programs and Designs

Scott Meyers, Effective STL: 50 Specific Ways to Improve Your Use of the Standard Template Library

Robert B. Murray, C++ Strategies and Tactics

David R. Musser/Gillmer J. Derge/Atul Saini, STL Tutorial and Reference Guide, Second Edition: C++

Programming with the Standard Template Library

John K. Ousterhout, Tcl and the Tk Toolkit

Craig Partridge, Gigabit Networking

Radia Perlman, Interconnections, Second Edition: Bridges, Routers, Switches, and Internetworking Protocols

Stephen A. Rago, UNIX®

System V Network Programming

Eric S. Raymond, The Art of UNIX Programming

Marc J. Rochkind, Advanced UNIX Programming, Second Edition

Curt Schimmel, UNIX®

Systems for Modern Architectures: Symmetric Multiprocessing and Caching for Kernel Programmers

W. Richard Stevens, TCP/IP Illustrated, Volume 1: The Protocols

W. Richard Stevens, TCP/IP Illustrated, Volume 3: TCP for Transactions, HTTP, NNTP, and the UNIX® Domain Protocols

W. Richard Stevens/Bill Fenner/Andrew M. Rudoff, UNIX Network Programming Volume 1, Third Edition: The Sockets Networking API

W. Richard Stevens/Stephen A. Rago, Advanced Programming in the UNIX®

Environment, Second Edition

W. Richard Stevens/Gary R. Wright, TCP/IP Illustrated Volumes 1-3 Boxed Set

John Viega/Gary McGraw, Building Secure Software: How to Avoid Security Problems the Right Way

Gary R. Wright/W. Richard Stevens, TCP/IP Illustrated, Volume 2: The Implementation

Ruixi Yuan/W. Timothy Strayer, Virtual Private Networks: Technologies and Solutions

Visit www.awprofessional.com/series/professionalcomputing for more information about these titles.

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(15)

Foreword

At some point during nearly every interview I give, as well as in question periods after talks, I get asked some variant of the same question: "Did you expect Unix to last for so long?" And of course the answer is always the same: No, we didn't quite anticipate what has happened. Even the observation that the system, in some form, has been around for well more than half the lifetime of the commercial computing industry is now dated.

The course of developments has been turbulent and complicated. Computer technology has changed greatly since the early 1970s, most notably in universal networking, ubiquitous graphics, and readily available personal computing, but the system has somehow managed to accommodate all of these phenomena. The commercial environment, although today dominated on the desktop by Microsoft and Intel, has in some ways moved from single-supplier to multiple sources and, in recent years, to increasing reliance on public standards and on freely available source.

Fortunately, Unix, considered as a phenomenon and not just a brand, has been able to move with and even lead this wave. AT&T in the 1970s and 1980s was protective of the actual Unix source code, but encouraged standardization efforts based on the system's interfaces and languages. For example, the SVIDthe System V Interface Definitionwas published by AT&T, and it became the basis for the POSIX work and its follow-ons. As it happened, Unix was able to adapt rather gracefully to a networked environment and, perhaps less elegantly, but still adequately, to a graphical one. And as it also happened, the basic Unix kernel interface and many of its characteristic user-level tools were incorporated into the technological foundations of the open-source movement.

It is important that papers and writings about the Unix system were always encouraged, even while the software of the system itself was proprietary, for example Maurice Bach's book, The Design of the Unix Operating System. In fact, I would claim that a central reason for the system's longevity has been that it has attracted remarkably talented writers to explain its beauties and mysteries. Brian

Kernighan is one of these; Rich Stevens is certainly another. The first edition of this book, along with his series of books about networking, are rightfully regarded as remarkably well-crafted works of exposition, and became hugely popular.

However, the first edition of this book was published before Linux and the several open-source renditions of the Unix interface that stemmed from the Berkeley CSRG became widespread, and also at a time when many people's networking consisted of a serial modem. Steve Rago has carefully updated this book to account for the technology changes, as well as developments in various ISO and IEEE standards since its first publication. Thus his examples are fresh, and freshly tested.

It's a most worthy second edition of a classic.

