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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

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Unit- IV

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Unit- IV

Inter-process Communication:


  • Inter-Process Communication

  • Process Tracing

  • Pipes

  • Sockets

  • System V IPC

  • Multiprocessor systems


  • 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

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

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;

  • }

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;

Pipes :







Fig 6-1 Data flow through a pipe.


  • A unidirectional, FIFO, unstructured data stream of fixed maximum size.

  • int pipe (int * filedes)

  • filedes[0] for read, filedes[1] for write


  • 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()

Sockets :

Sockets :

  • A socket is defined as an endpoint for communication

  • Concatenation of IP address and port

  • Communication needs a pair of sockets

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) ;

Socket Functions :

  • count = recv (sd, buf, length, flags) ;

  • shutdown (sd, mode)

  • getsockname(sd, name, length) ;

System V IPC

  • Messages: send formatted data streams to arbitrary process

  • Shared memory: share parts of virtual address space

  • Semaphores: synchronize execution

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

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


When msgget called

search msg queue to find id with given key

if no entry

allocate new queue struct

initialize it

return identifier


check permission & return


  • 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


  • 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


  • Id-msg descriptor

  • Cmd- type of command

  • Mstatbuf- address of user data structure that will contain control parameters or result of query

Shared Memory :

  • Sharing part of virtual address space for reading and writing.

  • Shmget()

  • Shmat()

  • Shmdt()

  • Shmctl()


  • 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


  • 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

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

Semaphores :

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

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)

Flag bits :








Multiprocessor configuration

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.

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 .

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.

Master Slave Arch :

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.


  • 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.

Mechanism for setting lock :

Solutions with semaphore :

  • Mechanism for setting lock :

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

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

Locking Mechanism:

Following sequence protects a resource

  • Pprim(semaphore)

    • Use resource here

  • Vprim (semaphore)

  • Disadvantage:- loops are slow so performance down

  • IBM 370 - atomic compare & swap instructions.

  • AT & T 3B20 –atomic read & clear instructions.

Algorithms implemented with semaphore:

  • Buffer allocation

  • Wait

  • Driver-locking scheme

  • Dummy process

Buffer allocation

  • Data structure for buffer allocation

    • Buffer Header

    • HQ of buffer

    • Free List Buffers

  • In multiprocessor two process could manipulate the linked list of hash queue, the semaphore for the hash queue permits only one processor at a time to manipulate the linked list.

  • Free list requires a semaphore as several process could corrupt it.


  • Parent should miss a zombie child as it executes the wait algorithm.

  • Parent must not sleep waiting forever.

Multiprocessor algorithm wait




search all child process;

if (status of child is zombie)


P(zombie_semaphore); /* initialized to 0*/



  • Perform V while exiting.

Drivers :

  • Avoided inserting semaphore into driver code.

  • P & V operations at the driver entry points.




  • Same semaphore for all entry points.

  • Different semaphore for each driver.

Dummy process :

  • In uniprocessor, if no process are ready to run, kernel idles in the context of the process that last run.

  • In multi processor, Kernel can not idle in context of the process executed most recently on the processor.

  • Create a dummy process per processor.

  • When processor has no work to do context switch is done to the dummy process & it idles the context of dummy process .

  • It consists of kernel stack only and can not be scheduled.

The Tunis system

  • User interface compatible to Unix.

  • Written in Concurrent Euclid.

  • Solves the mutual exclusion problem.

  • Kernel process are activated by queuing messages for input.

  • Concurrent Euclid implements monitor to prevent corruption of queues.

Performance limitation:

  • Methods to implement multiprocessor configuration.

    1: Master – slave

    2: Semaphore

  • The implementation of multiprocessor Unix system generalizes to any no of processor but throughput will not increase at linear rate with no of processors.

    1: Increased memory contention in h/w .

    2: Wait longer period of time to gain semaphore.

    3: Master processor becomes bottleneck.

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