IPC：管道之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 fifo1
prog3 < 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：

IPC：管道之FIFO

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);
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);
}

client.c

#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);
}

server.c

#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);
}
}

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.c
gcc server.c client.c mainfifo.c -o mainfifo

图解上面的程序如下：

IPC：管道之FIFO

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);
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);
}

client.c

#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);
}

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);
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);
}

server.c

#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);
}
}

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》