On Tue, Mar 3, 2009 at 8:22 AM, aepensky <apen...@gmail.com> wrote:

> I was reading through the code and I'm not sure I understand the
> thought behing the ZeroCopyInputStream implementations.
> It seems to me that if the underlying data source actually *is* a
> stream to begin with, "ZeroCopy" does not perform any fewer copies;
> using the copying adapter, it allocates an internal buffer, fills the
> internal buffer from the stream as needed, and then provides a
> ZeroCopy stream interface to that internal buffer.  So, the amount of
> copying appears to be the same as if you read the stream into memory
> yourself and then called ParseFromArray.
> What am I missing?

You're missing nothing.  ZeroCopyInputStream only provides advantages when
the data source is *not* a stream to begin with.  If you already happen to
have your data in in-memory data structures, then ZeroCopyInputStream
provides an abstract way to read that data without copying any bytes.

It may also be possible to write a true ZeroCopyInputStream for OS-level
streams.  For files you could do it easily using mmap(), or for general file
descriptors you might be able to do it with AIO.  But I don't know of anyone
who has bothered to do this.

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