QTcpSocket::waitForReadyRead continually reserved lots of memory

BernardZhu

I have a desktop application, which receving lots of data from TCP connection, the memory leak is found. I minimized it to a quite simple program it is still the same, i can't understand what happend. I used Visutal studio 2022 to debug it when the tcp is connected, the TCP server will send data to my desktop, I use visutal studio "diagnostic Tools && Heap profiing", take two snapshots, and diff it found lots of memory is reserved by the waitForReadyRead , the call stack look like below:

Screenshot 2025-01-03 223504.png

The main code is like below, it is just started the thread, and starting read the tcpsocket. The wired thing is if i commentd all the code related ostringstream, the memeory leak disappears(the two heap snapshot are the same, and proces memory keeps stable). As i understand the oss var will destructed every time, i can't understand why it can cause 'waitForReadyRead' function reserve many memory

Qt Vesion is Qt 6.8.1


void XCPWorkerThread::run()
{
    TestXCP();
}

static size_t readBytes(QAbstractSocket* socket, char* data, qint64 n)
{
    char* ptr = data;
    qint64 nleft = n;

    while (nleft > 0) {
        qint64 nread = socket->read(ptr, nleft);
        if (nread == -1) {
            qDebug() << "socket closed";
            break;
        } else if (nread == 0) {
            socket->waitForReadyRead(-1);
        }
        nleft -= nread;
        ptr += nread;
    }
    return n - nleft;
}

static bool readMessage(QAbstractSocket* socket, std::vector<uint8_t>& buffer)
{
    uint16_t packageLength = 0;
   // if i comment out this variable and related code use it, the memory leak disapper
    std::ostringstream oss;

    if (readBytes(socket, (char*)&packageLength, sizeof(packageLength)) != sizeof(packageLength)) {
        qDebug() << "can't get the xcp message length";
        return false;
    }
    oss << "Message received, length: " << packageLength;
    uint16_t messageCounter = 0;
    if (readBytes(socket, (char*)&messageCounter, sizeof(messageCounter))
        != sizeof(messageCounter)) {
        qDebug() << "can't get the message counter";
        return false;
    }

    oss << ", counter: " << (int)messageCounter;
    buffer.resize(packageLength + 4);
    memcpy((char*)&buffer[0], &packageLength, sizeof(packageLength));
    memcpy((char*)&buffer[2], &messageCounter, sizeof(messageCounter));

    if (readBytes(socket, (char*)&buffer[4], packageLength) != packageLength) {
        // qDebug() << "can't get the xcp package";
        return false;
    }
    oss << ", packet: [";
    for (int i = 0; i < packageLength + 4; i++) {
        oss << std::uppercase << std::setfill('0') << std::setw(2) << std::hex << (int)(buffer[i])
            << " ";
    }
    oss << "]";
    //  qDebug() << oss.str().c_str();
    return true;
}

void XCPWorkerThread::TestXCP()
{
    qDebug() << __FUNCTION__;
    s = new QTcpSocket(0);
    s->connectToHost("127.0.0.1", 17725);
    s->waitForConnected(-1);
    int i = 0;
    while (true) {
        if (s->waitForReadyRead(-1)) {
            std::vector<uint8_t> buffer(0xFF);
            if (!readMessage(s, buffer)) {
                qDebug() << "can't read message from socket";
                return;
            }
            qDebug() << "readed a pckage" << i++;
        }
    }
    delete s;
}

Christian Ehrlicher

Use signals and slots instead a blocking call. Then you also don't need a separate thread.