How to scan for virtual ports?

[dr3f-tools]

Hi, I'm wondering if it's possible to scan virtual and connect to them.

I was able to create two virtuals ports (one input, one output) and connect them with aconnect 128:0 129:0 from a terminal, but I was wondering if I could somehow create a virtual output port with libremidi, get its handle from a libremidi::observer, and connect to it without needing aconnect.

Thanks,

dr3f

[jcelerier]

hello!

You can pass an observer_configuration object to the libremidi::observer instance with only track_virtual set:

libremidi/include/libremidi/observer_configuration.hpp

Line 136 in d778326

uint32_t track_virtual : 1 = false;

alternatively, when you list ports with observers, they have the "flags" member which indicates the "kind" of port (if the API provides this information):

https://github.com/celtera/libremidi/blob/master/include/libremidi/observer_configuration.hpp#L27

you need to test like this:

 const port_information& p; 
 if(p.type & port_type::software) { ... }

[dr3f-tools]

Thank you! I missed that flag. I was able to create a loopback quite easily after that.

To anyone stumbling on this, virtual output ports created with libremidi::midi_out::open_virtual_port are actually listed as input ports, thus you can trivially connect to them with libremidi::midi_in::open_port() and send messages via libremidi::midi_out::send_message.

Virtual echo example

#include <libremidi/libremidi.hpp>

#include <iostream>

libremidi::input_port find_virtual_port_by_name(libremidi_api api, std::string_view const& name) {
    libremidi::observer obs(
        libremidi::observer_configuration{.track_hardware = false, .track_virtual = true},
        libremidi::observer_configuration_for(api)
    );

    for (auto const& port : obs.get_input_ports()) {
        if (port.port_name == name) {
            return port;
        }
    }

    return {};
}

int main() {
    constexpr std::string_view port_name = "My virtual output port";

    auto virtualOutput = libremidi::midi_out{{}, libremidi::alsa_seq::output_configuration{}};
    if (virtualOutput.open_virtual_port(port_name) != stdx::error{}) {
        std::cerr << "Error opening virtual port" << std::endl;
        return EXIT_FAILURE;
    }

    auto input = libremidi::midi_in{
        {.on_message =
             [](libremidi::message const& message) {
                 std::cout << "Received MIDI message with " << message.size() << " bytes : { ";
                 std::cout << std::hex;
                 for (auto byte : message) {
                     std::cout << "0x" << static_cast<int>(byte) << ", ";
                 }
                 std::cout << "} at timestamp " << std::dec << message.timestamp << std::endl;
             }},
        libremidi::alsa_seq::input_configuration{}
    };
    auto err = input.open_port(
        find_virtual_port_by_name(libremidi::API::ALSA_SEQ, port_name),
        "Virtual output loopback"
    );
    if (err != stdx::error{}) {
        std::cerr << "Error opening input port: " << err.message().data() << std::endl;
        return EXIT_FAILURE;
    } else {
        std::cout << "Successfully connected to virtual output port" << std::endl;
    }

    virtualOutput.send_message({0x90, 60, 127});
    virtualOutput.send_message({0x80, 60, 0});

    std::cout << "Press Enter to exit..." << std::endl;
    std::cin.get();

    return EXIT_SUCCESS;
}

Output

$ ./build/release/bin/virtual 
Successfully connected to virtual output port
Press Enter to exit...
Received MIDI message with 3 bytes : { 0x90, 0x3c, 0x7f, } at timestamp 73480
Received MIDI message with 3 bytes : { 0x80, 0x3c, 0x0, } at timestamp 74834

[jcelerier]

To anyone stumbling on this, virtual output ports created with libremidi::midi_out::open_virtual_port are actually listed as input ports, thus you can trivially connect to them with libremidi::midi_in::open_port() and send messages via libremidi::midi_out::send_message.

yes: if you create an output port and want to connect to this output port, from the point of view of the software that wants to connect it's going to read from it so it's a midi in

[jcelerier]

glad it could solve your issue!