本文关键字： 有名管道，linux有名管道

    1．实验目的

    通过编写有名管道多路通信实验，进一步掌握管道的创建、读写等操作，同时复习使用select()函数实现管道的通信。

    2．实验内容

    这里采用管道函数创建有名管道（并不是在控制台下输入命令），而且使用select()函数替代poll()函数实现多路复用（使用select()函数是出于以演示为目的）。

    3．实验步骤

    （1）画出流程图。该实验流程图如图1所示。

图1 实验流程图

    （2）编写代码。该实验源代码如下：

    /* pipe_select.c */
    #include <fcntl.h>
    #include <stdio.h>
    #include <unistd.h>
    #include <stdlib.h>
    #include <string.h>
    #include <time.h>
    #include <errno.h>

    #define FIFO1 "in1"
    #define FIFO2 "in2"
    #define MAX_BUFFER_SIZE 1024 /* 缓冲区大小 */
    #define IN_FILES 3 /* 多路复用输入文件数目 */
    #define TIME_DELAY 60 /* 超时值秒数 */
    #define MAX(a, b) ((a > b)?(a):(b))

    int main(void)
    {
        int fds[IN_FILES];
        char buf[MAX_BUFFER_SIZE];
        int i, res, real_read, maxfd;
        struct timeval tv;
        fd_set inset,tmp_inset;

        fds[0] = 0;

        /* 创建两个有名管道 */
        if (access(FIFO1, F_OK) == -1)
        {
            if ((mkfifo(FIFO1, 0666) < 0) && (errno != EEXIST))
            {
                printf("Cannot create fifo file\n");
                exit(1);
            }
        }
        if (access(FIFO2, F_OK) == -1)
        {
            if ((mkfifo(FIFO2, 0666) < 0) && (errno != EEXIST))
            {
                printf("Cannot create fifo file\n");
                exit(1);
            }
        }

        /* 以只读非阻塞方式打开两个管道文件 */
        if((fds[1] = open (FIFO1, O_RDONLY|O_NONBLOCK)) < 0)
        {
            printf("Open in1 error\n");
            return 1;
        }
        if((fds[2] = open (FIFO2, O_RDONLY|O_NONBLOCK)) < 0)
        {
            printf("Open in2 error\n");
            return 1;
        }

        /* 取出两个文件描述符中的较大者 */
        maxfd = MAX(MAX(fds[0], fds[1]), fds[2]);
        /* 初始化读集inset，并在读文件描述符集中加入相应的描述集 */
        FD_ZERO(&inset);
        for (i = 0; i < IN_FILES; i++)
        {
            FD_SET(fds[i], &inset);
        }
        FD_SET(0, &inset);

        tv.tv_sec = TIME_DELAY;
        tv.tv_usec = 0;
        /* 循环测试该文件描述符是否准备就绪，并调用select()函数对相关文件描述符做相应操作* /
        while(FD_ISSET(fds[0],&inset) || FD_ISSET(fds[1],&inset) || FD_ISSET(fds[2],
        &inset))
        {
            /* 文件描述符集的备份，以免每次都进行初始化 */
            tmp_inset = inset;
            res = select(maxfd + 1, &tmp_inset, NULL, NULL, &tv);
            switch(res)
            {
                case -1:
                {
                    printf("Select error\n");
                    return 1;
                }
                break;
                case 0: /* Timeout */
                {
                    printf("Time out\n");
                    return 1;
                }
                break;
                default:
                {
                    for (i = 0; i < IN_FILES; i++)
                    {
                        if (FD_ISSET(fds[i], &tmp_inset))
                        {
                            memset(buf, 0, MAX_BUFFER_SIZE);
                            real_read = read(fds[i], buf, MAX_BUFFER_SIZE);
                            if (real_read < 0)
                            {
                                if (errno != EAGAIN)
                                {
                                    return 1;
                                }
                            }
                            else if (!real_read)
                            {
                                close(fds[i]);
                                FD_CLR(fds[i], &inset);
                            }
                            else
                            {
                                if (i == 0)
                                { /* 主程序终端控制 */
                                    if ((buf[0] == 'q') || (buf[0] == 'Q'))
                                    {
                                        return 1;
                                    }
                                }
                                else
                                { /* 显示管道输入字符串 */
                                    buf[real_read] = '\0';
                                    printf("%s", buf);
                                }
                            }
                        } /* end of if */
                    } /* end of for */
                }
                break;
            } /* end of switch */
        } /* end of while */
        return 0;
    }

    （3）编译并运行该程序。

    （4）另外打开两个虚拟终端，分别输入“cat > in1”和“cat > in2”，接着在该管道中输入相关内容，并观察实验结果。

    4．实验结果

    实验运行结果如下：

    $ ./pipe_select （必须先运行主程序）
    SELECT CALL
    select call
    TEST PROGRAMME
    test programme
    END
    end
    q /* 在终端上输入“q”或“Q”立刻结束程序运行 */

    $ cat > in1
    SELECT CALL
    TEST PROGRAMME
    END

    $ cat > in2
    select call
    test programme
    end

    本文选自华清远见嵌入式培训教材《从实践中学嵌入式Linux应用程序开发》

   热点链接：

   1、有名管道（FIFO）
   2、linux 消息队列
   3、linux 共享内存
   4、linux下的信号量
   5、linux下的信号处理实例

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