Working with the Unix OS

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Processes II ! File and Record Locking UNIX line printer - Process has to place a job on the print queue. - has to assign a unique sequence number to each job - job exists long enough for process ID to be reused - file for each printer contains sequence number Each process that needs a sequence number - reads the sequence number file - uses the number - increments the number and writes it back The problem is that in the time it takes a single process to execute these three steps, another process can perform the same steps. Chaos results. ! Advisory Locking versus Mandatory Locking Advisory locking means that the operating system maintains information about which files have been locked and by which process. A process can ignore an advisory lock and write to it, if the process has adequate permissions. This is fine for cooperating processes. Mandatory locking means that the operating system check every read and write request to verify that the operation does not interfere with a lock held by a process. (System V Release 3 only - turn group-execute bit off and turn set-group-ID on for file) ! File Locking versus Record Locking File locking locks an entire file, while record locking allows a process to lock a specified portion of a file (on UNIX several records are locked this is called range locking). ! Other Unix Locking Techniques 1. The link system call fails if the name of the new link to the file already exists. 2. The creat system call fails if the file already exists and if the caller does not have write permission for the file. 3. Newer version of UNIX, support options to open system call that cause it to fail of the file already exists. - techniques 1 & 2 work on any version of UNIX. - all take longer to execute than actual file locking system calls. - an ancillary lock file is required. - remove ancillary lock files after a system crash - /tmp? cannot create links across file systems. - technique does not work for superuser. - instead of waiting one second - process wanting lock should be notified when lock is available. Simple Client-Server Example Pipes - Pipe in a single process stdin filename file client IPC contents stdout contents or error or error message user process read fd write fd server file kernel pipe ->flow of data-> 133
Processes II - Pipe in a single process, immediately after fork parent process fork child process read fd write fd read fd write fd kernel pipe ->flow of data-> - Pipe between three processes parent process write fd child process read fd kernel pipe who | sort | lpr ->flow of data-> - Pipe between three processes in a shell pipeline who process sort process lpr process read fd read fd write fd write fd kernel pipe 1 pipe 2 ->flow of data-> ->flow of data-> - Two pipes to provide a bi-directional flow of data - create pipe I, create pipe 2 - fork - parent closes read end of pipe 1 - parent closes write end of pipe 2 - child closes write end of pipe 1 - child closes read end of pipe 2 parent process read fd write fd kernel pipe 1 child process read fd write fd ->flow of data-> pipe 2 ->flow of data-> 134
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UNIVERSITY OF ATHENS DEPARTMENT OF
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Figure 27. Driver Entry Points.....
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Introduction to Unix ! Execution ma
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Introduction to Unix It works in th
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Introduction to Unix * matches 0 or
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Introduction to Unix vi file-name s
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Introduction to Unix 1,$s/^/>>/ 1,$
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Introduction to Unix ! Special Devi
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Introduction to Unix Working with F
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Introduction to Unix ! Communicatio
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Unix Shells 2. UNIX SHELLS Back Quo
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Unix Shells ! CASE Statement case t
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Unix Shells two lines !Y@Y@Z@Z@ZY c
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Unix Shells if $append then tee -a
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C Programming - Basics main() { int
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C Programming - Basics e.g. i += 2
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External Variables A C program cons
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C Programming - Basics main.c #incl
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C Programming - Basics default: pri
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C Programming - Basics struct tnode
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Unix Tools 4. UNIX TOOLS Tools of t
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Unix Tools Korn shell ! .profile HI
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Unix Tools grep "$name" $PHONEBOOK
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Unix Tools awk '$3 > 0 { print $1,
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Unix Tools END { print max, maxline
