Linux操作系统内核和设备文件对话

/* This function is called whenever a process which  
* has already opened the device file attempts to  
* read from it. */  
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)  
static ssize_t device_read(  
struct file *file,  
char *buffer, /* The buffer to fill with the data */  
size_t length, /* The length of the buffer */  
loff_t *offset) /* offset to the file */  
#else  
static int device_read(  
struct inode *inode,  
struct file *file,  
char *buffer, /* The buffer to fill with the data */  
int length) /* The length of the buffer  
* (mustnt write beyond that!) */  
#endif  
{  
/* Number of bytes actually written to the buffer */  
int bytes_read = 0;  

#ifdef DEBUG  
printk("device_read(%p,%p,%d)\n",  
file, buffer, length);  
#endif  

/* If were at the end of the message, return 0  
* (which signifies end of file) */  
if (*Message_Ptr == 0)  
return 0;  

/* Actually put the data into the buffer */  
while (length && *Message_Ptr) {  

/* Because the buffer is in the user data segment,  
* not the kernel data segment, assignment wouldnt  
* work. Instead, we have to use put_user which  
* copies data from the kernel data segment to the  
* user data segment. */  
put_user(*(Message_Ptr++), buffer++);  
length --;  
bytes_read ++;  
}  

#ifdef DEBUG  
printk ("Read %d bytes, %d left\n",  
bytes_read, length);  
#endif  

/* Read functions are supposed to return the number  
* of bytes actually inserted into the buffer */  
return bytes_read;  
}

/* This function is called when somebody tries to  
* write into our device file. */  
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)  
static ssize_t device_write(struct file *file,  
const char *buffer,  
size_t length,  
loff_t *offset)  
#else  
static int device_write(struct inode *inode,  
struct file *file,  
const char *buffer,  
int length)  
#endif  
{  
int i;  

#ifdef DEBUG  
printk ("device_write(%p,%s,%d)",  
file, buffer, length);  
#endif  

for(i=0; i  
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)  
get_user(Message, buffer+i);  
#else  
Message = get_user(buffer+i);  
#endif  

Message_Ptr = Message;  

/* Again, return the number of input characters used */  
return i;  
}