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Reading and writing Protobuf


“Writing” section adapted from the original: Recording Robocar Data with MCAP


To read Protobuf messages from an MCAP file using C++, we have two options:

  • Static – Use statically generated class definitions to deserialize the MCAP file

    Best when there is existing code that uses these Protobuf classes. For example, if you have a simulation that drives a planning module with recorded messages, you already have generated class definitions. It makes sense to take advantage of Protobuf’s existing compatibility mechanisms and use those definitions to deserialize the MCAP data.

  • Dynamic – Dynamically read fields using the schema definitions in the MCAP file

    Preferred for inspecting and debugging message content. For example, when building a visualization tool, we want to provide a full view of all fields in a message as it was originally recorded. We can use Protobuf’s DynamicMessage class to enumerate and inspect message fields in this way.

Statically generated class definitions

First, we generate our class definitions and include the relevant header:

#include "foxglove/PosesInFrame.pb.h"

We also include the MCAP reader implementation:

#include "mcap/reader.hpp"

Use the mcap::McapReader::open() method to open an MCAP file for reading:

mcap::McapReader reader;
  const auto res =;
  if (!res.ok()) {
    std::cerr << "Failed to open " << inputFilename << " for reading: " << res.message
              << std::endl;
    return 1;

Use a mcap::MessageView to iterate through all of the messages in the MCAP file:

auto messageView = reader.readMessages();
for (auto it = messageView.begin(); it != messageView.end(); it++) {
  // skip messages that we can't use
  if ((it->schema->encoding != "protobuf") || it->schema->name != "foxglove.PosesInFrame") {
  foxglove::PosesInFrame path;
  if (!path.ParseFromArray(static_cast<const void*>(it->,
                                  it->message.dataSize)) {
    std::cerr << "could not parse PosesInFrame" << std::endl;
    return 1;
  std::cout << "Found message: " << path.ShortDebugString() << std::endl;
  // print out the message

Finally, we close the reader:


Dynamically read fields

To read message fields dynamically, we must first include the relevant headers:

#include <google/protobuf/descriptor.pb.h>
#include <google/protobuf/descriptor_database.h>
#include <google/protobuf/dynamic_message.h>

#include "mcap/reader.hpp"

namespace gp = google::protobuf;

Then, we construct our mcap::McapReader and mcap::MessageView in the same way as before:

mcap::McapReader reader;
  const auto res =;
  if (!res.ok()) {
    std::cerr << "Failed to open " << inputFilename << " for reading: " << res.message
              << std::endl;
    return 1;
auto messageView = reader.readMessages();

Load schema definitions

We build a DynamicMessageFactory, using a google::Protobuf::SimpleDescriptorDatabase as the underlying descriptor database. By constructing this ourselves and retaining a reference to the database, we can more easily load that database with definitions from the MCAP file.

gp::SimpleDescriptorDatabase protoDb;
gp::DescriptorPool protoPool(&protoDb);
gp::DynamicMessageFactory protoFactory(&protoPool);

Now we’re ready to iterate through the messages in the MCAP file. We want to load every message’s FileDescriptorSet into the DescriptorDatabase, if it hasn’t been already:

for (auto it = messageView.begin(); it != messageView.end(); it++) {
  const gp::Descriptor* descriptor = protoPool.FindMessageTypeByName(it->schema->name);
  if (descriptor == nullptr) {
    if (!LoadSchema(it->schema, &protoDb)) {
      return 1;

Next, let’s define our LoadSchema() helper function:

bool LoadSchema(const mcap::SchemaPtr schema, gp::SimpleDescriptorDatabase* protoDb) {
  gp::FileDescriptorSet fdSet;
  if (!fdSet.ParseFromArray(static_cast<const void*>(schema->, schema->data.size())) {
    std::cerr << "failed to parse schema data" << std::endl;
    return false;
  gp::FileDescriptorProto unused;
  for (int i = 0; i < fdSet.file_size(); ++i) {
    const auto& file = fdSet.file(i);
    if (!protoDb->FindFileByName(, &unused)) {
      if (!protoDb->Add(file)) {
        std::cerr << "failed to add definition " << << "to protoDB" << std::endl;
        return false;
  return true;

