[Unix Programming] Interprocess Communication - PIPE
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[Unix Programming][Unix Programming]Interprocess Communication - Interprocess Communication -
PIPEPIPE
Young-Ju, HanYoung-Ju, Han
Email: Email: [email protected]
2007 UNIX Programming2007 UNIX Programming
ContentsContents
Pipes Using pipe a single process Using pipe a multiple processes Using pipe in the shell
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OverviewOverview
different forms of IPC
pipes(half duplex) FIFOs(named pipes) stream pipes(full duplex) named stream pipes message queues semaphores shared memory sockets streams
on the same host
on different host
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PipesPipes
only form of IPC on all UNIX implementations The oldest form of UNIX IPC most commonly used form of IPC
limitations half-duplex
one-way communication channel used only between processes
that have a common ancestor (related processes)
stream pipes
FIFOs & named stream pipes
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PipesPipes
Pipes connects the standard output of one program
to the standard input of another program
I/O Redirections and Pipelines
Stdout
stderrmyprog1
file2
stdinfile1
myprog2
stdoutstdin
stderr
$ (myprog1 | myprog2 ) < file1 > file2
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PipesPipes
Pipes at command level
(Example) I/O Redirection and Pipelines Method
1. I/O Redirection Method $ ls /home/kc > tempfile $ wc -l < tempfile 19 $ rm tempfile
2. Pipeline Method $ ls /home/kc | wc -l 19
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PipesPipes
(Example) cd /bin
1. I/O Redirection Method (Failure) $ ls > tempfile tempfile: permission denied
2. Pipeline Method ( Success) $ ls | wc -l 777
pipes at command level
Weak Point of I/O Redirection Method1. Cannot create temporary files if you don’t have permission2. May have a invalid result because of temporary file3. Easy to forget to remove temporary file
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pipespipes
%who | sort
pipewho sort
write pointer ofanother process
read pointer of one process
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PipesPipes
Create pipes pipe() system call
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 emptyautomatic blocking when full or empty FIFO basis, so lseek() don’t work
Types of pipes (unnamed) pipes named pipes
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PipesPipes
Programming with pipes create a pipe
#include <unistd.h>
int pipe (int fd[2]);
0 : ok-1 : errormore open than per-user process limit(EMFILE)kernel’s open file table overflow(ENFILE)
fd[0] : read only openfd[1] : write only open
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PipesPipes
ex) pipe on a single process#include <stdio.h>#include <unistd.h>#define MSGSIZE 1024char *msg1= “hello, world #1”;char *msg2= “hello, world #2”;char *msg3= “hello, world #3”;
int main(){
char inbuf[MSGSIZE];
int fd[2], j, ret;
if (pipe(fd) == -1) {perror(“pipe call”); exit(1);}
write(fd[1], msg1, strlen(msg1));
write(fd[1], msg2, strlen(msg2));
write(fd[1], msg3, strlen(msg3));
for (j = 0; j < 3; j++) {
ret = read(fd[0], inbuf, MSGSIZE);
inbuf[ret]=0;
printf(“%s\n”, inbuf);} }
$a.outhello, world #1hello, world #2hello, world #3
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PipesPipes
ex) pipe on a single process
fd[0] fd[1]
process
fd[0] fd[1]
process
kernel
pipe
read in the order in which they were written
first in/first out(FIFO) communication channel=> Since lseek don’t work on a pipe
or
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PipesPipes
kernel’s pipe buffer size constant PIPE_BUF rule
if a write overfill pipeoverfill pipe, then blockedblocked until reading