Murray Hill, New Jersey Dennis Ritchie

March 2005

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Preface

Introduction

Changes from the First Edition Acknowledgments

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Introduction

Rich Stevens and I first met through an e-mail exchange when I reported a typographical error in his first book, UNIX Network Programming. He used to kid me about being the person to send him his first errata notice for the book. Until his death in 1999, we exchanged e-mail irregularly, usually when one of us had a question we thought the other might be able to answer. We met for dinner at USENIX conferences and when Rich was teaching in the area.

Rich Stevens was a friend who always conducted himself as a gentleman. When I wrote UNIX System V Network Programming in 1993, I intended it to be a System V version of Rich's UNIX Network Programming. As was his nature, Rich gladly reviewed chapters for me, and treated me not as a competitor, but as a colleague. We often talked about collaborating on a STREAMS version of his TCP/IP Illustrated book. Had events been different, we might have actually done it, but since Rich is no longer with us, revising Advanced Programming in the UNIX Environment is the closest I'll ever get to writing a book with him.

When the editors at Addison-Wesley told me that they wanted to update Rich's book, I thought that there wouldn't be too much to change. Even after 13 years, Rich's work still holds up well. But the UNIX industry is vastly different today from what it was when the book was first published.

The System V variants are slowly being replaced by Linux. The major system vendors that ship their hardware with their own versions of the UNIX System have either made Linux ports available or announced support for Linux. Solaris is perhaps the last descendant of UNIX System V Release 4 with any appreciable market share.

After 4.4BSD was released, the Computing Science Research Group (CSRG) from the University of California at Berkeley decided to put an end to its development of the UNIX operating system, but several different groups of volunteers still maintain publicly available versions.

The introduction of Linux, supported by thousands of volunteers, has made it possible for anyone with a computer to run an operating system similar to the UNIX System, with freely available source code for the newest hardware devices. The success of Linux is something of a curiosity, given that several free BSD alternatives are readily available.

Continuing its trend as an innovative company, Apple Computer abandoned its old Mac operating system and replaced it with one based on Mach and FreeBSD.

Thus, I've tried to update the information presented in this book to reflect these four platforms.

After Rich wrote Advanced Programming in the UNIX Environment in 1992, I got rid of most of my UNIX programmer's manuals. To this day, the two books I keep closest to my desk are a dictionary and a copy of Advanced Programming in the UNIX Environment. I hope you find this revision equally useful.

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Changes from the First Edition

Rich's work holds up well. I've tried not to change his original vision for this book, but a lot has happened in 13 years. This is especially true with the standards that affect the UNIX programming interface.

Throughout the book, I've updated interfaces that have changed from the ongoing efforts in standards organizations. This is most noticeable in Chapter 2, since its primary topic is standards. The 2001 version of the POSIX.1 standard, which we use in this revision, is much more comprehensive than the 1990 version on which the first edition of this book was based. The 1990 ISO C standard was updated in 1999, and some changes affect the interfaces in the POSIX.1 standard.

A lot more interfaces are now covered by the POSIX.1 specification. The base specifications of the Single UNIX Specification (published by The Open Group, formerly X/Open) have been merged with POSIX.1. POSIX.1 now includes several 1003.1 standards and draft standards that were formerly published separately.

Accordingly, I've added chapters to cover some new topics. Threads and multithreaded programming are important concepts because they present a cleaner way for programmers to deal with concurrency and asynchrony.

The socket interface is now part of POSIX.1. It provides a single interface to interprocess communication (IPC), regardless of the location of the process, and is a natural extension of the IPC chapters.

I've omitted most of the real-time interfaces that appear in POSIX.1. These are best treated in a text devoted to real-time programming.

One such book appears in the bibliography.

I've updated the case studies in the last chapters to cover more relevant real-world examples. For example, few systems these days are connected to a PostScript printer via a serial or parallel port. Most PostScript printers today are accessed via a network, so I've changed the case study that deals with PostScript printer communication to take this into account.

The chapter on modem communication is less relevant these days. So that the original material is not lost, however, it is available on the book's Web site in two formats: PostScript (http://www.apuebook.com/lostchapter/modem.ps) and PDF

(http://www.apuebook.com/lostchapter/modem.pdf).

The source code for the examples shown in this book is also available at www.apuebook.com. Most of the examples have been run on four platforms:

FreeBSD 5.2.1, a derivative of the 4.4BSD release from the Computer Systems Research Group at the University of California at Berkeley, running on an Intel Pentium processor

1.

