On Wed, Sep 17, 2014 at 12:59:30PM -0400, Theodore Ts'o wrote:
> On Wed, Sep 17, 2014 at 12:05:23PM -0400, Milosz Tanski wrote:
> > Theodore,
> >
> > I might be missing understanding something, but... I already omitted
> > read2 and write2 which can be implemented in userspace by libc (as you
> > pointed out). In the case of readv vs. preadv there's an extra
> > positional argument (file offset) and preadv version doesn't change
> > the file location. I didn't want to overload the meaning of preadv2 to
> > take a special negative offset value that uses the current file
> > position but also changes the file position.
>
> off_t has to be signed, so having a magic negative value doesn't
> bother me that much. Or you could use a flag bitvalue which means to
> use the fd's offset and to ignore the positional value. (More
> bike-shedding :-)
splice has already set the precedent for an optionally specified offset
that falls back to the files's position.
static long do_splice(struct file *in, loff_t __user *off_in,
struct file *out, loff_t __user *off_out,
size_t len, unsigned int flags)
{
...
if (off_out) {
if (copy_from_user(&offset, off_out, sizeof(loff_t)))
return -EFAULT;
} else {
offset = out->f_pos;
}
...
if (!off_out)
out->f_pos = offset;
else if (copy_to_user(off_out, &offset, sizeof(loff_t)))
ret = -EFAULT;
It's nice and simple and lets you update the user's offset.
> So the suggestion was one of trying to (probably fruitlessly) trying
> to stem the expnoential increase in read/write system calls. :-)
I support this windmill tilting :).
- z
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