16. apue(interprocess communication)ysmoon/courses/2006_1/us/16.pdf · 2016-06-02 · 10 page 19...
TRANSCRIPT
1
컴퓨터컴퓨터 특강특강 (UNIX System Programming)(UNIX System Programming)
APUE(InterprocessAPUE(Interprocess Communication) Communication) [Ch. 14][Ch. 14]
20062006년년 봄학기봄학기
문양세문양세
강원대학교강원대학교 컴퓨터과학과컴퓨터과학과
Page 2UNIX System Programmingby Yang-Sae Moon
ContentsContents
Pipes
FIFOs
System V IPC
• Message Queues
• Shared Memory
• Semaphores
APUE (Interprocess Communication
2
Page 3UNIX System Programmingby Yang-Sae Moon
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()
APUE (Interprocess Communication
Page 4UNIX System Programmingby Yang-Sae Moon
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
APUE (Interprocess Communication
3
Page 5UNIX System Programmingby Yang-Sae Moon
Example (1/2)Example (1/2)
$ who | sort
APUE (Interprocess Communication
pipewho sort
write pointer ofanother process
read pointer of one process
Page 6UNIX System Programmingby Yang-Sae Moon
Example (2/2)Example (2/2)APUE (Interprocess Communication
4
Page 7UNIX System Programmingby Yang-Sae Moon
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)
APUE (Interprocess Communication
Page 8UNIX System Programmingby Yang-Sae Moon
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
APUE (Interprocess Communication
5
Page 9UNIX System Programmingby Yang-Sae Moon
Pipes (2/4)Pipes (2/4)
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.
APUE (Interprocess Communication
#include <unistd.h>
int pipe(int fd[2])Returns: 0 if OK, -1 on error
Page 10UNIX System Programmingby Yang-Sae Moon
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
6
Page 11UNIX System Programmingby Yang-Sae Moon
Pipes (4/4)Pipes (4/4)
Read from a pipe with write end closed: (fd[1]이 close된 경우)
• returns 0 to indicate EOF
Write to a pipe with read end closed: (fd[0]가 close된 경우)
• SIGPIPE generated,
• write() returns error (errno == EPIPE)
APUE (Interprocess Communication
Page 12UNIX System Programmingby Yang-Sae Moon
예제예제: : pipe.cpipe.c (1/2)(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, parent] write completed.\n", getpid());close(fd[WRITE]);
}else { // parent
close(fd[WRITE]);bytesRead = read(fd[READ], message, 100);fprintf(stdout, "[%d, child] read completed.\n", getpid());printf("Read %d bytes: %s\n", bytesRead,message);close(fd[READ]);
}}
APUE (Interprocess Communication
7
Page 13UNIX System Programmingby Yang-Sae Moon
예제예제: : pipe.cpipe.c (2/2)(2/2)APUE (Interprocess Communication
실행 결과
Page 14UNIX System Programmingby Yang-Sae Moon
ContentsContents
Pipes
FIFOs
System V IPC
• Message Queues
• Shared Memory
• Semaphores
APUE (Interprocess Communication
8
Page 15UNIX System Programmingby Yang-Sae Moon
FIFOsFIFOs (1/3)(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
APUE (Interprocess Communication
Page 16UNIX System Programmingby Yang-Sae Moon
FIFOsFIFOs (2/3)(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
9
Page 17UNIX System Programmingby Yang-Sae Moon
FIFOsFIFOs (3/3)(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.
APUE (Interprocess Communication
Page 18UNIX System Programmingby Yang-Sae Moon
Uses of Uses of FIFOsFIFOs
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.
APUE (Interprocess Communication
10
Page 19UNIX System Programmingby Yang-Sae Moon
Using Using FIFOsFIFOs to Duplicate Output Streamsto Duplicate Output StreamsAPUE (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
Page 20UNIX System Programmingby Yang-Sae Moon
An Example using a FIFOAn Example using a FIFOAPUE (Interprocess Communication
11
Page 21UNIX System Programmingby Yang-Sae Moon
ClientClient--Server Communication Using a FIFOServer Communication Using a FIFO
Server creates a “well-known” FIFO to communicate with clients.
