now0930 일지

[태그:] ipc

  • message queue, unix network programming 75p ~ 90p

    fifo는 먼저 들어온 순서로 동작하지만, message queue는 빠른 우선순위로 동작한다.

    signal handler를 정의하여 사용할 수도 있다. message noty는 queue가 비어있을 경우 한번 동작한다. queue가 차 있다면 signal을 무시한다. 한 번 사용하면 다시 시그널을 등록해야 한다.

    pi@raspberrypi:~/Project/cCode/IPC $ cat mesquereceive.c 
//message que header
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <mqueue.h>
#include <signal.h>
#define MAX_SIZE 100

//signal 처리하기 위한 hanlder
static void sig_usr1(int mysig);

struct sigevent sigev;
//signal hanlder로 사용하려고 global로 등록.
mqd_t	mqd;
struct mq_attr attr;
int priorecv;
char buff[100];

int main(){
	int		c, flags;
	char const *quename="/tmp_message";
	char messageptr[50]= "this is a test.";
	//메세지 큐 초기화
	attr.mq_flags = 0;
    attr.mq_maxmsg = 10;
    attr.mq_msgsize = MAX_SIZE;
    attr.mq_curmsgs = 0;


	//message를 넣기전에 mq_notify 정의
	signal(SIGUSR1, sig_usr1);
	sigev.sigev_notify = SIGEV_SIGNAL;
	sigev.sigev_signo = SIGUSR1;
	mq_notify(mqd, &sigev);

	flags = O_RDWR | O_CREAT;
	//gcc 컴파일시 gcc mesque.c -lrt
	//로 해야 컴파일 됨.
	mqd = mq_open(quename, flags, 0666, &attr);
	size_t len = sizeof(messageptr);
	//mq_send(mqd, messageptr, len, 10);
	//sleep(1);


	//넣은 메세지를 확인
	//mq_receive(mqd, buff, attr.mq_msgsize, &priorecv);
	//printf("받은 메세지는\n %s를 %d 우선 순위로\n", buff, priorecv);
	//mq_receive(mqd, buff, attr.mq_msgsize, &priorecv);
	//printf("받은 메세지는\n %s를 %d 우선 순위로\n", buff, priorecv);
	for(;;)
		pause();
	mq_close(mqd);
	return 0;
}

void sig_usr1(int mysig){
	//signal hanlder로 message receive..

	//한번 사용한 noty는 다시 등록.
	mq_notify(mqd, &sigev);
	mq_receive(mqd, buff, attr.mq_msgsize, &priorecv);
	printf("받은 메세지는\n %s를 %d 우선 순위로\n", buff, priorecv);
	return;



}

    pi@raspberrypi:~/Project/cCode/IPC $ cat mesquesender.c 
//message que header
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <mqueue.h>
#include <stdlib.h>
#include <signal.h>
#define MAX_SIZE 100


mqd_t	mqd;
struct mq_attr attr;
int priorecv;
char buff[100];

int main(int argc, char** argv){
	int		c, flags;
	int prio;
	char const *quename="/tmp_message";
	char messageptr[50]= "this is a test.";
	//메세지 큐 초기화
	attr.mq_flags = 0;
    attr.mq_maxmsg = 10;
    attr.mq_msgsize = MAX_SIZE;
    attr.mq_curmsgs = 0;

	flags = O_RDWR | O_CREAT;
	//gcc 컴파일시 gcc mesque.c -lrt
	//로 해야 컴파일 됨.
	mqd = mq_open(quename, flags, 0666, &attr);
	size_t len = sizeof(messageptr);

	prio = atoi(argv[1]);
	mq_send(mqd, messageptr, len, prio);
	mq_close(mqd);
	return 0;
}

