Acknowledgement : Prof. Y. Moon at Kangwon Nat’l Univ.

Inter-Process Communication , Advanced I/O(Chap 12, 14 in the book “Advanced Programming in the UNIX Environment”)

Acknowledgement : Prof. Y. Moon at Kangwon Nat’l Univ.

UNIX time

2

#include <time.h>

time_t time(time_t *calptr); Returns: value of time if OK, -1 on error

struct tm *gmtime(const time_t *calptr); Returns: pointer to time structure

time_t mktime(struct tm *tmptr); Returns : calendar time if OK, -1 on error

char *asctime(const struct tm *tmptr);char *ctime(const time_t *calptr); Returns : pointer to null terminated string

size_t strftime(char *buf, size_t maxsize, const char *format, const struct tm *tmptr); Returns : number of characters stored in array if room, else 0

struct tm{ int tm_sec; int tm_min; int tm_hour; int tm_mday; /* [1,31] */ int tm_mon; /* [0,11] */ int tm_year; /* years since 1900 */ int tm_wday; /* days since Sunday [0, 6] */ int tm_yday; /* [0, 365] */ int tm_isdst;};

Lock

a synchronization mechanism for enforcing limits on access to a resource in an environment where there are many threads of execution

Advisory lock Other processes can still access (read or write) to a locked resource There is a way to check whether a resource is locked

Mandatory lock Prevent access to a locked resource

3

http://en.wikipedia.org/wiki/Synchronization_%28computer_science%29

Advisory Locking

Applies or removes an advisory lock on the file associated with the file descriptor fd

Operation can be LOCK_SH LOCK_EX LOCK_NB LOCK_UN

Locks entire file

4

#include <fcntl.h>

int flock(int fd, int operation); Returns: 0 if OK, -1 otherwise

Advisory Record Locking

struct flock {short l_type; /* F_RDLCK, F_WRLCK, or F_UNLCK */off_t l_start; /* offset in bytes from l_whence */short l_whence; /* SEEK_SET, SEEK_CUR, SEEK_END */off_t l_len; /* length, in bytes; 0 means “lock to EOF */pid_t l_pid; /* returned by F_GETLK */

}

Lock types are : F_RDLCK : non-exclusive (read) lock; fails if write lock exists F_WRLCK : exclusive (write) lock; fails if any lock exists F_UNLCK : releases our lock on specified range

5

#include <fcntl.h>

int fcntl(int fd, int cmd, struct flock* lock); Returns: depends on cmd if OK, -1 on error

Setting and Clearing a lock

6

struct flock fl; int fd;

fl.l_type = F_WRLCK; /* F_RDLCK, F_WRLCK, F_UNLCK */ fl.l_whence = SEEK_SET; /* SEEK_SET, SEEK_CUR, SEEK_END */ fl.l_start = 0; /* Offset from l_whence */ fl.l_len = 0; /* length, 0 = to EOF */ fl.l_pid = getpid(); /* our PID */

fd = open("filename", O_WRONLY); fcntl(fd, F_SETLKW, &fl); /* F_GETLK, F_SETLK, F_SETLKW */

fl.l_type = F_UNLCK; /* tell it to unlock the region */ fcntl(fd, F_SETLK, &fl); /* set the region to unlocked */

Non-Blocking(Asynchronous) I/O

A form of I/O that permits subsequent processing to continue before current processing has finished.

Non Blocking I/O function returns immediately Two ways for specifying non-blocking I/O

When calling “open” and specify O_NONBLOCK flag For a file descriptor that is already open, call fcntl to

turn on O_NONBLOCK file status flag.

7

IPC

Pipes FIFOs

Message Queue Shared Memory Semaphores

ContentsContents

Pipes

FIFOs

System V IPC• Message Queues

• Shared Memory

• Semaphores

APUE (Interprocess Communication

IPC using PipesIPC using Pipes

IPC using regular files• unrelated processes can share

• fixed size

• lack of synchronization

IPC using pipes• for transmitting data between related processes

• can transmit an unlimited amount of data

• automatic synchronization on open()


Pipes in a UNIX ShellPipes in a UNIX Shell

In a UNIX shell, the pipe symbol is: | (the vertical bar)

In a shell, UNIX pipes look like:$ ls -alg | more

• where the standard output of the program at the left (i.e., the producer) becomes the standard input of the program at the right (i.e., the consumer).