APUE (Interprocess Communication
Problem: Server can't reply clients using a single “well-known” FIFO
client
well-knownFIFO
readrequest
client
server
write request
write request
.
.
.
Page 22UNIX System Programmingby Yang-Sae Moon
ContentsContents
Pipes
FIFOs
System V IPC
• Message Queues
• Shared Memory
• Semaphores
APUE (Interprocess Communication
12
Page 23UNIX System Programmingby Yang-Sae Moon
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.
APUE (Interprocess Communication
Page 24UNIX System Programmingby Yang-Sae Moon
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!
APUE (Interprocess Communication
13
Page 25UNIX System Programmingby Yang-Sae Moon
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)
APUE (Interprocess Communication
Page 26UNIX System Programmingby Yang-Sae Moon
Permission StructurePermission StructureAPUE (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 */
};
14
Page 27UNIX System Programmingby Yang-Sae Moon
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 exiting queue.
msgsnd
• Add a new message to a queue
msgrcv
• Receive a message from a queue
• Fetching order: based on type
APUE (Interprocess Communication
Page 28UNIX System Programmingby Yang-Sae Moon
Message Queues (2/2)Message Queues (2/2)
Each queue has a structure
APUE (Interprocess Communication
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.
15
Page 29UNIX System Programmingby Yang-Sae Moon
System Limits on QueuesSystem Limits on Queues
Each queue has a structure
APUE (Interprocess Communication
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.
Page 30UNIX System Programmingby Yang-Sae Moon
msggetmsgget()()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 queue
flag : ipc_perm.mode
Examplemsg_qid = msgget(DEFINED_KEY, IPC_CREAT | 0666);
16
Page 31UNIX System Programmingby Yang-Sae Moon
msgctlmsgctl()()APUE (Interprocess Communication
#include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h>
int msgctl(int msqid, int cmd, struct msqid_ds *buf); Returns: 0 if OK, -1 on error
Performs various operations on a queue
cmd = IPC_STAT:fetch the msqid_ds structure for this queue, storing it in buf
cmd = IPC_SET:set the following four fields from buf: msg_perm.uid, msg_perm.gid, msg_perm.mode, and msg_qbytes
cmd = IPC_RMID:remove the message queue.
Page 32UNIX System Programmingby Yang-Sae Moon
msgsndmsgsnd()()APUE (Interprocess Communication
#include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h>
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 */
};
17
Page 33UNIX System Programmingby Yang-Sae Moon
msgrcvmsgrcv()()APUE (Interprocess Communication
#include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h>
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 returned
type > 0: the first message on the queue whose message type equals type is returned
type < 0: the first message on the queue whose message type is the lowest value less than or equal to the absolute value of type isreturned
flag may be given by IPC_NOWAIT
Page 34UNIX System Programmingby Yang-Sae Moon
예제예제: : sender.csender.c receiver.creceiver.c (1/4)(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);}
}}
18
Page 35UNIX System Programmingby Yang-Sae Moon
예제예제: : sender.csender.c receiver.creceiver.c (2/4)(2/4)APUE (Interprocess Communication
#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);
}}
Page 36UNIX System Programmingby Yang-Sae Moon
예제예제: : sender.csender.c receiver.creceiver.c (3/4)(3/4)APUE (Interprocess Communication
실행 결과
19
Page 37UNIX System Programmingby Yang-Sae Moon
예제예제: : sender.csender.c receiver.creceiver.c (4/4)(4/4)APUE (Interprocess Communication
Message Queue 확인
Page 38UNIX System Programmingby Yang-Sae Moon
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
APUE (Interprocess Communication
20
Page 39UNIX System Programmingby Yang-Sae Moon
Shared Memory Segment StructureShared Memory Segment Structure
Each shared memory has a structure
APUE (Interprocess Communication
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.
