Re: [PATCH v6 0/5] Add preadv & pwritev system calls.

[Gerd Hoffmann <kraxel@redhat.com>]

Ulrich Drepper wrote:
> If they are in the kernel there is no reason not to export them from
> glibc.

Great.

> But I have a general comment about all kinds of read syscalls.
> If think they have been misdesigned from day one and if we are going to
> add new ones we might want to fix them.
> 
> The problem is that they don't allow for zero-copy operations in enough
> cases.  The kernel is not free to store the data wherever it wants even
> if the userlevel code is fine with that.

[ ... more text snipped ... ]

I do see the point in adding a interface like this ...

>    ssize_t readz (int fd, void *buf, size_t len, void **res)

... to help the kernel do zero-copy I/O.

I think system calls for vector I/O are *not* the right place for that
though.  Usually applications use vectored I/O because they *do* care
about the place the data is stored, because vectored I/O allows them to
avoid copying data within the application.

cheers,
  Gerd