How do I clear a stream in C++ for a nanoPB protocol buffer to use?

Question

I'm using nanopb in a project on ESP32, in platformIO. It's an arduino flavored C++ codebase.

I'm using some protobufs to encode data for transfer. And I've set up the memory that the protobufs will use at the root level to avoid re-allocating the memory every time a message is sent.

// variables to store the buffer/stream the data will render into...
uint8_t buffer[MESSAGE_BUFFER_SIZE];
pb_ostream_t stream = pb_ostream_from_buffer(buffer, sizeof(buffer));

// object to hold the data on its way into the encode action...
TestMessage abCounts = TestMessage_init_zero;

Then I've got my function that encodes data into this stream via protobufs (using nanoPB)...

void encodeABCounts(int32_t button_a, int32_t button_b, String message)
  {

    // populate our data structure...
    abCounts.a_count = button_a;
    abCounts.b_count = button_b;
    strcpy(abCounts.message, message.c_str());

    // encode the data!
    bool status = pb_encode(&stream, TestMessage_fields, &abCounts);

    if (!status)
    {
      Serial.println("Failed to encode");
      return;
    }

    // and here's some debug code I'll discuss below....
    Serial.print("Message Length: ");
    Serial.println(stream.bytes_written);
    for (int i = 0; i < stream.bytes_written; i++)
    {
      Serial.printf("%02X", buffer[i]);
    }
    Serial.println("");
  }

Ok. So the first time this encode action occurs this is the data I get in the serial monitor...

Message Length: 14
Message: 080110001A087370656369616C41

And that's great - everything looks good. But the second time I call encodeABCounts(), and the third time, and the forth, I get this...

Message Length: 28
Message: 080110001A087370656369616C41080210001A087370656369616C41

Message Length: 42
Message: 080110001A087370656369616C41080210001A087370656369616C41080310001A087370656369616C41

Message Length: 56
Message: 080110001A087370656369616C41080210001A087370656369616C41080310001A087370656369616C41080410001A087370656369616C41

...etc

So it didn't clear out the buffer/stream when the new data went in. Each time the buffer/stream is just getting longer as new data is appended.

How do I reset the stream/buffer to a state where it's ready for new data to be encoded and stuck in there, without reallocating the memory?

Thanks!

score 1 · Accepted Answer · answered Jan 08 '22 at 14:42

To reset the stream, simply re-create it. Now you have this:

pb_ostream_t stream = pb_ostream_from_buffer(buffer, sizeof(buffer));

You can recreate it by assigning again:

stream = pb_ostream_from_buffer(buffer, sizeof(buffer));

Though you can also move the initial stream declaration to inside encodeABCounts() to create it every time, if you don't have any particular reason to keep it around after use. The stream creation is very lightweight, as it just stores the location and size of the buffer.

How do I clear a stream in C++ for a nanoPB protocol buffer to use?

1 Answers1