Re: [fuse-devel] read return value

[Hans B. <han...@gm...>]

On 2012-03-05 8:25, Matt Turner wrote:
> On 5 March 2012 18:25, Hans Beckerus <han...@gm... 
> <mailto: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

Thanks. All that makes sense. But then, do we actually need to handle 
the direct_io=1 case specifically?
According to FUSE API reference when direct_io=1 the return from read 
should be equal to the system call read().
For normal case that should be ok with a common implementation. But I am 
a bit worried about the error cases? If the return
should match the system call, then a call to fuse read() should return 
-1 and set errno instead of returning -errno ?

Hans