Unix kernal

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  • 1. * Unix System Kernel by shehrevar davierwalaby shehrevar davierwala
  • 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. by shehrevar davierwalaby shehrevar davierwala
  • 3. * Architecture of Unix System hardwar e kernel sh who date ed wc grep as nroff ld cc cpp emacs Other apps ● OS interacts directly with the hardware ● Such OS is called system kernel by shehrevar davierwalaby shehrevar davierwala
  • 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 by shehrevar davierwalaby shehrevar davierwala
  • 5. * Block Diagram of System Kernel System Call Interface File Subsystem Inter-process communication Scheduler Memory management Process control subsystem Device drivers hardware control hardware Libraries User Programs User Level Kernel Level Hardware Level by shehrevar davierwalaby shehrevar davierwala
  • 6. * Process Control Subsystem ● Process Synchronization ● Interprocess communication ● Memory management: ● Scheduler: process scheduling (allocate CPU to Processes) by shehrevar davierwalaby shehrevar davierwala
  • 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. by shehrevar davierwalaby shehrevar davierwala
  • 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. by shehrevar davierwalaby shehrevar davierwala
  • 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 & $ ps PID TTY TIME CMD 12893 pts/4 0:00 tcsh 12581 pts/4 0:01 emacs 12582 pts/4 0:01by shehrevar davierwalaby shehrevar davierwala
  • 10. * 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 by shehrevar davierwalaby shehrevar davierwala
  • 11. * 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 by shehrevar davierwalaby shehrevar davierwala
  • 12. * Process Table ● Process table: an entry in process table has the following information: ● process state: ●A. running in user mode or kernel mode ●B. Ready in memory or Ready but swapped ●C. 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 by shehrevar davierwalaby shehrevar davierwala
  • 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. by shehrevar davierwalaby shehrevar davierwala
  • 14. * u area Active process resident swappable data stack text Process table Per-process region table Region table Kernel address space user address space by shehrevar davierwalaby shehrevar davierwala
  • 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 programsby shehrevar davierwalaby shehrevar davierwala
  • 16. * Active process data stack text Process table Per-process region table Region table data stack text Reference count = 2 by shehrevar davierwalaby shehrevar davierwala
  • 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 by shehrevar davierwalaby shehrevar davierwala
  • 18. * 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;by shehrevar davierwalaby shehrevar davierwala
  • 19. * /* forkEx1.c */ #include <stdio.h> main() { int fpid; printf("Before forking ...n"); fpid = fork(); if (fpid == 0) { printf("Child Process fpid=%dn", fpid); } else { printf("Parent Process fpid=%dn", fpid); } printf("After forking fpid=%dn", fpid); } $ cc forkEx1.c -o forkEx1 $ forkEx1 Before forking ... Child Process fpid=0 After forking fpid=0 Parent Process fpid=14707 After forking fpid=14707 $ by shehrevar davierwalaby shehrevar davierwala
  • 20. * /* forkEx2.c */ #include <stdio.h> main() { int fpid; printf("Before forking ...n"); system("ps"); fpid = fork(); system("ps"); printf("After forking fpid=%dn", fpid); } $ forkEx2 Before 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 forkEx2 After forking fpid=14780 $ PID TTY TIME CMD 14781 pts/9 0:00 sh 14759 pts/9 0:00 tcsh 14780 pts/9 0:00 forkEx2 After forking fpid=0 $ ps PID TTY TIME CMD 14759 pts/9 0:00 tcsh $ by shehrevar davierwalaby shehrevar davierwala
  • 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(). by shehrevar davierwalaby shehrevar davierwala
  • 22. * /* pid.c */ #include <stdio.h> #include <sys/types.h> #include <unistd.h> main() { printf("pid=%d ppid=%dn",getpid(), getppid()); } $ cc pid.c -o pid $ pid pid=14935 ppid=14759 $ by shehrevar davierwalaby shehrevar davierwala
  • 23. * /* 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=%dn", fpid, getpid(), getppid()); } else { printf("Parent Process fpid=%d pid=%d ppid=%dn", fpid, getpid(), getppid()); } printf("After forking fpid=%d pid=%d ppid=%dn", fpid, getpid(), getppid()); } by shehrevar davierwalaby shehrevar davierwala
  • 24. * $ cc forkEx3.c -o forkEx3 $ forkEx3 Before forking ... Parent Process fpid=14942 pid=14941 ppid=14759 After forking fpid=14942 pid=14941 ppid=14759 $ Child Process fpid=0 pid=14942 ppid=1 After forking fpid=0 pid=14942 ppid=1 $ ps PID TTY TIME CMD 14759 pts/9 0:00 tcsh by shehrevar davierwalaby shehrevar davierwala
