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1 Unix System Kernel Instructor: S. Kiptoo Computer Science & IT athi University College of Technolo

Unix System Calls

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1

Unix System Kernel

Instructor: S. Kiptoo

Computer Science & ITKimathi University College of Technology

2

Unix: Introduction

• Operating System: a system that manages the resources of a computer.• Resources: CPUs, Memory, I/O devices, Network• Kernel: the memory resident portion of Unix system• File system and process control system are two major components of Unix Kernel.

3

Architecture of Unix System

hardware

kernel

sh whodate

ed

wc

grep

as

nroff

ld

cc

cpp

emacs

Other apps

• OS interacts directly with the hardware • Such OS is called system kernel

4

Unix System Kernel

• Three major tasks of kernel: Process Management Device Management File Management

• Three additional Services for Kernel: Virtual Memory Networking Network File Systems

• Experimental Kernel Features: Multiprocessor support Lightweight process (thread) support

5

Block Diagram of System Kernel

System Call Interface

File SubsystemInter-process

communication

Scheduler

Memorymanagement

Process

control

subsystemDevice drivers

hardware control

hardware

LibrariesUser Programs

User LevelKernel Level

Hardware Level

6

Process Control Subsystem

• Process Synchronization• Interprocess communication• Memory management: • Scheduler: process scheduling

(allocate CPU to Processes)

7

File subsystem

• A file system is a collection of files and directories on a disk or tape in standard UNIX file system format. • Kernel’s file sybsystem regulates data flow between the kernel and secondary storage devices.

8

Hardware Control

• Hardware control is responsible for handling interrupts and for communicating with the machine.• Devices such as disks or terminals may interrupt the CPU while a process is executing.• The kernel may resume execution of the interrupted process after servicing the interrupt.

9

Processes• A program is an executable file.• A process is an instance of the program in execution.• For example: create two active processes

$ emacs &$ emacs & $ psPID TTY TIME CMD12893 pts/4 0:00 tcsh12581 pts/4 0:01 emacs12582 pts/4 0:01 emacs$

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Processes• A process has

text: machine instructions (may be shared by other processes)

data stack

• Process may execute either in user mode and in kernel mode.• Process information are stored in two places:

Process table User table

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User mode and Kernel mode

• At any given instant a computer running the Unix system is either executing a process or the kernel itself is running• The computer is in user mode when it is executing instructions in a user process and it is in kernel mode when it is executing instructions in the kernel.• Executing System call ==> User mode to Kernel mode perform I/O operations system clock interrupt

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Process Table • Process table: an entry in process table has the following information:

process state: A. running in user mode or kernel modeB. Ready in memory or Ready but swappedC. Sleep in memory or sleep and swapped

PID: process id UID: user id scheduling information signals that is sent to the process but not yet handled a pointer to per-process-region table

• There is a single process table for the entire system

13

User Table (u area)• Each process has only one private user table.• User table contains information that must be accessible while the process is in execution.

A pointer to the process table slot parameters of the current system call, return values error codes file descriptors for all open files current directory and current root process and file size limits.

• User table is an extension of the process table.

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u area

Active process

residentswappable

data

stack

text

Processtable

Per-processregion table

Regiontable

Kerneladdressspace

useraddressspace

15

Shared Program Text and Software Libraries

• Many programs, such as shell, are often being executed by several users simultaneously. • The text (program) part can be shared.• In order to be shared, a program must be compiled using a special option that arranges the process image so that the variable part(data and stack) and the fixed part (text) are cleanly separated. • An extension to the idea of sharing text is sharing libraries.• Without shared libraries, all the executing programs contain their own copies.

