NOTE TO THE READER In the 25 years since this book was published, the electronic files used to produce the camera-ready copy have been lost to the bit-bucket. Other than scanning in page images, there’s no good way to provide an electronic copy of exactly what you could have purchased at the bookstore. To provide something better than a bunch of page images, I had to go back to my original troff manuscript files, which were themselves almost lost to technological obsolescence. I owe a special debt of gratitude to Curt Freeland, who managed to get an old Exabyte tape drive working long enough to read in an 8mm tape archive that contained the only remaining usable copy of these files. This document is the result of importing the original troff manuscript files into Microsoft® Word, and then applying appropriate paragraph styles and fonts to make the text look reasonably similar to the original book. I didn’t try to reproduce the original format in every detail, but it’s pretty close. With regard to content, numerous copy- editing changes were made during the original production process that I have no record of, other than in the printed book itself. I tried to catch as many of these as I could by comparing this document to the book side-by-side, but I may have missed a few. The index has been omitted; use the search function. Please note that I have not made any attempt to update the text to match current UNIX (or Linux) systems. While most of the material is still accurate, you should expect to encounter some (usually minor) differences in include file locations, names of constants, and so forth. Appendix A, Calling FORTRAN From C describes compilers that are no longer in use. The details may be different for whatever compilers you’re using. Appendix C, Reading Kernel Data Structures, was written before the development of multi-processor systems and dynamically-loaded kernel modules. Although the general concepts are still accurate, the programming details are somewhat different on modern systems. This document is for your personal, non-commercial use only. You may also use it as a bibliographic reference in any works that you are writing. Any commercial use of this document, including printing and distribution to groups of people (such as a classroom) is prohibited without my prior written permission. I hope you find the information in this book useful, even if it’s older than some of you who are reading it. In 1989, UNIX had only been commercially available for a handful of years, and the open source movement was in its infancy—if you needed new functionality, you couldn’t buy it or download it—instead, you had to write it yourself. If nothing else, this book provides a historical look at how those of us who were around back then did just that. David A. Curry August 2014 FOR PERSONAL, NON-COMMERCIAL USE ONLY Using C on the UNIX System A Guide to System Programming David A. Curry O’Reilly & Associates, Inc. 981 Chestnut Street Newton, MA 02164 FOR PERSONAL, NON-COMMERCIAL USE ONLY Using C on the UNIX System–A Guide to System Programming By David A. Curry Nutshell Series Editor Tim O’Reilly Copyright © 1985, 1987, 1988 Purdue Research Foundation Internet Download Edition Copyright © 2008, 2009, 2012, 2014 David A. Curry Additional material copyright © 1989 O’Reilly & Associates, Inc. All rights reserved. Printed in the United States of America Nutshell Handbook is a trademark of O’Reilly & Associates, Inc. UNIX is a registered trademark of AT&T SunWindows is a trademark of Sun Microsystems, Inc. The X Window System is a trademark of the Massachusetts Institute of Technology. This documentation is based in part on the Fourth Berkeley Software Distribution under license from The Regents of the University of California. First Printing: January 1989 First Internet Download Edition: April 2009 Second Internet Download Edition: August 2014 While every precaution has been taken in the preparation of this book, the publisher assumes no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. Please address comments and questions in care of the author. FOR PERSONAL, NON-COMMERCIAL USE ONLY TABLE OF CONTENTS Preface .............................................................................................. xiii Documentation Conventions ............................................................................. xiv Acknowledgements ........................................................................................... xv Chapter 1 Introduction ....................................................................... 1 System Calls vs. Library Routines ....................................................................... 2 Versions of UNIX ............................................................................................... 2 Error Handling .................................................................................................... 3 Chapter 2 The Standard I/O Library ................................................ 5 File Pointers ........................................................................................................ 5 Opening and Creating Files.................................................................................. 6 Closing Files ....................................................................................................... 7 Reading and Writing Files ................................................................................... 7 The getc and putc Routines ........................................................................... 7 The fgets and fputs Routines ......................................................................... 9 The fread and fwrite Routines ..................................................................... 10 The fscanf and fprintf Routines ................................................................... 12 The sscanf and sprintf Routines ............................................................ 14 Moving Around in Files..................................................................................... 14 Chapter 3 Low-Level I/O .................................................................. 17 File Descriptors ................................................................................................. 17 Opening and Creating Files................................................................................ 17 Opening and Creating Files on Older UNIX Systems .................................. 18 Closing Files ..................................................................................................... 18 Reading and Writing Files ................................................................................. 19 Moving Around in Files..................................................................................... 20 Duplicating File Descriptors .............................................................................. 23 Converting File Descriptors to File Pointers ....................................................... 23 Chapter 4 Files and Directories ........................................................ 25 File System Concepts ........................................................................................ 25 FOR PERSONAL, NON-COMMERCIAL USE ONLY iii Ordinary Files ............................................................................................ 25 Directories .................................................................................................. 25 Special Files ............................................................................................... 26 Removable File Systems ............................................................................. 26 Device Numbers ......................................................................................... 27 I-Numbers, the I-List, and I-Nodes .............................................................. 27 Hard Links .......................................................................................... 28 Symbolic Links.................................................................................... 28 Determining the Accessibility of a File .............................................................. 28 Getting Information From an I-Node.................................................................. 29 Reading Directories ........................................................................................... 31 Modifying File Attributes .................................................................................. 37 Miscellaneous File System Routines .................................................................. 38 Changing Directories .................................................................................. 38 Deleting and Truncating Files ..................................................................... 