linux驱动28：定位设备llseek

        llseek方法实现了lseek和llseek系统调用，如果设备操作未定义llseek方法，内核默认通过修改filp->f_pos执行定位，filp->f_pos是文件的当前读写位置。为了使lseek系统调用正常工作，read和write方法需要更新它们收到的偏移量参数。

        然而大多数设备定位是没有意义的，这些设备只提供数据流，而不是数据区。如果不需要llseek方法，不能简单地不声明llseek，因为默认是允许定位的。应该在open方法中调用nonseekable_open通知内核设备不支持llseek，filp将被标记为不可定位；为了完整，还应该将file_operations中的llseek设置为no_llseek（定义在<linux/fs.h>）。

int nonseekable_open(struct inode *inode, struct file *filp);

#include <linux/init.h>
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/kdev_t.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/uaccess.h>

#define MAX_BUF_LEN 1024

static int major = 277;
static int minor = 0;
static dev_t helloNum;
struct class *helloClass = NULL;
struct device *helloDev = NULL;
static char helloBuf[MAX_BUF_LEN] = {0};
static loff_t helloBufPos = 0;
static int helloBufSize = 0;

int hello_open(struct inode *pinode, struct file *pfile)
{
	printk("hello_open, minor:%d, major:%d\n", iminor(pinode), imajor(pinode));
	return 0;
}

int hello_release(struct inode *pinode, struct file *pfile)
{
	printk("hello_release, minor:%d, major:%d\n", iminor(pinode), imajor(pinode));	
	return 0;
}

ssize_t hello_read(struct file *filep, char __user *buf, size_t size, loff_t *pos)
{
	int copySize = size;

	if (helloBufSize <= helloBufPos)
	{
		return 0;
	}

	if (copySize > helloBufSize - helloBufPos)
	{
		copySize = helloBufSize - helloBufPos;
	}

	if(copy_to_user(buf, helloBuf + helloBufPos, copySize))
	{
		return -EFAULT;
	}

	*pos += copySize;
	helloBufPos = *pos;
	return copySize;
}

ssize_t hello_write(struct file *filep, const char __user *buf, size_t size, loff_t *pos)
{
	
	if(copy_from_user(helloBuf + helloBufPos, buf, size))
	{
		return -EFAULT;
	}
	
	helloBufSize = *pos + size;
	*pos += size;
	helloBufPos = *pos;
	printk("write:%s\n", helloBuf);
	return size;
}


loff_t hello_llseek(struct file *filep, loff_t off, int whence)
{
	loff_t newPos = 0;

	switch(whence)
	{
		case SEEK_SET:
			newPos = off;
			break;
		case SEEK_CUR:
			newPos = helloBufPos + off;
			break;
		case SEEK_END:
			newPos = helloBufSize + off;
			break;
		default:
			return -EINVAL;
	}

	if (newPos < 0)
	{
		return -EINVAL;
	}
	
	helloBufPos = newPos;
	return newPos;
}

static struct file_operations hello_ops = {
	.open = hello_open,
	.release = hello_release,
	.read = hello_read,
	.write = hello_write,
	.llseek = hello_llseek,
};

static int hello_init(void)
{
	int ret = 0;
	printk("hello_init\n");

	ret = register_chrdev( major, "hello", &hello_ops);
	if(ret < 0)
	{
		printk("register_chrdev failed.\n");
		return ret;
	}

	helloClass = class_create(THIS_MODULE, "hellocls");
	if (IS_ERR(helloClass)) 
	{
		printk("class_create failed.\n");
		ret = PTR_ERR(helloClass);
		goto error_exit1;
	}

	helloNum = MKDEV(major,minor);
	printk("major:%d, minor:%d\n", MAJOR(helloNum), MINOR(helloNum));

	helloDev = device_create(helloClass, NULL, helloNum, NULL, "hello0");
	if (IS_ERR(helloDev)) 
	{
		printk("device_create failed.\n");
		ret = PTR_ERR(helloDev);
		goto error_exit2;
	}
	
	return 0;

error_exit2:
	class_destroy(helloClass);

error_exit1:
	unregister_chrdev(major,"hello");

	return ret;
}

static void hello_exit(void)
{
	printk("hello_exit\n");
	device_destroy(helloClass, helloNum);
	class_destroy(helloClass);
	unregister_chrdev(major,"hello");
}

MODULE_LICENSE("GPL");
module_init(hello_init);
module_exit(hello_exit);

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char * argv [ ])
{
	int fd = open("/dev/hello0", O_RDWR, 666);
	if (fd < 0)
	{
		perror("open faied");
		return -1;
	}
	printf("open successed: %d\n", fd);

	write(fd, "hello1", 6);
	lseek(fd, 0, SEEK_SET);
	char readBuf[32] = {0};
	read(fd, readBuf, sizeof(readBuf));
	printf("read:%s\n", readBuf); 

	lseek(fd, 0, SEEK_END);
	write(fd, "hello2", 6);
	lseek(fd, 0, SEEK_SET);
	read(fd, readBuf, sizeof(readBuf));
	printf("read:%s\n", readBuf); 
	
	close(fd);
	return 0;
}