1、阻塞型字元裝置驅動的功能
當一個裝置無法立刻滿足使用者的讀寫請求時應當如何處理? 例如:調用read時沒有資料可讀, 但以後可能會有;或者一個程序試圖向裝置寫入資料,但是裝置暫時沒有準備好接收資料。應用程式通常不關心這種問題,應用程式隻是調用 read 或 write 并得到傳回值。驅動程式應當(預設地)阻塞程序,使它進入睡眠,直到請求可以得到滿足。
2、阻塞方式
1)在阻塞型驅動程式中,Read實作方式如下:如果程序調用read,但裝置沒有資料或資料不足,程序阻塞。當新資料到達後,喚醒被阻塞程序。
2)在阻塞型驅動程式中,Write實作方式如下:如果程序調用了write,但裝置沒有足夠的空間供其寫入資料,程序阻塞。當裝置中的資料被讀走後,緩沖區中空出部分空間,則喚醒程序。
3、非阻塞方式
阻塞方式是檔案讀寫操作的預設方式,但應用程式員可通過使用O_NONBLOCK标志來人為的設定讀寫操作為非阻塞方式(該标志定義在<linux/fcntl.h>中,在打開檔案時指定)。
如果設定了O_NONBLOCK标志,read和write的行為是不同的。如果程序在沒有資料就緒時調用了read,或者在緩沖區沒有空間時調用了write,系統隻是簡單地傳回-EAGAIN,而不會阻塞程序。
4、執行個體分析
程式實作的功能當程序讀檔案時,沒有資料可讀,則該程序阻塞。
1)memdev.h源代碼
#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)阻塞型字元驅動memdev.c
#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 "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;
}
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,
};
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));
}
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("yinjiabin");
MODULE_LICENSE("GPL");
module_init(memdev_init);
module_exit(memdev_exit);
3)測試程式源碼app-read.c
#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;
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);
FD_SET(fd, &rds);
strcpy(Buf,"Buf is NULL!");
printf("Read BUF1: %s\n",Buf);
ret = select(fd + 1, &rds, NULL, NULL, NULL);
if (ret < 0)
{
printf("select error!\n");
exit(1);
}
if (FD_ISSET(fd, &rds))
read(fd, Buf, sizeof(Buf));
printf("Read BUF2: %s\n",Buf);
close(fd);
return 0;
}