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Re: [HELP] (bug?) Saving a buffer without any conversion?

From: Eli Zaretskii
Subject: Re: [HELP] (bug?) Saving a buffer without any conversion?
Date: Tue, 14 Jan 2003 08:06:02 +0200 (IST)

On 14 Jan 2003, Kim F. Storm wrote:

> > We're receiving binary content via a network process.  After the
> > transfer is complete, this buffer should be saved to a file.
> > 
> > The effect I'm having is that we receive 1372422 bytes via the process
> > filter function STRING argument, and after insertion into a buffer,
> > we have a buffer with buffer-size 1372422, but after calling (save-buffer)
> > we get this:
> > 
> > -rw-r--r--    1 root     root      1865264 Jan 13 18:35 blah28.mp3
> > 
> > I'm using:
> > 
> >       (set-process-coding-system proc 'binary 'binary)
> >       (set-buffer-file-coding-system 'no-conversion t)
> > 
> I have looked at Mario's data before sending it to emacs and after
> emacs has written it to a file.
> It seems that every byte in the range 0xa0 .. 0xff that were in the
> original file is prefixed with an 0x81 byte in the file containing the
> received data.  To me, that looks like the internal multi-byte
> representation for the binary data.

Yes.  That's what no-conversion does: it prevents encoding of the 
internal buffer's contents.

I suggest to use raw-text for both coding systems above, and see if that 

An alternative approach is to (set-buffer-multibyte nil) before reading 
the data into it and before saving it.

> The buffer's coding system for save is no-conversion.  How did
> that internal data end up in the file?

Probably because the buffer was a multibyte buffer, in which case 
no-conversion writes out the internal representation.  That's why I 
suggested using raw-text to save the buffer.

The reason you seem to see the right size is that Emacs tries very hard 
to conceal the fact that some characters in the 128-255 code range are 
stored in a multibyte buffer as multibyte sequences.  Using no-conversion 
to save such a buffer exposes the internal representation, so it is 
exactly the thing _not_to_do_ in this case.

> What coding systems should be set on the network process and on the
> buffer to make it possible to have the received binary data in the
> buffer make its way unmangled into the file on the disk?

As I said above, two ways: either force the receiving buffer to be 
unibyte, or use raw-text to save it.  Both ways should have the same 
effect; however, I'm personally biased towards not using unibyte buffers, 
so my preference would be to try the raw-text approach first.

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