进程间通信(二)：FIFO

一.FIFO 文件的创建，创建一个fifo在文件系统上

int ret = mkfifo("/tmp/fifo", mode）;

二.打开FIFO

open("/tmp/fifo", flag);

根据flag的不同有四种打开方式：

O_RDONLY:阻塞直到，此fifo被O_WRONLY方式打开

O_WRONLY:阻塞直到，此fifo被O_RDONLY方式打开

O_RDONLY|O_NONBLOCK：立即返回

O_WRONLY|O_NONBLOCK：如果此fifo已经被只读打开，立即返回文件描述符，如果没有，立即返回-1

FIFO 支持open,close,read,write.unlink函数，O_RDONLY和O_WRONLY组合打开的结果是未定义的

四种打开方式测试代码：

fifo_read.c

#include<sys/stat.h>

#include<stdio.h>

#include <errno.h>

#include <fcntl.h>

int main()

{

char line[1024+1] = {'\0'};

mode_t mode;

mode = S_IRWXU|S_IRWXG;

if(mkfifo("/tmp/fifo", mode) < 0 && errno!=EEXIST)

{

printf("mkfifo failed !\n");

}

printf("\n");

int fd = open("/tmp/fifo", O_RDONLY);

if(fd<0)

{

printf("reader:first open fifo failed !\n");

}

else

{

printf("reader:open fifo secceed !\n");

}

sleep(1);

fd = open("/tmp/fifo", O_RDONLY );

if(fd<0)

{

printf("reader:second open fifo failed !\n");

}

else

{

printf("reader:second open fifo secceed !\n");

}

if(read(fd, line, 1024) < 0)

{

printf("reader:read fifo failed !\n");

}

printf("get msg:%s \n", line);

return 0;

}

fifo_writer.c

#include<sys/stat.h>

#include<stdio.h>

#include <errno.h>

#include <fcntl.h>

#include <unistd.h>

#define MSG "hello world !"

int main()

{

char line[1024] = {'\0'};

mode_t mode;

mode = S_IRWXU|S_IRWXG;

if(mkfifo("/tmp/fifo", mode) < 0 && errno!=EEXIST)

{

printf("mkfifo failed !");

}

printf("\n");

int fd = open("/tmp/fifo", O_WRONLY);

if(fd<0)

{

printf("open fifo failed !");

}

printf("writer:open secceed !\n");

close(fd);

fd = open("/tmp/fifo", O_WRONLY|O_NONBLOCK);

if(fd < 0)

{

printf("writer:second open failed !\n");

}

else

{

printf("writer:second open failed !\n");

}

sleep(2);

fd = open("/tmp/fifo", O_WRONLY|O_NONBLOCK);

if(fd < 0)

{

printf("writer:third open failed !\n");

}

else

{

printf("writer:third open secceed !\n");

}

if(write(fd, MSG, strlen(MSG)) < 0)

{

printf("write fifo failed !");

}

return 0;

}

fifo_read.c 生成可执行文件reader

fifo_writer.c 生成可执行文件writer

测试结果：

[read_ng@wukong workspace]$ ./reader &

[1] 17862

[read_ng@wukong workspace]$ ./writer &

[2] 17863

[read_ng@wukong workspace]$

writer:open secceed !

reader:open fifo secceed !

writer:second open failed !

reader:second open fifo secceed !

writer:third open secceed !

get msg:hello world !

[1]- Done ./reader

[2]+ Done ./writer

[read_ng@wukong workspace]$

三.FIFO 的读写

1.如果一个FIFO，所有打开的描述符关闭后，内容会清空，否则不会清空

2.read时，如果FIFO中有数据，会读出来，如果没有，而且没有被写方式打开返回0，表示结束

3.write 时，如果FIFO没有被读方式打开，产生信号SIGPIPE，导致进程终止