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Active process

data

stack

text

Processtable

Per-processregion table

Regiontable

data

stack

text

Referencecount = 2

17

System Call• A process accesses system resources through system call.• System call for

Process Control: fork: create a new process wait: allow a parent process to synchronize its

execution with the exit of a child process. exec: invoke a new program. exit: terminate process execution File system: File: open, read, write, lseek, close inode: chdir, chown chmod, stat fstat others: pipe dup, mount, unmount, link, unlink

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System call: fork()

• fork: the only way for a user to create a process in Unix operating system.• The process that invokes fork is called parent process and the newly created process is called child process.• The syntax of fork system call:

newpid = fork();• On return from fork system call, the two processes have identical copies of their user-level context except for the return value pid. • In parent process, newpid = child process id• In child process, newpid = 0;

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/* forkEx1.c */#include <stdio.h>

main(){ int fpid; printf("Before forking ...\n"); fpid = fork(); if (fpid == 0) { printf("Child Process fpid=%d\n", fpid); } else { printf("Parent Process fpid=%d\n", fpid); } printf("After forking fpid=%d\n", fpid); }

$ cc forkEx1.c -o forkEx1$ forkEx1Before forking ...Child Process fpid=0After forking fpid=0Parent Process fpid=14707After forking fpid=14707$

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/* forkEx2.c */#include <stdio.h>

main(){ int fpid; printf("Before forking ...\n"); system("ps"); fpid = fork(); system("ps"); printf("After forking

fpid=%d\n", fpid);}

$ forkEx2Before forking ... PID TTY TIME CMD 14759 pts/9 0:00 tcsh 14778 pts/9 0:00 sh 14777 pts/9 0:00 forkEx2 PID TTY TIME CMD 14781 pts/9 0:00 sh 14759 pts/9 0:00 tcsh 14782 pts/9 0:00 sh 14780 pts/9 0:00 forkEx2 14777 pts/9 0:00 forkEx2After forking fpid=14780$ PID TTY TIME CMD 14781 pts/9 0:00 sh 14759 pts/9 0:00 tcsh 14780 pts/9 0:00 forkEx2After forking fpid=0

$ ps PID TTY TIME CMD 14759 pts/9 0:00 tcsh$

21

System Call: getpid() getppid()

• Each process has a unique process id (PID). • PID is an integer, typically in the range 0 through 30000.• Kernel assigns the PID when a new process is created.• Processes can obtain their PID by calling getpid().• Each process has a parent process and a corresponding parent process ID.• Processes can obtain their parent’s PID by calling getppid().

22

/* pid.c */#include <stdio.h>#include <sys/types.h>#include <unistd.h>

main(){ printf("pid=%d ppid=%d\n",getpid(), getppid());}

$ cc pid.c -o pid$ pidpid=14935 ppid=14759$

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/* forkEx3.c */#include <stdio.h>#include <sys/types.h>#include <unistd.h>main(){ int fpid; printf("Before forking ...\n"); fpid = fork(); if (fpid == 0) { printf("Child Process fpid=%d pid=%d ppid=%d\n", fpid, getpid(), getppid()); } else { printf("Parent Process fpid=%d pid=%d ppid=%d\n", fpid, getpid(), getppid()); } printf("After forking fpid=%d pid=%d ppid=%d\n", fpid, getpid(), getppid());}

24

$ cc forkEx3.c -o forkEx3$ forkEx3Before forking ...Parent Process fpid=14942 pid=14941 ppid=14759After forking fpid=14942 pid=14941 ppid=14759$ Child Process fpid=0 pid=14942 ppid=1After forking fpid=0 pid=14942 ppid=1

$ ps PID TTY TIME CMD 14759 pts/9 0:00 tcsh

25

System Call: wait()

• wait system call allows a parent process to wait for the demise of a child process. • See forkEx4.c

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#include <stdio.h>#include <sys/types.h>#include <unistd.h>main(){ int fpid, status; printf("Before forking ...\n"); fpid = fork(); if (fpid == 0) { printf("Child Process fpid=%d pid=%d ppid=%d\n",

fpid, getpid(), getppid()); } else { printf("Parent Process fpid=%d pid=%d ppid=%d\n",

fpid, getpid(), getppid()); } wait(&status); printf("After forking fpid=%d pid=%d ppid=%d\n",

fpid, getpid(), getppid());}

27

$ cc forkEx4.c -o forkEx4$ forkEx4Before forking ...Parent Process fpid=14980 pid=14979 ppid=14759Child Process fpid=0 pid=14980 ppid=14979After forking fpid=0 pid=14980 ppid=14979After forking fpid=14980 pid=14979 ppid=14759$

28

System Call: exec()

• exec() system call invokes another program by replacing the current process• No new process table entry is created for exec() program. Thus, the total number of processes in the system isn’t changed.• Six different exec functions: execlp, execvp, execl, execv, execle, execve, (see man page for more detail.)• exec system call allows a process to choose its successor.

