设备文件指针操控llseek

    llseek主要用来操作文件的指针.其实质是内核依赖filp->f_ops执行文件的定位.

 

1.原型:

llseek(struct file *filp, loff_t off, int whence);

    各参数意义如下:

filp:用户空间打开的设备文件描述符;off:偏移量;whence:基址.一共有三个标识:SEEK_SET、SEEK_CUR和SEEK_END.这三个标识一般和off配合使用.如下:SEEK_SET:off即为新的读写位置;SEEK_CUR:当前读写位置后增加off个偏移量;SEEK_END:将读写位置指向文件尾后再增加off个偏移量.

    如下:

static loff_t mem_llseek(struct file *filp, loff_t offset, int whence){    loff_t newpos;    switch(whence) {      case 0: /* SEEK_SET */        newpos = offset;        break;      case 1: /* SEEK_CUR */        newpos = filp->f_pos + offset;        break;      case 2: /* SEEK_END */        newpos = MEMDEV_SIZE -1 + offset;        break;      default: /* can't happen */        return -EINVAL;    }    if ((newpos<0) || (newpos>MEMDEV_SIZE))        return -EINVAL;    filp->f_pos = newpos;    return newpos;}

2.llseek对write/read的影响:

    当实现了llseek系统调用,在进行读写时,必须对文件位置指针进行"校准".如:

    read:

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;  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);  }  return ret;}

    write:

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);  }  return ret;}

3.实际的使用情况:

    在驱动中,很少会用到llseek系统调用.相反,为了严谨编程,需要对其禁用.需要:

1).把filp标记为不可定位--通过在open方法中调用nonseekable_open;2).设置fops的llseek域为no_llseek;

    如下:

static int scull_p_open(struct inode *inode, struct file *filp){    struct scull_pipe *dev;    dev = container_of(inode->i_cdev, struct scull_pipe, cdev);    filp->private_data = dev;    if (down_interruptible(&dev->sem))        return -ERESTARTSYS;    if (!dev->buffer) {        /* allocate the buffer */        dev->buffer = kmalloc(scull_p_buffer, GFP_KERNEL);        if (!dev->buffer) {            up(&dev->sem);            return -ENOMEM;        }    }    dev->buffersize = scull_p_buffer;    dev->end = dev->buffer + dev->buffersize;    dev->rp = dev->wp = dev->buffer; /* rd and wr from the beginning */    /* use f_mode,not  f_flags: it's cleaner (fs/open.c tells why) */    if (filp->f_mode & FMODE_READ)        dev->nreaders++;    if (filp->f_mode & FMODE_WRITE)        dev->nwriters++;    up(&dev->sem);    return nonseekable_open(inode, filp);}

struct file_operations scull_pipe_fops = {    .owner =    THIS_MODULE,    .llseek =   no_llseek,    .read =     scull_p_read,    .write =    scull_p_write,    .poll =     scull_p_poll,    .ioctl =    scull_ioctl,    .open =     scull_p_open,    .release =  scull_p_release,    .fasync =   scull_p_fasync,};

4.实例:

    下面实例源于国嵌:

    app:

#include <stdio.h>int main(){FILE *fp0 = NULL;char Buf[4096];strcpy(Buf,"Mem is char dev!");printf("BUF: %s\n",Buf);fp0 = fopen("/dev/memdev0","r+");if (fp0 == NULL){printf("Open Memdev0 Error!\n");return -1;}fwrite(Buf, sizeof(Buf), 1, fp0);fseek(fp0,0,SEEK_SET);strcpy(Buf,"Buf is NULL!");printf("BUF: %s\n",Buf);fread(Buf, sizeof(Buf), 1, fp0);printf("BUF: %s\n",Buf);return 0;}

    drv:

    memdev.h

#ifndef _MEMDEV_H_#define _MEMDEV_H_#ifndef MEMDEV_MAJOR#define MEMDEV_MAJOR 251 #endif#ifndef MEMDEV_NR_DEVS#define MEMDEV_NR_DEVS 2    #endif#ifndef MEMDEV_SIZE#define MEMDEV_SIZE 4096#endifstruct mem_dev                                     {                                                          char *data;                        unsigned long size;       };#endif /* _MEMDEV_H_ */

    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 <linux/device.h> #include "memdev.h"static int mem_major = MEMDEV_MAJOR;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;    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);  }  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);  }  return ret;}static loff_t mem_llseek(struct file *filp, loff_t offset, int whence){     loff_t newpos;    switch(whence) {      case 0: /* SEEK_SET */        newpos = offset;        break;      case 1: /* SEEK_CUR */        newpos = filp->f_pos + offset;        break;      case 2: /* SEEK_END */        newpos = MEMDEV_SIZE -1 + offset;        break;      default: /* can't happen */        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){struct class *myclass;  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);  }myclass = class_create(THIS_MODULE,"test_char"); device_create(myclass, NULL, MKDEV(mem_major,0), NULL, "memdev0");     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("David Xie");MODULE_LICENSE("GPL");module_init(memdev_init);module_exit(memdev_exit);