    pi@raspberrypi:~/Project/cCode/IPC $ mv mesque.c mesquereceive.c
pi@raspberrypi:~/Project/cCode/IPC $ gcc -lrt mesquereceive.c -o recv
pi@raspberrypi:~/Project/cCode/IPC $ gcc -lrt mesquesender.c -o send
pi@raspberrypi:~/Project/cCode/IPC $ cat /dev/mqueue/tmp_message 
QSIZE:0          NOTIFY:0     SIGNO:0     NOTIFY_PID:0     
pi@raspberrypi:~/Project/cCode/IPC $ ./send 
세그멘테이션 오류
pi@raspberrypi:~/Project/cCode/IPC $ ./send 10
pi@raspberrypi:~/Project/cCode/IPC $ ./send 20
pi@raspberrypi:~/Project/cCode/IPC $ cat /dev/mqueue/tmp_message 
QSIZE:100        NOTIFY:0     SIGNO:0     NOTIFY_PID:0     
pi@raspberrypi:~/Project/cCode/IPC $ ./send 100
pi@raspberrypi:~/Project/cCode/IPC $ ./recv 
^Z
[1]+  Stopped                 ./recv
pi@raspberrypi:~/Project/cCode/IPC $ bg
[1]+ ./recv &
pi@raspberrypi:~/Project/cCode/IPC $ cat /dev/mqueue/tmp_message 
QSIZE:150        NOTIFY:0     SIGNO:0     NOTIFY_PID:0     
pi@raspberrypi:~/Project/cCode/IPC $ ./send 15
pi@raspberrypi:~/Project/cCode/IPC $ bg
-bash: bg: job 1 already in background
pi@raspberrypi:~/Project/cCode/IPC $ jobs
[1]+  Running                 ./recv &
pi@raspberrypi:~/Project/cCode/IPC $ ./send 20
pi@raspberrypi:~/Project/cCode/IPC $ pidof recv 
16120
pi@raspberrypi:~/Project/cCode/IPC $ pidof recv |xargs kill -10
받은 메세지는
 this is a test.를 100 우선 순위로
pi@raspberrypi:~/Project/cCode/IPC $ pidof recv |xargs kill -10
받은 메세지는
 this is a test.를 20 우선 순위로
pi@raspberrypi:~/Project/cCode/IPC $ pidof recv |xargs kill -10
받은 메세지는
 this is a test.를 20 우선 순위로
pi@raspberrypi:~/Project/cCode/IPC $ pidof recv |xargs kill -10
받은 메세지는
 this is a test.를 15 우선 순위로
pi@raspberrypi:~/Project/cCode/IPC $ pidof recv |xargs kill -10
받은 메세지는
 this is a test.를 10 우선 순위로
pi@raspberrypi:~/Project/cCode/IPC $ pidof recv |xargs kill -10
pi@raspberrypi:~/Project/cCode/IPC $ ./send 100
받은 메세지는
 this is a test.를 100 우선 순위로
pi@raspberrypi:~/Project/cCode/IPC $ cat /dev/mqueue/tmp_message 
QSIZE:0          NOTIFY:0     SIGNO:10    NOTIFY_PID:16120 
pi@raspberrypi:~/Project/cCode/IPC $ ./send 10
받은 메세지는
 this is a test.를 10 우선 순위로
pi@raspberrypi:~/Project/cCode/IPC $ cat /dev/mqueue/tmp_message 
QSIZE:0          NOTIFY:0     SIGNO:10    NOTIFY_PID:16120 
pi@raspberrypi:~/Project/cCode/IPC $
    https://www.joinc.co.kr/w/Site/system_programing/IPC/MessageQueue

    gcc 기본 컴파일하면 에러를 본다. 옵션 넣어야 한다. -lrt.

    https://stackoverflow.com/questions/19964206/weird-posix-message-queue-linking-issue-sometimes-it-doesnt-link-correctly

  • fifo 예제. unix network programming 55p

    pipe는 이름을 없어 parent, child process만 통신이 가능하다. 이를 보안한게 named pipe인데, fifo라고도 한다. 책은 다른 유닉스 버전으로 작성하여, linux에 맞게 수정했다. 결론적으로 pipe와 비슷하고 더 쉽다.

