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。

#include <sys/types.h>#include <sys/stat.h>int mkfifo(const char *pathname, mode_t mode);

在创建出一个FIFO后，它必须是或者打开来读，或者打开来写，可由open或fopen打开函数。FIFO不能打开来既读又写，因为它是半双工的。

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：

mkfifo fifo1prog3 < fifo1 &prog1 < infile | tee fifof1 | prog2

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

#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);intmain(int argc, char **argv){intreadfd, writefd;pid_tpid;/*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);}

client.c

#include "fifo.h"intclient(int readfd, int writefd){size_t len;ssize_tn;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);}

server.c

#include "fifo.h"void server(int readfd, int writefd){intfd;ssize_tn;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);}}

fifo.h

#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>

Makefile

all:mainfifo mainfifo:mainfifo.c server.c client.cgcc  server.c client.c mainfifo.c -o mainfifo

图解上面的程序如下：

图 使用两个FIFO的客户-服务器例子

上面的例子送虽然使用了FIFO，但仍然是有亲缘关系的进程间通信，下面给出无亲缘关系的客户和服务器的例子。

客户程序client_main.c如下：

#include "fifo.h"extern client(int, int);intmain(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);}

client.c

#include "fifo.h"intclient(int readfd, int writefd){size_t len;ssize_tn;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);}

服务器程序server_main.c如下：

#include "fifo.h"#include <sys/stat.h>#define FILE_MODE (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)extern void server(int, int);intmain(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);}

server.c

#include "fifo.h"void server(int readfd, int writefd){intfd;ssize_tn;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);}}

fifo.h

#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

参考：《UNIX环境高级编程》、《UNIX网络编程 卷2：进程间通信》