Messages in this thread | | | Date | Mon, 10 Jul 2006 14:51:50 +0200 | From | Jens Axboe <> | Subject | Re: splice() and file offsets |
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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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