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1
2）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>fd2
switch(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;
poll_wait(filp,&dev->inq,wait);
if (有数据可读)
mask = POLLIN |POLLRDNORM;
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
#endif
#ifndef MEMDEV_NR_DEVS
#define MEMDEV_NR_DEVS 2
#endif
#ifndef MEMDEV_SIZE
#define MEMDEV_SIZE 4096
#endif
struct mem_dev
{
char *data;
unsigned long size;
wait_queue_head_t inq;
};
#endif
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;
module_param(mem_major, int, S_IRUGO);
struct mem_dev *mem_devp;
struct cdev cdev;
int mem_open(struct inode *inode, struct file *filp)
{
struct mem_dev *dev;
int num = MINOR(inode->i_rdev);
if (num >= MEMDEV_NR_DEVS)
return -ENODEV;
dev = &mem_devp[num];
filp->private_data = dev;
return 0;
}
int mem_release(struct inode *inode, struct file *filp)
{
return 0;
}
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;
if (p >= MEMDEV_SIZE)
return 0;
if (count > MEMDEV_SIZE - p)
count = MEMDEV_SIZE - p;
while (!have_data)
{
if (filp->f_flags & O_NONBLOCK)
return -EAGAIN;
wait_event_interruptible(dev->inq,have_data);
}
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;
return ret;
}
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;
if (p >= MEMDEV_SIZE)
return 0;
if (count > MEMDEV_SIZE - p)
count = MEMDEV_SIZE - p;
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;
wake_up(&(dev->inq));
return ret;
}
static loff_t mem_llseek(struct file *filp, loff_t offset, int whence)
{
loff_t newpos;
switch(whence) {
case 0:
newpos = offset;
break;
case 1:
newpos = filp->f_pos + offset;
break;
case 2:
newpos = MEMDEV_SIZE -1 + offset;
break;
default:
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;
poll_wait(filp, &dev->inq, wait);
if (have_data)
mask |= POLLIN | POLLRDNORM;
return mask;
}
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,
};
static int memdev_init(void)
{
int result;
int i;
dev_t devno = MKDEV(mem_major, 0);
if (mem_major)
result = register_chrdev_region(devno, 2, "memdev");
else
{
result = alloc_chrdev_region(&devno, 0, 2, "memdev");
mem_major = MAJOR(devno);
}
if (result < 0)
return result;
cdev_init(&cdev, &mem_fops);
cdev.owner = THIS_MODULE;
cdev.ops = &mem_fops;
cdev_add(&cdev, MKDEV(mem_major, 0), MEMDEV_NR_DEVS);
mem_devp = kmalloc(MEMDEV_NR_DEVS * sizeof(struct mem_dev), GFP_KERNEL);
if (!mem_devp)
{
result = - ENOMEM;
goto fail_malloc;
}
memset(mem_devp, 0, sizeof(struct mem_dev));
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);
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;
}
static void memdev_exit(void)
{
cdev_del(&cdev);
kfree(mem_devp);
unregister_chrdev_region(MKDEV(mem_major, 0), 2);
}
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];
strcpy(Buf,"memdev is char dev!");
printf("BUF: %s\n",Buf);
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
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));
printf("Read BUF2: %s\n",Buf);
close(fd);
return 0;
}
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