[Rd] Using rawConnection() + gzfile() to obtain a raw vector of compressed data does not appear to work.

[Pavel Krivitsky]

Thanks, Ivan!

On Sun, 2026-09-13 at 18:48 +0300, Ivan Krylov wrote:
> В Sat, 12 Sep 2026 23:49:47 +0000
> Pavel Krivitsky via R-devel <r-devel using r-project.org> пишет:
>
> > In practice, the gzcon() actually modifies `compressed` in place,
> > which is fine if it's a file or similar, but apparently
> > rawConnectionValue() no longer works on it.
>
> There's also another problem: gzcon buffers the compressed content
> internally and only writes it into the underlying connection when the
> buffer is full or when the connection is closed, but then there's no
> valid connection to call rawConnectionValue() on. The feature request
> with a candidate patch is now at
> https://bugs.r-project.org/show_bug.cgi?id=19172.

Ah, that explains why all the generative AIs were saying to close the
connection---and it still didn't work.

> > Am I using the wrong approach? Is there a workaround? memCompress()
> > uses an incompatible format.
>
> What's the intended use for the compressed buffer? In theory, it may
> be
> possible to chop off the zlib header and construct the RFC1952 header
> and trailer by hand, though CRC32 may be annoying to calculate.

Long story short, I wanted to get a raw vector containing the output of
saveRDS() without actually writing a file to the file system. Since
saveRDS() ignores compression settings when writing to a connection, I
needed to do the gzipping myself.

I thought it would be good to flag this issue, to either fix the
problem or fix the documentation; myself, I eventually ended up just
using temporary files. (This was a one-time thing, so not worth
optimising.)

                                        Best,
                                        Pavel

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