29

/* execEx1.c */#include <stdio.h>#include <unistd.h>

main(){ printf("Before execing ...\n"); execl("/bin/date", "date", 0); printf("After exec\n"); }

$ execEx1Before execing ...Sun May 9 16:39:17 CST 1999$

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/* execEx2.c */#include <sys/types.h>#include <unistd.h> #include <stdio.h>

main(){ int fpid; printf("Before execing ...\n"); fpid = fork(); if (fpid == 0) { execl("/bin/date", "date", 0); } printf("After exec and fpid=%d\n",fpid); }

$ execEx2Before execing ...After exec and fpid=14903$ Sun May 9 16:47:08 CST 1999$

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Handling Signal

• A signal is a message from one process to another. • Signal are sometime called “software interrupt” • Signals usually occur asynchronously.• Signals can be sent A. by one process to anther (or to itself) B. by the kernel to a process.• Unix signals are content-free. That is the only thing that can be said about a signal is “it has arrived or not”

32

Handling Signal

• Most signals have predefined meanings: A. sighup (HangUp): when a terminal is closed, the hangup signal is sent to every process in control terminal. B. sigint (interrupt): ask politely a process to terminate. C. sigquit (quit): ask a process to terminate and produce a codedump. D. sigkill (kill): force a process to terminate.• See signEx1.c

33

#include <stdio.h>#include <sys/types.h>#include <unistd.h>main() { int fpid, *status; printf("Before forking ...\n"); fpid = fork(); if (fpid == 0) { printf("Child Process fpid=%d pid=%d ppid=%d\n",

fpid, getpid(), getppid()); for(;;); /* loop forever */ } else { printf("Parent Process fpid=%d pid=%d ppid=%d\n",

fpid, getpid(), getppid()); } wait(status); /* wait for child process */ printf("After forking fpid=%d pid=%d ppid=%d\n",

fpid, getpid(), getppid());}

34

$ cc sigEx1.c -o sigEx1$ sigEx1 &Before forking ...Parent Process fpid=14989 pid=14988 ppid=14759Child Process fpid=0 pid=14989 ppid=14988$ ps PID TTY TIME CMD 14988 pts/9 0:00 sigEx1 14759 pts/9 0:01 tcsh 14989 pts/9 0:09 sigEx1$ kill -9 14989$ ps ...

35

Scheduling Processes

• On a time sharing system, the kernel allocates the CPU to a process for a period of time (time slice or time quantum) preempts the process and schedules another one when time slice expired, and reschedules the process to continue execution at a later time.• The scheduler use round-robin with multilevel feedback algorithm to choose which process to be executed: A. Kernel allocates the CPU to a process for a time slice. B. preempts a process that exceeds its time slice. C. feeds it back into one of the several priority queues.

36

Process Priority

swapperwait for Disk IOwait for bufferwait for inode

...

wait for child exitUser level 0User level 1

User level n

...Kernel ModeUser Mode

ProcessesPriority Levels

37

Process Scheduling (Unix System V)

• There are 3 processes A, B, C under the following assumptions: A. they are created simultaneously with initial priority 60. B. the clock interrupt the system 60 times per second. C. these processes make no system call. D. No other process are ready to run E. CPU usage calculation: CPU = decay(CPU) = CPU/2 F. Process priority calculation: priority = CPU/2 + 60. G. Rescheduling Calculation is done once per second.

38

Process A Priority CPU count

Process B Priority CPU count

Process C Priority CPU count

60 0 … 60

75 30

67 15

63 7 …

6776 33

60 0

60 0 … 60

75 30

67 15

63 7 ...

