Linux环境编程之IPC进程间通信(三)：FIFO

管道是没有名字的，因此它只能在有亲缘关系的进程间使用，给管道加上名字，我们称之为有名管道FIFO，当然FIFO与管道之间不止有没有名字的区别，还有其他区别下面会提到。与管道类似的是，FIFO是一个单向（半双工）数据流。不同于管道的是，每个FIFO有一个路径名与之关联，从而允许无亲缘关系的进程之间访问同一个FIFO。FIFO是一种文件类型。stat结构成员st_mode的编码指明文件是否是FIFO类型，可以用S_ISFIFO宏对此进行测试。

FIFO由mkfifo函数创建，它已经隐含指定了O_CREAT|O_EXCL。也就是说，如果FIFO存在就返回一个EEXIST错误，如果不存在就创建一个新的FIFO。FIFO由open打开而不是mkfifo。因此要打开一个已存在的FIFO或创建一个新的FIFO，应先调用mkfifo，再检查它是否返回EEXIST错误，若返回该错误则改为调用open。

<span style="font-size:14px;">#include <sys/types.h>
#include <sys/stat.h>
int mkfifo(const char *pathname, mode_t mode);</span>

FIFO有下面两种用途：

1、FIFO由shell命令使用以便将数据从一条管道线传送到另一条，为此无需创建中间临时文件。FIFO可被用于复制串行管道命令之间的输出流，于是也就不需要写数据到中间磁盘文件中。如：

<span style="font-size:14px;">mkfifo fifo1
prog3 < fifo1 &
prog1 < infile | tee fifof1 | prog2</span>

创建FIFO，然后在后台启动prog3，它从FIFO读数据。然后启动prog1，用tee将输出发送到FIFO和prog2。图解如下：

图 使用FIFO和tee将一个数据发送到两个进程

2、FIFO用于客户进程-服务器进程应用程序中，以在客户进程和服务器进程之间传递数据。对于客户-服务器程序，改用两个FIFO代替两个管道的代码如下：

mainfifo.c

<span style="font-size:14px;">#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/stat.h>
#include "fifo.h"

#define FIFO1 "/tmp/fifo.1"
#define FIFO2 "/tmp/fifo.2"
#define FILE_MODE 	(S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)

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

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

	/*creat tow FIFOs; OK if they already exist*/
	if((mkfifo(FIFO1, FILE_MODE) < 0) && (errno = EEXIST))
		printf("can't creat %s.\n", FIFO1);
	if((mkfifo(FIFO2, FILE_MODE) < 0) && (errno = EEXIST)){
		unlink(FIFO1);
		printf("can't creat %s.\n", FIFO2);
	}

	if((pid = fork()) < 0){
		printf("can't fork.\n");	
		return -1;
	}else if(pid == 0){			/*child*/
		readfd  = open(FIFO1, O_RDONLY, 0);
		writefd = open(FIFO2, O_WRONLY, 0);

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

	client(readfd, writefd);

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

	close(readfd);
	close(writefd);

	unlink(FIFO1);
	unlink(FIFO2);
	exit(0);
}</span>

<span style="font-size:14px;">#include "fifo.h"

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

	/*read pathname*/
	fgets(buff, MAXLINE, stdin);
	len = strlen(buff);
	if(buff[len-1] == '\n')
		len--;

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

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

<span style="font-size:14px;">#include "fifo.h"

void 
server(int readfd, int writefd)
{
	int		fd;
	ssize_t	n;
	char 	buff[MAXLINE+1];

	/*read pathname frome IPC channel*/
	if((n = read(readfd, buff, MAXLINE)) == 0){
		printf("end-of-file while reading pathname.\n");
		return ;
	}
	buff[n] = '\0';			/*null terminate pathname*/

	if((fd = open(buff, O_RDONLY)) < 0){
		/*error:must tell client*/	
		snprintf(buff+n, sizeof(buff)-n, ":can't open, %s\n", strerror(errno));
		n = strlen(buff);
		write(writefd, buff, n);
	}else{
		/*open succeeded:copy file to IPC channel*/	
		while((n = read(fd, buff, MAXLINE)) > 0)
			write(writefd, buff, n);
		close(fd);
	}

}</span>

<span style="font-size:14px;">#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <errno.h>
</span>

<span style="font-size:14px;">all:mainfifo 
mainfifo:mainfifo.c server.c client.c
	gcc  server.c client.c mainfifo.c -o mainfifo</span>

<span style="font-size:14px;">#include "fifo.h"

extern client(int, int);

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

	writefd = open(FIFO1, O_WRONLY, 0);
	readfd  = open(FIFO2, O_RDONLY, 0);

	client(readfd, writefd);

	close(readfd);
	close(writefd);

	unlink(FIFO1);
	unlink(FIFO2);
	exit(0);
}</span>

<span style="font-size:14px;">#include "fifo.h"

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

	/*read pathname*/
	fgets(buff, MAXLINE, stdin);
	len = strlen(buff);
	if(buff[len-1] == '\n')
		len--;

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

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

<span style="font-size:14px;">#include "fifo.h"
#include <sys/stat.h>
#define FILE_MODE 	(S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)

extern void server(int, int);

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

	/*creat two FIFOs:OK if they already exist*/
	if((mkfifo(FIFO1, FILE_MODE) < 0) && (errno != EEXIST)){
		printf("can't create %s.\n", FIFO1);
	}
	if((mkfifo(FIFO1, FILE_MODE) < 0) && (errno != EEXIST)){
		unlink(FIFO1);
	 	printf("can't create %s.\n", FIFO1);
	}

	readfd  = open(FIFO1, O_RDONLY, 0);
	writefd = open(FIFO2, O_WRONLY, 0);

	server(readfd, writefd);
	exit(0);
}</span>

<span style="font-size:14px;">#include "fifo.h"

void 
server(int readfd, int writefd)
{
	int		fd;
	ssize_t	n;
	char 	buff[MAXLINE+1];

	/*read pathname frome IPC channel*/
	if((n = read(readfd, buff, MAXLINE)) == 0){
		printf("end-of-file while reading pathname.\n");
		return ;
	}
	buff[n] = '\0';			/*null terminate pathname*/

	if((fd = open(buff, O_RDONLY)) < 0){
		/*error:must tell client*/	
		snprintf(buff+n, sizeof(buff)-n, ":can't open, %s\n", strerror(errno));
		n = strlen(buff);
		write(writefd, buff, n);
	}else{
		/*open succeeded:copy file to IPC channel*/	
		while((n = read(fd, buff, MAXLINE)) > 0)
			write(writefd, buff, n);
		close(fd);
	}

}</span>

<span style="font-size:14px;">#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <errno.h>

#define FIFO1 	"/tmp/fifo.1"	
#define FIFO2 	"/tmp/fifo.2"	
#define MAXLINE 4096</span>

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