fseek char linux device

I'm trying to write a simple char device for linux, I need to read and write the device by fread/fwrite and use fopen and fseek. I've written a simple test program to use my device and I've noted that fpos in fseek function doesn't work and unknow fread after fseek appears. I cannot get the device working because fseek doesn't get a correct file position when opened in r+ mode, everything works except fread if I use file opened in w mode instead.

Thanks folks

/* Makefile */

obj-m += char.o

all:
        make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
        make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean


/* char device char.c */
// to complie:
// # make
// # insmod char.ko
// # dmesg 
// (to get majornumber)
// # mknod /dev/dp0 c majornumber 0
// # ./test
// # dmesg

#include <linux/init.h>
#include <linux/module.h>
#include <linux/types.h>
#include <linux/errno.h>
#include <linux/kdev_t.h>
#include <linux/cdev.h>
#include <linux/fs.h>
#include <asm/uaccess.h>
#include <asm/atomic.h>

int char_init (void);
void char_exit (void);

int char_open (struct inode *, struct file *);
int char_release (struct inode *, struct file *);
ssize_t char_read (struct file *, char __user *, size_t, loff_t *);
ssize_t char_write (struct file *, const char __user *, size_t, loff_t *);
loff_t char_llseek (struct file *, loff_t, int);



int char_open (struct inode *inode, struct file *fp)
{

  printk ("f_mode=%d f_pos=%d\n", (int) fp->f_mode, (int)fp->f_pos);
  return 0;
} 



int char_release (struct inode *inode, struct file *fp)
{
  return 0;
} 


ssize_t char_read (struct file *fp, char __user *buff, size_t count, loff_t *fpos)
{
  printk ("char_read() fp->f_pos=%d\n", (int) fp->f_pos);
  return count;
}


ssize_t char_write (struct file *fp, const char __user *buff, size_t count, loff_t *fpos)
{
  printk ("char_write() fp->f_pos=%d\n", (int) fp->f_pos);
  return count;
}


loff_t char_llseek (struct file *fp, loff_t fpos, int whe)
{
  printk ("char_llseek() fpos=%d\n ", (int) fpos);
  fp->f_pos = fpos;

  return fpos;
}


struct file_operations char_fops =
  {
    .owner = THIS_MODULE,
    .open = char_open,
    .release = char_release,
    .read = char_read,
    .write = char_write,
    .llseek = char_llseek,
    .ioctl = NULL,
  };


/* the char device */
struct cdev cdev;


int char_init ()
{
  dev_t mm;
  int mj, mi, ret;

  mi = 0;
  ret = alloc_chrdev_region (&mm, mi, 1, "dp0");
  mj = MAJOR(mm);
  cdev_init (&cdev, &char_fops);
  ret = cdev_add (&cdev, mm, 1);

  if (ret<0)
    {
      printk ("char: unable to add device\n");
      return -1;
    }
  printk ("char: device added major=%d\n", mj);

  return 0;
}


void char_exit ()
{
  cdev_del(&cdev);
  printk ("char: device deleted\n");
}

module_init(char_init);
module_exit(开发者_如何转开发char_exit);



/* test program test.c */

#include <stdio.h>
#include <string.h>

int main (int argc, char *argv[])
{
  int ret;
  FILE *fp;

  fp = fopen ("/dev/dp0", "r+");

  if (fp == NULL)
    {
      printf ("open failed\n");
      return -1;
    }


  int x;

  x= fseek (fp, 1, SEEK_SET);
  printf("ret fseek=%d\n", x);

  fseek (fp, 1, SEEK_SET);

  fwrite ("\0", 1, 1, fp);

  fseek (fp, 2, SEEK_SET);

  fread (&ret, 1, 1, fp);

  printf ("DONE\n");

  fclose (fp);
  return 0;
}

// this is my dmesg:

f_mode=31 f_pos=0 // this is my fopen
char_llseek() fpos=0 // this is my first fseek but why fpos=0? it should be 1
 char_read() fp->f_pos=0 // what's that?
char_llseek() fpos=0 //this is my second fseek but why fpos is 0 again?
 char_read() fp->f_pos=0 // I suppose this is my fread fpos=0?
char_llseek() fpos=-4095 // what's that?
 char_llseek() fpos=-4095 // What's that too?
 char_llseek() fpos=-4095 // What's that too?

You are using stdio (fread, fseek, etc.) which generally has a buffer of sorts. It may not necessarily translate directly to the syscalls, hence the "unexpected" reads and seeks.