  • 25. * System Call: wait() ● wait system call allows a parent process to wait for the demise of a child process. ● See forkEx4.c by shehrevar davierwalaby shehrevar davierwala
  • 26. * #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=%dn", fpid, getpid(), getppid()); } else { printf("Parent Process fpid=%d pid=%d ppid=%dn", fpid, getpid(), getppid()); } wait(&status); printf("After forking fpid=%d pid=%d ppid=%dn", fpid, getpid(), getppid()); } by shehrevar davierwalaby shehrevar davierwala
  • 27. * $ cc forkEx4.c -o forkEx4 $ forkEx4 Before forking ... Parent Process fpid=14980 pid=14979 ppid=14759 Child Process fpid=0 pid=14980 ppid=14979 After forking fpid=0 pid=14980 ppid=14979 After forking fpid=14980 pid=14979 ppid=14759 $ by shehrevar davierwalaby shehrevar davierwala
  • 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. by shehrevar davierwalaby shehrevar davierwala
  • 29. * /* execEx1.c */ #include <stdio.h> #include <unistd.h> main() { printf("Before execing ...n"); execl("/bin/date", "date", 0); printf("After execn"); } $ execEx1 Before execing ... Sun May 9 16:39:17 CST 1999 $ by shehrevar davierwalaby shehrevar davierwala
  • 30. * /* 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=%dn",fpid); } $ execEx2 Before execing ... After exec and fpid=14903 $ Sun May 9 16:47:08 CST 1999 $ by shehrevar davierwalaby shehrevar davierwala
  • 31. * 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” by shehrevar davierwalaby shehrevar davierwala
  • 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 by shehrevar davierwalaby shehrevar davierwala
  • 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=%dn", fpid, getpid(), getppid()); for(;;); /* loop forever */ } else { printf("Parent Process fpid=%d pid=%d ppid=%dn", fpid, getpid(), getppid()); } wait(status); /* wait for child process */ printf("After forking fpid=%d pid=%d ppid=%dn", fpid, getpid(), getppid()); } by shehrevar davierwalaby shehrevar davierwala
  • 34. * $ cc sigEx1.c -o sigEx1 $ sigEx1 & Before forking ... Parent Process fpid=14989 pid=14988 ppid=14759 Child 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 ... by shehrevar davierwalaby shehrevar davierwala
  • 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. by shehrevar davierwalaby shehrevar davierwala
  • 36. * Process Priority swapper wait for Disk IO wait for buffer wait for inode ... wait for child exit User level 0 User level 1 User level n ... Kernel Mode User Mode ProcessesPriority Levels by shehrevar davierwalaby shehrevar davierwala
  • 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. by shehrevar davierwalaby shehrevar davierwala
  • 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 … 67 76 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 by shehrevar davierwalaby shehrevar davierwala
  • 39. * Unix System Kernel Instructors: Fu-Chiung Cheng ( 鄭福炯 ) Associate Professor Computer Science & Engineering Tatung Institute of Technology by shehrevar davierwalaby shehrevar davierwala
  • 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. by shehrevar davierwalaby shehrevar davierwala
  • 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).by shehrevar davierwalaby shehrevar davierwala
  • 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. by shehrevar davierwalaby shehrevar davierwala
  • 43. * Init fork/exec a getty progrma to manage the line Getty prints “login:” message and waits for someone to login The login process prints the password message, read the password then check the password The shell runs programs for the user unitl the user logs off When the shell dies, init wakes up and fork/exec a getty for the line by shehrevar davierwalaby shehrevar davierwala
  • 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 by shehrevar davierwalaby shehrevar davierwala
  • 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 by shehrevar davierwalaby shehrevar davierwala
  • 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. by shehrevar davierwalaby shehrevar davierwala
  • 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 system size: the number of logic blocks date: the last modification date of super block. B. information of i-nodes the number of i-nodes the number of free i-nodes C. information of data block: free data blocks.by shehrevar davierwalaby shehrevar davierwala
  • 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 by shehrevar davierwalaby shehrevar davierwala