    #include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <unistd.h>
#include <sys/types.h>
#include <fcntl.h>
#include <sys/wait.h>
#include <string.h>
#include <sys/stat.h>
#include <unistd.h>


#define	FIFO1	"/tmp/fifo.1"
#define	FIFO2	"/tmp/fifo.2"
#define MAXLINE 100

void	client(int, int), server(int, int);

void server(int readfd, int writefd){
	/*
	char	buff[MAXLINE];
	int n;
	n = read(readfd, buff, MAXLINE);
	printf("서버가 읽은 data는 %s\n",buff);
	// 샘플 테스트용..

	*/
	int fd;
	ssize_t n;
	char	buff[MAXLINE+1];

	/* 4read pathname from IPC channel */
	if ( (n = read(readfd, buff, MAXLINE)) == 0)
		perror("end-of-file while reading pathname");
	buff[n] = '\0';		/* null terminate pathname */

	if ( (fd = open(buff, O_RDONLY)) < 0) {
			/* 4error: must tell client */
		snprintf(buff + n, sizeof(buff) - n, ": can't open, %s\n",
				 strerror(errno));
		n = strlen(buff);
		write(writefd, buff, n);

	} else {
			/* 4open succeeded: copy file to IPC channel */
		while ( (n = read(fd, buff, MAXLINE)) > 0)
			write(writefd, buff, n);
		close(fd);
	}

}

void client(int readfd, int writefd)
{
	size_t	len;
	ssize_t	n;
	char	buff[MAXLINE];

	/* 4read pathname */
	fgets(buff, MAXLINE, stdin);
	len = strlen(buff);		/* fgets() guarantees null byte at end */
	if (buff[len-1] == '\n')
		len--;				/* delete newline from fgets() */

	/* 4write pathname to IPC channel */
	write(writefd, buff, len);

	/* 4read from IPC, write to standard output */
	while ( (n = read(readfd, buff, MAXLINE)) > 0)
		write(STDOUT_FILENO, buff, n);
}


int
main(int argc, char **argv)
{
	int		readfd, writefd;
	pid_t	childpid;

		/* 4create two FIFOs; OK if they already exist */
	if ((mkfifo(FIFO1, 0666) < 0) && (errno != EEXIST))
		printf("can't create %s", FIFO1);
	if ((mkfifo(FIFO2, 0666) < 0) && (errno != EEXIST)) {
		unlink(FIFO1);
		printf("can't create %s", FIFO2);
	}

	if ( (childpid = fork()) == 0) {		/* child */
		readfd = open(FIFO1, O_RDONLY, 0);
		writefd = open(FIFO2, O_WRONLY, 0);

		server(readfd, writefd);
		exit(0);
	}
		/* 4parent */
	writefd = open(FIFO1, O_WRONLY, 0);
	readfd = open(FIFO2, O_RDONLY, 0);

	client(readfd, writefd);

	waitpid(childpid, NULL, 0);		/* wait for child to terminate */

	close(readfd);
	close(writefd);

	unlink(FIFO1);
	unlink(FIFO2);
	exit(0);
}

    pi@raspberrypi:~/Project/cCode/IPC $ ls
a.out      fifo.c  simplepipe.c  unpv22e
fduplex.c  pipe.c  test.txt      unpv22e.tar
pi@raspberrypi:~/Project/cCode/IPC $ cat test.txt 
안녕하세요.
hello.
pi@raspberrypi:~/Project/cCode/IPC $ ./a.out 
./test.txt
안녕하세요.
hello.
pi@raspberrypi:~/Project/cCode/IPC $

    이 장에 여러 내용도 많은데 일단 다른 유닉스 버전도 보이고 시간도 없어 다음 장으로 넘어간다.

    https://www.geeksforgeeks.org/named-pipe-fifo-example-c-program/

  • pipe 예제. unix network programming 47p

    pipe를 이해하기 전, 일단 gdb를 사용하는 방법을 알아야 했다. pipe가 서로 다른 프로세스를 연결하는 수단이라, gdb 기본 설정으로는 pipe로 어떤 내용을 확인하기 어려웠다. 아래 코드를 디버그 하기로 했다.