60 0

60 0

60 0 … 60

75 30

67 15

1

2

3

4

0

39

Unix System Kernel

Part 2

40

Booting

• When the computer is powered on or rebooted, a short built-in program (maybe store in ROM) reads the first block or two of the disk into memory. These blocks contain a loader program, which was placed on the disk when disk is formatted.• The loader is started. The loader searches the root directory for /unix or /root/unix and load the file into memory• The kernel starts to execute.

41

The first processes

• The kernel initializes its internal data structures: it constructs linked list of free inodes, regions, page table• The kernel creates u area and initializes slot 0 of process table• Process 0 is created• Process 0 forks, invoking the fork algorithm directly from the Kernel. Process 1 is created.• In kernel mode, Process 1 creates user-level context (regions) and copy code (/etc/init) to the new region.• Process 1 calls exec (executes init).

42

init process

• The init process is a process dispatcher:spawning processes, allow users to login.• Init reads /etc/inittab and spawns getty• when a user login successfully, getty goes through a login procedure and execs a login shell.• Init executes the wait system call, monitoring the death of its child processes and the death of orphaned processes by exiting parent.

43

Init fork/execa getty progrma

to manage the line

Getty prints “login:” message and

waits for someoneto login

The login processprints the

password message, read the password

then check the password

The shell runsprograms for the

user unitl the user logs off

When the shelldies, init wakes up

and fork/exec a getty for the line

44

File Subsystem

• A file system is a collection of files and directories on a disk or tape in standard UNIX file system format. • Each UNIX file system contains four major parts: A. boot block: B. superblock: C. i-node table: D. data block: file storage

45

File System Layout

Block 0: bootstrap

Block 1: superblock

Block 2

Block n

...

Block n+1

The last Block

...

Block 2 - n:i-nodes

Block n+1 - last:Files

46

Boot Block

• A boot block may contains several physical blocks.• Note that a physical block contains 512 bytes

(or 1K or 2KB)• A boot block contains a short loader program for

booting• It is blank on other file systems.

47

Superblock

• Superblock contains key information about a file system• Superblock information: A. Size of a file system and status:

label: name of this file systemsize: the number of logic blocksdate: the last modification date of super block.

B. information of i-nodesthe number of i-nodesthe number of free i-nodes

C. information of data block: free data blocks.• The information of a superblock is loaded into memory.

48

I-nodes

• i-node: index node (information node)• i-list: the list of i-nodes • i-number: the index of i-list.• The size of an i-node: 64 bytes. • i-node 0 is reserved.• i-node 1 is the root directory.• i-node structure: next page

49

I-node structuremode

owner

timestamp

Size

Block count

Direct blocks0-9

Double indirect

Triple indirect

Single indirect

Data block

Data block

Data block

Indirect block

...

Data block

Data block

Data block

...

Indirect blockIndirect block

Indirect block

...

Reference count

50

I-node structure

• mode: A. type: file, directory, pipe, symbolic link B. Access: read/write/execute (owner, group,)

• owner: who own this I-node (file, directory, ...)• timestamp: creation, modification, access time• size: the number of bytes• block count: the number of data blocks• direct blocks: pointers to the data• single indirect: pointer to a data block which

pointers to the data blocks (128 data blocks).• Double indirect: (128*128=16384 data blocks)• Triple indirect: (128*128*128 data blocks)

51

Data Block

• A data block has 512 bytes. A. Some FS has 1K or 2k bytes per blocks.B. See blocks size effect (next page)

• A data block may contains data of files or data of a directory.• File: a stream of bytes. • Directory format:

i-# Next size File name pad

52

Report.txt

home

john

bin

find

alex jenny

notes

grep

i-# Next 10 Report.txt pad i-# Next 3

bin pad i-# Next 5 notes pad 0 Next

53

Boot Block

SuperBlock

i-node

i-node

i-node

i-node

...

...

...

Current Dir

Report.txt

source

notes...

...

...

...

i-nodes

Data Blocks

Report.txt

home

kc

source

find

alex

notes

grep

Device driver &

Hardwarecontrol

Currentdirectory

inode

u areai-node

i-node

i-node

...