  • 49. * I-node structure mode owner timestamp Size Block count Direct blocks 0-9 Double indirect Triple indirect Single indirect Data block Data block Data block Indirect block ... Data block Data block Data block ... Indirect block Indirect block Indirect block ... Reference count by shehrevar davierwalaby shehrevar davierwala
  • 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)by shehrevar davierwalaby shehrevar davierwala
  • 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 by shehrevar davierwalaby shehrevar davierwala
  • 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 Nextby shehrevar davierwalaby shehrevar davierwala
  • 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 & Hardware control Current director y inode u area i-node i-node i-node ... ... In-core inodes by shehrevar davierwalaby shehrevar davierwala
  • 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. by shehrevar davierwalaby shehrevar davierwala
  • 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 by shehrevar davierwalaby shehrevar davierwala
  • 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. by shehrevar davierwalaby shehrevar davierwala
  • 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 by shehrevar davierwalaby shehrevar davierwala
  • 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 by shehrevar davierwalaby shehrevar davierwala
  • 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 $ openEx1 Before open ... fd1=3 fd2=4 fd3=5 $ by shehrevar davierwalaby shehrevar davierwala
  • 60. * … CNT=2 /etc/passwd CNT=1 ./openEx2.c in-core inodes Pointer to Descript or table U area User file descriptor table 0 1 2 3 4 5 6 7 . . . ... ... CNT=1 R CNT=1 W ... CNT=1 R file table ... ... ... by shehrevar davierwalaby shehrevar davierwala
  • 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 by shehrevar davierwalaby shehrevar davierwala
  • 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-Us fd1=3 buf2=er:/:/sbin/sh daemo ======= $ by shehrevar davierwalaby shehrevar davierwala
  • 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 ====== $ by shehrevar davierwalaby shehrevar davierwala
  • 64. * … CNT=2 /etc/passwd ... in-core inodes Descript or table U area User file descriptor table 0 1 2 3 4 5 6 7 . . . ... ... CNT=1 R ... ... CNT=1 R file table ... ... ... by shehrevar davierwalaby shehrevar davierwala
  • 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 by shehrevar davierwalaby shehrevar davierwala
  • 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/sh daemo ====== $ by shehrevar davierwalaby shehrevar davierwala
  • 67. * … CNT=1 /etc/passwd ... in-core inodes Descript or table U area User file descriptor table 0 1 2 3 4 5 6 7 . . . ... ... CNT=2 R ... ... ... file table ... ... ... by shehrevar davierwalaby shehrevar davierwala
  • 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 by shehrevar davierwalaby shehrevar davierwala
  • 69. * System Call: close ● close: A process may close a file by close system call ● syntax: close(fd); A. fd: file descriptor Example by shehrevar davierwalaby shehrevar davierwala
  • 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 by shehrevar davierwalaby shehrevar davierwala
  • 71. * /* creatEx1.c */ #include <stdio.h> #include <sys/types.h> #include <fcntl.h> main() { int fd1; char *buf1="I am a stringn"; char *buf2="second linen"; 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"); } by shehrevar davierwalaby shehrevar davierwala
  • 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 ... by shehrevar davierwalaby shehrevar davierwala
  • 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 by shehrevar davierwalaby shehrevar davierwala
  • 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=%dn", fd1, bufStat1.st_ino,bufStat1.st_blksize, bufStat1.st_blocks); printf("fd2=%d inode no=%d block size=%d blocks=%dn", fd2, bufStat2.st_ino,bufStat2.st_blksize, bufStat2.st_blocks); printf("======n"); } by shehrevar davierwalaby shehrevar davierwala
  • 75. * $ cc statEx1.c -o statEx1 $ statEx1 ====== fd1=3 inode no=21954 block size=8192 blocks=6 fd2=4 inode no=190611 block size=8192 blocks= ====== ... by shehrevar davierwalaby shehrevar davierwala
  • 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. by shehrevar davierwalaby shehrevar davierwala
  • 77. * System Call: chdir ● chdir: A process may change the current directory of a processl ● syntax: chdir(pathname); A. pathname: file name Example by shehrevar davierwalaby shehrevar davierwala
  • 78. * #include <stdio.h> #include <sys/types.h> #include <fcntl.h> main() { chdir("/usr/bin"); system("ls -l"); } $ ls -l /usr/bin $ by shehrevar davierwalaby shehrevar davierwala
  • 79. * End of System Kernel Lecture by shehrevar davierwalaby shehrevar davierwala