Protobuf notes
Contents
Overview
There's a lot going on with Protobuf, therefore this local Neela Nurseries page started to capture some links to online docs and tutorials.
Also a good more detailed introduction, the following documentation page is one of a full collection of documentations. It contains example C code to "get the size of the message without storing it anywhere":
Another reference with possible good detail:
Encoding details of protobuf "on the wire":
^ Factoring Protobug Message Definitions
The following online guide speaks to defining multiple related protobuf messages in a single .proto file:
^ 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/
Cmake script to locate Nanopb headers and sources:
^ Protobuf C Code Examples
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:
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:
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;
}
A search for calls to `pack_batched_periodic_data()`:
$ grep -nr pack_batched_periodic_data ./*
./commands.c:1069: pack_batched_periodic_data(&data_batched, &periodicdata);
./proto_utils.c:158:void pack_batched_periodic_data(batched_periodic_data* batched, periodic_data_to_encode* encode_wrapper)
./proto_utils.h:29:void pack_batched_periodic_data(batched_periodic_data* batched, periodic_data_to_encode* encode_wrapper);
Tracing yet further back kitsune project commands.c has following routine which declares and uses a `periodic_data` type:
1038 void thread_tx(void* unused) {
1039 batched_periodic_data data_batched = {0};
1040 #ifdef UPLOAD_AP_INFO
1041 batched_periodic_data_wifi_access_point ap;
1042 #endif
1043 periodic_data forced_data;
1044 bool got_forced_data = false;
1045
1046 LOGI(" Start polling \n");
1047 while (1) {
1048 if (uxQueueMessagesWaiting(data_queue) >= data_queue_batch_size
1049 || got_forced_data ) {
1050 LOGI( "sending data\n" );
1051
1052 periodic_data_to_encode periodicdata;
1053 periodicdata.num_data = 0;
1054 periodicdata.data = (periodic_data*)pvPortMalloc(MAX_BATCH_SIZE*sizeof(periodic_data));
1055
1056 if( !periodicdata.data ) {
1057 LOGI( "failed to alloc periodicdata\n" );
1058 vTaskDelay(1000);
1059 continue;
1060 }
1061 if( got_forced_data ) {
1062 memcpy( &periodicdata.data[periodicdata.num_data], &forced_data, sizeof(forced_data) );
1063 ++periodicdata.num_data;
1064 }
1065 while( periodicdata.num_data < MAX_BATCH_SIZE && xQueueReceive(data_queue, &periodicdata.data[periodicdata.num_ data], 1 ) ) {
1066 ++periodicdata.num_data;
1067 }
1068
1069 pack_batched_periodic_data(&data_batched, &periodicdata);
1070
1071 data_batched.has_uptime_in_second = true;
1072 data_batched.uptime_in_second = xTaskGetTickCount() / configTICK_RATE_HZ;
1073
1074 if( !is_test_boot() && provisioning_mode ) {
. . .
^ Length Prefixing
One way to send large data sets via protobuf is to break them into smaller pieces, and apply protobuf definition to give these pieces a meaning both sender and receiver can understand. See one Mr. Eli's article on this strategy:
^ 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
JSON supported data types:
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.