Linux 2.4.22 (the Mandrake 9.2 distribution), a free UNIX-like operating system, running on Intel Pentium processors 2.

Solaris 9, a derivative of System V Release 4 from Sun Microsystems, running on a 64-bit UltraSPARC IIi processor 3.

Darwin 7.4.0, an operating environment based on FreeBSD and Mach, supported by Apple Mac OS X, version 10.3, on a PowerPC processor

4.

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Acknowledgments

Rich Stevens wrote the first edition of this book on his own, and it became an instant classic.

I couldn't have updated this book without the support of my family. They put up with piles of papers scattered about the house (well, more so than usual), my monopolizing most of the computers in the house, and lots of hours with my face buried behind a computer terminal. My wife, Jeanne, even helped out by installing Linux for me on one of the test machines.

The technical reviewers suggested many improvements and helped make sure that the content was accurate. Many thanks to David Bausum, David Boreham, Keith Bostic, Mark Ellis, Phil Howard, Andrew Josey, Mukesh Kacker, Brian Kernighan, Bengt Kleberg, Ben Kuperman, Eric Raymond, and Andy Rudoff.

I'd also like to thank Andy Rudoff for answering questions about Solaris and Dennis Ritchie for digging up old papers and answering history questions. Once again, the staff at Addison-Wesley was great to work with. Thanks to Tyrrell Albaugh, Mary Franz, John Fuller, Karen Gettman, Jessica Goldstein, Noreen Regina, and John Wait. My thanks to Evelyn Pyle for the fine job of copyediting.

As Rich did, I also welcome electronic mail from any readers with comments, suggestions, or bug fixes.

Warren, New Jersey Stephen A. Rago

April 2005 sar@apuebook.com

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Preface to the First Edition

Introduction Unix Standards Organization of the Book Examples in the Text

Systems Used to Test the Examples Acknowledgments

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Introduction

This book describes the programming interface to the Unix systemthe system call interface and many of the functions provided in the standard C library. It is intended for anyone writing programs that run under Unix.

Like most operating systems, Unix provides numerous services to the programs that are runningopen a file, read a file, start a new program, allocate a region of memory, get the current time-of-day, and so on. This has been termed the system call interface.

Additionally, the standard C library provides numerous functions that are used by almost every C program (format a variable's value for output, compare two strings, etc.).

The system call interface and the library routines have traditionally been described in Sections 2 and 3 of the Unix Programmer's Manual. This book is not a duplication of these sections. Examples and rationale are missing from the Unix Programmer's Manual, and that's what this book provides.

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Unix Standards

The proliferation of different versions of Unix during the 1980s has been tempered by the various international standards that were started during the late 1980s. These include the ANSI standard for the C programming language, the IEEE POSIX family (still being developed), and the X/Open portability guide.

This book also describes these standards. But instead of just describing the standards by themselves, we describe them in relation to popular implementations of the standardsSystem V Release 4 and the forthcoming 4.4BSD. This provides a real-world description, which is often lacking from the standard itself and from books that describe only the standard.

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Organization of the Book

This book is divided into six parts:

An overview and introduction to basic Unix programming concepts and terminology (Chapter 1), with a discussion of the various Unix standardization efforts and different Unix implementations (Chapter 2).

1.

I/Ounbuffered I/O (Chapter 3), properties of files and directories (Chapter 4), the standard I/O library (Chapter 5), and the standard system data files (Chapter 6).

2.

Processesthe environment of a Unix process (Chapter 7), process control (Chapter 8), the relationships between different processes (Chapter 9), and signals (Chapter 10).

3.

More I/Oterminal I/O (Chapter 11), advanced I/O (Chapter 12), and daemon processes (Chapter 13).

4.

IPCInterprocess communication (Chapters 14 and 15).

5.

Examplesa database library (Chapter 16), communicating with a PostScript printer (Chapter 17), a modem dialing program (Chapter 18), and using pseudo terminals (Chapter 19).

6.

A reading familiarity with C would be beneficial as would some experience using Unix. No prior programming experience with Unix is assumed. This text is intended for programmers familiar with Unix and programmers familiar with some other operating system who wish to learn the details of the services provided by most Unix systems.