...

In-core inodes

54

In-core inode table

• UNIX system keeps regular files and directories on block devices such as disk or tape, • Such disk space are called physical device address space.• The kernel deals on a logical level with file system (logical device address space) rather than with disks.• Disk driver can transfer logical addresses into physical device addresses.• In-core (memory resident) inode table stores the inode information in kernel space.

55

In-core inode table

• An in-core inode contains A. all the information of inode in disks. B. status of in-core inode inode is locked, inode data changed file data changed. C. the logic device number of the file system. D. inode number E. reference count

56

File table

• The kernel have a global data structure, called file table, to store information of file access.• Each entry in file table contains: A. a pointer to in-core inode table B. the offset of next read or write in the file C. access rights (r/w) allowed to the opening process. D. reference count.

57

User File Descriptor table

• Each process has a user file descriptor table to identify all opened files.• An entry in user file descriptor table pointer to an entry of kernel’s global file table.• Entry 0: standard input• Entry 1: standard output• Entry 2: error output

58

System Call: open

• open: A process may open a existing file to read or write• syntax: fd = open(pathname, mode); A. pathname is the filename to be opened B. mode: read/write• Example

59

#include <stdio.h>#include <sys/types.h>#include <fcntl.h>

main(){ int fd1, fd2, fd3; printf("Before open ...\n"); fd1 = open("/etc/passwd", O_RDONLY); fd2 = open("./openEx1.c", O_WRONLY); fd3 = open("/etc/passwd", O_RDONLY); printf("fd1=%d fd2=%d fd3=%d \n", fd1, fd2, fd3);}

$ cc openEx1.c -o openEx1$ openEx1Before open ...fd1=3 fd2=4 fd3=5 $

60

CNT=2/etc/passwd

CNT=1./openEx2.c

in-core inodes

Pointer to Descriptor table

U area

User filedescriptor table

01234567

.

.

....

...

CNT=1 R

CNT=1 W

...

CNT=1 R

file table

...

...

...

61

System Call: read

• read: A process may read an opened file• syntax: fd = read(fd, buffer, count); A. fd: file descriptor B. buffer: data to be stored in C. count: the number (count) of byte• Example

62

#include <stdio.h>#include <sys/types.h>#include <fcntl.h>

main(){ int fd1, fd2, fd3; char buf1[20], buf2[20]; buf1[19]='\0'; buf2[19]='\0'; printf("=======\n"); fd1 = open("/etc/passwd", O_RDONLY); read(fd1, buf1, 19); printf("fd1=%d buf1=%s \n",fd1, buf1); read(fd1, buf2, 19); printf("fd1=%d buf2=%s \n",fd1, buf2); printf("=======\n");}

$ cc openEx2.c -o openEx2$ openEx2=======fd1=3 buf1=root:x:0:1:Super-Usfd1=3 buf2=er:/:/sbin/shdaemo =======$

63

#include <stdio.h>#include <sys/types.h>#include <fcntl.h>main(){ int fd1, fd2, fd3; char buf1[20], buf2[20]; buf1[19]='\0'; buf2[19]='\0'; printf("======\n"); fd1 = open("/etc/passwd", O_RDONLY); fd2 = open("/etc/passwd", O_RDONLY); read(fd1, buf1, 19); printf("fd1=%d buf1=%s \n",fd1, buf1); read(fd2, buf2, 19); printf("fd2=%d buf2=%s \n",fd2, buf2); printf("======\n");}

$ cc openEx3.c -o openEx3$ openEx3======fd1=3 buf1=root:x:0:1:Super-Us fd2=4 buf2=root:x:0:1:Super-Us ======$

64

CNT=2/etc/passwd

...

in-core inodes

Descriptortable

U area

User filedescriptor table

01234567

.

.

....

...

CNT=1 R

...

...

CNT=1 R

file table

...

...

...