We can have longer pipes:$ pic paper.ms | tbl | eqn | ditroff -ms


Example (1/2)Example (1/2)

$ who | sort


pipewho sort

write pointer ofanother process

read pointer of one process

Example (2/2)Example (2/2) APUE (Interprocess Communication

IPC using PipesIPC using Pipes

Data transmitting• data is written into pipes using the write() system call

• data is read from a pipe using the read() system call

• automatic blocking when full or empty

Types of pipes• (unnamed) pipes

• named pipes (FIFOs)


Pipes (1/4)Pipes (1/4)

In UNIX, pipes are the oldest form of IPC.

Limitations of Pipes:• Half duplex (data flows in one direction)

• Can only be used between processes that have a common ancestor (Usually used between the parent and child processes)

• Processes cannot pass pipes and must inherit them from their parent

• If a process creates a pipe, all its children will inherit it



Two file descriptors are returned through the fd argument• fd[0]: can be used to read from the pipe, and

• fd[1]: can be used to write to the pipe

Anything that is written on fd[1] may be read by fd[0].• This is of no use in a single process.

• However, between processes, it gives a method of communication

The pipe() system call gives parent-child processes a way to communicate with each other.


#include <unistd.h>

int pipe(int fd[2]) Returns: 0 if OK, -1 on error

Pipes (3/4)Pipes (3/4) APUE (Interprocess Communication

parent child: parent closes fd[0] child closes fd[1]

parent child: parent closes fd[1] child closes fd[0]

pipe

kernel

fd[1]

parent

fd[0]

child

pipe

kernel

fd[0]

parent

fd[1]

child


Read from a pipe with write end closed: (fd[1] 이 close 된 경우 )

• returns 0 to indicate EOF

Write to a pipe with read end closed: (fd[0] 가 close 된 경우 )

• SIGPIPE generated,

• write() returns error (errno == EPIPE)


example: pipe.c (1/2)example: pipe.c (1/2)

#include <stdio.h> // pipe.c

#define READ 0#define WRITE 1

char* phrase = "Stuff this in your pipe and smoke it";

main( ) { int fd[2], bytesRead; char message[100]; pipe(fd); if (fork() == 0) { // child close(fd[READ]); write(fd[WRITE], phrase, strlen(phrase)+1); fprintf(stdout, "[%d, child] write completed.\n", getpid()); close(fd[WRITE]); } else { // parent close(fd[WRITE]); bytesRead = read(fd[READ], message, 100); fprintf(stdout, "[%d, parent] read completed.\n", getpid()); printf("Read %d bytes: %s\n", bytesRead,message); close(fd[READ]); }}


example: pipe.c (2/2)example: pipe.c (2/2) APUE (Interprocess Communication

실행 결과

ContentsContents

Pipes

FIFOs


• Shared Memory

• Semaphores


FIFOs (1/3)FIFOs (1/3)

Pipes can be used only between related processes.(e.g., parent and child processes)

FIFOs are "named pipes" that can be used between unrelated processes.

A type of file• stat.st_mode == FIFO

• Test with S_ISFIFO() macro


FIFOs (2/3)FIFOs (2/3) APUE (Interprocess Communication

Creating FIFOs is similar to creating a file.• pathname: filename

• mode: permissons, same as for open() function

Using a FIFO is similar to using a file.• we can open, close, read, write, unlink, etc., to the FIFO.

#include <sys/types.h>#include <sys/stat.h>

int mkfifo(const char *pathname, mode_t mode); Returns: 0 if OK, -1 on error

FIFOs (3/3)FIFOs (3/3)

if FIFO opened without O_NONBLOCK flag• an open for read-only blocks until some other process opens the FIFO for writing

• an open for write-only blocks until some other process opens the FIFO for reading

if O_NONBLOCK is specified (nonblocking)• an open for read-only returns immediately if no process has the FIFO open for writing

• an open for write-only returns an error (errno=ENXIO) if no process has the FIFO open for reading

Like a pipe, if we write to a FIFO that no process has open for reading, the signal SIGPIPE is generated.

When the last writer for a FIFO closes the FIFO, an end of file (EOF) is generated for the reader of the FIFO.


Uses of FIFOsUses of FIFOs

Used by shell commands to pass data from one shell pipeline to another, without creating intermediate files.

Used in client-server application to pass data between clients and server.


Using FIFOs to Duplicate Output StreamsUsing FIFOs to Duplicate Output Streams APUE (Interprocess Communication

tee(1) copies its standard input to both its standard output and to the file named on its command line.