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Examples in the Text

This book contains many examplesapproximately 10,000 lines of source code. All the examples are in the C programming language.

Furthermore, these examples are in ANSI C. You should have a copy of the Unix Programmer's Manual for your system handy while reading this book, since reference is made to it for some of the more esoteric and implementation-dependent features.

Almost every function and system call is demonstrated with a small, complete program. This lets us see the arguments and return values and is often easier to comprehend than the use of the function in a much larger program. But since some of the small programs are contrived examples, a few bigger examples are also included (Chapters 16, 17, 18, and 19). These larger examples demonstrate the programming techniques in larger, real-world examples.

All the examples have been included in the text directly from their source files. A machine-readable copy of all the examples is available via anonymous FTP from the Internet host ftp.uu.net in the file published/books/stevens.advprog.tar.Z. Obtaining the source code allows you to modify the programs from this text and experiment with them on your system.

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Systems Used to Test the Examples

Unfortunately all operating systems are moving targets. Unix is no exception. The following diagram shows the recent evolution of the various versions of System V and 4.xBSD.

[View full size image]

4.xBSD are the various systems from the Computer Systems Research Group at the University of California at Berkeley. This group also distributes the BSD Net 1 and BSD Net 2 releasespublicly available source code from the 4.xBSD systems. SVRx refers to System V Release x from AT&T. XPG3 is the X/Open Portability Guide, Issue 3, and ANSI C is the ANSI standard for the C programming language.

POSIX.1 is the IEEE and ISO standard for the interface to a Unix-like system. We'll have more to say about these different standards and the various versions of Unix in Sections 2.2 and 2.3.

In this text we use the term 4.3+BSD to refer to the Unix system from Berkeley that is somewhere between the BSD Net 2 release and 4.4BSD.

At the time of this writing, 4.4BSD was not released, so the system could not be called 4.4BSD. Nevertheless a simple name was needed to refer to this system and 4.3+BSD is used throughout the text.

Most of the examples in this text have been run on four different versions of Unix:

Unix System V/386 Release 4.0 Version 2.0 ("vanilla SVR4") from U.H. Corp. (UHC), on an Intel 80386 processor.

1.

4.3+BSD at the Computer Systems Research Group, Computer Science Division, University of California at Berkeley, on a Hewlett Packard workstation.

2.

BSD/386 (a derivative of the BSD Net 2 release) from Berkeley Software Design, Inc., on an Intel 80386 processor. This system is almost identical to what we call 4.3+BSD.

3.

SunOS 4.1.1 and 4.1.2 (systems with a strong Berkeley heritage but many System V features) from Sun Microsystems, on a SPARCstation SLC.

4.

Numerous timing tests are provided in the text and the systems used for the test are identified.

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Acknowledgments

Once again I am indebted to my family for their love, support, and many lost weekends over the past year and a half.

Writing a book is, in many ways, a family affair. Thank you Sally, Bill, Ellen, and David.

I am especially grateful to Brian Kernighan for his help in the book. His numerous thorough reviews of the entire manuscript and his gentle prodding for better prose hopefully show in the final result. Steve Rago was also a great resource, both in reviewing the entire manuscript and answering many questions about the details and history of System V. My thanks to the other technical reviewers used by Addison- Wesley, who provided valuable comments on various portions of the manuscript: Maury Bach, Mark Ellis, Jeff Gitlin, Peter Honeyman, John Linderman, Doug McIlroy, Evi Nemeth, Craig Partridge, Dave Presotto, Gary Wilson, and Gary Wright.

Keith Bostic and Kirk McKusick at the U.C. Berkeley CSRG provided an account that was used to test the examples on the latest BSD system. (Many thanks to Peter Salus too.) Sam Nataros and Joachim Sacksen at UHC provided the copy of SVR4 used to test the examples. Trent Hein helped obtain the alpha and beta copies of BSD/386.

Other friends have helped in many small, but significant ways over the past few years: Paul Lucchina, Joe Godsil, Jim Hogue, Ed Tankus, and Gary Wright. My editor at Addison-Wesley, John Wait, has been a great friend through it all. He never complained when the due date slipped and the page count kept increasing. A special thanks to the National Optical Astronomy Observatories (NOAO), especially Sidney Wolff, Richard Wolff, and Steve Grandi, for providing computer time.

