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Unit- IV

Unit- IV. Inter-process Communication:. Contents:. Inter-Process Communication Process Tracing Pipes Sockets System V IPC Multiprocessor systems. IPC:. Independent processes & co-operating processes Communicating two processes related or unrelated. Purposes for IPC Information Sharing

matthew-randall
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Unit- IV

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  1. Unit- IV Inter-process Communication:

  2. Contents: • Inter-Process Communication • Process Tracing • Pipes • Sockets • System V IPC • Multiprocessor systems

  3. IPC: • Independent processes & co-operating processes • Communicating two processes related or unrelated. • Purposes for IPC • Information Sharing • Data Transfer • Synchronization • Event Notification • IPC mechanism: • Signal • Message Passing • Pipes • Sockets • Shared memory

  4. Process Tracing • ptrace(cmd, pid, addr, data) • pid is the process ID • addr refers to a location • cmd: r/w, intercept, set or delete watchpoints, resume the execution • Data: interpreted by cmd • Used by debuggers sdb or dbx • Global trace data structure

  5. Debugging Process : • if ((pid = fork() ) == 0) • { • //child traced process • ptrace(0, 0, 0, 0) ; • exec("name of traced process here") ; • // debugger process continues here • for (;;) • { • wait( (int *) 0) ; • read (input for tracing instructions) • ptrace(cmd,pid,addr,data) ; • if (quitting trace) • break; • }

  6. Process Tracing • drawbacks and limitations - child must allow tracing explicitly by invoking ptrace; - parent can control the execution of its direct children only; - extremely inefficient (requires several context switches); - cannot control a process already running; - security problems when tracing a setuid programs (user can obtain super user privileges) – to avoid this UNIX disables tracing such programs;

  7. Pipes :

  8. P P P Data P P Fig 6-1 Data flow through a pipe. Pipes • A unidirectional, FIFO, unstructured data stream of fixed maximum size. • int pipe (int * filedes) • filedes[0] for read, filedes[1] for write

  9. Pipes • Applications: in shell – passing output of one program to another program – e.g. cat fil1 file2 | sort • Limitations: cannot be used for broadcasting; • Data in pipe is a byte stream • No way to distinguish between several readers or writers • Named Pipes: • Use mknod(),mkfifo()

  10. Sockets :

  11. Sockets : • A socket is defined as an endpoint for communication • Concatenation of IP address and port • Communication needs a pair of sockets

  12. Socket Functions : • sd = socket (format, type, protocol) ; • bind (sd, address, length) ; • connect (sd, address, length) ; • listen (sd, qlength) • nsd = accept (sd, address, addrlen) ; • count = send (sd, msg, iength, flags) ;

  13. Socket Functions : • count = recv (sd, buf, length, flags) ; • shutdown (sd, mode) • getsockname(sd, name, length) ;

  14. System V IPC • Messages: send formatted data streams to arbitrary process • Shared memory: share parts of virtual address space • Semaphores: synchronize execution

  15. Common properties : • Table whose entries describes all instances of mechanism • Each entry contains user chosen numeric key • Each mech. contains get system call • IPC_CREAT bit in flag: new entry with given key • IPC_CREAT and IPC_EXCL bit in flag: error if entry for given key is present • To find index into table • index=descriptor modulo (no of entries) • Permission structure: UID, GID • Status information: PID and time of last access/modification • Control system call: set/return status info, remove entry

  16. Messages : • Msgqid=msgget(key,flag) • Fields of queue structure - ptr to 1st and last msg on linked list - no of msg & total no of data byte on linked list - max no of bytes of data byte on linked list - process id of last process - timestamp of last system call

  17. msgqid=msgget(key,flag) When msgget called search msg queue to find id with given key if no entry allocate new queue struct initialize it return identifier else check permission & return

  18. Msgsnd(msgqid,msg,count,flag) • Msgqid-descriptor of msg queue returned by msgget • Msg- ptr to structure int type and char array • Count- size of data array • Flag-action taken if it runs of internal buffer space

  19. Count=msgrcv(id,msg,maxcount,type,flag) • id- msg descriptor • Msg-address of user structure to contain rcvmsg • Maxcount-size of data array in msg • Type-msg type user wants to read • Flag- what kernel should do if no msg are on the queue • Count- no of bytes returned to user