38 Making Directories ..................................................................................... 39 Linking and Renaming Files ....................................................................... 39 Symbolic Links .......................................................................................... 39 The umask Value ........................................................................................ 40 Chapter 5 Device I/O Control........................................................... 41 The Version 7 and Berkeley UNIX ioctl ............................................................ 42 Line Disciplines.......................................................................................... 42 The sgttyb Structure.................................................................................... 43 The tchars Structure .................................................................................... 45 The ltchars Structure ................................................................................... 45 The Local Mode Word ................................................................................ 46 The winsize Structure ................................................................................. 46 Miscellaneous Operations ........................................................................... 47 The System V ioctl ............................................................................................ 50 c_iflag ........................................................................................................ 51 c_oflag ....................................................................................................... 51 c_cflag ....................................................................................................... 52 c_lflag ........................................................................................................ 52 c_cc............................................................................................................ 52 The fcntl System Call ........................................................................................ 55 Non-Blocking I/O .............................................................................................. 56 The select System Call ................................................................................ 56 iv FOR PERSONAL, NON-COMMERCIAL USE ONLY Chapter 6 Information About Users ................................................ 61 The Login Name ............................................................................................... 61 The User Id ....................................................................................................... 61 The Group Id ..................................................................................................... 63 The Berkeley UNIX Group Mechanism ...................................................... 63 Reading the Password File ................................................................................. 64 Reading the Group File ...................................................................................... 65 Reading the utmp File........................................................................................ 66 Chapter 7 Telling Time and Timing Things .................................... 69 Telling Time ..................................................................................................... 69 Obtaining the Time ..................................................................................... 69 Timezones .................................................................................................. 70 Converting the Time to ASCII .................................................................... 70 Time Differences ........................................................................................ 71 Sleeping and Alarm Clocks................................................................................ 71 Sleeping ..................................................................................................... 71 The Alarm Clock ........................................................................................ 72 Interval Timers ........................................................................................... 72 Process Timing .................................................................................................. 73 Changing File Times ......................................................................................... 74 Chapter 8 Processing Signals ........................................................... 75 Overview of Signal Handling............................................................................. 75 Resetting Signals ........................................................................................ 76 Restarting System Calls .............................................................................. 76 The Signals ....................................................................................................... 76 Sending Signals ................................................................................................. 78 Catching and Ignoring Signals ........................................................................... 79 Ignoring Signals ......................................................................................... 79 Catching Signals ......................................................................................... 80 Using Signals for Timeouts................................................................................ 84 The setjmp and longjmp Routines ............................................................... 84 The New Berkeley UNIX Signal Mechanism ..................................................... 86 Handler Calling Conventions ...................................................................... 87 The Signal Mask......................................................................................... 87 The Signal Stack......................................................................................... 88 FOR PERSONAL, NON-COMMERCIAL USE ONLY v Chapter 9 Executing Programs ........................................................ 93 The system Library Routine ............................................................................... 93 Executing Programs Directly ............................................................................. 94 Creating Processes ...................................................................................... 94 Executing Programs .................................................................................... 94 Waiting for Processes to Terminate ............................................................. 96 Redirecting Input and Output ............................................................................. 98 Setting Up Pipelines ........................................................................................ 100 The popen Library Routine ....................................................................... 100 Creating Pipes Directly ............................................................................. 100 Chapter 10 Job Control .................................................................. 103 Preliminary Concepts ...................................................................................... 103 The Controlling Terminal ......................................................................... 103 Process Groups ......................................................................................... 104 System Calls............................................................................................. 104 ioctl ................................................................................................... 105 setpgrp .............................................................................................. 105 killpg ................................................................................................. 105 wait3 ................................................................................................. 105 The JOB and PROC Data Types ............................................................... 106 Job Control in the Shell ................................................................................... 106 Setting Up for Job Control ........................................................................ 106 Executing a Program ................................................................................ 107 Stopping a Job .......................................................................................... 107 Backgrounding a Job ................................................................................ 