Real Unix books are written using troff and this book follows that time-honored tradition. Camera-ready copy of the book was produced by the author using the groff package written by James Clark. Many thanks to James Clark for providing this excellent system and for his rapid response to bug fixes. Perhaps someday I will really understand troff footer traps.

I welcome electronic mail from any readers with comments, suggestions, or bug fixes.

Tucson, Arizona W. Richard Stevens

April 1992 rstevens@kohala.com

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Chapter 1. UNIX System Overview

Section 1.1. Introduction Section 1.2. UNIX Architecture Section 1.3. Logging In

Section 1.4. Files and Directories Section 1.5. Input and Output Section 1.6. Programs and Processes Section 1.7. Error Handling

Section 1.8. User Identification Section 1.9. Signals

Section 1.10. Time Values

Section 1.11. System Calls and Library Functions Section 1.12. Summary

Exercises

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1.1. Introduction

All operating systems provide services for programs they run. Typical services include executing a new program, opening a file, reading a file, allocating a region of memory, getting the current time of day, and so on. The focus of this text is to describe the services provided by various versions of the UNIX operating system.

Describing the UNIX System in a strictly linear fashion, without any forward references to terms that haven't been described yet, is nearly impossible (and would probably be boring). This chapter provides a whirlwind tour of the UNIX System from a programmer's perspective.

We'll give some brief descriptions and examples of terms and concepts that appear throughout the text. We describe these features in much more detail in later chapters. This chapter also provides an introduction and overview of the services provided by the UNIX System, for programmers new to this environment.

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1.2. UNIX Architecture

In a strict sense, an operating system can be defined as the software that controls the hardware resources of the computer and provides an environment under which programs can run. Generally, we call this software the kernel, since it is relatively small and resides at the core of the environment. Figure 1.1 shows a diagram of the UNIX System architecture.

Figure 1.1. Architecture of the UNIX operating system

The interface to the kernel is a layer of software called the system calls (the shaded portion in Figure 1.1). Libraries of common functions are built on top of the system call interface, but applications are free to use both. (We talk more about system calls and library functions in Section 1.11.) The shell is a special application that provides an interface for running other applications.

In a broad sense, an operating system is the kernel and all the other software that makes a computer useful and gives the computer its personality. This other software includes system utilities, applications, shells, libraries of common functions, and so on.

For example, Linux is the kernel used by the GNU operating system. Some people refer to this as the GNU/Linux operating system, but it is more commonly referred to as simply Linux. Although this usage may not be correct in a strict sense, it is understandable, given the dual meaning of the phrase operating system. (It also has the advantage of being more succinct.)

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1.3. Logging In

Login Name

When we log in to a UNIX system, we enter our login name, followed by our password. The system then looks up our login name in its password file, usually the file /etc/passwd. If we look at our entry in the password file we see that it's composed of seven colon-separated fields: the login name, encrypted password, numeric user ID (205), numeric group ID (105), a comment field, home directory (/home/sar), and shell program (/bin/ksh).

sar:x:205:105:Stephen Rago:/home/sar:/bin/ksh

All contemporary systems have moved the encrypted password to a different file. In Chapter 6, we'll look at these files and some functions to access them.

Shells

Once we log in, some system information messages are typically displayed, and then we can type commands to the shell program. (Some systems start a window management program when you log in, but you generally end up with a shell running in one of the windows.) A shell is a command-line interpreter that reads user input and executes commands. The user input to a shell is normally from the terminal (an interactive shell) or sometimes from a file (called a shell script). The common shells in use are summarized in Figure 1.2.

Figure 1.2. Common shells used on UNIX systems

Name Path FreeBSD 5.2.1 Linux 2.4.22 Mac OS X 10.3 Solaris 9

Bourne shell /bin/sh • link to bash link to bash

Bourne-again shell /bin/bash optional • • •

C shell /bin/csh link to tcsh link to tcsh link to tcsh

Korn shell /bin/ksh

TENEX C shell /bin/tcsh • • • •

The system knows which shell to execute for us from the final field in our entry in the password file.

The Bourne shell, developed by Steve Bourne at Bell Labs, has been in use since Version 7 and is provided with almost every UNIX system in existence. The control-flow constructs of the Bourne shell are reminiscent of Algol 68.