if a read when emptyempty, then blockedblocked until writing if a write > pipe, then block after write if a read > pipe, then return after read
pipe_size = fpathconf( p[0], _PC_PIPE_BUF );
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PipesPipes
rules (when one end of a pipe is closed) If we read from a pipe whose write end has been closed,
after all the data has been read, read return 0 to indicate an end of file
End of file is not generated until there are no more writers for the pipe
If we write to a pipe whose read end has been closed, the signal SIGPIPESIGPIPE is generated
Default action : terminate
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PipesPipes
ex) pipe from child to parentint main(){
char inbuf[MSGSIZE];
int fd[2], j;
pid_t pid;
if (pipe(fd) == -1) {perror(“pipe call”); exit(1);}
switch(pid = fork()) {
case -1 : perror(“fork error”); exit(1);
case 0 : write(fd[1], msg1, strlen(msg1));
write(fd[1], msg2, strlen(msg1));
write(fd[1], msg3, strlen(msg1)); break;
default : for (j = 0; j < 3; j++) {
read(fd[0], inbuf, MSGSIZE);
inbuf[ret] = 0;
printf(“%s\n”, inbuf);
}
wait(NULL);
} }
$a.outhello, world #1hello, world #2hello, world #3=> 무한 대기
무한대기
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PipesPipes
ex) pipe from child to parent
fd[0] fd[1]
parent
fd[0] fd[1]
child
kernel
pipe
fork
write()
read()
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PipesPipes
ex) pipe from child to parent (recommended)
switch(pid = fork()) {
case -1 : perror(“fork error”); exit(1);
case 0 : close(fd[0]);
write(fd[1], msg1, MSGSIZE);
write(fd[1], msg2, MSGSIZE);
write(fd[1], msg3, MSGSIZE); break;
default : close(fd[1]);
for (j = 0; j < 3; j++) {
read(fd[0], inbuf, MSGSIZE);
printf(“%s\n”, inbuf);
}
wait(NULL);
}
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PipesPipes
pipe from child to parent (recommended)
parent child
kernel
pipe
fork
fd[0] fd[1]
write()read()
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PipesPipes
Non-blocking reads and writes to use fstat
If st_size > 0 , and then read() from pipe st_size : number of characters currently in the
pipe problem : if several processes are reading the
pipe, another process could read from a pipe in the gap between fstat and read
to use fcntl ( recommend) O_NONBLOCK flag
errno -1 : EAGAIN
#include <fcntl.h>
if( fcntl(fd, F_SETFL, O_NONBLOCK) == -1 ) perror(“fcntl”);
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PipesPipes
Non-blocking read and write#include <fcntl.h>#include <errno.h>#define MSGSIZE 6int parent (int *);int child (int *);char *msg1 = "hello";char *msg2 = "bye!!";void fatal(char* msg) { fprintf(stderr,”%s\n”,msg);exit(1);}int main(){ int pfd[2]; /* 파이프를 개방한다 */ if(pipe (pfd) == -1) fatal ("pipe call");
/* p[0] 의 O_NONBLOCK 플래그를 1 로 설정한다 */ if (fcntl (pfd[0], F_SETFL, O_NONBLOCK) == -1) fatal ("fcntl call"); switch(fork()){ case -1: fatal("fork call"); /* 오류 */ case 0: child(pfd); /* 자식 */ default: parent (pfd); /* 부모 */ } }
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PipesPipes
Non-blocking read and writeint parent (int p[2]) { /* 부모의 코드 */ int nread; char buf[MSGSIZE]; close (p[1]); for(;;) { switch (nread = read(p[0], buf, MSGSIZE)){ case -1: /* 파이프에 아무것도 없는지 검사한다 . */ if (errno == EAGAIN){ printf ("(pipe empty)\n"); sleep (1); break; } else { perror("read call"); exit(1);} case 0: /* 파이프가 닫혔음 . */ printf ("End of conversation\n"); exit (0); default: printf ("MSG=%s\n", buf); } } }
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PipesPipes
Non-blocking read and write
int child(int p[2]) { int count;
close (p[0]);
for (count= 0; count < 3; count++) { write (p[1], msg1, MSGSIZE); sleep(3); }
/* 마지막 메시지를 보낸다 */ write (p[1], msg2, MSGSIZE); exit (0); }
$a.out(pipe empty)MSG=hello(pipe empty)(pipe empty)(pipe empty)MSG=hello(pipe empty)(pipe empty)(pipe empty)MSG=hello(pipe empty)(pipe empty)MSG=bye!!(pipe empty)End of conversation
