I'm not sure what you mean by "work on the input file", but in general, no.. you cannot read/write/anything a file without opening it (well, ok.. you can do file tests and stat without opening, but I don't think that's what you are talking about) If you can explain a bit more about what you are trying to do and why you see it as a problem to open a large file, we might be able to provide a more complete answer