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Unix Tools Let an example input fil
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Unix Tools Brown eqn 1 # output Jon
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Development Tools Make looks at the
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Development Tools /* compute size o
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Development Tools #endif #line cons
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Development Tools 1.1 5 lines Retri
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Development Tools In Emacs, you may
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Development Tools */ %{ #define YYS
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C Libraries 6. C LIBRARIES Input an
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C Libraries } while (!feof(fp1)) pu
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C Libraries } free(p); /* what is w
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C Libraries } if ((fp->flag&( _READ
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C Libraries /* dirwalk: apply fcn t
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C Libraries } switch (pid=fork()){
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C Libraries What is curses? - libra
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C Libraries } if ((fd=fopen(argv[1]
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Introduction to kernel 7. INTRODUCT
Page 85 and 86: 82 Introduction to kernel Programs
Page 87 and 88: Introduction to kernel } { printf("
Page 89 and 90: Introduction to kernel The kernel r
Page 91 and 92: Introduction to kernel UNIX Interna
Page 93 and 94: Introduction to kernel File System
Page 95 and 96: Introduction to kernel Every proces
Page 97 and 98: Processes I Process States 1. Execu
Page 99 and 100: Processes I } printf("%d %s\n", cnt
Page 101 and 102: Processes I dup (and other similar
Page 103 and 104: Processes I The u area contains - p
Page 105 and 106: Processes I The Context of a Proces
Page 107 and 108: Processes I Process Control use and
Page 109 and 110: Processes I Although the processes
Page 111 and 112: Processes I Handling Signals - proc
Page 113 and 114: Processes I USER=neville PATH=/user
Page 115 and 116: Processes I ! Shadow Password /etc/
Page 117 and 118: struct timeval { long tv_sec; /* se
Page 119 and 120: Processes I F_SETFL set status flag
Page 121 and 122: Processes I process is the child pr
Page 123 and 124: Processes I Rules about sharing of
Page 125 and 126: Processes II 9. PROCESSES (II) FORK
Page 127 and 128: Processes II /* background.c */ #in
Page 129 and 130: Processes II } while (1){ printf("p
Page 131 and 132: Processes II } /* child */ signal (
Page 133 and 134: Processes II int readline(int fd, c
Page 135: Processes II { } char process_name[
Page 139 and 140: Processes II System V IPC - message
Page 141 and 142: Processes II int msgctl(int msqid,
Page 143 and 144: Processes II For every semop operat
Page 145 and 146: Processes II /* client loop */ mesg
Page 147 and 148: I/O Subsystem Drivers frequently sl
Page 149 and 150: I/O Subsystem Terminal Drivers - In
Page 151 and 152: I/O Subsystem Canonical Mode algori
Page 153 and 154: I/O Subsystem Raw Mode Raw mode is
Page 155 and 156: Figure 43. Pushing a Module onto a
Page 157 and 158: Interprocess Communication } { ptra
Page 159 and 160: Interprocess Communication T ID KEY
Page 161 and 162: Interprocess Communication }; int m
Page 163 and 164: Interprocess Communication for (i=0
Page 165 and 166: Interprocess Communication /* struc
Page 167 and 168: Interprocess Communication ! Compar
Page 169 and 170: … connect(sockfd,(struct sockaddr
Page 171 and 172: Interprocess Communication Address
Page 173 and 174: Process Scheduling Page Tables and
Page 176 and 177: Figure 49. Tie breaker rule Process
Page 178 and 179: Process Scheduling Clock - restart
Page 180 and 181: Process Scheduling Figure 57. Mappi
Page 182 and 183: Figure 62. Sequence of Swapping Ope
Page 184 and 185: Buffer Cache The kernel takes buffe
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Buffer Cache The kernel places the
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Buffer Cache Process B will find th
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Unix Administration 14. UNIX ADMINI
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Unix Administration Example output
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Unix Administration *) IN=-mtime -2
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Unix Administration Load /Unix kern
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Unix Administration for (i=0; i&i d
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Unix Administration fi done echo $s
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Unix Administration New Software !
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Unix Security Example: group name:g
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Unix Security $ crypt < exam320.Z >
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Unix Security - exported filesystem
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Unix Security system(cmdstr); $ ech
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Unix Security - repairs damaged use
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