UNIT III- INTER PROCESS COMMUNICATIONS Part A

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1 UNIT III- INTER PROCESS COMMUNICATIONS Part A 1 What are the different communications supported by UNIX? Inter process communication and network communication 2 What do you mean by Inter process communication? UNIX operating system supports multiple processes simultaneously The communications between these processes are referred as Inter process communication 3 What are the different inter process communication methods supported by UNIX V? Message Queues Named and unnamed pipes Share memory Semaphore 4 Write down the difference between named and unnamed pipes A major difference between theses IPC mechanism is the related processes are allowed to communicate using unnamed pipes but the unrelated processes are also communicate in named pipes 5 What the Inter process communication mechanisms available in System V? Messages allow processes to send formatted data streams to arbitrary processes Shared memory allows processes to share parts of their virtual address space Semaphores allow processes to synchronize execution 6 List down the common properties of system V IPC Each mechanism contains a table whose entries describe all instances of the mechanism Each entry contains a numeric key Each mechanism contains get system call to create a new entry or to retrieve an existing entry The kernel uses the following formula to find the index into the table Index = descriptor modulo (number of entries in table) CHETTINAD COLLEGE OF ENGINEERING AND TECHNOLOGY 1

2 Each IPC entry has a permission structure that includes the user ID and group ID, read_write_execute permissions for owner, group and others Each entry contains other status information like time of last access or update etc Each mechanism contains a control system call to query status of an entry, to set status information or to remove the entry from the system 7 What are the system calls related to message queues? msgget returns a message descriptor that designates a message queue for use in other system calls msgctl has options to set and return parameters associated with a message descriptor and an option to remove descriptors msgsnd sends a message msgrcv receives a message 8 What do you mean by shared memory? Processes can communicate directly with each other by sharing parts of their virtual address space and then reading and writing the data stored in the shared memory 9 What are the elements of system V semaphore? A value of a semaphore The process ID of the last process to manipulate the semaphore The number of processes waiting for the semaphore value to increase The number of processes waiting for the semaphore value equal to 0 10 What is the format of each element of oplist? The semaphore number identifying the semaphore array entry being operated on The operation and flags 11 What are the different types of pipes? Unnamed pipe, and named pipe(fifo) 12 What are the different forms of IPC? Pipes Message queue Remote procedure call 13 What are the three ways to share information between Unix process? CHETTINAD COLLEGE OF ENGINEERING AND TECHNOLOGY 2

3 Sharing the file that resides in the files system Sharing some information that resides within the kernel Sharing information in a region of shared memory that each process can reference 14 What do you mean by persistence of IPC objects? Persistence of any type of IPC is defined as how long an object of that type remains in existence 15 What are the three types of Persistent IPC? Process Persistent IPC Kernel Persistent IPC File system Persistent IPC 16 Define file system Persistent IPC object? A file system persistent IPC object remains in existence until the object is explicitly deleted The object retains its value even if the kernel reboots Example Semaphores 17 What is wrapper function? Wrapper function is a function that performs the actual function call, tests the return value, and terminates on an error Wrapper function name begins with a capital letter, that is a wrapper function of our own It calls a function whose name is the same but begins with the lowercase letter Example sunc( ) original function func( ) original function 18 What is an errno? errno is a global variable It is set to a positive value, indicating the type of error, when an error occurs in a unix function All positive error values are constant with an all upper case name beginning with E and are normally defined in the <sys/errorh> header No error has the value of 0 19 Explain the term Posix? CHETTINAD COLLEGE OF ENGINEERING AND TECHNOLOGY 3

4 Posix is expanded into POrtable Operating System Interface It is a family of standards developed by IEEE 20 Define name space of an IPC? The set of possible names for a given type of IPC is called its name space It is also called as IPC key 21 What is the purpose of ftok function? The function ftok converts an existing pathname and an integer identifier into a key-t value(called an IPC key) name/id of a system V IPC key-t ftok(const char *pathname, int id); Return: IPC key if OK, -1 on error 22 Explain ipc perm structure? ipc perm is a structure of information for each IPC object which is maintained by kernel 23 Define Pipe? A Pipe is a one way communication channel Input from one end becomes output at the other end 24 What are the drawbacks of pipes? Pipes can only be used on processes That have a common ancestry like a parent and child process They are not permanent A process creates them and the termination of the process leads to their destruction 25 Define named Pipes/FIFOS? It is one way communication channels But unlike pipes, a named pipe is a permanent fixture Unix treats it just like a file Giving it size, owner and access permissions It can be opened/closed or deleted like any other file FIFO stands for First In, First Out 26 Define Iterative Server? Iterative server is a server program that iterates through the client requests, completely handling each client s request before proceeding to the next client 27 Define Concurrent Server? Concurrent server call fork to create a new child each time a client request arrives It is also called as a one-child-per-client server The new child handles the client request to CHETTINAD COLLEGE OF ENGINEERING AND TECHNOLOGY 4

5 completion The multiprogramming features of Unix allows all child to execute concurrently 28 Explain the POPEN( ) function? POPEN( ), a function provided by standard I/O library It creates a pipe and initializes a process that either reads from the pipe or writes to it 29 Explain the function to create a new/access(existing) message queue? Using msgget, a new message queue is created, or an existing message queue is accessed int msgget(key-t key, int oflay); 30 What is the use of IPC NOWAIT value to flag argument to msgsnd function? This flag makes the call to msgsnd Non blocking : the function returns immediately if no room is available for the new message This condition can occur if Too many bytes are already on the specified queue Too many messages exist system wide 31 What are the differences between POSIX message Queue and system V message Queue? A read on a POSIX message queue always returns the oldest message of the highest priority, where as a read on a system message queue can return a message of any desired priority POSIX message queues allow the generation of a signal or the initial of a thread when a message is placed on to an empty queue, whereas nothing similar is provided by system V message queues 32 How do you close a open message queue? A open message queue is closed by mq-close #include<mqueueh> int mq-close (mqd-t mqdes); CHETTINAD COLLEGE OF ENGINEERING AND TECHNOLOGY 5