$ mkfifo fifo1$ prog3 < fifo1 &$ prog1 < infile | tee fifo1 | prog2

prog1 tee

prog2

prog3fifo1

infile

An Example using a FIFOAn Example using a FIFO APUE (Interprocess Communication

Client-Server Communication Using a FIFOClient-Server Communication Using a FIFO

Server creates a “well-known” FIFO to communicate with clients.


Problem: Server can't reply clients using a single “well-known” FIFO

client

well-knownFIFO

readrequest

client

server

write request

write request

.

.

.

ContentsContents

Pipes

FIFOs


• Shared Memory

• Semaphores ( 간단히 )


System V IPCSystem V IPC

Message Queues• Send and receive amount of data called “messages”.

• The sender classifies each message with a type.

Shared Memory• Shared memory allows two or more processes to share a given region of memory.

• Readers and writers may use semaphore for synchronization.

Semaphores• Process synchronization and resource management

• For example, a semaphore might be used to control access to a device like printer.


Identifiers & KeysIdentifiers & Keys

Identifier: each IPC structure has a nonnegative integer

Key: when creating an IPC structure, a key must be specified (key_t)

id = xxxget(key, …)

How to access the same IPC? key in a common header• Define a key in a common header

• Client and server agree to use that key

• Server creates a new IPC structure using that key

• Problem when the key is already in use

− (msgget, semget, shmget returns error)

− Solution: delete existing key, create a new one again!


IPC System CallsIPC System Calls

msg/sem/shm get• Create new or open existing IPC structure.

• Returns an IPC identifier

msg/sem/shm ctl• Determine status, set options and/or permissions

• Remove an IPC identifier

msg/sem/shm op• Operate on an IPC identifier

• For example(Message queue)

− add new msg to a queue (msgsnd)

− receive msg from a queue (msgrcv)


Permission StructurePermission Structure APUE (Interprocess Communication

ipc_perm is associated with each IPC structure.

Defines the permissions and owner.

struct ipc_perm { uid_t uid; /* owner's effective user id */ gid_t gid; /* owner's effective group id */ uid_t cuid; /* creator's effective user id */ gid_t cgid; /* creator’s effective group id */ mode_t mode; /* access modes */ ulong seq; /* slot usage sequence number */ key_t key; /* key */};

Message Queues (1/2)Message Queues (1/2)

Linked list of messages• Stored in kernel• Identified by message queue identifier (in kernel)

msgget• Create a new queue or open existing queue.

msgsnd• Add a new message to a queue

msgrcv• Receive a message from a queue• Fetching order: based on type


Message Queues (2/2)Message Queues (2/2)

Each queue has a structure


struct msqid_ds { struct ipc_perm msg_perm; struct msg *msg_first; /* ptr to first msg on queue */ struct msg *msg_last; /* ptr to last msg on queue */ ulong msg_cbytes; /* current # bytes on queue */ ulong msg_qnum; /* # msgs on queue */ ulong msg_qbytes; /* max # 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 */ };

We can get the structure using msgctl() function.Actually, however, we don’t need to know the structure in detail.

msgget()msgget() APUE (Interprocess Communication

#include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h>

int msgget(key_t key, int flag); Returns: msg queue ID if OK, -1 on error

Create new or open existing queueflag : ipc_perm.modeExamplemsg_qid = msgget(DEFINED_KEY, IPC_CREAT | 0666);

msgctl()msgctl() APUE (Interprocess Communication


int msgctl(int msqid, int cmd, struct msqid_ds *buf); Returns: 0 if OK, -1 on error

Performs various operations on a queuecmd = IPC_STAT:fetch the msqid_ds structure for this queue, storing it in bufcmd = IPC_SET:set the following four fields from buf: msg_perm.uid, msg_perm.gid, msg_perm.mode, and msg_qbytescmd = IPC_RMID:remove the message queue.

msgsnd()msgsnd() APUE (Interprocess Communication


int msgsnd(int msqid, const void *ptr, size_t nbytes, int flag); Returns: 0 if OK, -1 on error

msgsnd() places a message at the end of the queue.• ptr: pointer that points to a message• nbytes: length of message data• if flag = IPC_NOWAIT: IPC_NOWAIT is similar to the nonblocking I/O flag for file

I/O.

