Re: splice() and file offsets

[Michael Kerrisk]

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.

I'm still not clear here.  Let me phrase my question another way:
why is it that the presence or absence of off_out affects whether
or not splice() changes the current file offset for fd_out?

> It's identical to how sendfile() works.

But it isn't: sendfile() never changes the file offset 
of its 'in_fd'.

Cheers,

Michael
-- 
Michael Kerrisk
maintainer of Linux man pages Sections 2, 3, 4, 5, and 7 

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