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The C shell, developed by Bill Joy at Berkeley, is provided with all the BSD releases. Additionally, the C shell was provided by AT&T with System V/386 Release 3.2 and is also in System V Release 4 (SVR4). (We'll have more to say about these different versions of the UNIX System in the next chapter.) The C shell was built on the 6th Edition shell, not the Bourne shell. Its control flow looks more like the C language, and it supports additional features that weren't provided by the Bourne shell: job control, a history mechanism, and command line editing.

The Korn shell is considered a successor to the Bourne shell and was first provided with SVR4. The Korn shell, developed by David Korn at Bell Labs, runs on most UNIX systems, but before SVR4 was usually an extra-cost add-on, so it is not as widespread as the other two shells. It is upward compatible with the Bourne shell and includes those features that made the C shell popular: job control, command line editing, and so on.

The Bourne-again shell is the GNU shell provided with all Linux systems. It was designed to be POSIX-conformant, while still remaining compatible with the Bourne shell. It supports features from both the C shell and the Korn shell.

The TENEX C shell is an enhanced version of the C shell. It borrows several features, such as command completion, from the TENEX operating system (developed in 1972 at Bolt Beranek and Newman). The TENEX C shell adds many features to the C shell and is often used as a replacement for the C shell.

Linux uses the Bourne-again shell for its default shell. In fact, /bin/sh is a link to /bin/bash. The default user shell in FreeBSD and Mac OS X is the TENEX C shell, but they use the Bourne shell for their administrative shell scripts because the C shell's programming language is notoriously difficult to use. Solaris, having its heritage in both BSD and System V, provides all the shells shown in Figure 1.2. Free ports of most of the shells are available on the Internet.

Throughout the text, we will use parenthetical notes such as this to describe historical notes and to compare different implementations of the UNIX System. Often the reason for a particular implementation technique becomes clear when the historical reasons are described.

Throughout this text, we'll show interactive shell examples to execute a program that we've developed. These examples use features common to the Bourne shell, the Korn shell, and the Bourne-again shell.

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1.4. Files and Directories

File System

The UNIX file system is a hierarchical arrangement of directories and files. Everything starts in the directory called root whose name is the single character /.

A directory is a file that contains directory entries. Logically, we can think of each directory entry as containing a filename along with a structure of information describing the attributes of the file. The attributes of a file are such things as type of fileregular file, directorythe size of the file, the owner of the file, permissions for the filewhether other users may access this fileand when the file was last modified.

The stat and fstat functions return a structure of information containing all the attributes of a file. In Chapter 4, we'll examine all the attributes of a file in great detail.

We make a distinction between the logical view of a directory entry and the way it is actually stored on disk. Most implementations of UNIX file systems don't store attributes in the directory entries themselves, because of the difficulty of keeping them in synch when a file has multiple hard links. This will become clear when we discuss hard links in Chapter 4.

Filename

The names in a directory are called filenames. The only two characters that cannot appear in a filename are the slash character (/) and the null character. The slash separates the filenames that form a pathname (described next) and the null character terminates a pathname.

Nevertheless, it's good practice to restrict the characters in a filename to a subset of the normal printing characters. (We restrict the characters because if we use some of the shell's special characters in the filename, we have to use the shell's quoting mechanism to reference the filename, and this can get complicated.)

Two filenames are automatically created whenever a new directory is created: . (called dot) and .. (called dot-dot). Dot refers to the current directory, and dot-dot refers to the parent directory. In the root directory, dot-dot is the same as dot.

The Research UNIX System and some older UNIX System V file systems restricted a filename to 14 characters. BSD versions extended this limit to 255 characters. Today, almost all commercial UNIX file systems support at least 255-character filenames.

Pathname

A sequence of one or more filenames, separated by slashes and optionally starting with a slash, forms a pathname. A pathname that begins with a slash is called an absolute pathname; otherwise, it's called a relative pathname. Relative pathnames refer to files relative to the current directory. The name for the root of the file system (/) is a special-case absolute pathname that has no filename component.

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Example

Listing the names of all the files in a directory is not difficult. Figure 1.3 shows a bare-bones implementation of the ls(1) command.

The notation ls(1) is the normal way to reference a particular entry in the UNIX system manuals. It refers to the entry for ls in Section 1.

