Linux IPC实践(3) --具名FIFO-白红宇

FIFO具名/命名管道

(匿名)管道应用的一个限制就是只能在具有共同祖先(具有亲缘关系)的进程间通信。

如果我们想在不相关的进程之间交换数据,可以使用FIFO文件来做这项工作，它经常被称为命名管道;命名管道是一种特殊类型的文件.

创建一个命名管道

1)命名管道可以从命令行上创建:

$ mkfifo <filename>

2)命名管道在程序里创建：

#include 
     
             #include 
      
              int mkfifo(const char *pathname, mode_t mode);

//示例int main(){    if (mkfifo("p2", 0644) == -1)        err_exit("mkfifo error");}

FIFO与PIPE的区别:

1) 匿名管道由pipe函数创建并打开。

命名管道由mkfifo函数创建，打开用open

2) FIFO（命名管道）与pipe（匿名管道）之间唯一的区别在它们创建与打开的方式不同，一但这些工作完成之后，它们具有相同的语义 (The only difference between pipes and FIFOs is the manner in which they are created and opened.

Once these tasks have been accomplished, I/O on pipes and FIFOs has exactly the same semantics.)。

命名管道的打开规则

1)读打开FIFO

O_NONBLOCK disable(阻塞)：阻塞直到有相应进程为写而打开该FIFO

O_NONBLOCK enable(非阻塞)：立刻返回成功

//示例1: 阻塞, 只读打开int main(){    int fd = open("fifo", O_RDONLY);    if (fd == -1)        err_exit("FIFO open error");    cout << "fifo O_RDONLY open success" << endl;}

//示例2: 只读, 非阻塞打开int main(){    int fd = open("fifo", O_RDONLY|O_NONBLOCK);    if (fd == -1)        err_exit("FIFO open error");    cout << "fifo O_RDONLY open success" << endl;}

2)写打开FIFO

O_NONBLOCK disable(阻塞)：阻塞直到有相应进程为读而打开该FIFO

O_NONBLOCK enable(非阻塞)：立刻返回失败，错误码为ENXIO

//示例1: 阻塞, 只写打开int main(){    int fd = open("fifo", O_WRONLY);    if (fd == -1)        err_exit("FIFO open error");    cout << "FIFO O_WRONLY open success" << endl;}

//示例2: 非阻塞, 只写打开int main(){    int fd = open("fifo", O_WRONLY|O_NONBLOCK);    if (fd == -1)        err_exit("FIFO open error");    cout << "FIFO O_WRONLY open success" << endl;}

命名管道的读写规则

同匿名管道

/**FIFO示例, 两个进程通过FIFO对拷数据:利用管道,两个进程间进行文件复制。1.进程writefifo:(1)读文件(文件名从命令行参数中获取)(2)写入管道myFifo(管道由该进程创建)(3)关闭文件及管道2.进程readfifo:	(1)读管道myFifo(2)写入文件[该文件有进程创建并打开](3)关闭文件(4)删除管道*/

//1:writefifoint main(int argc, char *argv[]){    if (argc < 2)        err_quit("Usage: ./writefifo 
     
      ");    // 创建管道    if (mkfifo("myFifo", 0644) == -1)        err_exit("mkfifo error");    int outfd = open("myFifo", O_WRONLY);   //打开FIFO    int infd = open(argv[1], O_RDONLY);     //打开文件    if (outfd == -1 || infd == -1)        err_exit("open file/fifo error");    char buf[BUFSIZ];    int readBytes;    while ((readBytes = read(infd, buf, sizeof(buf))) > 0)    {        write(outfd, buf, readBytes);    }    close(infd);    close(outfd);}

//2:readfifoint main(int argc, char *argv[]){    if (argc < 2)        err_quit("Usage: ./writefifo 
     
      ");    int outfd = open(argv[1], O_WRONLY|O_CREAT|O_TRUNC, 0644);  //创建并打卡文件    int infd = open("myFifo", O_RDONLY);    //打开FIFO    if (infd == -1 || outfd == -1)        err_exit("open file/fifo error");    char buf[BUFSIZ];    int readBytes;    while ((readBytes = read(infd, buf, sizeof(buf))) > 0)    {        write(outfd, buf, readBytes);    }    close(outfd);    unlink("myFifo");   //删除FIFO}