f_pos定义在file结构体(定义在<linux/fs.h>),表示文件当前的读写位置:

struct file {
    ......
    loff_t          f_pos;
    ......
}

LDD3中关于f_pos的描述:

loff_t f_pos;

The current reading or writing position. loff_t is a 64-bit value on all platforms ( long long in gcc terminology). The driver can read this value if it needs to know the current position in the file but should not normally change it; read and write should update a position using the pointer they receive as the last argument instead of acting on filp->f_pos directly. The one exception to this rule is in the llseek method, the purpose of which is to change the file position.

驱动的读写操作不需要直接更新filp->f_pos。关于其中原因,可参考这篇笔记