At Tuesday 16/1/2007 20:05, David Hirschfield wrote:
I'm implementing a relatively simple inter-application communication
system that uses asyncore/asynchat to send messages back and forth.
The messages are prefixed by a length value and terminator string, to
signal that a message is incoming, and an integer value specifying the
size of the message, followed by the message data.
My question is: how can I produce a short terminator string that won't
show up (or has an extremely small chance of showing up) in the binary
data that I send as messages?
If you send previously the size of data to follow, there is no need
for a terminator. On the receiving side, just read so many bytes;
it's a lot easier. Anyway you can add a terminator, and check for it
after the data arrives, just a small consistency test.
If you can't do that for whatever reason, usually it's done
backwards: ensure the chosen (fixed) terminator never happens inside
the message.
Example: encoding using || as terminator, % as quote character:
- any | inside the message is prefixed by the quote character: | -> %|
- any % inside the message is doubled: % -> %%
This way, any terminator inside the message would be converted to
%|%| and could never be confused.
At the receiving side, decode as follows:
- search for any % character; when found, delete it and keep the
following character; keep searching until no more % are found.
--
Gabriel Genellina
Softlab SRL
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