65

System Call: dup

• dup: copy a file descriptor into the first free slot of the user file descriptor table.• syntax: newfd = dup(fd); A. fd: file descriptor Example

66

#include <stdio.h>#include <sys/types.h>#include <fcntl.h>main(){ int fd1, fd2, fd3; char buf1[20], buf2[20]; buf1[19]='\0'; buf2[19]='\0'; printf("======\n"); fd1 = open("/etc/passwd", O_RDONLY); fd2 = dup(fd1); read(fd1, buf1, 19); printf("fd1=%d buf1=%s \n",fd1, buf1); read(fd2, buf2, 19); printf("fd2=%d buf2=%s \n",fd2, buf2); printf("======\n"); char buf1[20], buf2[20];}

$ cc openEx4.c -o openEx4$ openEx4======fd1=3 buf1=root:x:0:1:Super-Us fd2=4 buf2=er:/:/sbin/shdaemo ======$

67

CNT=1/etc/passwd

...

in-core inodes

Descriptortable

U area

User filedescriptor table

01234567

.

.

....

...

CNT=2 R

...

...

...

file table

...

...

...

68

System Call: creat

• creat: A process may create a new file by creat system call• syntax: fd = write(pathname, mode); A. pathname: file name B. mode: read/write Example

69

System Call: close

• close: A process may close a file by close system call• syntax: close(fd); A. fd: file descriptor Example

70

System Call: write

• write: A process may write data to an opened file• syntax: fd = write(fd, buffer, count); A. fd: file descriptor B. buffer: data to be stored in C. count: the number (count) of byte• Example

71

/* creatEx1.c */#include <stdio.h>#include <sys/types.h>#include <fcntl.h>main(){ int fd1; char *buf1="I am a string\n"; char *buf2="second line\n"; printf("======\n"); fd1 = creat("./testCreat.txt", O_WRONLY); write(fd1, buf1, 20); write(fd1, buf2, 30); printf("fd1=%d buf1=%s \n",fd1, buf1); close(fd1); chmod("./testCreat.txt", 0666); printf("======\n");}

72

$ cc creatEx1.c -o creatEx1$ creatEx1======fd1=3 buf1=I am a string ======$ ls -l testCreat.txt-rw-rw-rw- 1 cheng staff 50 May 10 20:37 testCreat.txt$ more testCreat.txt...

73

System Call: stat/fstat

• stat/fstat: A process may query the status of a file (locked) file type, file owner, access permission. file size, number of links, inode number, access time.• syntax: stat(pathname, statbuffer); fstat(fd, statbuffer); A. pathname: file name B. statbuffer: read in data C. fd: file descriptor Example

74

/* statEx1.c */#include <sys/stat.h>main(){ int fd1, fd2, fd3; struct stat bufStat1, bufStat2; char buf1[20], buf2[20]; printf("======\n"); fd1 = open("/etc/passwd", O_RDONLY); fd2 = open("./statEx1", O_RDONLY); fstat(fd1, &bufStat1); fstat(fd2, &bufStat2); printf("fd1=%d inode no=%d block size=%d blocks=%d\n", fd1, bufStat1.st_ino,bufStat1.st_blksize, bufStat1.st_blocks); printf("fd2=%d inode no=%d block size=%d blocks=%d\n", fd2, bufStat2.st_ino,bufStat2.st_blksize, bufStat2.st_blocks); printf("======\n");}

75

$ cc statEx1.c -o statEx1$ statEx1======fd1=3 inode no=21954 block size=8192 blocks=6fd2=4 inode no=190611 block size=8192 blocks=======...

76

System Call: link/unlink

• link: hardlink a file to another • syntax: link(sourceFile, targetFile); unlink(file) A. sourceFile targetFile, file: file name Example: Lab exercise: write a c program which use link/unlink system call. Use ls -l to see the reference count.

77

System Call: chdir

• chdir: A process may change the current directory of a processl• syntax: chdir(pathname); A. pathname: file name Example

78

#include <stdio.h>#include <sys/types.h>#include <fcntl.h>

main(){ chdir("/usr/bin"); system("ls -l");}

$ ls -l /usr/bin$

79

End of System Kernel Lecture