Interprocess Communication. Originally multiple approaches Today more standard some differences between distributions still exist

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1 Interprocess Communication Originally multiple approaches Today more standard some differences between distributions still exist

2 Pipes Oldest form of IPC provided by all distros Limitations Historically half-duplex (data moves in only 1 direction) Can only be used between processes that have a common ancester Normally created by parent parent forks child used between parent & child Most commonly used form of IPC

3 Pipes Sequence of commands in a pipeline for shell to execute Shell creates a separate process for each command Links standard output of one process to standard input of next process using a pipe int pipe (int fd[2]); 2 file descriptors are returned through fd argument fd[0] open for reading fd[1] open for writing

4 Pipes

5 Pipes fstat function returns file type of pipe (FIFO) Can test for a pipe with S_ISFIFO macro

6 Pipes For a pipe from child to parent, parent closes fd[1] & child closes fd[0]

7 Pipes When 1 end of the pipe is closed If read is done from a pipe whose write end has been closed read returns a 0 If write is done to a pipe whose read end has been closed, signal SIGPIPE is generate PIPE_BUF specifies pipe buffer size

8 Pipes Can use 2 pipes for parent/child synchronization

9 popen & pclose functions FILE* popen (const char* cmdsring, const char* type); int pclose (FILE* f); Functions in stdio that open & close pipes handle all the details (fork, exec, ) Pclose closes standard I/O stream waits for command to terminate returns termination status to shell

10 FIFOs Named pipes Unnamed pipes can only be used between related processes with a common ancestor creating the pipe FIFOs unrelated processes can exchange data int mkfifo (const char* path, mode_t mode); int mkfifoat (int fd, const char* path, mode_t, mode); Similar to creating a file After fifo has been created, open function to open it Normal file I/O functions used with FIFOs

11 FIFOs Nonblocking flag (O_NONBLOCK) Not used when opening Open for read-only blocks until a different process opens FIFO for writing Open for write-only blocks until a different process opens FIFO for reading Used when opening Open for read-only returns immediately Open for write-only returns -1 & sets error flat if no process has the FIFO open for reading Write to FIFO that has no process open for reading SIGPIPE is generated

12 FIFOs Common to have multiple writers for a FIFO Have to worry about atomic writes Uses for FIFOS Shell commands to pass data from one shell pipeline to another Rendezvous points in client-server applications

13 Using FIFOs to Duplicate Output Streams FIFOs cannot be used for nonlinear connections since they have names Tee copies standard input to both standard output & file named on command line mkfifo fifo1 prog3 < fifo1 & prog1 < infile tee fifo1 prog2

14 Client-Server Communication using a FIFO Each client can write its request to a known FIFO for the server Requests need to be < PIPE_BUF bytes

15 Client-Server Communication using a FIFO Problem how to send responses back to client Cannot use a single FIFO clients would not know when to get response One possibility send process id with request server create a FIFO for each client Server cannot tell whether client crashes

16 XSI IPC IPC structures Message queues Semaphores Shared memory segment Each IPC structure has non-negative integer identifier internal name for IPC object Cooperating processes need an external naming scheme to be able to access each other Each IPC object has a key acts as external name

17 XSI IPC Client server rendezvous techniques Server can create a new IPC structure by specifying a key of IPC_PRIVATE store IPC_PRIVATE guarantees new IPC structure is created Disadvantage file system operations are required for server to write integer identifier to a file & for clients to retrieve this identifier Client & server agree on a key by defining the key in a common header Server creates IPC structure specifying this key Potential problem possible for key to already be associated with an IPC structure Client & server agree on a pathname & project ID call ftok to convert these values to a key

18 Permission Structure ipc_perm structure associated with each IPC structure struct ipc_perm { uid_t uid; /* owner's effective user id */ gid_t gid; /* owner's effice group id */ uid_t cuid; /* creator's effective user id */ git_t cgid; /* creator's effective group id */ mode_t mode; /* access modes */... } Each implementation has additional members Fields initialized when IPC structure is created

19 Advantages & Disadvantages IPC structures are systemwide & do not have reference count Remain in system until explicitly deleted IPC structures not known by names in file system Cannot access them using file operations

20 Message Queues Linked list of messages stored with kernel Identified by message queue identifier (queue ID) Created by msgget function Messages added at end by msgsnd function Messages fetched by msgrcv function Can fetch messages by type instead of FIFO Message consists of Positive long integer type field Non-negative length Actual data

21 System Limits

22 Message Queues int msgget (key_t key, int flag); Open existing queue or create new queue Queue ID is returned msgctl (int msqid, int cmd, struct msquid_ds *buf); Performs various operations on a queue cmd IPC_STAT fetch msqid_ds structure for queue storing in buf IPC_SET copy fields from buf to msqid_ds structure IPC_RMID remove message queue from system includes any data still on the queue int msgsnd (int msqid, const soid* ptr, size_t nbytes, int flag); Data placed on message queue ptr points to long integer followed by message data