  20. Msgctl(id,cmd,mstatbuf) • Id-msg descriptor • Cmd- type of command • Mstatbuf- address of user data structure that will contain control parameters or result of query

  21. Shared Memory : • Sharing part of virtual address space for reading and writing. • Shmget() • Shmat() • Shmdt() • Shmctl()

  22. Shmid=shmget(key,size,flag) • Size-no of bytes in region • Search for given key if entry present & permission return descriptor for entry if no entry & IPC_CREAT flag verify size & allocate region using allocreg saves permission modes, size, ptr to region table allocate mem when attach region

  23. virtaddr=shmat(id,addr,flags) • Id-returned by shmget • Addr-virtual address where user wants to attach • Flags-whether region read only, etc. • Virtaddr- Virtual address where kernel attached region

  24. Shared Memory detach and control : • shmdt(addr) addr- address returned by shmat • shmctl(id,cmd,shmstatbuf) id-shared mem table entry cmd- type of operation shmstatbuf-address of user level structure that contain status information

  25. Semaphores :

  26. Semaphore Functions: • id=semget(key,count,flag) • No of semaphore entries • Time of last semop and semctl • Oldval=semop(id,oplist,count); • Oplist format:- • Sem_op • Sem_no • Flags • Semctl(id,number,cmd,arg) • Args:-union • Sem_val • Sem_ds • Array ptr

  27. Change in Semaphore value a/c to operation value: • If Positive: - • Increment semaphore • Wake up processes waiting for sem value to increase • If 0: - • If sem value=0 :- continue with other operation with array • Else put process to sleep (process waiting for sem value to be 0) • If Negative: - • Decrements semaphore by op value • If semaphore = 0 • Wakeup all processes waiting for sem to be 0 • Else • process goes to sleep (process waiting for sem value to increase)

  28. Flag bits : • IPC_CREAT • IPC_EXCL • IPC_NOWAIT • IPC_RMID • IPC_PRIVATE • MSG_NOERROR • SEM_UNDO

  29. Multiprocessor configuration

  30. Multiprocessor configuration • Greater throughput as processes run concurrently. • Each CPU executes independently but all of them execute one copy of kernel. • Semantics for system call are same. • Drawback: Several process execute simultaneously in kernel mode causes integrity problems.

  31. Problem of multiprocessor system In uniprocessor Unix system integrity of kernel data structure is maintained by two policies. • Kernel cannot preempt a process and switch context to another process while executing in kernel mode. • Kernel masks out interrupts when executing a critical region of code .

  32. There are 3 methods for preventing such corruption • Execute all critical activity on one processor • Serialize access to critical regions of code with locking primitives. • Redesign algorithms to avoid contention of data structure.

  33. Master Slave Arch :

  34. Solution with master and slave processors • One processor called master can execute in kernel mode and other called slave execute only in user mode. • Master processor- handle all system calls and interrupt. • Slave processor-execute process in user mode and inform master when makes system calls.

  35. Drawback: • Corruption of kernel data structure possible in scheduler algorithm because it does not protect against having a process selected for execution on two processor. • Master can specify the slave processor on which the process should execute (more than one process can be assign –load balancing problem). • Kernel can allow only one processor to execute the scheduling loop at a time.

  36. Mechanism for setting lock : Solutions with semaphore : • Mechanism for setting lock :

  37. Solution with semaphore • Partition the kernel into critical region such that at most one processor can execute code in critical region at a time. Operations on semaphores: • Initialization :- to a nonnegative value. • P operation :- • decrements the value of semaphore. • If value of semaphore < 0 (process that did the p goes to sleep) • V operation :- I • incrementsthe value of semaphore. • If the value of semaphore >= 0 (one process that has been sleeping as a result of p operation wakes up.) • Conditional P (CP) :- • Decrements the value of semaphore • True: if its value > 0 • False: if value <= 0

  38. Implementation of semaphore • Dijkstra- possible to implement semaphore without sp. m/c instruction. • Pprim locks semaphore by checking value of array val. • When lock sem. It checks to see if processor already locked(val=2) or if processor with lower ID trying to lock(val=1). • If either cond true, resets entry =1 & tries again

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