Structure of messages

struct mymesg { long mtype; /* positive message type */ char mtext[512]; /* message data, of length nbytes */};

msgrcv()msgrcv() APUE (Interprocess Communication


int msgrcv(int msqid, void *ptr, size_t nbytes, long type, int flag); Returns: data size in message if OK, -1 on error

msgrcv() retrieves a message from a queue.type == 0: the first message on the queue is returnedtype > 0: the first message on the queue whose message type equals type is returnedtype < 0: the first message on the queue whose message type is the lowest value less than or equal to the absolute value of type is returnedflag may be given by IPC_NOWAIT

example: sender.c receiver.c (1/4)example: sender.c receiver.c (1/4) APUE (Interprocess Communication

#include <stdio.h> // sender.c#include <sys/types.h>#include <sys/ipc.h>#include <sys/msg.h>

#define DEFINED_KEY 0x10101010

main(int argc, char **argv){ int msg_qid; struct { long mtype; char content[256]; } msg;

fprintf(stdout, "=========SENDER==========\n");

if((msg_qid = msgget(DEFINED_KEY, IPC_CREAT | 0666)) < 0) { perror("msgget: "); exit(-1); }

msg.mtype = 1; while(1) { memset(msg.content, 0x0, 256); gets(msg.content); if(msgsnd(msg_qid, &msg, sizeof(msg.content), 0) < 0) { perror("msgsnd: "); exit(-1); } }}


#include <stdio.h> // receiver.c#include <sys/types.h>#include <sys/ipc.h>#include <sys/msg.h>

#define DEFINED_KEY 0x10101010

main(int argc, char **argv){ int msg_qid; struct { long mtype; char content[256]; } msg;

fprintf(stdout, "=========RECEIVER==========\n");

if((msg_qid = msgget(DEFINED_KEY, IPC_CREAT | 0666)) < 0) { perror("msgget: "); exit(-1); }

while(1) { memset(msg.content, 0x0, 256); if(msgrcv(msg_qid, &msg, 256, 0, 0) < 0) { perror("msgrcv: "); exit(-1); } puts(msg.content); }}


Message Queue

Shared MemoryShared Memory

Allows multiple processes to share a region of memory• Fastest form of IPC: no need of data copying between client & server

If a shared memory segment is attached• It become a part of a process data space, and shared among multiple processes

Readers and writers may use semaphore to• synchronize access to a shared memory segment


Shared Memory Segment StructureShared Memory Segment Structure

Each shared memory has a structure


struct shmid_ds { struct ipc_perm shm_perm; struct anon_map *shm_amp; /* pointer in kernel */ int shm_segsz; /* size of segment in bytes */ ushort shm_lkcnt; /* # of times segment is being locked */ pid_t shm_lpid; /* pid of last shmop() */ pid_t shm_cpid; /* pid of creator */ ulong shm_nattch; /* # of current attaches */ ulong shm_cnattch; /* used only for shminfo() */ time_t shm_atime; /* last-attach time */ time_t shm_dtime; /* last-detach time */ time_t shm_ctime; /* last-change time */ };

We can get the structure using shmctl() function.Actually, however, we don’t need to know the structure in detail.

shmget()shmget() APUE (Interprocess Communication

#include <sys/types.h> #include <sys/ipc.h> #include <sys/shm.h>

int shmget(key_t key, int size, int flag); Returns: shared memory ID if OK, -1 on error

Obtain a shared memory identifiersize: is the size of the shared memory segmentflag: ipc_perm.modeExampleshmId = shmget(key, size, PERM|IPC_CREAT|IPC_EXCL|0666);

shmctl()shmctl() APUE (Interprocess Communication


int shmctl(int shmid, int cmd, struct shmid_ds *buf); Returns: 0 if OK, -1 on error

Performs various shared memory operationscmd = IPC_STAT:fetch the shmid_ds structure into bufcmd = IPC_SET:set the following three fields from buf: shm_perm.uid, shm_perm.gid, and shm_perm.modecmd = IPC_RMID:remove the shared memory segment set from the system

shmat()shmat() APUE (Interprocess Communication


void *shmat (int shmid, void *addr, int flag); Returns: pointer to shared memory segment if OK, -1 on error

Attached a shared memory to an address

flag = SHM_RDONLY: the segment is read-only

addr==0: at the first address selected by the kernel (recommended!)

