On Tue, Oct 8, 2013 at 7:06 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Oct 08, 2013 at 06:38:47PM -0700, Linus Torvalds wrote:
>> On Tue, Oct 8, 2013 at 6:18 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>> >
>> > Point, but I would argue that we should yell very loud if we get 0 from
>> > vfs_write() for non-zero size. I'm not sure if POSIX allows write(2)
>> > to return that, but a lot of userland code won't be expecting that and
>> > won't be able to cope...
>>
>> Actually POSIX very much allows zero returns. O_NDELAY is mentioned as
>> a possible cause, in addition to zero-sized writes themselves, of
>> course.
>
> Umm... What it says is "If some data can be written without blocking the
> thread, write() shall write what it can and return the number of bytes
> written. Otherwise, it shall return -1 and set errno to EAGAIN."
Look closer.
".. most historical implementations return zero (with the O_NDELAY
flag set, which is the historical predecessor of O_NONBLOCK .."
>> Also, writing to (but not past) the end of a block device returns 0
>> for "end of device", iirc.
>
> What do you mean? If the starting position is below the end of device,
> we get a non-zero length write, not exceeding the end. If it's at
> the end of device, we get -ENOSPC. It's out of scope for POSIX, but
> Linux is definitely acting that way...
Hmm. I'm pretty sure I've seen zero returns for EOF somewhere..
Linus
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