20201317 LYX Linux进程间通信学习

Linux进程间通信

1、匿名管道：pipe
2、命名管道：fifo
3、内存映射：mmap
4、信号

进程是程序运行资源分配的最小单位。每个进程各自有不同的用户地址空间，任何一个进程的全局变量在另一个进程中都看不到，所以进程之间要交换数据必须通过内核，在内核中开辟一块缓冲区，进程1把数据从用户空间拷到内核缓冲区，进程2再从内核缓冲区把数据读走，内核提供的这种机制称为进程间通信。

1、匿名管道：pipe

pipe只能用于有血缘关系的进程进行单向通信。调用 pipe 函数时在内核中开辟一块缓冲区（称为管道）用于通信，它有一个读端一个写端，然后通过 fd 参数传出给用户程序两个文件描述符， fd[0] 指向管道的读端， fd[1] 指向管道的写端。支持多端读或多端写，但不支持一端同时读写。看图会更加直观一点：

那么在使用pipe通信的时候可能会遇到以下的几种情况：

​ 1. 当读管道时，如果管道中没有数据，则会阻塞，直到管道另一端写入数据。

​ 2. 当写管道时，如果管道中已经满了，则会阻塞，直到管道另一端读出数据(可见读出数据时，管道中将不会保留该数据)。

​ 3. 当管道写端关闭时，读端读完管道内的数据时，如果再次去读没有数据的管道会返回0，相当于读到了EOF。

​ 4. 当管道读端关闭时，如果写端在写入数据时，产生SIGPIPE信号，写进程默认情况下会终止进程。

#include	<stdio.h>
#include    <stdlib.h>
#include	<unistd.h>

#define	oops(m,x)	{ perror(m); exit(x); }

int main(int ac, char **av)
{
	int	thepipe[2],			
		newfd,				
		pid;				

	if ( ac != 3 ){
		fprintf(stderr, "usage: pipe cmd1 cmd2\n");
		exit(1);
	}
	if ( pipe( thepipe ) == -1 )		
		oops("Cannot get a pipe", 1);

	if ( (pid = fork()) == -1 )			
		oops("Cannot fork", 2);

	if ( pid > 0 ){			
		close(thepipe[1]);	

		if ( dup2(thepipe[0], 0) == -1 )
			oops("could not redirect stdin",3);

		close(thepipe[0]);	
		execlp( av[2], av[2], NULL);
		oops(av[2], 4);
	}

	close(thepipe[0]);		

	if ( dup2(thepipe[1], 1) == -1 )
		oops("could not redirect stdout", 4);

	close(thepipe[1]);		
	execlp( av[1], av[1], NULL);
	oops(av[1], 5);
}

#include	<stdio.h>

main( int ac, char *av[] )
{
	int	i;

	printf("Number of args: %d, Args are:\n", ac);
	for(i=0;i<ac;i++)
		printf("args[%d] %s\n", i, av[i]);

	fprintf(stderr,"This message is sent to stderr.\n");
}

#include	<stdio.h>
#include    <stdlib.h>
#include    <string.h>
#include	<unistd.h>

int main()
{
	int	len, i, apipe[2];	
	char	buf[BUFSIZ];		
	
	if ( pipe ( apipe ) == -1 ){
		perror("could not make pipe");
		exit(1);
	}
	printf("Got a pipe! It is file descriptors: { %d %d }\n", 
							apipe[0], apipe[1]);


	while ( fgets(buf, BUFSIZ, stdin) ){
		len = strlen( buf );
		if (  write( apipe[1], buf, len) != len ){	
			perror("writing to pipe");		
			break;					
		}
		for ( i = 0 ; i<len ; i++ )                     
			buf[i] = 'X' ;
		len = read( apipe[0], buf, BUFSIZ ) ;		
		if ( len == -1 ){				
			perror("reading from pipe");		
			break;
		}
		if ( write( 1 , buf, len ) != len ){		
			perror("writing to stdout");		
			break;					
		}
	}
}

