Re: splice() and file offsets

[Jens Axboe <axboe@suse.de>]

On Mon, Jul 10 2006, Michael Kerrisk wrote:
> Jens,
> 
> What are the semantics of splice() supposed to be with respect 
> to the current file offsets of 'fd_in' and 'fd_out', and how
> is the presence or absence (NULL) of 'off_in' and 'off_out'
> supposed to affect things.
> 
> Using the program below, here is what I observe for 
> fd_out/off_out:
> 
> 1. If off_out is NULL, then 
>    a) splice() changes the current file offset of fd_out.
> 
> 2. If off_out is not NULL, then splice() 
>    a) does not change the current file offset of fd_out, but 
>    b) treats off_out as a value result parameter, returning 
>       an updated offset of the file.
> 
> It is "2 a)" that surprises me.  But perhaps it's expected 
> behaviour; or I'm doing something dumb in my test program.

Not sure why you find that surprising, that is exactly what is supposed
to happen :-)

If you don't give off_out, we use the current position. For most people,
that's probably what they want. If you are sharing the fd, that doesn't
work though. So you pass off_in/off_out as you please, and the kernel
uses those and passes the updated parameter back out so you don't have
to update it manually.

It's identical to how sendfile() works.

-- 
Jens Axboe