    #include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <string.h>
#define MAXLINE 100

void client(int, int), server(int, int);

void server(int readfd, int writefd){
	char	buff[MAXLINE];
	int n;
	n = read(readfd, buff, MAXLINE);
	printf("서버가 읽은 data는 %s\n",buff);
}

void client(int readfd, int writefd)
{
	size_t	len;
	ssize_t	n;
	char	buff[MAXLINE];

	/* 4read pathname */
	fgets(buff, MAXLINE, stdin);
	len = strlen(buff);		/* fgets() guarantees null byte at end */
	if (buff[len-1] == '\n')
		len--;				/* delete newline from fgets() */

	/* 4write pathname to IPC channel */
	write(writefd, buff, len);

	/* 4read from IPC, write to standard output */
	while ( (n = read(readfd, buff, MAXLINE)) > 0)
		write(STDOUT_FILENO, buff, n);
}


int main(int argc, char **argv)
{
	int		pipe1[2], pipe2[2];
	pid_t	childpid, childpid2;
	pid_t zombieFlag;

	if(	pipe(pipe1) < 0)
		exit(1);
	if (pipe(pipe2) < 0)
		exit(1);


	printf("made pipe1, pipe2\n");
	//child process pipe 닫음.
	if( (childpid = fork()) == 0) {		//난 자식..
		close(pipe1[1]);
		close(pipe2[0]);
		childpid2 = getpid();

		printf("자식 process는 %d\n",childpid2);
		server(pipe1[0], pipe2[1]);

		//    pipe1 ----> server, server process 입력은 pipe1 출력.
		//    pipe2 <---- server, server process 출력은 pipi2 입력.
		exit(0);
	}
	// 난 부모.
	close(pipe1[0]);
	close(pipe2[1]);
	client(pipe2[0], pipe1[1]);

	zombieFlag = waitpid(childpid, NULL, 0);
	printf("종료된 자식 process는 %d\n",zombieFlag);
	//sleep(1);

	exit(0);
}

    이 내용을 gdb로 확인했다. gdb를 그냥 실행하면 detach-on-fork 옵션을 on으로 갖는다. child process로 분기하면 기존 프로세스를 끝까지 실행하는 옵션인 듯 하다. parent 프로세스를 정지시키고, inferior로 child 프로세스로 이동하여 나머지를 실행하고 싶었다. 그러나 detach-on-fork off 에서는 프로세스가 끝나 inferior로 이동해도 실행할 수 없다. detatch-on-fork off로 기존 프로세스를 멈추고 inferior로 다른 프로세스로 옮겨 확인했다.

    pi@raspberrypi:~/Project/cCode/IPC $ gdb ./a.out 
GNU gdb (Raspbian 8.2.1-2) 8.2.1
Copyright (C) 2018 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "arm-linux-gnueabihf".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from ./a.out...done.
(gdb) show detach-on-fork 
Whether gdb will detach the child of a fork is on.
(gdb) set detach-on-fork off
(gdb) show follow-fork-mode 
Debugger response to a program call of fork or vfork is "parent".
(gdb) l
31		write(writefd, buff, len);
32	
33		/* 4read from IPC, write to standard output */
34		while ( (n = read(readfd, buff, MAXLINE)) > 0)
35			write(STDOUT_FILENO, buff, n);
36	}
37	
38	
39	int main(int argc, char **argv)
40	{
(gdb) l 2
1	#include <stdlib.h>
2	#include <stdio.h>
3	#include <unistd.h>
4	#include <sys/types.h>
5	#include <sys/wait.h>
6	#include <string.h>
7	#define MAXLINE 100
8	
9	void client(int, int), server(int, int);
10	
(gdb) l
11	void server(int readfd, int writefd){
12		char	buff[MAXLINE];
13		int n;
14		n = read(readfd, buff, MAXLINE);
15		printf("서버가 읽은 data는 %s\n",buff);
16	}
17	
18	void client(int readfd, int writefd)
19	{
20		size_t	len;
(gdb) b 14
Breakpoint 1 at 0x10658: file pipe.c, line 14.
(gdb) l
21		ssize_t	n;
22		char	buff[MAXLINE];
23	
24		/* 4read pathname */
25		fgets(buff, MAXLINE, stdin);
26		len = strlen(buff);		/* fgets() guarantees null byte at end */
27		if (buff[len-1] == '\n')
28			len--;				/* delete newline from fgets() */
29	
30		/* 4write pathname to IPC channel */
(gdb) b 25
Breakpoint 2 at 0x106a4: file pipe.c, line 25.
(gdb) r
Starting program: /home/pi/Project/cCode/IPC/a.out 
made pipe1, pipe2
[New inferior 2 (process 2918)]
Reading symbols from /home/pi/Project/cCode/IPC/a.out...done.
Reading symbols from /usr/lib/debug/.build-id/ef/dd27c16f5283e5c53dcbd1bbc3ef136e312d1b.debug...done.
Reading symbols from /usr/lib/debug/.build-id/fb/85e699c11db06c7b24f74de2cdada3146442a8.debug...done.