108 Foregrounding a Job ................................................................................. 109 The jobs Command................................................................................... 110 Waiting for Jobs ....................................................................................... 110 Asynchronous Process Notification ........................................................... 113 Job Control Outside the Shell .......................................................................... 113 Important Points .............................................................................................. 114 Chapter 11 Interprocess Communication ...................................... 117 Berkeley UNIX IPC ........................................................................................ 117 The socket System Call............................................................................. 118 The bind System Call................................................................................ 119 The send and recv System Calls ................................................................ 120 The listen System Call .............................................................................. 120 vi FOR PERSONAL, NON-COMMERCIAL USE ONLY The shutdown System Call........................................................................ 121 Connection-Based Sockets ........................................................................ 121 The accept System Call ...................................................................... 121 The connect System Call .................................................................... 121 Connectionless Sockets............................................................................. 122 The sendto System Call...................................................................... 122 The recvfrom System Call.................................................................. 122 Connecting Datagram Sockets............................................................ 122 A Small Client Program ............................................................................ 122 A Small Server Program ........................................................................... 124 System V IPC .................................................................................................. 126 Message Queues ....................................................................................... 127 The msgget System Call..................................................................... 128 The msgctl System Call ..................................................................... 128 The msgsnd and msgrcv System Calls ................................................ 128 Semaphores .............................................................................................. 132 The semget System Call ..................................................................... 132 The semctl System Call...................................................................... 132 The semop System Call...................................................................... 133 Shared Memory ........................................................................................ 134 The shmget System Call..................................................................... 135 The shmctl System Call ..................................................................... 135 The shmat System Call ...................................................................... 135 The shmdt System Call ...................................................................... 136 Chapter 12 Networking .................................................................. 139 Addresses ........................................................................................................ 139 Translating Hostnames Into Network Numbers ................................................ 140 Obtaining Port Numbers .................................................................................. 141 Network Byte Order ........................................................................................ 141 Networking System Calls ................................................................................ 142 Chapter 13 The File System............................................................ 151 Disk Terminology ........................................................................................... 151 The “Standard” UNIX File System .................................................................. 153 The Berkeley Fast File System......................................................................... 160 Reading Data Blocks From the File System ..................................................... 169 Chapter 14 Miscellaneous Routines ............................................... 173 Resource Limits .............................................................................................. 173 FOR PERSONAL, NON-COMMERCIAL USE ONLY vii The getrlimit System Call ......................................................................... 173 The setrlimit System Call .......................................................................... 174 Obtaining Resource Usage Information ............................................................ 175 Manipulating Byte Strings ............................................................................... 177 The bcmp and memcmp Library Routines ................................................. 177 The bcopy and memcpy Library Routines ................................................. 177 The bzero and memset Library Routines ................................................... 177 Environment Variables .................................................................................... 178 The Current Working Directory ....................................................................... 178 Searching for Characters in Strings .................................................................. 178 Determining Whether a File is a Terminal ........................................................ 179 The isatty Library Routine ........................................................................ 179 The ttyname Library Routine .................................................................... 179 The /dev/tty Device .................................................................................. 179 Printing Error Messages .................................................................................. 179 The perror Library Routine ....................................................................... 179 The psignal Library Routine ..................................................................... 180 Sorting Arrays in Memory ............................................................................... 180 Appendix A Calling FORTRAN From C ....................................... 183 Data Representation ........................................................................................ 183 Procedure Naming ........................................................................................... 184 Naming C Routines to be Called From FORTRAN ................................... 184 Naming FORTRAN Routines to be Called From C ................................... 184 Returning Values from Functions .................................................................... 185 Integer, Logical, Real, Double Precision ................................................... 185 Complex and Double Complex ................................................................. 185 Character Strings ...................................................................................... 186 Passing Arguments .......................................................................................... 186 Integers, Floats (Reals), and Doubles ........................................................ 186 Characters and Logicals ............................................................................ 187 Character Strings ...................................................................................... 187 Functions.................................................................................................. 188 Overall Argument Sequence ..................................................................... 188 Input and Output ............................................................................................. 188 From C Programs ..................................................................................... 188 From FORTRAN Programs ...................................................................... 189 Libraries .......................................................................................................... 189 viii FOR PERSONAL, NON-COMMERCIAL USE ONLY