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ex_lsl

LSL library: publish a stream, discover it and receive its samples over loopback.

Libraries: Lab Streaming Layer

Build and run​

cmake --build build/developer-dynamic --target ex_lsl
./out/Release/examples/ex_lsl

Used in the API reference​

Source​

src/examples/ex_lsl/main.cpp
//=============================================================================================================
/**
 * SPDX-License-Identifier: BSD-3-Clause
 * Copyright (c) 2026 MNE-CPP Authors
 *
 * @file     main.cpp
 * @author   Christoph Dinh <christoph.dinh@mne-cpp.org>
 * @since    2.4.0
 * @date     October 2026
 * @brief    LSL library: publish a stream, discover it and receive its samples over loopback.
 *
 * An outlet publishes a 3-channel stream; an inlet connected to it on
 * 127.0.0.1 must receive every pushed sample in order. Discovery by name is
 * checked when multicast is available. Exits non-zero on any mismatch.
 */

//=============================================================================================================
// INCLUDES
//=============================================================================================================

#include <lsl/lsl_stream_discovery.h>
#include <lsl/lsl_stream_info.h>
#include <lsl/lsl_stream_inlet.h>
#include <lsl/lsl_stream_outlet.h>

//=============================================================================================================
// QT INCLUDES
//=============================================================================================================

#include <QCoreApplication>
#include <QDebug>
#include <QElapsedTimer>
#include <QThread>

//=============================================================================================================
// STL INCLUDES
//=============================================================================================================

#include <cstdlib>
#include <vector>

//=============================================================================================================
// USED NAMESPACES
//=============================================================================================================

using namespace LSLLIB;

//=============================================================================================================
// DEFINE GLOBAL METHODS
//=============================================================================================================

namespace
{

bool expect(bool condition, const QString& what)
{
    qInfo().noquote() << (condition ? "  ok  " : "  FAIL") << what;
    return condition;
}

} // namespace

//=============================================================================================================
// MAIN
//=============================================================================================================

int main(int argc, char* argv[])
{
    QCoreApplication app(argc, argv);
    bool ok = true;

    //! [lsl_stream_outlet_usage]
    stream_info info("ex_lsl_eeg", "EEG", 3, 250.0, ChannelFormat::Float32, "ex_lsl"); // name, type, channels, Hz
    stream_outlet outlet(info);                                                        // starts serving immediately
    //! [lsl_stream_outlet_usage]
    ok &= expect(outlet.info().name() == "ex_lsl_eeg" && outlet.info().channel_count() == 3 && outlet.info().nominal_srate() == 250.0 && outlet.info().data_port() > 0 && !outlet.info().uid().empty(),
                 QString("stream_info: 3 channels at 250 Hz, served on port %1").arg(outlet.info().data_port()));

    //! [lsl_stream_inlet_usage]
    stream_info source = outlet.info(); // normally an entry of resolve_stream("name", "ex_lsl_eeg")
    source.set_data_host("127.0.0.1");
    stream_inlet inlet(source);
    inlet.open_stream();
    QElapsedTimer clock;
    clock.start();
    while (!outlet.have_consumers() && clock.elapsed() < 5000) { // samples pushed before the outlet accepts the inlet are not delivered
        QThread::msleep(5);
    }
    outlet.push_sample({1.0f, 2.0f, 3.0f});
    outlet.push_chunk({{4.0f, 5.0f, 6.0f}, {7.0f, 8.0f, 9.0f}});

    std::vector<std::vector<float>> received;
    clock.restart();
    while (received.size() < 3 && clock.elapsed() < 5000) {
        QThread::msleep(20);
        for (const std::vector<float>& sample : inlet.pull_chunk<float>()) { // one vector per sample
            received.push_back(sample);
        }
    }
    inlet.close_stream();
    //! [lsl_stream_inlet_usage]
    ok &= expect(received == std::vector<std::vector<float>>({{1.0f, 2.0f, 3.0f}, {4.0f, 5.0f, 6.0f}, {7.0f, 8.0f, 9.0f}}),
                 QString("stream_inlet received %1 of 3 samples in order").arg(received.size()));

    //! [lsl_resolve_stream_usage]
    const std::vector<stream_info> found = resolve_stream("name", "ex_lsl_eeg", 1.5); // multicast discovery
    //! [lsl_resolve_stream_usage]
    if (found.empty()) {
        qInfo().noquote() << "  skip  resolve_stream (multicast discovery not available here)";
    } else {
        ok &= expect(found.front().uid() == outlet.info().uid() && found.front().data_port() == outlet.info().data_port(),
                     "resolve_stream finds the outlet by name");
    }

    qInfo().noquote() << (ok ? "All lsl checks passed." : "lsl checks FAILED.");
    return ok ? EXIT_SUCCESS : EXIT_FAILURE;
}