Thread 1.1 "a.out" hit Breakpoint 2, client (readfd=5, writefd=4)
    at pipe.c:25
25		fgets(buff, MAXLINE, stdin);
(gdb) n
this is a test.
26		len = strlen(buff);		/* fgets() guarantees null byte at end */
(gdb) display buff
1: buff = "this is a test.\n\000\000\000\000\001\000\000\000\000\360\377\266$\364\377\276T\332\375\266 \230\377\266\001\000\000\000\001\000\000\000\000\000\000\000\370\234\377\266\354j\346\266f\v\000\000\000\000\000\000p\b\001\000T\005\001", '\000' <repeats 13 times>, "D;\376\266\003\000\000\000\000\000\000"
(gdb) n
27		if (buff[len-1] == '\n')
1: buff = "this is a test.\n\000\000\000\000\001\000\000\000\000\360\377\266$\364\377\276T\332\375\266 \230\377\266\001\000\000\000\001\000\000\000\000\000\000\000\370\234\377\266\354j\346\266f\v\000\000\000\000\000\000p\b\001\000T\005\001", '\000' <repeats 13 times>, "D;\376\266\003\000\000\000\000\000\000"
(gdb) n
28			len--;				/* delete newline from fgets() */
1: buff = "this is a test.\n\000\000\000\000\001\000\000\000\000\360\377\266$\364\377\276T\332\375\266 \230\377\266\001\000\000\000\001\000\000\000\000\000\000\000\370\234\377\266\354j\346\266f\v\000\000\000\000\000\000p\b\001\000T\005\001", '\000' <repeats 13 times>, "D;\376\266\003\000\000\000\000\000\000"
(gdb) n
31		write(writefd, buff, len);
1: buff = "this is a test.\n\000\000\000\000\001\000\000\000\000\360\377\266$\364\377\276T\332\375\266 \230\377\266\001\000\000\000\001\000\000\000\000\000\000\000\370\234\377\266\354j\346\266f\v\000\000\000\000\000\000p\b\001\000T\005\001", '\000' <repeats 13 times>, "D;\376\266\003\000\000\000\000\000\000"
(gdb) n
34		while ( (n = read(readfd, buff, MAXLINE)) > 0)
1: buff = "this is a test.\n\000\000\000\000\001\000\000\000\000\360\377\266$\364\377\276T\332\375\266 \230\377\266\001\000\000\000\001\000\000\000\000\000\000\000\370\234\377\266\354j\346\266f\v\000\000\000\000\000\000p\b\001\000T\005\001", '\000' <repeats 13 times>, "D;\376\266\003\000\000\000\000\000\000"
(gdb) n
^Z
Thread 1.1 "a.out" received signal SIGTSTP, Stopped (user).
0xb6f2187c in __GI___libc_read (fd=5, buf=0xbefff384, nbytes=100)
    at ../sysdeps/unix/sysv/linux/read.c:26
26	../sysdeps/unix/sysv/linux/read.c: 그런 파일이나 디렉터리가 없습니다.
(gdb) info inferiors
  Num  Description       Executable        
* 1    process 2915      /home/pi/Project/cCode/IPC/a.out 
  2    process 2918      /home/pi/Project/cCode/IPC/a.out 
(gdb) inferior 2
[Switching to inferior 2 [process 2918] (/home/pi/Project/cCode/IPC/a.out)]
[Switching to thread 2.1 (process 2918)]
#0  arch_fork (ctid=0xb6ff9cf8) at ../sysdeps/unix/sysv/linux/arch-fork.h:40
40	../sysdeps/unix/sysv/linux/arch-fork.h: 그런 파일이나 디렉터리가 없습니다.
(gdb) c
Continuing.