6 33 How do you notify a process when a message placed onto a queue? POSIX message queues allow for an asynchronous event notification when a message is placed on to an empty message queue This notification can be either The generation of a signal or The creation of a thread to execute a specified function 34 Explain mutex? A mutex which stands for mutual exclusion is the most basic form of synchronization A mutex is used to protect a critical region, to make certain that only one thread at a time executes the code within the region or that only one process at a time executes the code within the region 35 How will you wakeup a multiple thread? Pthread-cond-broadcast will wake up all threads that are blocked on the condition variable int pthread cond-broadcast(pthread-cond-t *cptr); 36 Describe Shared Lock? Shared lock is a read- write lock for reading Any number of thread can have read access to a given piece of data as long as no thread is reading or modifying that piece of data 37 Describe Exclusive lock? Exclusive is a read-write lock for writing A read-write lock can be allocated for writing only if no thread holds the read-write lock for reading or writing 38 What is thread Cancellation? A thread may be cancelled by any other thread in the same process when other thread calls pthread cancel a function whose argument is the thread ID to cancel int pthread-cancel (pthread-t tid); 39 Explain about advisory locking? (DEC 04) POSIX record locking is called advisory locking This means the kernel maintains correct knowledge of all files that have been locked by each process, but it does not prevent a process form writing to a file that is read locked by another process Similarly, the kernel does not prevent a process from reading from a file that is CHETTINAD COLLEGE OF ENGINEERING AND TECHNOLOGY 6

7 write-locked by another process Advisory locks are fine for co-operating processes 40 What is range locking? Range locking is defined as locking or unlocking a range of bytes within the file 41 Define Semaphore? (DEC 05) A semaphore is a primitive used to provide synchronization between various processes or between the various thread in a given process 42 What are the three operations that a process can perform on a semaphore? Created a semaphore Wait for a semaphore Post to a semaphore 43 Define counting semaphore? Counting semaphore is a semaphore whose value is between 0 and some limit(at least for posix) We used these to indicate number of resources available 44 What are the two types of POSIX semaphores? Named semaphore Memory based semaphore 45 Which function is used to remove the semaphore from the system? A named semaphore is removed from the system by sem-unlink int sem-unlink(const char *name); 46 How do you remove memory mapping? To remove a mapping from the address space of the process, we call munmap int munmap(void *addr, size-t len); 47 What are the two ways of memory sharing between two unrelated process in POSIX1? Memory mapped files Shared memory objects CHETTINAD COLLEGE OF ENGINEERING AND TECHNOLOGY 7

8 48 How do you create a shared memory segment using system V function? A shared memory segment is created, or an existing one is accessed, by the shmget function int shmget(key-t key, size-t size, int oflag); 49 What is the use of ftruncate function? When dealing with mmap, memory map function the size of either regular file or a shared memory object can be charged by calling truncate int ftruncate(int fd, off-t length); 50 What is the purpose of shmdt function? When a process is finished with a shared memory segment, it detaches the segment by calling shmdt int shmdt(const void *snmaddr); 51 How do you get the number of threads waiting for semaphore to be unlocked? Sem-getvalue funtion is used to get number of threads waiting for the semaphore to be unlocked and or current value of semaphore int sem-getvalue(sem-t *sem, int *valp); 52 Write the message queue structure struct msqid_ds { struct ipc_perm msg_perm; msgqnum_t msg_qnum; /* # of messages on queue */ msglen_t msg_qbytes; /* max # of bytes on queue */ pid_t msg_lspid; /* pid of last msgsnd() */ pid_t msg_lrpid; /* pid of last msgrcv() */ time_t msg_stime; /* last-msgsnd() time */ time_t msg_rtime; /* last-msgrcv() time */ time_t msg_ctime; /* last-change time */ CHETTINAD COLLEGE OF ENGINEERING AND TECHNOLOGY 8

9 }; When a new queue is created, the following members of the msqid_ds structure are initialized The ipc_perm structure is initialized msg_qnum, msg_lspid, msg_lrpid, msg_stime, and msg_rtime are all set to 0 msg_ctime is set to the current time msg_qbytes is set to the system limit 53 Write the shared memory structure struct shmid_ds { struct ipc_perm shm_perm; /* see Section 1562 */ size_t shm_segsz; /* size of segment in bytes */ pid_t shm_lpid; /* pid of last shmop() */ pid_t shm_cpid; /* pid of creator */ shmatt_t shm_nattch; /* number of current attaches */ time_t shm_atime; /* last-attach time */ time_t shm_dtime; /* last-detach time */ time_t shm_ctime; /* last-change time */ }; 54 How the data can be read from/write to pipe? 1 If we read from a pipe whose write end has been closed, read returns 0 to indicate an end of file after all the data has been read 2 If we write to a pipe whose read end has been closed, the signal SIGPIPE is generated If we either ignore the signal or catch it and return from the signal handler, write returns 1 with errno set to EPIPE CHETTINAD COLLEGE OF ENGINEERING AND TECHNOLOGY 9

10 55 List out the uses of FIFO There are two uses for FIFOs 1 FIFOs are used by shell commands to pass data from one shell pipeline to another without creating intermediate temporary files 2 FIFOs are used as rendezvous points in client server applications to pass data between the clients and the servers CHETTINAD COLLEGE OF ENGINEERING AND TECHNOLOGY 10