Portable exchange of binary/packed data

Thad Smith wibbled on Saturday 17 July 2010 22:02

I've used this for network communications (actual network and unix domain sockets). It's reasonably pleasant, lots of high level language libraries exist for various languages and it's efficient.

To deal with the problem of "packetising" data over a stream, I wrapped the whole lot in SLIP.

I suggest adding an explicit byte order to the message specification. The processor can load or write the proper order. This can done, if you wish, in portable code that works on either endian machine.

ASN.1 BER encoding is a reasonable standardized binary encoding technique. As an option, you can formally specify the message in ASN.1.

I don't know what you mean there.

the byte order is specified.

I was not clear in my question. I was looking for maintainable C code to abstract the access to message elements in the different targets.

These targets range from tiny and "slow" 8 bit controllers where I need direct access to the elements up to 32 bit controllers with plenty of computing power to use indirect methods to access the message elements, e.g. a table.

Direct access could be a "struct" or a bunch of defines with offsets and type casts. That's rather simple to maintain in code and works for all targets if:

• the endianness is the same,

• the target supports misaligned access (if the message requires for). And of course the "struct" method only works if the compiler (for the larger processors) has extensions to control the packing. So this method would result in some limitation on future targets.

Larger, faster targets have enough resources to convert the message to and from a working structure using an abstract description of the message. But I'm afraid that there is no maintainable solution to have _one_ message description for the tiny and the abstract format, IOW I hade to maintain different versions of the message description/abstraction sources.

Oliver