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PipesPipes
Using select to handle multiple pipes Server & client Applications
parent
Child 1
pipe
fork
fd1[0]
fd1[1]
write()
read()
Child 2
fd2[0] fd3[0]
fd2[1] fd3[1]
pip
e pipe
Child 3
fork
fork
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PipesPipes
Using select to handle multiple pipes
nfds : specifies the range of file descriptors to be tested. 0 ~ nfds-1
readfds : the specified file descriptors is ready for reading writefds : FDs is ready for writing errorfds : FDs is ready for error checking Timeout
Timeout = NULL : block forever or until something of interest turns up
timeout.tv_sec = 0 && timeout.tv_usec = 0: returns immediately without blocking
Timeout.tv_sec != 0 || timeout.tv_usec != 0 : return after that number of tv_sec or tv_usec specified if there is no FDs activity
#include <sys/time.h>
int select (int nfds, fd_set *readfds, fd_set *writefds, fd_set* errorfds, struct timeval *timeout);
#include <sys/time.h>
struct timeval { long tv_sec; /*second */ long tv_usec; /*microseconds */ }
0 : timeout, -1 : error>0 : the number of “interesting” FDs
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PipesPipes
Macro for manipulating sets of file descriptor (bit mask)
#include <sys/time.h>
/* initialize the mask pointed to by fdset */void FD_ZERO(fd_set *fdset);/* set the bit, fd, in the mask pointed to by fdset */void FD_SET(int fd, fd_set* fdset);/* is the bit, fd, set in the mask pointed to by fdset */void FD_ISSET(int fd, fd_set* fdset);/* turn off the bit, fd, in the mask pointed to by fdset */void FD_CLR(int fd, fd_set *fdset);
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PipesPipes
Using select to handle multiple regular files
#include <sys/time.h>#include <sys/types.h>#include <fcntl.h>...
int fd1, fd2;fd_set readset;
fd1 = open(“file1”, O_RDONLY);fd2 = open(“file2”, O_RDONLY);
FD_ZERO(&readset);FD_SET(fd1, &readset);FD_SET(fd2, &readset);
switch(select(5,&readset,NULL,NULL,NULL)){ /* logic */}
select(fd2+1,&readset,NULL,NULL,NULL)
Not recommended
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pipespipes
Examples (Using select to handle multiple pipes)
#include <sys/time.h>#include <sys/wait.h>#include <stdio.h>#include <unistd.h>#define MSGSIZE 6char *msg1 = "hello"; char *msg2 = "bye!!";void parent(int a[][]); int child(int b[]);void fatal(char* msg){ fprintf(stderr,"%s\n",msg);exit(1);}int main(){ int pip[3][2]; int i; for (i = 0; i < 3; i++) { if (pipe(pip[i]) == -1) fatal("pipe call"); switch (fork()){ case -1: /* 오류 */ fatal("fork call"); case 0: /* 자식 */ child(pip[i]); } } parent(pip); /* 부모 */ exit(0); }
2007 UNIX Programming2007 UNIX Programming
pipespipes
Examples (parent process)void parent(int p[3][2]){ char buf[MSGSIZE], ch; fd_set set, master; int i; for (i = 0; i < 3; i++) close(p[i][1]); FD_ZERO(&master); FD_SET(0, &master); for (i = 0; i <3; i++) FD_SET(p[i][0], &master); while(set=master, select (p[2][0]+1, &set, NULL, NULL, NULL) > 0){ if(FD_ISSET(0, &set)){ /* standard input check */ printf ("From standard input..."); read (0, &ch, 1); printf("%c\n", ch); } for (i = 0; i < 3; i++) { if(FD_ISSET(p[i][0], &set)) { if(read(p[i][0], buf, MSGSIZE) > 0) { printf ("Message from child%d\n", i); printf ("MSG=%s\n",buf); } } }
if(waitpid (-1, NULL,WNOHANG) == -1) return; }}
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pipespipes
Examples (child process)
int child(int p[2]){ int count; close(p[0]);
for (count = 0; count < 2; count++) { write (p[1], msg1, MSGSIZE); sleep (getpid()%4); } write (p[1], msg2, MSGSIZE); exit (0);}
$a.outMessage from child0MSG=helloMessage from child1MSG=helloMessage from child2MSG=helloMessage from child2MSG=helloMessage from child2MSG=bye!!dFrom standard input...dFrom standard input...Message from child0MSG=hello...