Thread 2.1 "a.out" received signal SIGTSTP, Stopped (user).
arch_fork (ctid=0xb6ff9cf8) at ../sysdeps/unix/sysv/linux/arch-fork.h:40
40	in ../sysdeps/unix/sysv/linux/arch-fork.h
(gdb) c
Continuing.

Thread 2.1 "a.out" received signal SIGTSTP, Stopped (user).
arch_fork (ctid=0xb6ff9cf8) at ../sysdeps/unix/sysv/linux/arch-fork.h:40
40	in ../sysdeps/unix/sysv/linux/arch-fork.h
(gdb) 
Continuing.
자식 process는 2918

Thread 2.1 "a.out" hit Breakpoint 1, server (readfd=3, writefd=6)
    at pipe.c:14
14		n = read(readfd, buff, MAXLINE);
(gdb) display buff
2: buff = "x\371\377\266,\003\001\000P\241\377\266\000\000\000\000\001\000\000\000\000\360\377\266$\364\377\276T\332\375\266\062\071\061\070\000:\215\371\001\000\000\000h\351\377\266p\b\001\000T\005\001", '\000' <repeats 14 times>, "\360\377\266$\364\377\276pD\352\266\354\363\377\276\000:\215\371\000\000\000\000\364\a\001\000\024\t\001"
(gdb) n
15		printf("서버가 읽은 data는 %s\n",buff);
2: buff = "this is a test.\000\001\000\000\000\000\360\377\266$\364\377\276T\332\375\266\062\071\061\070\000:\215\371\001\000\000\000h\351\377\266p\b\001\000T\005\001", '\000' <repeats 14 times>, "\360\377\266$\364\377\276pD\352\266\354\363\377\276\000:\215\371\000\000\000\000\364\a\001\000\024\t\001"
(gdb) n
서버가 읽은 data는 this is a test.
16	}
2: buff = "this is a test.\000\001\000\000\000\000\360\377\266$\364\377\276T\332\375\266\062\071\061\070\000:\215\371\001\000\000\000h\351\377\266p\b\001\000T\005\001", '\000' <repeats 14 times>, "\360\377\266$\364\377\276pD\352\266\354\363\377\276\000:\215\371\000\000\000\000\364\a\001\000\024\t\001"
(gdb) n
main (argc=1, argv=0xbefff574) at pipe.c:63
63			exit(0);
(gdb) c
Continuing.
[Inferior 2 (process 2918) exited normally]
(gdb) c
Continuing.

Thread 1 "a.out" received signal SIGTSTP, Stopped (user).
0xb6f2187c in __GI___libc_read (fd=5, buf=0xbefff384, nbytes=100)
    at ../sysdeps/unix/sysv/linux/read.c:26
26	../sysdeps/unix/sysv/linux/read.c: 그런 파일이나 디렉터리가 없습니다.
(gdb) c
Continuing.
종료된 자식 process는 2918
[Inferior 1 (process 2915) exited normally]

    다음은 수정된 코드이다.

