Lecture Eleven

1
Lecture Eleven

• Unix Programming System Calls

2
Client-Server Model

• An application model consisting of multiple
  concurrent processes, 1 server process and n
  client procesesses.
• The server process provides support services to
  the clients
• The clients contains the main application logic.

3
Client-Server Model
4
Direct Access I/O

• Avoid sequentially scan through a file
• int fseek(fp, offset, mode)
• FILE fp
• int offset / in bytes /
• in mode / 0 (SEEK_SET) -- set position to
  offset.
• 1 (SEEK_CUR) -- set pos to
  (current pos) offset.
• 2 (SEEK_END) -- set pos to
  EOFoffset
• fp fopen("myfile", "r")

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Direct Access I/O

• if (fseek(fp, nsizeof(record), 0) ! 0)
• perror("cant seek")
• return
• Use with
• fread((char )record, sizeof(record), 1, fp)
• / record represents an array. Read a
  record. Return of recs read /
• fwrite((char) record, sizeof(record), 1, fp)
• / return written /

6
File Property

• include ltsys/stat.hgt
• int stat (Path, Buffer )
• const char Path
• struct stat Buffer
• int fstat (FileDescriptor, Buffer)
• int FileDescriptor
• struct stat Buffer

7
Structure stat

• struct stat
• mode_t st_mode / File mode /
• short st_nlink / Number of links /
• uid_t st_uid / User ID of the
  file's owner /
• gid_t st_gid / Group ID of the
  file's group /
• off_t st_size / File size in bytes
  /
• time_t st_atime / Time of last access /
• time_t st_mtime / Time of last data
  modification /

8
Define Game Board

• / board.h /
• typedef enum Blank, Red, Blue CellStatus
• typedef struct
• CellStatus board9
• char redPlayer65
• char bluePlayer65
• GameBoard

9
Manipulate Game Board

• / board.c /
• include ltstdio.hgt
• include ltstrings.hgt
• include "board.h"
• static int newGameNumber()
• int r
• / to do /
• return r

10
Manipulate Game Board

• int addNewGame(char redPlayer, char bluePlayer,
  int boardSize)
• int i
• GameBoard game
• if (redPlayer NULL bluePlayer NULL
  boardSize lt 3)
• return -1 / invalid input /
• for (i 0 i lt 9 i)
• game.boardi Blank

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Manipulate Game Board

• strcpy(game.redPlayer,redPlayer)
• strcpy(game.bluePlayer,bluePlayer)
• static int findGameBoard(int number, GameBoard
  game )
• / to do /

12
Manipulate Game Board

• static int won(GameBoard game,CellStatus player)
• / To Do /

13
Manipulate Game Board

• static int tie(GameBoard game)
• int i
• if (won(game,Red))
• return 0
• if (won(game,Blue))
• return 0
• for (i 0 i lt game-gtsizegame-gtsize i)
• if (game-gtboardi Blank)
• return 0
• return 1

14
Manipulate Game Board

• void watch(int gameNumber, int strm)
• GameBoard game
• if (findGameBoard(gameNumber,game) lt 0)
• invalidGame(strm)
• return
• gennerateBoardLayout(strm,game)
• void move(int gameNumber, int x, int y,
  CellStatus player,int strm)
• GameBoard game
• if (findGameBoard(gameNumber,game) lt 0)

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Manipulate Game Board

• invalidGame(strm)
• return
• if (x gt 3 y gt 3 x lt 0 y lt 0)
• invalidMove(strm)
• return
• game.boardx3y player
• gennerateBoardLayout(strm,game)

16
Calling Command via Shell

• Use system() . ( include ltstdlib.hgt )
• Example the list of current directory and print
  it.
• mail( )
• FILE fp, fopen() char c
• system("ls -l gt tmp") / invoking bourne
  shell /
• fp fopen("tmp", "r")
• while ( (c getc(fp)) ! EOF)) putchar(c)
• fclose(fp)

17
Use pipe

• Use pipe to read the results of a shell command
  or provide input to a shell command.
• main( )
• FILE infp, outfp, popen( ) int c
• infp popen("ls -l", "r") / equivelant to
  ls -l thisprogram /
• outfp popen("mail qli", "w")
• while ( ( c getc(infp)) ! EOF)
  fprintf(outfp, "c", c)
• pclose(infp)
• pclose(outfp)

18
Access Shell Environment

• User change the behavior of a program through
  environment variables.
• char getenv(char name) return a pointer to
  the value of the shell variable named by the
  argument
• include ltstdlib.hgt
• main( )
• char value, getenv( )
• value getenv("HOME")
• if (value ! NULL) printf("s\n", value)

19
Process Control

• A process is the execution of a program.
• A program by itself is not a process. (A
  university's catalog describes what you need to
  study, but it is not your study. Your study is
  the "process" while the catalog is the program).
• Multitasking system has more than one processes
  running at a time, to get higher resourse
  utilization and more user frientliness.

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Process Control

• Example
• Mutliple users can use a machine at the same time
  and each has his/her own machine (apperantly).
• When one process is doing IO (10's of
  milliseconds), the processor can run other
  processes.
• A process has code (text), static data, dynamic
  data (heap and stack).

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Process Control (cont.)