Page 40UNIX System Programmingby Yang-Sae Moon
shmgetshmget()()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 identifier
size: is the size of the shared memory segment
flag: ipc_perm.mode
Example
shmId = shmget(key, size, PERM|IPC_CREAT|IPC_EXCL|0666);
21
Page 41UNIX System Programmingby Yang-Sae Moon
shmctlshmctl()()APUE (Interprocess Communication
#include <sys/types.h> #include <sys/ipc.h> #include <sys/shm.h>
int shmctl(int shmid, int cmd, struct shmid_ds *buf);Returns: 0 if OK, -1 on error
Performs various shared memory operations
cmd = IPC_STAT:fetch the shmid_ds structure into buf
cmd = IPC_SET:set the following three fields from buf: shm_perm.uid, shm_perm.gid, and shm_perm.mode
cmd = IPC_RMID:remove the shared memory segment set from the system
Page 42UNIX System Programmingby Yang-Sae Moon
shmatshmat()()APUE (Interprocess Communication
#include <sys/types.h> #include <sys/ipc.h> #include <sys/shm.h>
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
22
Page 43UNIX System Programmingby Yang-Sae Moon
shared memory
Memory LayoutMemory LayoutAPUE (Interprocess Communication
uninitialized data(bss)
stack
heap
initialized data
text
high address
low address
command-line argumentsand environment variables0xf7fffb2c
0xf77e86a00xf77d0000
shared memory of 100,000 bytes
0x0003d2c80x00024c28
malloc of 100,000 bytes
0x0003d2c80x00024c28 array[] of 40,000 bytes
Page 44UNIX System Programmingby Yang-Sae Moon
shmdtshmdt()()APUE (Interprocess Communication
#include <sys/types.h> #include <sys/ipc.h> #include <sys/shm.h>
void shmdt (void *addr); Returns: 0 if OK, -1 on error
Detach a shared memory segment
23
Page 45UNIX System Programmingby Yang-Sae Moon
예제예제: : tshm.ctshm.c (1/2)(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);}
Page 46UNIX System Programmingby Yang-Sae Moon
APUE (Interprocess Communication
실행 결과
예제예제: : tshm.ctshm.c (2/2)(2/2)
24
Page 47UNIX System Programmingby Yang-Sae Moon
SemaphoresSemaphores
A counter to provide access to shared data object for multiple processes (복수의 프로세스가 데이터를 공유하는데 사용하는 카운터)
To obtain a shared resource:
• 1. Test semaphore that controls the resource (확인하여)
• 2. If value > 0, value--, grant use (양수이면, 감소시키고 사용하고)
• 3. If value == 0, sleep until value > 0 (0이면 기다림)
• 4. Release resource, value ++ (다 쓴 후에는 다시 양수로 만듦)
Step 1, 2 must be an atomic operation
APUE (Interprocess Communication
Page 48UNIX System Programmingby Yang-Sae Moon
Semaphore StructureSemaphore StructureAPUE (Interprocess Communication
Each semaphore has a structure
struct 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.
25
Page 49UNIX System Programmingby Yang-Sae Moon
semgetsemget()()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 ID
nsems: sem_nsens (# of semaphores in set)
flag: ipc_perm.mode
Page 50UNIX System Programmingby Yang-Sae Moon
semctlsemctl()()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.
26
Page 51UNIX System Programmingby Yang-Sae Moon
ipcsipcs, , ipcrmipcrm
ipcs:System V IPC의 상태를 확인하는 명령어
• $ ipcs // IPC 정보를 확인 (q, m, s 모두)
• $ ipcs –q ($ ipcs –qa) // Message Queue 정보를 확인
• $ ipcs –m ($ ipcs –ma) // Shared Memory 정보를 확인
• $ ipcs –s ($ ipcs –sa) // Semaphore 정보를 확인
ipcrm: 정의된(생성된)IPC를 삭제함
• $ ipcrm –q id // Message Queue를 삭제
• $ ipcrm –m id // Shared Memory를 삭제
• $ ipcrm –s id // Semaphore를 삭제
APUE (Interprocess Communication