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Sell pipe implementationSell pipe implementation
Pipes and the exec system call
%ls –l | wc
Shell/bin/sh
parent
childls
childwc
pipe
fd[1] fd[0]
/* stdin 에서 command read *//* command parsing *//* 2 개의 command 이고 둘이 pipe 로 연결되어 있다면 */int fd[2], c1;pipe(fd);c1 = fork ();If(!c1) { /*first child*/
close( fd[0] );close(1); dup2( fd[1],1 ); close( fd[1]);execlp(/*ls –l first command */);
}c2= fork ();If(!c2) { /*second child*/
close( fd[1] );close(0); dup2( fd[0],0 ); close( fd[0]);execlp(/*wc second command */);
} close( fd[0] ); close( fd[1] );
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Sell pipe implementationSell pipe implementation
dup(), dup2()
사용중인 파일 디스크립터의 복사본을 만듦 dup() 는 새 파일 디스크립터가 할당 dup2() 는 fd2 를 사용
리턴 값 성공하면 복사된 새 파일 디스크립터 , 실패하면 -1 dup() 는 할당 가능한 가장 작은 번호를 리턴 dup2() 는 fd2 를 리턴 dup2() 는 복사본을 만들기 전에 기존의 fd2 를 close
함
#include <unistd.h>
int dup(int fd);int dup2( int fd, int fd2);
2007 UNIX Programming2007 UNIX Programming
Sell pipe implementationSell pipe implementation
Another method: The Join program
parent
Child 2
(com1)
wait()
write()
(stdout)
read()
(stdin)pipe
Child 1
(com2)
P[1] P[0]
2007 UNIX Programming2007 UNIX Programming
Sell pipe implementationSell pipe implementation
Pseudo-code form of the Join program
Process forks, parent waits for childAnd child continues
Child create a pipe
Child then forks
In child created by second fork (child 2):standard output is coupled towrite end of pipe using dup2excess file descriptors are closed
program described by ‘com1’ is exec’ed
In child of first fork (child 1):standard input is coupled toread end of pipe using dup2
excess file descriptors are closed
program described by ‘com2’ is exec’ed
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Sell pipe implementationSell pipe implementation
The Join program
int join(char* com1[], char* com2[]){int p[2], status;
/* 명령을 수행할 자식을 생성한다 . */ switch (fork()){ case -1: /* 오류 */ fatal ("1st fork call in join"); case 0: /* 자식 */ break; default: /* 부모 */ wait(&status); return (status); }
/* 루틴의 나머지 부분으로 자식에 의해 수행된다 . */ /* 파이프를 만든다 . */ if (pipe(p) == -1)
fatal ("pipe call in join");
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Sell pipe implementationSell pipe implementation
The Join program
switch (fork()){/* 다른 프로세스를 생성한다 . */
case -1: /* 오류 */ fatal ("2nd fork call in join"); case 0: /* 쓰는 프로세스 */ dup2 (p[1],1); /* 표준 출력이 파이프로 가게 한다 . */
close (p[0]); /* 화일 기술자를 절약한다 . */ close (p[1]); execvp (com1[0], com1); /* execvp 가 복귀하면 , 오류가 발생한 것임 . */ fatal("1st execvp call in join");default: /* 읽는 프로세스 */ dup2(p[0], 0); /* 표준 입력이 파이프로부터 오게 한다 */ close (p[0]); close (p[1]); execvp (com2[0], com2); fatal ("2nd execvp call in join"); }}
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Sell pipe implementationSell pipe implementation
The Join program
앞서 join routine 은 다음 프로그램을 이용하여 호출될 수 있다 :
#include <stdio.h>
main(){ char *one[4] = {"ls", "-l", "/usr/lib", NULL}; char *two[3] = {"grep", " d", NULL};∧ int ret;
ret = join (one, two); printf ("join returned %d\n", ret); exit (0);}