On 7/13/06, Om. <om.turyx@xxxxxxxxx> wrote:
On 7/11/06, Deepak Joshi <deepak_cins@xxxxxxxxxxx> wrote:
> Hi all,
>
Hi,
> While reading device drivers by rubini, i found following statement related
> to loff_t fops.
>
> The current reading or writing position. loff_t is a 64-bit value (long long
> in gcc terminology). The driver can read this value if it needs to know the
> current position in the file, but should never 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).
vfs subsystem increments the f_pos immediately after the call to
VFS doesn't increment f_pos, but it just assigns the value of pos.
static inline void file_pos_write(struct file *file, loff_t pos)
{
file->f_pos = pos;
}
f_ops->read()/write(). If you increment f_pos in read()/write(), this
value would be incorrect. (would be modified twice)
Its because the kernel writes to file->f_pos with the value that is
contained in pos after we return from the read/write. See the code
below:
loff_t pos = file_pos_read(file);
ret = vfs_read(file, buf, count, &pos);
file_pos_write(file, pos);
The first and third statements could be avoided if the one who writes
the device driver can be more responsible for modifying the
file->f_pos in each call to read/write.
Look in fs/read_write.c for sys_read/sys_write implementations.
Jinesh.
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