Once the FileDescriptorSet is loaded, we can get the descriptor by name:

descriptor = protoPool.FindMessageTypeByName(it->schema->name);

We can use this descriptor to parse our message:

gp::Message* message = protoFactory.GetPrototype(descriptor)->New();
if (!message->ParseFromArray(static_cast<const void*>(it->,
                              it->message.dataSize)) {
  std::cerr << "failed to parse message using included schema" << std::endl;
  return 1;
std::cout << message->ShortDebugString() << std::endl;

Finally, we close the reader:



Create an MCAP writer to start writing your Protobuf messages:

mcap::McapWriter writer;
mcap::McapWriterOptions opts("protobuf");
auto s ="output.mcap");
if (!s.ok) {
  std::cerr << "Failed to open mcap writer: " << status.message << "\n";
  throw std::runtime_error("could not open mcap writer");

Configure it to your desired specifications using McapWriterOptions. For example, opts.compressionLevel = mcap::CompressionLevel::Fast will customize your writer to use a faster compression level.

Register schema

Before we can write messages, we need to register a schema.

You must use the fully-qualified name of the message type (e.g. foxglove.PosesInFrame) and provide a serialized google::protobuf::FileDescriptorSet for the schema itself. Generated Protobuf messages will contain enough information to reconstruct this FileDescriptorSet schema at runtime:

// Recursively adds all `fd` dependencies to `fd_set`.
void fdSetInternal(google::protobuf::FileDescriptorSet& fd_set,
                   std::unordered_set<std::string>& files,
                   const google::protobuf::FileDescriptor* fd) {
  for (int i = 0; i < fd->dependency_count(); ++i) {
    const auto* dep = fd->dependency(i);
    auto [_, inserted] = files.insert(dep->name());
    if (!inserted) continue;
    fdSetInternal(fd_set, files, fd->dependency(i));

// Returns a serialized google::protobuf::FileDescriptorSet containing
// the necessary google::protobuf::FileDescriptor's to describe d.
std::string fdSet(const google::protobuf::Descriptor* d) {
  std::string res;
  std::unordered_set<std::string> files;
  google::protobuf::FileDescriptorSet fd_set;
  fdSetInternal(fd_set, files, d->file());
  return fd_set.SerializeAsString();

mcap::Schema createSchema(const google::protobuf::Descriptor* d) {
  mcap::Schema schema(d->full_name(), "protobuf", fdSet(d));
  return schema;

// Create a schema for the foxglove.PosesInFrame message.
mcap::Schema path_schema = createSchema(foxglove::PosesInFrame::descriptor());
writer.addSchema(path_schema);  // Assigned schema id is written to

Register channel

Next, we’ll register a channel to write our messages to:

mcap::Channel path_channel("/planner/path", "protobuf",;
mcap.addChannel(path_channel);  // Assigned channel id written to

Write messages

We can now finally write messages to the channel using its ID:

foxglove::PosesInFrame poses_msg;
// Fill in path_msg.
uint64_t timestamp_ns = std::chrono::duration_cast<std::chrono::nanoseconds>(
poses_msg.mutable_timestamp()->set_seconds(timestamp_ns / 1'000'000'000ull)
poses_msg.mutable_timestamp()->set_nanos(timestamp_ns % 1'000'000'000ull)
// Example path in a straight line down the X axis
for (int i = 0; i < 10; ++i) {
  auto pose = poses_msg.add_poses();

std::string data = poses_msg.SerializeAsString();
mcap::Message msg;
msg.channelId =;
msg.logTime = timestamp_ns;
msg.publishTime = msg.logTime; = reinterpret_cast<const std::byte*>(;
msg.dataSize = data.size();


Don’t forget to close the writer when you’re done:


Inspect MCAP file

Now, we can inspect our output MCAP file’s messages. Use the Data source dialog in Foxglove Studio to “Open local file”.

Add a few relevant panels (Plot, Image, Raw Messages, 3D) to visualize the robot’s performance.