linux驱动---file_operations之llseek

简述：
file_operations中llseek是用于定位设备文件位置。
lseek,sys_lseek（在include/unistd.h,如果是在宿主机上，ubuntu所在路径/usr/include/unistd.h）这两个系统调用，会调用到file_operations中的llseek.

file_operations结构体定义：

这是Linux内核3.10版本
定义在include/linux/fs.h中

struct file_operations {    struct module *owner;    loff_t (*llseek) (struct file *, loff_t, int);    ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);    ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);    ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);    ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);    int (*readdir) (struct file *, void *, filldir_t);    unsigned int (*poll) (struct file *, struct poll_table_struct *);    long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);    long (*compat_ioctl) (struct file *, unsigned int, unsigned long);    int (*mmap) (struct file *, struct vm_area_struct *);    int (*open) (struct inode *, struct file *);    int (*flush) (struct file *, fl_owner_t id);    int (*release) (struct inode *, struct file *);    int (*fsync) (struct file *, loff_t, loff_t, int datasync);    int (*aio_fsync) (struct kiocb *, int datasync);    int (*fasync) (int, struct file *, int);    int (*lock) (struct file *, int, struct file_lock *);    ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);    unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);    int (*check_flags)(int);    int (*flock) (struct file *, int, struct file_lock *);    ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);    ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);    int (*setlease)(struct file *, long, struct file_lock **);    long (*fallocate)(struct file *file, int mode, loff_t offset,              loff_t len);    int (*show_fdinfo)(struct seq_file *m, struct file *f);};

其中：
loff_t (llseek) (struct file , loff_t, int);
功能：定位文件位置
第一个参数：打开的文件指针（文件句柄）
第二个参数：文件偏移量（偏移量单位是字节）
第三个参数：文件起止位置标志

文件起止位置标志：
一般有下面几个标志：

#define SEEK_SET    0   /* seek relative to beginning of file */#define SEEK_CUR    1   /* seek relative to current file position */#define SEEK_END    2   /* seek relative to end of file */#define SEEK_DATA   3   /* seek to the next data */#define SEEK_HOLE   4   /* seek to the next hole */#define SEEK_MAX    SEEK_HOLE

前面三个是常用的：
SEEK_SET：表从文件开始位置加上偏移量处，（偏移量单位是字节）
SEEK_CUR:表示当前文件位置加上偏移量处，（偏移量单位是字节）
SEEK_END：表示从文件结束位置加上偏移量，（偏移量单位是字节）

在系统调用lseek中：

extern __off_t lseek(int __fd , __off_t __offset, int __whence);

第三个参数__whence就是传递这个标志到驱动的。

文件位置是由struct file结构体成员f_ops表示的:
定义在include/linux/fs.h中

loff_t          f_pos;

驱动程序中llseek例子：

loff_t d_llseek(struct file *f, loff_t off, int whence){    ....    ...    switch(whence)    {        case SEEK_SET:            f->f_pos = off;//文件开始位置是0            break;        case SEEK_CUR:            f->f_pos += off;            break;        case SEEK_END:            f->f_pos = dev->size + off;        default：            break;    }}

dev->size是指设备文件最大空间，文件开始位置是0，自然结束位置就是最大空间的数字。

这样就达到了对文件位置f->f_pos修改，从而llseek实现了定位功能。
(其实可以随意修改f_pos来定位文件位置，但是不能这么做，会乱套。)

ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);

其中read,write的第四个参数也是偏移量。
如驱动有下面的read函数：

char buf[1000];ssize_t d_read(struct file *f, char __user *up, size_t s, loff_t *off){    char *p = buf;    p += (f->f_ops+(*off));    copy_to_user(up,p,s);}

p += (f->f_ops+(*p))表示定位文件位置。上面这段代码，就是从指定的位置处，读取s个字节到用户空间up。

对于不支持llseek的驱动，应该在open中调用nonseekable_open():

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

并且将file_operations中的llseek设置成no_llseek,这样当有对这个文件调用lseek，系统会拒绝。

如下驱动例子：

int d_open(struct inode *in, struct file *f){    ...    ..    nonseekable_open(in,f);    ...    ..}struct file_operations op = {    .llseek = no_llseek;    .open = d_open;    ...    ..};

可以参考如下博客：
LDD3源码分析之llseek分析
lseek及llseek介绍