Nanopb example "using_union_messages" ===================================== Union messages is a common technique in Google Protocol Buffers used to represent a group of messages, only one of which is passed at a time. It is described in Google's documentation: https://developers.google.com/protocol-buffers/docs/techniques#union This directory contains an example on how to encode and decode union messages with minimal memory usage. Usually, nanopb would allocate space to store all of the possible messages at the same time, even though at most one of them will be used at a time. By using some of the lower level nanopb APIs, we can manually generate the top level message, so that we only need to allocate the one submessage that we actually want. Similarly when decoding, we can manually read the tag of the top level message, and only then allocate the memory for the submessage after we already know its type. NOTE: There is a newer protobuf feature called `oneof` that is also supported by nanopb. It might be a better option for new code. Example usage ------------- Type `make` to run the example. It will build it and run commands like following: ./encode 1 | ./decode Got MsgType1: 42 ./encode 2 | ./decode Got MsgType2: true ./encode 3 | ./decode Got MsgType3: 3 1415 This simply demonstrates that the "decode" program has correctly identified the type of the received message, and managed to decode it. Details of implementation ------------------------- unionproto.proto contains the protocol used in the example. It consists of three messages: MsgType1, MsgType2 and MsgType3, which are collected together into UnionMessage. encode.c takes one command line argument, which should be a number 1-3. It then fills in and encodes the corresponding message, and writes it to stdout. decode.c reads a UnionMessage from stdin. Then it calls the function decode_unionmessage_type() to determine the type of the message. After that, the corresponding message is decoded and the contents of it printed to the screen.