Re: [fuse-devel] read return value

[Matt T. <mat...@gm...>]

On 5 March 2012 18:25, Hans Beckerus <han...@gm...> wrote:

> Is it possible that someone could elaborate on what to return from read
> calls?
> If direct_io=0, my understanding is that a read should always return the
> exact number of requested bytes.
> But what is confusing me is the case when offset+size exceeds the file
> size? Is it still required to return the number of requested bytes but
> pad buffer with zeros or is it ok to return only the size actually read
> until EOF without padding?
> Also, if the read request offset is equal or greater than the file size,
> should I simply return 0 to indicate EOF?
>

Importantly the return value of read() and the number of *valid* bytes
written into buf should be equal. read() should return the number of bytes
successfully read. If offset + size exceeds the length of the file then
read() should return the number of bytes it was able to satisfy. That much
of buf should be filled with the data; the rest can be left uninitialised
or zero'd or something.

read() can also chose not to return all the bytes asked for because of an
error or because it simply doesn't want to perform an expensive operation
yet. Again, just be sure to indicate this with the return value. Good
userspace code should notice this and re-request those bytes.

In the general case, read() will be able to return all the bytes requested,
and hence should return count.

I used to assume that offset would always be < file size, and asserted this
in my filesystem. However you do see the odd request with offset >= file
size. I've found that just returning 0 works fine. The read() man page
implies this with "zero indicates end of file".

matt