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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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