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SubjectRe: splice() and file offsets
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

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