Segmentation Fault when reading from a TCPSocket

onurcevik

Hey I am trying to send data continuously through TCPSocket and when I debug I see the line below gives me SIGSEGV Segmentation Fault crash. Can you tell me how can I fix this problem?

qint64 bytes = buffer->write(socket->readAll()); //The program crashes at this line

Here is sender the code:

#include "tcpsender.h"
#include "ui_tcpsender.h"

#include <QtWidgets>
#include <QtNetwork>
#include <QtCore>

#include <QDebug>
#include <QBuffer>

#define XRES 640
#define YRES 480
#define SIZE 640*480*3


TCPSender::TCPSender(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::TCPSender)
{
    ui->setupUi(this);
    statusLabel = new QLabel(tr("Ready to send frames on port 6667"));
    statusLabel->setWordWrap(true);
    startButton = new QPushButton(tr("&Start"));
    auto quitButton = new QPushButton(tr("&Quit"));
    auto buttonBox = new QDialogButtonBox;
    buttonBox->addButton(startButton, QDialogButtonBox::ActionRole);
    buttonBox->addButton(quitButton, QDialogButtonBox::RejectRole);

    socket = new QTcpSocket(this);
    connect(startButton, &QPushButton::clicked, this, &TCPSender::startConnection);
    connect(quitButton, &QPushButton::clicked, this, &TCPSender::close);
    connect(socket, SIGNAL(connected()), SLOT(startSending()));
    connect(&timer, &QTimer::timeout, this, &TCPSender::sendFrame);

    auto mainLayout = new QVBoxLayout;
    mainLayout->addWidget(statusLabel);
    mainLayout->addWidget(buttonBox);
    setLayout(mainLayout);

    setWindowTitle(tr("Broadcast Sender"));
    camera = new Camera("/dev/video0", XRES, YRES);

    time = QTime::currentTime();

}

TCPSender::~TCPSender()
{
    delete ui;
}

void TCPSender::startConnection()
{
    socket->connectToHost(ui->lineEdit->text(), 6667, QIODevice::WriteOnly);
}

void TCPSender::startSending()
{
    startButton->setEnabled(false);
    //timer.start(1000/(24*1800));
    timer.start(1000/24);
    qDebug()<<"Timer start";
}

void TCPSender::sendFrame()
{
    if(socket->state()==QAbstractSocket::ConnectedState){
        auto frame = camera->frame();

        image = new QImage(frame.data,XRES,YRES,QImage::Format_RGB888);
        QImage im = image->convertToFormat(QImage::Format_Grayscale8);
        QByteArray ba;
        QBuffer buffer(&ba);


        qDebug()<<"writing socket";
        socket->write(ba);

        int speed = time.msecsTo(QTime::currentTime());
        time = QTime::currentTime();
        speed = 1000*300/speed;
        ui->label->setText(QString("%1 kb/s").arg(speed));
        delete image;

   }
   
}

And here is the reciever code:

#include "reciever.h"

#include <QBuffer>
#include <QTcpSocket>
#include <QImage>
#include <QDebug>
#include <iostream>
#include <fstream>
#include <string>
#include <sstream>
#include <unistd.h>

#define XRES 640
#define YRES 480
#define SIZE 640*480*3

Reciever::Reciever(QObject* parent): QTcpServer(parent)
{
    connect(this, SIGNAL(newConnection()), this, SLOT(addConnection()));
    qDebug()<<"Const";
}


void Reciever::addConnection()
{
    qDebug()<<"Selam";
    QTcpSocket* connection = nextPendingConnection();
    connect(connection, SIGNAL(readyRead()),
            this, SLOT(receiveImage()));


}



void Reciever::receiveImage()
{
    qDebug()<<"RECIEVE";
    QTcpSocket* socket = static_cast<QTcpSocket*>(sender());
    QBuffer* buffer = buffers.value(socket);
    
    qint64 bytes = buffer->write(socket->readAll());

    emit sendBuffer(buffer,bytes);
}

Please note that I only posted the relevant parts of the code.

jsulm

@onurcevik said in Segmentation Fault when reading from a TCPSocket:

QTcpSocket* socket = static_cast<QTcpSocket*>(sender());
QBuffer* buffer = buffers.value(socket);
qint64 bytes = buffer->write(socket->readAll());

In such cases you ALWAYS have to check the pointers!
Either socket is an invalid pointer or buffer.

onurcevik

@jsulm Code was working just fine in the previous version where I was creating new connection and ending the previous one whenever I wanted to send new data. Which wasn't really optimized way to do it. But it started to give me this error when I switched to this version(single-connection) Where I create the connection and don't end it until program exits. So I am not sure if its really the pointers.

jsulm

@onurcevik Please simply print out socket to see whether it is nullptr or not, else we can guess here a lot about something you can easily check.

QTcpSocket* socket = static_cast<QTcpSocket*>(sender());
qDebug() << socket;
QBuffer* buffer = buffers.value(socket);
qDebug() << buffer;

onurcevik

@jsulm said in Segmentation Fault when reading from a TCPSocket:

QTcpSocket* socket = static_cast<QTcpSocket*>(sender());
qDebug() << socket;
QBuffer* buffer = buffers.value(socket);
qDebug() << buffer;

Oh I see it now

QBuffer* buffer = buffers.value(socket);

causes the problem because I commented out the lines

    connections.append(connection);
    QBuffer* buffer = new QBuffer(this);
    buffer->open(QIODevice::ReadWrite);
    buffers.insert(connection, buffer);

I removed this lines since I will only have single connection. Thanks for the help @jsulm . Since its just a comment line that causes problem should I delete this thread ?

aha_1980

@onurcevik

Since its just a comment line that causes problem should I delete this thread ?

No! It may help others, or yourself later. You only learn from mistakes ;)

Just mark the topic as SOLVED.

Thanks

SGaist

Hi,

One small note, use qobject_cast when casting QObject based classes. This will return a null pointer if either the source is already a null pointer or not from the correct class.