23 Message Queues ssize_t msgrcv (int msqid, void* ptr, size_t nbytes, long type, int flag) Retrieve message from queue type specify which message type == 0 first message type > 0 first message whose type is argument type type < 0 first message whose type is lower than argument type flag IPC_NOWAIT operation nonblocking When operations succeeds kernel updates msqid_ds structure

24 Semaphores Counter used to provide access to shared resource Test semaphore that controls resource If value > 0 process can use resource Value decremented If value = 0 Process goes to sleep until semaphore value > 0 Returns to first step When process is finished with shared resource value incremented & processes waiting for resources are awakened

25 Semaphores Test of semaphores value & decrementing value must be atomic Normally implemented in kernel Binary semaphore controls single resource value initialized to 1

26 XSI Semaphores More complicated Not simply a single non-negative value Set of 1 or more semaphores Creation of semaphore is independent of its initialization Have to worry about a program that terminates without releasing semaphores it has been allocated

27 XSI Semaphores int semctl (int semid, int semnum, int cmd, ); Catchall for various semaphore operations Last argument union of command specific arguments Optional int semop (int semid, struct sembuf semoparray[], size_t nops); Atomically performs an array of operations on a semaphore set semoparray pointer to an array of semaphore operations nops number of operations

28 XSI Semaphores Operation on each member of the set specified by sem_op value Positive number of resources being returned by the process - value of sem_op added to semaphore's value Negative want to obtain resources that semaphore controls Zero process wants to wait until semaphore's value becomes 0 Semaphore adjustment on exit If SEM_UNDO flag is set, kernel remembers the resources allocated from that semaphore When process terminates kernel makes adjustments to semaphore counts

29 XSI Semaphores Methods of handling shared resources Semaphores Record locking mutex

30 Shared Memory Allows 2 or more processes to share a region of memory Fastest form of IPC data does not need to be copied Need to synchronize access XSI shared memory anonymous memory segments

31 Shared Memory int shmget (key_t key, size_t size, int flag); Obtain a shared memory identifier int shmctl (int shmid, int cmd, struct shmid_ds *buf); Catchall for various shared memory operations cmd IPC_STAT fetch shmid_ds structure for segment & store it in buf IPT_SET set 3 fields in structure pointed to by buf IPC_RMID remove shared memory segment from system SHM_LOCK lock shared memory segment in memory SHM_UNLOCK unlock shared memory segment in memory

32 Shared Memory void* shmat (int shmid, const void* addr, int flag); Process attaches to shared memory segment to its address space Addr == 0 segment is attached to first available address selected by kernel addr!= 0 & SHM_RND not specified segment is attached at address given by addr addr!= 0 & SHM_RND is specified segment is attached at address given by (addr (addr modulus SHMLBA)) SHMBLA low boundary address multiple power of 2 for portability should not specify address instead put 0 & let system choose address Address is returned

33 Shared Memory int shmdt (const void* addr); Detaches memory segment

34 Shared Memory /dev/zero Provides unbounded supply of null characters Writing to it has no effect Memory mapping of /dev/zero Unnamed memory region created whose size is the second argument to mmap rounded to nearest page size Memory region is initialized to 0 Multiple processes can share this region if a common ancestor specifies th MAP_SHARED flag to mmap

35 POSIX Semaphores Address several deficiencies with XSI semaphores Higher performance implementations Simpler to use Behave better when removed Named & unnamed versions Differ in how they are created & destroyed Unnamed can only be used by threads in the same process

36 POSIX Semaphores sem_t sem_open (const char* name, int oflag,, unsigned int value); Only first 2 arguments unless creating a semaphore then need 2 more (mode & initial value) Portability naming requirements First character should be '/' No other characters should be '/' Max length implementation defined

37 POSIX Semaphores int sem_close (sem_t* sem); Kernel will close open semaphores if this method is not called int sem_unlink (const char* name); Destroys a semaphore deferred until last open reference is closed int sem_trywait (sem_t* sem); int sem_wait (sem_t* sem); int sem_timedwait (sem_t* restrict sem, const struct timespec* restrict tsprt); Decrements value of semaphore sem_wait blocks but sem_trywait does not sem_timedwait block for a specified amount of time

38 POSIX Semaphores int sem_post (sem_t* sem); Increments value int sem_init (sem_t* sem, int pshared, unsigned int value); Creates unnamed semaphore pshared whether semaphore is to be used with multiple processes Int sem_destroy (sem_t* sem); Destroys unnamed semaphore Int sem_getvalue (sem_t* restrict sem, int* restrict valp); Retrieves value of semaphore valp Need to use synchronization

39 POSIX Semaphores

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