Messages in this thread | | | Date | Wed, 17 Sep 2014 10:24:17 -0700 | From | Zach Brown <> | Subject | Re: [PATCH 2/7] Define new syscalls readv2,preadv2,writev2,pwritev2 |
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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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