Protobuf notes

Introduction:

• https://protobuf.dev/getting-started/cpptutorial/
• https://protobuf.dev/programming-guides/proto3/

Also a good more detailed introduction:

• https://chromium.googlesource.com/external/github.com/nanopb/nanopb/+/refs/heads/master/docs/concepts.md

Another reference with possible good detail:

• https://www.swi-prolog.org/pldoc/man?section=protobufs-tags

When compiling nanopb Protobuf library as part of C language programs, nested Protobuf messages require use of nanopb defined function type `pb_callback_t` in order to encode and to decode those nested messages. Some examples of this on github:

• https://github.com/particle-iot/device-os/blob/9338b13b1e611f1b57f71f702795c2ca71142c1f/proto_defs/src/cloud/describe.pb.h#L9

• https://github.com/hello/kitsune/blob/6a28afa80dd4547907e58341a84bd0b5bec5d88e/kitsune/protobuf/periodic.pb.h#L11

In the first example an early on file instance of `pb_callback_t` occurs on line 56. Looking further this project has a few dozen protoc generated files . . . switching to a possible smaller project:

• https://github.com/hello/kitsune

In kitsune project, looking at:

(1) file kitsune/kitsune/audio_features_upload_task.c function setup_protbuf( . . . )
(2) file audio_features_upload_task_helpers.c function encode_repeated_streaming_bytes_and_mark_done(pb_ostream_t *stream, const pb_field_t *field, void * const *arg)
(3) in same file reviewing function write_streams(pb_ostream_t *stream, const pb_field_t *field,hlo_stream_t * hlo_stream)

Here is an excerpt from proto_utils.c which appears to contain a pb_callback_t definition:

147 bool encode_device_id_string(pb_ostream_t *stream, const pb_field_t *field, void * const *arg) {
148     //char are twice the size, extra 1 for null terminator
149     char hex_device_id[2*DEVICE_ID_SZ+1] = {0};
150     if(!get_device_id(hex_device_id, sizeof(hex_device_id)))
151     {
152         return false;
153     }
154 
155     return pb_encode_tag_for_field(stream, field) && pb_encode_string(stream, (uint8_t*)hex_device_id, strlen(hex_device_id));
156 }

Same routine no line numbers, plus following routine which references first routine in function point assignment:

bool encode_device_id_string(pb_ostream_t *stream, const pb_field_t *field, void * const *arg) {
    //char are twice the size, extra 1 for null terminator
    char hex_device_id[2*DEVICE_ID_SZ+1] = {0};
    if(!get_device_id(hex_device_id, sizeof(hex_device_id)))
    {   
        return false;
    }   

    return pb_encode_tag_for_field(stream, field) && pb_encode_string(stream, (uint8_t*)hex_device_id, strlen(hex_device_id));
}

void pack_batched_periodic_data(batched_periodic_data* batched, periodic_data_to_encode* encode_wrapper)
{
    if(NULL == batched || NULL == encode_wrapper)
    {   
        LOGE("null param\n");
        return;
    }   

    batched->data.funcs.encode = encode_all_periodic_data;  // This is smart :D
    batched->data.arg = encode_wrapper;
    batched->firmware_version = KIT_VER;
    batched->device_id.funcs.encode = encode_device_id_string;
}

^ References To Sort

Protobuf references, somewhat arbitrary starting point yet introduces some key topics of Protobuf standard and use cases:

• https://www.crankuptheamps.com/blog/posts/2017/10/12/protobuf-battle-of-the-syntaxes/
• https://www.educative.io/edpresso/what-is-the-difference-between-protocol-buffers-and-json

• https://developers.google.com/protocol-buffers/docs/proto
• https://spline.de/talks/protobuf.pdf

JSON supported data types:

• https://www.w3schools.com/js/js_json_datatypes.asp

First Protobuf .proto file, compiles using `protoc-c`, part of a package available with Ubuntu 20.04:

// syntax = "proto3";
syntax = "proto2";

// Notes:
// $ protoc-c --c_out=. ./first.proto

message sensorUpdates {
  required int32 message_id = 1;
  optional float vrms = 2;
}

. . . It appears that the integer values which message elements are assigned as tantamount to key names in JSON.

^ Nanopb

2022-01-08 Saturday

• https://github.com/nanopb/nanopb/blob/master/generator/proto/nanopb.proto
• https://jpa.kapsi.fi/nanopb/docs/whats_new.html
• https://jpa.kapsi.fi/nanopb/docs/

• https://docs.python.org/3/tutorial/modules.html

Cmake script to locate Nanopb headers and sources:

• https://chromium.googlesource.com/external/github.com/nanopb/nanopb/+/nanopb-0.2.9.1/extra/FindNanopb.cmake