Jens,
> > 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?
>
> The logic is simple - either you don't give an explicit offset, and the
> current position is used and updated. Or you give an offset, and the
> current position is ignored (not read, not updated).
Yes, I understand what the code is doing, but *why* do
things this way? (To put things another way: why not *always
have splice() update the file offset?) I realise there may be
some good reason for this, and if there is, it will go into the
man page!
> > > It's identical to how sendfile() works.
> >
> > But it isn't: sendfile() never changes the file offset
> > of its 'in_fd'.
>
> Ehm, yes it does. Would you expect the app to do an appropriate lseek()
> on every sendfile() call?
No! It does not! See the sendfile.2 man page: "sendfile()
does not modify the current file offset of in_fd."
(You had me worried -- I just now went and *tested*
the operation of sendfile().) The app does not need to
do an lseek() call because the 'offset' argument is *always*
updated with the new "virtual" offset. This is part of why I
am disturbed/confused: sendfile() always updates its 'offset'
argument and *never* changes the file offset; splice() only
does that if its 'offset' argument is non-NULL.
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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