Ros2 bag compatiblity

[lvanroye]

Hello,

I am experiencing problems writing to an mcap file (in cpp) and using it as a rosbag.
This i how i am writing to the ros2 bag:

#include <mcap/writer.hpp>
#include <fstream>
#include <cstddef>

int main()
{
    std::ofstream out("test.mcap", std::ios::binary);

    // Create MCAP writer
    ::mcap::McapWriter writer;
    ::mcap::McapWriterOptions options("ros2");
    writer.open(out, options);

    // Add schema
    ::mcap::Schema schema("std_msgs/Float64", "ros2idl", "float64 data");
    writer.addSchema(schema);

    // Add channel
    ::mcap::Channel channel("my_topic", "cdr", schema.id);
    writer.addChannel(channel);

    // Write some doubles in a loop
    for (int i = 0; i < 10; ++i)
    {
        double value = i * 1.1;
        ::mcap::Message msg;
        msg.channelId = channel.id;
        msg.sequence = i;
        msg.logTime = i * 1000000;
        msg.publishTime = i * 1000000;
        msg.dataSize = sizeof(double);
        msg.data = reinterpret_cast<const std::byte*>(&value);
        if (!writer.write(msg).ok())
            return 1;
    }

    writer.close();
    return 0;
}

when i run

ros2 bag info test.mcap

i get as expected

Files:             test.mcap
Bag size:          1.1 KiB
Storage id:        mcap
ROS Distro:        unknown
Duration:          0.009000000s
Start:             Jan  1 1970 01:00:00.000000000 (0.000000000)
End:               Jan  1 1970 01:00:00.009000000 (0.009000000)
Messages:          10
Topic information: Topic: my_topic | Type: std_msgs/Float64 | Count: 10 | Serialization Format: cdr
Service:           0
Service information: 

but when i run

ros2 bag play test.mcap

i get the following error:

>>> [rcutils|error_handling.c:108] rcutils_set_error_state()
This error state is being overwritten:

  'Handle's typesupport identifier (rosidl_typesupport_cpp) is not supported by this library, at ./src/type_support_dispatch.hpp:114'

with this new error message:

  'Fast CDR exception deserializing message of type std_msgs::msg::dds_::Float64_., at ./src/type_support_common.cpp:118'

rcutils_reset_error() should be called after error handling to avoid this.
<<<
[ERROR] [1759487382.344016844] [rosbag2_player]: Failed to publish message on 'my_topic' topic. 
Error: failed to deserialize msg: Fast CDR exception deserializing message of type std_msgs::msg::dds_::Float64_., at ./src/type_support_common.cpp:118

Am i missing something? Help would really be appreciated.

Best regards,
Lander

[jtbandes]

msg.data = reinterpret_cast<const std::byte*>(&value) is not going to produce a valid CDR encoded message.

If you want to produce a mcap that works well with ros2 bag play, it would be best to use the rosbag2 APIs:

https://docs.ros.org/en/iron/p/rosbag2_cpp/generated/classrosbag2__cpp_1_1Writer.html
example: https://daobook.github.io/ros2-docs/xin/Tutorials/Ros2bag/Recording-A-Bag-From-Your-Own-Node-Cpp.html#write-the-c-node
Python: https://mcap.dev/guides/python/ros2#writing