• Process creation/termination
• fork( ) exit( )
• execl( ) excelc( ) exclv( ) execv( ) execve( )
  execvp( )
• Communication/synchronization
• wait( ) pipe( ) sleep( ) alarm( ) signal( )
  kill( )
• setjmp( ) longjmp( )
• The System V IPC
• msgget( ) msgsnd( ) msgrcv( ) msgctl( )
• semget( ) semop( ) semctl( )
• shmget( ) shmat( ) shmdt( ) shmctl( )

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fork( )

• int fork( )
• creates a new (child) process which is identical
  to the caller (the parent process).
• the child process inherits
• the code
• the environment
• all signal setting
• set user and group ids
• time left one the alarm clock
• current working directory
• all open file descriptors (copies)

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fork()

• fork() returns -1 if a child cannot be created.
  Otherwise it returns0 -- if the process is the
  childthe process id of the child -- if the
  processs is the parent.

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fork() ( cont.)

• Example
• .
• if ( (pid fork( )) ! 0 )
• printf("I am the parent process\n")
• other stuff
• wait(status)
• else
• printf("I am the child process\n")
• . other child process code

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fork() ( cont.)

• How many new processes will be greated if the
  following sequence is executed fork( )
  fork( )

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wait( ) and exit( )

• int wait(int status)
• wait for a child process to terminate
• void exit(int status)
• terminate the calling process and return the
  status code.

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wait( ) and exit( )

• If the child process ends with exit( ) system
  call then the second lowest byte of status is set
  to the arg given to exit( ).
• If the child process ended because of a signal,
  the lowest byte of status contains the singal
  number.

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Event Detection (Signal)

• Signals (interrupts) can be sent to processes.
  You can ignore a signal, tak default action on a
  signal, or catch the signal.
• During the execution of a process an interrupt
  (singal) can be sent to the process by "kill"
  command, by kill( ) system call, and alarm( )
  system call.

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Event Detection (Signal)

• If you program does not specify anything about
  signal, the default action will be taken by the
  system, normally termination.
• You can set up traps to catch the signals and
  invoke your own signal handler routines.

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Event Detection (Signal)

• Set a trap (calling signal() function)
• include ltsignal.hgt
• main( )
• .
• signal(SIGINT, handler)
• / call hander() if SIGINT arrives /

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Event Detection (Signal)

• The handler is a function that you define
  ashandler(int signal) check the signal
  type take action accordingly
• Upon return from the handler, the execution
  continues from the interrupted point.

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Event Detection (Signal)

• You can use signal(SIG???, SIG_IGN) to ignore the
  signal SIG???.In this case your program is not
  disturbed.
• signal(SIG???, SIG_DFL) take default action.
  Normally terminate.
• There are about 30 signals. Look at "man 5
  signal". E.g., HUP, INT, KILL, STOP, ALRM, etc.

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Event Detection (Signal)

• Example
• kill -HUP pid
• Many system daemons catch this signal and
  reinitialize themselves.

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Event Detection (Signal)

• User may generate a signal by using kill() and
  alarm() system call
• int kill(int process_id, int signal_name)
• kill() function can generate any signal and send
  to the processs specified.

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Event Detection (Signal)

• unsigned alarm(int seconds)
• wait the specified number of seconds and send
  ALRM signal to the process who called this
  function.
• Example set timeout for a process
• ..
• signal(SIGALRM, handletimeout)
• ..
• alarm(20)
• .
• void handletimeout() .

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Shared Memory

• Shared memory allows processes to share a piece
  of memory
• use semaphores to control the synchronization

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Shared Memory Example

• Sever
• include ltstdio.hgt
• include ltsys/types.hgt
• include ltsys/ipc.hgt
• include ltsys/shm.hgt
• define SHMKEY ((key_t) 7890) /base value for
  shmem key /
• define PERMS 0666
• int shmid

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Shared Memory Example

• main()
• Gameboard game
• if (shmid shmget(SHMKEY, sizeof(Gameboard),
  PERMS IPC_CREAT)) lt 0)
• error handling
• if (game (GameBoard)shmat(shmid,(char)0,0))
  (GameBoard)-1)
• error handling
• if (shmdt(game) lt 0)
• error handling
• if (shmctl(shmid,IPC_RMID,(struct shmid_ds )0)
  lt 0)
• error hanlding

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Shared Memory Example

• Client
• include ltstdio.hgt
• include ltsys/types.hgt
• include ltsys/ipc.hgt
• include ltsys/shm.hgt
• define SHMKEY ((key_t) 7890) /base value for
  shmem key /
• define PERMS 0666
• int shmid

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Shared Memory Example

• main()
• Gameboard game
• if (shmid shmget(SHMKEY, sizeof(Gameboard),
  0)) lt 0)
• error handling
• if (game (GameBoard)shmat(shmid,(char)0,0))
  (GameBoard)-1)
• error handling
• if (shmdt(game) lt 0)
• error handling

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Semaphores

• A synchronization primitive
• Oprations on a semaphore are atomic
• Not Used for exchanging data
• Let multiple processes/threads synchronize their
  operations
• lock a file
• synchronize access to shared memory

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Semaphore Example

• include ltsys/types.hgt
• include ltsys/ipc.hgt
• include ltsys/sem.hgt
• define SEMKEY 123456L
• define PERMS 0666
• static struct sembuf op_lock2
• 0,0,0, / wait for sem 0 to become 0 /
• 0,1,SEM_UNDO / then increment sem 0
  by 1 /

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Semaphore Example

• static struct sembuf op_unlock1
• 0,-1,IPC_NOWAIT SEM_UNDO / decrement
  sem 0 by 1 /
• int semid -1
• void lockResource()
• if (semid semget(SEMKEY,1,IPC_CREAT PERMS))
  lt 0)
• error handling
• if (semop(semid,op_lock0,2) lt 0)
• error handling

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Semaphore Example

• void unlockResource()
• if (semop(semid,op_unlock0,1) lt 0)
• error handling

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Communicating Through Sockets
Machine A
Machine B
listen
Network