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
Want to help with man page maintenance?
Grab the latest tarball at
ftp://ftp.win.tue.nl/pub/linux-local/manpages/,
read the HOWTOHELP file and grep the source
files for 'FIXME'.
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