Linux操作系统内核和设备文件对话(1)(2)

/* Device Declarations ******************************** */

/* The name for our device, as it will appear in

* /proc/devices */

#define DEVICE_NAME "char_dev"

/* The maximum length of the message for the device */

#define BUF_LEN 80

/* Is the device open right now? Used to prevent

* concurent access into the same device */

static int Device_Open = 0;

/* The message the device will give when asked */

static char Message[BUF_LEN];

/* How far did the process reading the message get?

* Useful if the message is larger than the size of the

* buffer we get to fill in device_read. */

static char *Message_Ptr;

/* This function is called whenever a process attempts

* to open the device file */

static int device_open(struct inode *inode,

struct file *file)

{

#ifdef DEBUG

printk ("device_open(%p)\n", file);

#endif

/* We dont want to talk to two processes at the

* same time */

if (Device_Open)

return -EBUSY;

/* If this was a process, we would have had to be

* more careful here, because one process might have

* checked Device_Open right before the other one

* tried to increment it. However, were in the

* kernel, so were protected against context switches.

*

* This is NOT the right attitude to take, because we

* might be running on an SMP box, but well deal with

* SMP in a later chapter.

*/

Device_Open++;

/* Initialize the message */

Message_Ptr = Message;

MOD_INC_USE_COUNT;

return SUCCESS;

}

/* This function is called when a process closes the

* device file. It doesnt have a return value because

* it cannot fail. Regardless of what else happens, you

* should always be able to close a device (in 2.0, a 2.2

* device file could be impossible to close). */

#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)

static int device_release(struct inode *inode,

struct file *file)

#else

static void device_release(struct inode *inode,

struct file *file)

#endif

{

#ifdef DEBUG

printk ("device_release(%p,%p)\n", inode, file);

#endif

/* Were now ready for our next caller */

Device_Open --;

MOD_DEC_USE_COUNT;

#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)

return 0;

#endif

}

/* 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