The sections are normally numbered 1 through 8, and all the entries within each section are arranged alphabetically. Throughout this text, we assume that you have a copy of the manuals for your UNIX system.

Historically, UNIX systems lumped all eight sections together into what was called the UNIX Programmer's Manual. As the page count increased, the trend changed to distributing the sections among separate manuals:

one for users, one for programmers, and one for system administrators, for example.

Some UNIX systems further divide the manual pages within a given section, using an uppercase letter. For example, all the standard input/output (I/O) functions in AT&T [1990e] are indicated as being in Section 3S, as in fopen(3S). Other systems have replaced the numeric sections with alphabetic ones, such as C for commands.

Today, most manuals are distributed in electronic form. If your manuals are online, the way to see the manual pages for the ls command would be something like

man 1 ls

or

man -s1 ls

Figure 1.3 is a program that just prints the name of every file in a directory, and nothing else. If the source file is named myls.c, we compile it into the default a.out executable file by

cc myls.c

Historically, cc(1) is the C compiler. On systems with the GNU C compilation system, the C compiler is gcc(1). Here, cc is often linked to gcc.

Some sample output is

$ ./a.out /dev .

..

console tty mem kmem null mouse stdin stdout stderr zero

many more lines that aren't shown cdrom

$ ./a.out /var/spool/cron

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can't open /var/spool/cron: Permission denied $ ./a.out /dev/tty

can't open /dev/tty: Not a directory

Throughout this text, we'll show commands that we run and the resulting output in this fashion: Characters that we type are shown in this font, whereas output from programs is shown like this. If we need to add comments to this output, we'll show the comments in italics. The dollar sign that precedes our input is the prompt that is printed by the shell. We'll always show the shell prompt as a dollar sign.

Note that the directory listing is not in alphabetical order. The ls command sorts the names before printing them.

There are many details to consider in this 20-line program.

First, we include a header of our own: apue.h. We include this header in almost every program in this text. This header includes some standard system headers and defines numerous constants and function prototypes that we use throughout the examples in the text. A listing of this header is in Appendix B.

The declaration of the main function uses the style supported by the ISO C standard. (We'll have more to say about the ISO C standard in the next chapter.)

We take an argument from the command line, argv[1], as the name of the directory to list. In Chapter 7, we'll look at how the main function is called and how the command-line arguments and environment variables are accessible to the program.

Because the actual format of directory entries varies from one UNIX system to another, we use the functions opendir, readdir, and closedir to manipulate the directory.

The opendir function returns a pointer to a DIR structure, and we pass this pointer to the readdir function. We don't care what's in the DIR structure. We then call readdir in a loop, to read each directory entry. The readdir function returns a pointer to a dirent structure or, when it's finished with the directory, a null pointer. All we examine in the dirent structure is the name of each directory entry (d_name). Using this name, we could then call the stat function (Section 4.2) to determine all the attributes of the file.

We call two functions of our own to handle the errors: err_sys and err_quit. We can see from the preceding output that the err_sys function prints an informative message describing what type of error was encountered ("Permission denied" or "Not a directory"). These two error functions are shown and described in Appendix B. We also talk more about error handling in Section 1.7.

When the program is done, it calls the function exit with an argument of 0. The function exit terminates a program. By convention, an argument of 0 means OK, and an argument between 1 and 255 means that an error occurred. In Section 8.5, we show how any program, such as a shell or a program that we write, can obtain the exit status of a program that it executes.

Figure 1.3. List all the files in a directory

#include "apue.h"

#include <dirent.h>

int

main(int argc, char *argv[]) {

DIR *dp;

struct dirent *dirp;

if (argc != 2)

err_quit("usage: ls directory_name");

if ((dp = opendir(argv[1])) == NULL) err_sys("can't open %s", argv[1]);

while ((dirp = readdir(dp)) != NULL)

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printf("%s\n", dirp->d_name);

closedir(dp);

exit(0);

}

Working Directory

Every process has a working directory, sometimes called the current working directory. This is the directory from which all relative pathnames are interpreted. A process can change its working directory with the chdir function.

For example, the relative pathname doc/memo/joe refers to the file or directory joe, in the directory memo, in the directory doc, which must be a directory within the working directory. From looking just at this pathname, we know that both doc and memo have to be directories, but we can't tell whether joe is a file or a directory. The pathname /usr/lib/lint is an absolute pathname that refers to the file or directory lint in the directory lib, in the directory usr, which is in the root directory.

