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Re: Look for data serialisation format to implement communication betwee

From: Helmut Eller
Subject: Re: Look for data serialisation format to implement communication between Emacs and external program.
Date: Mon, 07 Jan 2013 08:53:34 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3.50 (gnu/linux)

On Sun, Jan 06 2013, Oleksandr Gavenko wrote:

> Is that right to use ASN.1 BER as serialisation data format for communication
> between Emacs and external program?

S-expressions is the only format that Emacs can write and parse quickly
because the printer and reader are implemented in C.  This is likely 10
times faster than any parser that you write in Emacs Lisp.  The downside
is that the external program needs to be able to do the same.  Not such
a bad tradeoff as S-expressions are fairly easy to parse.

For communication with an external format I recommend a "framed" format:
a frame is a fixed sized header followed by a variable length payload.
The header describes the length of the frame.  The length should be in
bytes (not characters as counting characters in UTF8 strings is
uneccessary complicated).  Knowing the length of the frame is very
useful because that makes it easy to wait for a complete frame.  After
you received a complete frame, parsing is simpler because you don't have
to worry about incomplete input.

I also recommend to limit the frame length to 24 bits (not 32 bit)
because Emacs fixnums are limited to 29 bits on 32 bit machines.

The payload can then be an S-expression printed with the Emacs prin1 and
parsed back with the read function.  The encoding of the payload can be
utf-8.  But use the Emacs 'binary coding system for communication with
the external process and unibyte buffers for parsing.  For the
binary-to-utf8 conversion of the payload use something like
decode-coding-string (which is written C and should be fast).

If you like, you can also use extra bits in the header to indicate the
format of the payload.  E.g. it might be useful to have frames that
contain only plain strings (not encoded as S-expr).


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