Multiple Vehicle Information Logging in VEINS

Question

I am trying to log data from multiple vehicles into the same CSV file in VEINS, but it stops logging after the second vehicle spawns, i.e., it only works for a single-vehicle scenario.

void MyVeinsApp::handlePositionUpdate(cObject* obj) {
    DemoBaseApplLayer::handlePositionUpdate(obj);

    simtime_t currentTime = simTime();
    double offsetX = 0.0, offsetY = 0.0, offsetZ = 0.0;
    int isOffsetApplied = 0, isAttacker = attackerVehicles.count(vehicleId) ? 1 : 0;
    double currentSpeed = mobility->getSpeed();

    if (isAttacker && offsetDistribution(generator)) {
        offsetX = offsetValue(generator);
        offsetY = offsetValue(generator);
        offsetZ = offsetValue(generator);
        isOffsetApplied = 1;
        currentSpeed /= 2.0;
    }

    vehicleData[vehicleId] = std::make_tuple(curPosition.x + offsetX, curPosition.y + offsetY,
                                             curPosition.z + offsetZ, isOffsetApplied,
                                             isAttacker, currentSpeed);

    logFile << std::fixed << std::setprecision(2) << currentTime.dbl();
    for (int i = 0; i < 50; ++i) {
        if (vehicleData.count(i)) {
            // Vehicle data exists; log it
            auto tupleData = vehicleData[i];
            double x = std::get<0>(tupleData);
            double y = std::get<1>(tupleData);
            double z = std::get<2>(tupleData);
            int offset = std::get<3>(tupleData);
            int attacker = std::get<4>(tupleData);
            double speed = std::get<5>(tupleData);

            logFile << "," << x << "," << y << "," << z << "," << offset << "," << attacker << "," << speed;
        } else {
            // Vehicle data is missing; log blanks
            logFile << ",,,," << "0,0,";
        }
    }
    logFile << std::endl;
}

Could this be due to a race condition, where multiple instances of the application layer for each vehicle are trying to write to the CSV file simultaneously? If so, how can it be fixed?

Answer 1

Yes. This looks like a race condition where multiple vehicles are trying to write to the same file at the same time, which can lead to file corruption or incomplete writes. Below is a thread-safe solution using mutex to synchronise file access:

#include <mutex>

class MyVeinsApp : public DemoBaseApplLayer {
private:
    // Add mutex as class member
    static std::mutex logMutex;
    
    // Other existing class members...

public:
    void handlePositionUpdate(cObject* obj) {
        DemoBaseApplLayer::handlePositionUpdate(obj);

        simtime_t currentTime = simTime();
        double offsetX = 0.0, offsetY = 0.0, offsetZ = 0.0;
        int isOffsetApplied = 0, isAttacker = attackerVehicles.count(vehicleId) ? 1 : 0;
        double currentSpeed = mobility->getSpeed();

        if (isAttacker && offsetDistribution(generator)) {
            offsetX = offsetValue(generator);
            offsetY = offsetValue(generator);
            offsetZ = offsetValue(generator);
            isOffsetApplied = 1;
            currentSpeed /= 2.0;
        }

        // Update vehicle data
        {
            std::lock_guard<std::mutex> lock(logMutex);  // Scope-based lock for vehicle data
            vehicleData[vehicleId] = std::make_tuple(curPosition.x + offsetX, curPosition.y + offsetY,
                                                   curPosition.z + offsetZ, isOffsetApplied,
                                                   isAttacker, currentSpeed);
        }

        // Write to file with mutex protection
        {
            std::lock_guard<std::mutex> lock(logMutex);
            logFile << std::fixed << std::setprecision(2) << currentTime.dbl();
            
            for (int i = 0; i < 50; ++i) {
                if (vehicleData.count(i)) {
                    // Vehicle data exists; log it
                    auto tupleData = vehicleData[i];
                    double x = std::get<0>(tupleData);
                    double y = std::get<1>(tupleData);
                    double z = std::get<2>(tupleData);
                    int offset = std::get<3>(tupleData);
                    int attacker = std::get<4>(tupleData);
                    double speed = std::get<5>(tupleData);

                    logFile << "," << x << "," << y << "," << z << "," 
                           << offset << "," << attacker << "," << speed;
                } else {
                    // Vehicle data is missing; log blanks
                    logFile << ",,,," << "0,0,";
                }
            }
            logFile << std::endl;
            logFile.flush();  // Ensure data is written to disk
        }
    }
};

// Define the static mutex
std::mutex MyVeinsApp::logMutex;

The changes made above are:

• Added a static mutex (logMutex) as a class member to synchronize access across all instances

• Used std::lock_guard to RAII-style locking to ensure the mutex is always released

• Protected both the vehicleData update and file writing operations

• Added separate scoping blocks for the locks to minimise the critical section

• Added logFile.flush() to ensure data is written to disk immediately

The following is also recommended:

Consider using a buffered approach where data is collected in memory and written periodically to reduce I/O overhead

You might want to add error handling for file operations

Consider using a more efficient data structure than checking 50 sequential IDs