Linux高级字符设备驱动 poll方法（select多路监控原理与实现）

1、什么是Poll方法,功能是什么?2、Select系统调用(功能)Select系统调用用于多路监控,当没有一个文件满足要求时,select将阻塞调用进程。int select(int maxfd, fd_set *readfds, fd_set *writefds, fe_set *exceptfds, const struct timeval *timeout)Select系统调用(参数)1）Maxfd:文件描述符的范围,比待检测的最大文件描述符大12）Readfds:被读监控的文件描述符集3）Writefds:被写监控的文件描述符集4）Exceptfds:被异常监控的文件描述符集;5）Timeout:定时器，Timeout取不同的值,该调用有不同的表现:1>Timeout值为0,不管是否有文件满足要求,都立刻返回,无文件满足要求返回0,有文件满足要求返回一个正值。2>Timeout为NULL,select将阻塞进程,直到某个文件满足要求3>Timeout 值 为 正 整 数 , 就 是 等 待 的 最 长 时 间 , 即select在timeout时间内阻塞进程。3、Select系统调用(返回值)Select调用返回时,返回值有如下情况:1）正常情况下返回满足要求的文件描述符个数;2）经过了timeout等待后仍无文件满足要求,返回值为0;3）如果select被某个信号中断,它将返回-1并设置errno为EINTR。4）如果出错,返回-1并设置相应的errno。4、Select系统调用(使用方法)1）将要监控的文件添加到文件描述符集2）调用Select开始监控3）判断文件是否发生变化系统提供了4个宏对描述符集进行操作:#include <sys/select.h>void FD_SET(int fd, fd_set *fdset)void FD_CLR(int fd, fd_set *fdset)void FD_ZERO(fd_set *fdset)void FD_ISSET(int fd, fd_set *fdset)宏FD_SET将文件描述符fd添加到文件描述符集fdset中;宏FD_CLR从文件描述符集fdset中清除文件描述符fd;宏FD_ZERO清空文件描述符集fdset;在调用select后使用FD_ISSET来检测文件描述符集fdset中的文件fd发生了变化。FD_ZERO(&fds); //清空集合FD_SET(fd1,&fds); //设置描述符FD_SET(fd2,&fds); //设置描述符maxfdp=fd1+1; //描述符最大值加1,假设fd1>fd2switch(select(maxfdp,&fds,NULL,NULL,&timeout))case -1: exit(-1);break; //select错误,退出程序case 0:break;default:if(FD_ISSET(fd1,&fds)) //测试fd1是否可读5、poll方法应用程序常常使用select系统调用,它可能会阻塞进程。这个调用由驱动的 poll 方法实现,原型为:unsigned int (*poll)(struct file *filp,poll_table *wait)Poll设备方法负责完成:1）使用poll_wait将等待队列添加到poll_table中。2）返回描述设备是否可读或可写的掩码。位掩码1>POLLIN 设备可读2>POLLRDNORM数据可读3>POLLOUT\设备可写4>POLLWRNORM数据可写设备可读通常返回(POLLIN|POLLRDNORM )设备可写通常返回(POLLOUT|POLLWRNORM )6、范例static unsigned int mem_poll(struct file *filp,poll_table *wait){struct scull_pipe *dev =filp->private_data;unsigned int mask =0;/* 把等待队列添加到poll_table */poll_wait(filp,&dev->inq,wait);/*返回掩码*/if (有数据可读)mask = POLLIN |POLLRDNORM;/*设备可读*/return mask;}7、工作原理Poll方法只是做一个登记,真正的阻塞发生在select.c 中的 do_select函数。8、实例分析1）poll型设备驱动memdev.h源码#ifndef _MEMDEV_H_#define _MEMDEV_H_#ifndef MEMDEV_MAJOR#define MEMDEV_MAJOR 0 /*预设的mem的主设备号*/#endif#ifndef MEMDEV_NR_DEVS#define MEMDEV_NR_DEVS 2 /*设备数*/#endif#ifndef MEMDEV_SIZE#define MEMDEV_SIZE 4096#endif/*mem设备描述结构体*/struct mem_dev{char *data;unsigned long size;wait_queue_head_t inq;};#endif /* _MEMDEV_H_ */2）Poll型设备驱动memdev.c源码#include <linux/module.h>#include <linux/types.h>#include <linux/fs.h>#include <linux/errno.h>#include <linux/mm.h>#include <linux/sched.h>#include <linux/init.h>#include <linux/cdev.h>#include <asm/io.h>#include <asm/system.h>#include <asm/uaccess.h>#include <linux/poll.h>#include "memdev.h"static mem_major = MEMDEV_MAJOR;bool have_data = false; /*表明设备有足够数据可供读*/module_param(mem_major, int, S_IRUGO);struct mem_dev *mem_devp; /*设备结构体指针*/struct cdev cdev;/*文件打开函数*/int mem_open(struct inode *inode, struct file *filp){struct mem_dev *dev;/*获取次设备号*/int num = MINOR(inode->i_rdev);if (num >= MEMDEV_NR_DEVS)return -ENODEV;dev = &mem_devp[num];/*将设备描述结构指针赋值给文件私有数据指针*/filp->private_data = dev;return 0;}/*文件释放函数*/int mem_release(struct inode *inode, struct file *filp){return 0;}/*读函数*/static ssize_t mem_read(struct file *filp, char __user *buf, size_t size, loff_t *ppos){unsigned long p = *ppos;unsigned int count = size;int ret = 0;struct mem_dev *dev = filp->private_data; /*获得设备结构体指针*//*判断读位置是否有效*/if (p >= MEMDEV_SIZE)return 0;if (count > MEMDEV_SIZE - p)count = MEMDEV_SIZE - p;while (!have_data) /* 没有数据可读，考虑为什么不用if，而用while */{if (filp->f_flags & O_NONBLOCK)return -EAGAIN;wait_event_interruptible(dev->inq,have_data);}/*读数据到用户空间*/if (copy_to_user(buf, (void*)(dev->data + p), count)){ret = - EFAULT;}else{*ppos += count;ret = count;printk(KERN_INFO "read %d bytes(s) from %d\n", count, p);}have_data = false; /* 表明不再有数据可读 *//* 唤醒写进程 */return ret;}/*写函数*/static ssize_t mem_write(struct file *filp, const char __user *buf, size_t size, loff_t *ppos){unsigned long p = *ppos;unsigned int count = size;int ret = 0;struct mem_dev *dev = filp->private_data; /*获得设备结构体指针*//*分析和获取有效的写长度*/if (p >= MEMDEV_SIZE)return 0;if (count > MEMDEV_SIZE - p)count = MEMDEV_SIZE - p;/*从用户空间写入数据*/if (copy_from_user(dev->data + p, buf, count))ret = - EFAULT;else{*ppos += count;ret = count;printk(KERN_INFO "written %d bytes(s) from %d\n", count, p);}have_data = true; /* 有新的数据可读 *//* 唤醒读进程 */wake_up(&(dev->inq));return ret;}/* seek文件定位函数 */static loff_t mem_llseek(struct file *filp, loff_t offset, int whence){loff_t newpos;switch(whence) {case 0: /* SEEK_SET */newpos = offset;break;case 1: /* SEEK_CUR */newpos = filp->f_pos + offset;break;case 2: /* SEEK_END */newpos = MEMDEV_SIZE -1 + offset;break;default: /* can't happen */return -EINVAL;}if ((newpos<0) || (newpos>MEMDEV_SIZE))return -EINVAL;filp->f_pos = newpos;return newpos;}unsigned int mem_poll(struct file *filp, poll_table *wait){struct mem_dev *dev = filp->private_data;unsigned int mask = 0;/*将等待队列添加到poll_table表中 */poll_wait(filp, &dev->inq, wait);if (have_data)mask |= POLLIN | POLLRDNORM; /* readable */return mask;}/*文件操作结构体*/static const struct file_operations mem_fops ={.owner = THIS_MODULE,.llseek = mem_llseek,.read = mem_read,.write = mem_write,.open = mem_open,.release = mem_release,.poll = mem_poll,};/*设备驱动模块加载函数*/static int memdev_init(void){int result;int i;dev_t devno = MKDEV(mem_major, 0);/* 静态申请设备号*/if (mem_major)result = register_chrdev_region(devno, 2, "memdev");else /* 动态分配设备号 */{result = alloc_chrdev_region(&devno, 0, 2, "memdev");mem_major = MAJOR(devno);}if (result < 0)return result;/*初始化cdev结构*/cdev_init(&cdev, &mem_fops);cdev.owner = THIS_MODULE;cdev.ops = &mem_fops;/* 注册字符设备 */cdev_add(&cdev, MKDEV(mem_major, 0), MEMDEV_NR_DEVS);/* 为设备描述结构分配内存*/mem_devp = kmalloc(MEMDEV_NR_DEVS * sizeof(struct mem_dev), GFP_KERNEL);if (!mem_devp) /*申请失败*/{result = - ENOMEM;goto fail_malloc;}memset(mem_devp, 0, sizeof(struct mem_dev));/*为设备分配内存*/for (i=0; i < MEMDEV_NR_DEVS; i++){mem_devp[i].size = MEMDEV_SIZE;mem_devp[i].data = kmalloc(MEMDEV_SIZE, GFP_KERNEL);memset(mem_devp[i].data, 0, MEMDEV_SIZE);/*初始化等待队列*/init_waitqueue_head(&(mem_devp[i].inq));//init_waitqueue_head(&(mem_devp[i].outq));}return 0;fail_malloc:unregister_chrdev_region(devno, 1);return result;}/*模块卸载函数*/static void memdev_exit(void){cdev_del(&cdev); /*注销设备*/kfree(mem_devp); /*释放设备结构体内存*/unregister_chrdev_region(MKDEV(mem_major, 0), 2); /*释放设备号*/}MODULE_AUTHOR("David Xie");MODULE_LICENSE("GPL");module_init(memdev_init);module_exit(memdev_exit);3）测试程序app-read.c源码#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <sys/ioctl.h>#include <sys/types.h>#include <sys/stat.h>#include <fcntl.h>#include <sys/select.h>#include <sys/time.h>#include <errno.h>int main(){int fd;fd_set rds; //声明描述符集合int ret;char Buf[128];/*初始化Buf*/strcpy(Buf,"memdev is char dev!");printf("BUF: %s\n",Buf);/*打开设备文件*/fd = open("/dev/memdev0",O_RDWR);FD_ZERO(&rds); //清空描述符集合FD_SET(fd, &rds); //设置描述符集合/*清除Buf*/strcpy(Buf,"Buf is NULL!");printf("Read BUF1: %s\n",Buf);ret = select(fd + 1, &rds, NULL, NULL, NULL);//调用select（）监控函数if (ret < 0){printf("select error!\n");exit(1);}if (FD_ISSET(fd, &rds)) //测试fd1是否可读read(fd, Buf, sizeof(Buf));/*检测结果*/printf("Read BUF2: %s\n",Buf);close(fd);return 0;}