Asti's Notes
Decoding a oneof case in nanopb
Working around callback limitations

Decoding callbacks

Say you have a message type with variable fields:

message KeyValue {
   string key     = 1;
   string value   = 2;

This generates:

typedef struct _KeyValue {
    pb_callback_t key;
    pb_callback_t value;
} KeyValue;

Decoding this in nanopb isn’t so bad -

KeyValue decodeKV(char* key, char* value)
	KeyValue kv;
	kv.key = readStringCallback(key);
	kv.value = readStringCallback(value);
    return kv;

Call pb_decode and you’re done.

The callback and the union

If your type happens to be inside a oneof like so:

message Request {
	oneof type {
		Version version = 1;
		KeyValue setting = 2;

suddenly, the KeyValue struct is in an union type - which itself is a field in another struct.

Now when you use decodeKV try to decode Request (the container message), you will find that nothing happens, and your callbacks are never called. This is because during the decode, the union type itself is initialized and so the pb_callbacks are set to NULL - no callbacks set, no call back (pardon the alliteration).

If you don’t succeed at first, decode again

For this to work out, we need to create an environment where we decode the inner struct as a standalone message - like in our very first scenario.

First we need to figure out which was our oneof case - to be more specific, the message descriptor corresponding to our oneof case.

We need a type to collect our findings, so:

	typedef struct pb_union_s
		const uint32_t tag;
		const pb_msgdesc_t* submsg_desc;
		pb_istream_t stream;
	} pb_union_t;	

I’ll put in the full implementation in a gist below, but for now:

	//iterate and decode tag till we get our union desc 
	const pb_union_t getUnionType(uint8_t buffer[], size_t size);
	//actually decode our inner message
	bool decodeUnion(void* message, pb_union_t& unionType);


pb_union_t oneof = getUnionType(packet, packet_size);


Now we have our union message type, so - now what? Now you can check when oneof.tag matches Request_setting_tag and we have all the information we need to decode KeyValue.

KeyValue kv = decodeKV(key, value);
decodeUnion(&kv, oneof);

and that’s it really.

Last modified on 2019-08-03