Comment 1:

Labels changed: added repo-main, release-go1.5.

Comment 2:

There is a way.
f, _ := os.Open(filename)
f.Seek(offset, 0)
c.ReadFrom(io.LimitReader(f, count))

Status changed to Invalid.

Comment 3:

That doesn't currently work either, because an io.LimitedReader isn't an *io.File, and
fails the type assertion in sendfile_linux.go. It would be nice if that worked too.
Using Seek though still doesn't quite cover the sendfile api. If you call sendfile with
an offset, it does not modify the current file offset.

Comment 4:

(Sorry, I missed the re-assignment of r from the LimitedReader)
Though using Seek though still doesn't quite cover the sendfile api. If you call
sendfile with an offset, it does not modify the current file offset.

Comment 5:

We're not trying to cover the sendfile API. We're trying to make it possible to write
meaningful portable code that uses the sendfile optimization automatically. And we have.
ReadFrom changes the file offset. You can't avoid that and still have ReadFrom. So if
you really can't modify the file offset you're asking us to introduce a completely
different way to get to sendfile on top of what's already there. And I'd really rather
not. It's easy to open a file multiple times.