Home Directory

When we log in, the working directory is set to our home directory. Our home directory is obtained from our entry in the password file (Section 1.3).

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1.5. Input and Output

File Descriptors

File descriptors are normally small non-negative integers that the kernel uses to identify the files being accessed by a particular process. Whenever it opens an existing file or creates a new file, the kernel returns a file descriptor that we use when we want to read or write the file.

Standard Input, Standard Output, and Standard Error

By convention, all shells open three descriptors whenever a new program is run: standard input, standard output, and standard error. If nothing special is done, as in the simple command

ls

then all three are connected to the terminal. Most shells provide a way to redirect any or all of these three descriptors to any file. For example,

ls > file.list

executes the ls command with its standard output redirected to the file named file.list.

Unbuffered I/O

Unbuffered I/O is provided by the functions open, read, write, lseek, and close. These functions all work with file descriptors.

Example

If we're willing to read from the standard input and write to the standard output, then the program in Figure 1.4 copies any regular file on a UNIX system.

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The <unistd.h> header, included by apue.h, and the two constants STDIN_FILENO and STDOUT_FILENO are part of the POSIX standard (about which we'll have a lot more to say in the next chapter). In this header are function prototypes for many of the UNIX system services, such as the read and write functions that we call.

The constants STDIN_FILENO and STDOUT_FILENO are defined in <unistd.h> and specify the file descriptors for standard input and standard output. These values are typically 0 and 1, respectively, but we'll use the new names for portability.

In Section 3.9, we'll examine the BUFFSIZE constant in detail, seeing how various values affect the efficiency of the program. Regardless of the value of this constant, however, this program still copies any regular file.

The read function returns the number of bytes that are read, and this value is used as the number of bytes to write. When the end of the input file is encountered, read returns 0 and the program stops. If a read error occurs, read returns -1. Most of the system functions return 1 when an error occurs.

If we compile the program into the standard name (a.out) and execute it as

./a.out > data

standard input is the terminal, standard output is redirected to the file data, and standard error is also the terminal. If this output file doesn't exist, the shell creates it by default. The program copies lines that we type to the standard output until we type the end-of-file character (usually Control-D).

If we run

./a.out < infile > outfile

then the file named infile will be copied to the file named outfile.

Figure 1.4. List all the files in a directory

#include "apue.h"

#define BUFFSIZE 4096

int main(void) { int n;

char buf[BUFFSIZE];

while ((n = read(STDIN_FILENO, buf, BUFFSIZE)) > 0) if (write(STDOUT_FILENO, buf, n) != n)

err_sys("write error");

if (n < 0)

err_sys("read error");

exit(0);

}

In Chapter 3, we describe the unbuffered I/O functions in more detail.

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Standard I/O

The standard I/O functions provide a buffered interface to the unbuffered I/O functions. Using standard I/O prevents us from having to worry about choosing optimal buffer sizes, such as the BUFFSIZE constant in Figure 1.4. Another advantage of using the standard I/O functions is that they simplify dealing with lines of input (a common occurrence in UNIX applications). The fgets function, for example, reads an entire line. The read function, on the other hand, reads a specified number of bytes. As we shall see in Section 5.4, the standard I/O library provides functions that let us control the style of buffering used by the library.

The most common standard I/O function is printf. In programs that call printf, we'll always include <stdio.h>normally by including apue.has this header contains the function prototypes for all the standard I/O functions.

Example

The program in Figure 1.5, which we'll examine in more detail in Section 5.8, is like the previous program that called read and write. This program copies standard input to standard output and can copy any regular file.

The function getc reads one character at a time, and this character is written by putc. After the last byte of input has been read, getc returns the constant EOF (defined in <stdio.h>). The standard I/O constants stdin and stdout are also defined in the <stdio.h> header and refer to the standard input and standard output.

Figure 1.5. Copy standard input to standard output, using standard I/O

#include "apue.h"

int main(void) { int c;

while ((c = getc(stdin)) != EOF) if (putc(c, stdout) == EOF) err_sys("output error");

if (ferror(stdin)) err_sys("input error");

exit(0);

}

Figure

Updating...

References

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