    #include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <fcntl.h>
#include <sys/wait.h>
#include <string.h>
#include <errno.h>
#define MAXLINE 100

void client(int, int), server(int, int);

void server(int readfd, int writefd){
	/*
	char	buff[MAXLINE];
	int n;
	n = read(readfd, buff, MAXLINE);
	printf("서버가 읽은 data는 %s\n",buff);
	// 샘플 테스트용..

	*/
	int fd;
	ssize_t n;
	char	buff[MAXLINE+1];

	/* 4read pathname from IPC channel */
	if ( (n = read(readfd, buff, MAXLINE)) == 0)
		perror("end-of-file while reading pathname");
	buff[n] = '\0';		/* null terminate pathname */

	if ( (fd = open(buff, O_RDONLY)) < 0) {
			/* 4error: must tell client */
		snprintf(buff + n, sizeof(buff) - n, ": can't open, %s\n",
				 strerror(errno));
		n = strlen(buff);
		write(writefd, buff, n);

	} else {
			/* 4open succeeded: copy file to IPC channel */
		while ( (n = read(fd, buff, MAXLINE)) > 0)
			write(writefd, buff, n);
		close(fd);
	}

}

void client(int readfd, int writefd)
{
	size_t	len;
	ssize_t	n;
	char	buff[MAXLINE];

	/* 4read pathname */
	fgets(buff, MAXLINE, stdin);
	len = strlen(buff);		/* fgets() guarantees null byte at end */
	if (buff[len-1] == '\n')
		len--;				/* delete newline from fgets() */

	/* 4write pathname to IPC channel */
	write(writefd, buff, len);

	/* 4read from IPC, write to standard output */
	while ( (n = read(readfd, buff, MAXLINE)) > 0)
		write(STDOUT_FILENO, buff, n);
}


int main(int argc, char **argv)
{
	int		pipe1[2], pipe2[2];
	pid_t	childpid, childpid2;
	pid_t zombieFlag;

	if(	pipe(pipe1) < 0)
		exit(1);
	if (pipe(pipe2) < 0)
		exit(1);


	printf("made pipe1, pipe2\n");
	//child process pipe 닫음.
	if( (childpid = fork()) == 0) {		//난 자식..
		close(pipe1[1]);
		close(pipe2[0]);
		childpid2 = getpid();

		printf("자식 process는 %d\n",childpid2);
		server(pipe1[0], pipe2[1]);

		//    pipe1 ----> server, server process 입력은 pipe1 출력.
		//    pipe2 <---- server, server process 출력은 pipi2 입력.
		exit(0);
	}
	// 난 부모.
	close(pipe1[0]);
	close(pipe2[1]);
	client(pipe2[0], pipe1[1]);

	zombieFlag = waitpid(childpid, NULL, 0);
	printf("종료된 자식 process는 %d\n",zombieFlag);
	//sleep(1);

	exit(0);
}
    pi@raspberrypi:~/Project/cCode/IPC $ cat test.txt 
안녕하세요.
hello.
pi@raspberrypi:~/Project/cCode/IPC $ ./a.out 
made pipe1, pipe2
자식 process는 4890
this is a test.
this is a test.: can't open, No such file or directory
종료된 자식 process는 4890
pi@raspberrypi:~/Project/cCode/IPC $ ./a.out 
made pipe1, pipe2
자식 process는 4893
./test
./test: can't open, No such file or directory
종료된 자식 process는 4893
pi@raspberrypi:~/Project/cCode/IPC $ ./a.out 
made pipe1, pipe2
자식 process는 4895
./test.txt
안녕하세요.
hello.
종료된 자식 process는 4895

    잘 실행된다. pipe를 만들고 파이프 양 끝에 입력, 출력으로 생각했는데, 이러면 헷갈린다. 프로세스 기준으로 입력, 출력을 생각하고 파이프로 연결한다 생각하면 쉽다.

    하… 가벼운 마음으로 들어왔는데 IPC를 책 한 권에 걸쳐 설명하네.

    https://www.geeksforgeeks.org/pipe-system-call/
    https://woosunbi.tistory.com/94
    https://moss.cs.iit.edu/cs351/gdb-inferiors.html