addr!=0: at the address given by addr

shared memory

Memory LayoutMemory Layout APUE (Interprocess Communication

uninitialized data(bss)

stack

heap

initialized data

text

high address

low address

command-line argumentsand environment variables

0xf7fffb2c

0xf77e86a00xf77d0000 shared memory of 100,000 bytes

0x0003d2c80x00024c28

malloc of 100,000 bytes

0x0003d2c80x00024c28 array[] of 40,000 bytes

shmdt()shmdt() APUE (Interprocess Communication


void shmdt (void *addr); Returns: 0 if OK, -1 on error

Detach a shared memory segment

example: tshm.c (1/2)example: tshm.c (1/2) APUE (Interprocess Communication

#include <sys/types.h> // tshm.c#include <sys/ipc.h>#include <sys/shm.h>

#define ARRAY_SIZE 100000#define MALLOC_SIZE 100000#define SHM_SIZE 100000

err_sys(char *p) { perror(p); exit(-1); }

char array[ARRAY_SIZE]; /* uninitialized data = bss */

int main(void) { int shmid; char *ptr, *shmptr;

printf("array[] from %x to %x\n", &array[0], &array[ARRAY_SIZE]); printf("stack around %x\n", &shmid);

if ((ptr = malloc(MALLOC_SIZE)) == NULL) err_sys("malloc error"); printf("malloced from %x to %x\n", ptr, ptr+MALLOC_SIZE);

if ((shmid = shmget(0x01010101, SHM_SIZE, IPC_CREAT | 0666)) < 0) err_sys("shmget error");

if ((shmptr = shmat(shmid, 0, 0)) == (void *) -1) err_sys("shmat error"); printf("shared memory attached from %x to %x\n", shmptr, shmptr+SHM_SIZE);

// if (shmctl(shmid, IPC_RMID, 0) < 0) err_sys("shmctl error");

exit(0);}

APUE (Interprocess Communicationexample: tshm.c (2/2)example: tshm.c (2/2)

SemaphoresSemaphores

A counter to provide access to shared data object for multiple processes

To obtain a shared resource:• 1. Test semaphore that controls the resource• 2. If value > 0, value--, grant use • 3. If value == 0, sleep until value > 0 • 4. Release resource, value ++

Step 1, 2 must be an atomic operation


Semaphore StructureSemaphore Structure APUE (Interprocess Communication

Each semaphore has a structurestruct semid_ds { struct ipc_perm sem_perm; struct sem *sem_base; /*ptr to first semaphore in set */ ushort sem_nsems; /* # of semaphors in set */ time_t sem_otime; /* last-semop() time */ time_t sem_ctime; /* last-change time */ };

struct sem { ushort semval; /* semaphore value, always >= 0 */ pid_t sempid; /* pid for last operation */ ushort semncnt; /* # processes awaiting semval > currval */ ushort semzcnt; /* # processes awaiting semval = 0 */ };

We can get the structure using semctl() function.Actually, however, we don’t need to know the structure in detail.

semget()semget() APUE (Interprocess Communication

#include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h>

int semget(key_t key, int nsems, int flag); Returns: semaphore ID if OK, -1 on error

Obtain a semaphore IDnsems: sem_nsens (# of semaphores in set)flag: ipc_perm.mode

semctl()semctl() APUE (Interprocess Communication

#include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h>

int semctl(int semid, int semnum, int cmd, union semun arg);

union semun { int val; /* for SETVAL */ struct semid_ds *buf; /* for IPC_START and IPC_SET */ ushort *array; /* for GETALL and SETALL */};

To use semaphore, please refer to the textbook and manuals related semaphore.

ipcs, ipcrmipcs, ipcrm

ipcs: checking status of System V IPC• $ ipcs // check information of IPC (q, m, s)• $ ipcs –q ($ ipcs –qa) // check information of Message Queue • $ ipcs –m ($ ipcs –ma) // check information of Shared Memory• $ ipcs –s ($ ipcs –sa) // check information of Semaphore

ipcrm: delete a defined IPC• $ ipcrm –q id // Delete Message Queue• $ ipcrm –m id // Delete Shared Memory• $ ipcrm –s id // Delete Semaphore


Memory mapped file I/OMemory mapped file I/O APUE (Interprocess Communication

#include <sys/types.h> #include <sys/mman.h>

caddr_t mmap (caddr_t addr, size_t len, int prot, int flag, int filedes, off_t off); Returns: starting address of mapped region if OK, -1 on error

int munmap(caddr_t addr, size_t len);Returns : 0 if OK, -1 on error

Memory mapped I/O lets us map a file on disk into a buffer in memory so that when we fetch bytes from the buffer, the corresponding bytes of the file are read. Similarly, when we store data in the buffer, the corresponding bytes are automatically written to the file. This lets us perform I/O without using read or write.

To use this feature we have to tell the kernel to map a given file to a region in memory.

Acknowledgement : Prof. Y. Moon at Kangwon Nat’l Univ.

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Transcript of Acknowledgement : Prof. Y. Moon at Kangwon Nat’l Univ.