#include	<stdio.h>
#include    <stdlib.h>
#include    <string.h>
#include    <unistd.h>


#define	CHILD_MESS	"I want a cookie\n"
#define	PAR_MESS	"testing..\n"
#define	oops(m,x)	{ perror(m); exit(x); }

main()
{
	int	pipefd[2];		
	int	len;			
	char	buf[BUFSIZ];		
	int	read_len;

	if ( pipe( pipefd ) == -1 )
		oops("cannot get a pipe", 1);

	switch( fork() ){
		case -1:
			oops("cannot fork", 2);
	
		case 0:			
			len = strlen(CHILD_MESS);
			while ( 1 ){
				if (write( pipefd[1], CHILD_MESS, len) != len )
					oops("write", 3);
				sleep(5);
			}
		
		default:		
			len = strlen( PAR_MESS );
			while ( 1 ){
				if ( write( pipefd[1], PAR_MESS, len)!=len )
					oops("write", 4);
				sleep(1);
				read_len = read( pipefd[0], buf, BUFSIZ );
				if ( read_len <= 0 )
					break;
				write( 1 , buf, read_len );
			}
	}
}

#include	<stdio.h>
#include	<fcntl.h>

int main()
{
	int	fd ;
	char	line[100];

	fgets( line, 100, stdin ); printf("%s", line );
	fgets( line, 100, stdin ); printf("%s", line );
	fgets( line, 100, stdin ); printf("%s", line );

	close(0);
	fd = open("/etc/passwd", O_RDONLY);
	if ( fd != 0 ){
		fprintf(stderr,"Could not open data as fd 0\n");
		exit(1);
	}

	fgets( line, 100, stdin ); printf("%s", line );
	fgets( line, 100, stdin ); printf("%s", line );
	fgets( line, 100, stdin ); printf("%s", line );
}

#include	<stdio.h>
#include    <stdlib.h>
#include	<fcntl.h>

//#define	CLOSE_DUP		
//#define	USE_DUP2	

main()
{
	int	fd ;
	int	newfd;
	char	line[100];

	fgets( line, 100, stdin ); printf("%s", line );
	fgets( line, 100, stdin ); printf("%s", line );
	fgets( line, 100, stdin ); printf("%s", line );

	fd = open("data", O_RDONLY);	
#ifdef CLOSE_DUP
	close(0);
	newfd = dup(fd);		
#else
	newfd = dup2(fd,0);		
#endif
	if ( newfd != 0 ){
		fprintf(stderr,"Could not duplicate fd to 0\n");
		exit(1);
	}
	close(fd);			

	fgets( line, 100, stdin ); printf("%s", line );
	fgets( line, 100, stdin ); printf("%s", line );
	fgets( line, 100, stdin ); printf("%s", line );
}

#include	<stdio.h>
#include    <stdlib.h>
#include    <unistd.h>

int main()
{
	int	pid ;
	int	fd;

	printf("About to run who into a file\n");

	if( (pid = fork() ) == -1 ){
		perror("fork"); exit(1);
	}
	if ( pid == 0 ){
		close(1);				/* close, */
		fd = creat( "userlist", 0644 );		/* then open */
		execlp( "who", "who", NULL );		/* and run	*/
		perror("execlp");
		exit(1);
	}
	if ( pid != 0 ){
		wait(NULL);
		printf("Done running who.  results in userlist\n");
	}

	return 0;
}

#include	<stdio.h>
#include    <stdlib.h>
#include    <unistd.h>

int main()
{
	int	pid ;
	int	fd;

	printf("About to run who into a file\n");

	if( (pid = fork() ) == -1 ){
		perror("fork"); exit(1);
	}
	if ( pid == 0 ){
		close(1);				/* close, */
		fd = creat( "userlist", 0644 );		/* then open */
		execlp( "who", "who", NULL );		/* and run	*/
		perror("execlp");
		exit(1);
	}
	if ( pid != 0 ){
		wait(NULL);
		printf("Done running who.  results in userlist\n");
	}

	return 0;
}