Problems about a connection to a tcp server with domain!!

[coderKnight]

I‘m using QTcpSocket with Qt version 5.9.9. In my system,I have a 4G module ,I powered it on when my program started and wait for it attched to my system as a netcard by usb.When I try calling "connectToHost" to connect to the tcp server(a server with a domain) in three threads，I sometimes got "host not found" error,and this is not recoverable unless I restart my program, I was really confused for this for serveral days,any suggestions? thanks

[Ronel_qtmaster]

@coderKnight please could you send your code?especially the connect part

[coderKnight]

@Ronel_qtmaster
1. I just create a QTcpSocket like this
clientInstance = new QTcpSocket(this);
2. then create a timer ，in the timer slot,I try connect to
server periodicly if connect to server failed,or stop the
timer
bool result = false;
clientInstance->connectToHost("domain",10000);
result = clientInstance->waitForConnected(10000);
if(result == false){
qDebug() << "got error:" << clientInstance>error()
}
The procedure happens In three diffrent threads,did I do something wrong？

[Pl45m4]

@coderKnight

What thread approach do you use?

And please use the code tags to format the code you post here.

[coderKnight]

@Pl45m4
Sorry for reply late，here is the code

#include "dnsparsetask.h"
#include <QHostInfo>
namespace Communication
{
    DnsParseTask::DnsParseTask(QObject *parent) : QObject(parent)
    {

    }

    DnsParseTask *DnsParseTask::instance()
    {
        static QMutex mutex;
        static QScopedPointer<DnsParseTask> inst;
        QMutexLocker locker(&mutex);

        if (Q_UNLIKELY(!inst)) {
            if (!inst) {
                inst.reset(new DnsParseTask);
            }
        }
        return inst.data();
    }

    void DnsParseTask::init(QString hostname,int port,int period)
    {
        _self = new QThread();
        if(_self == nullptr){
            qWarning() << "create  thread failed";
            return;
        }
        _hostname = hostname;
        _port = port;
        _periodTime = period;
        this->moveToThread(_self);
        connect(_self, &QThread::finished, _self, &QObject::deleteLater);
        connect(_self, &QThread::finished, this, &QObject::deleteLater);
        connect(_self, SIGNAL(started()), this, SLOT(startRun()));

        _self->start();

        return;
    }

    int DnsParseTask::setDomainToParse(QString domain)
    {
        QMutexLocker locker(&_lock);

        if(domain.isEmpty()){
            return -1;
        }
        if(_domainMap.contains(domain)){
            qWarning() << "domain[" << domain << "] has been in map";
            return 0;
        }
        _domainMap.insert(domain,"");
        if(_parseTimer->isActive() == false){
            qWarning() << "start parse domain";
            _parseTimer->start();
        }
        return 0;
    }

    QString DnsParseTask::getDomainIpAddr(QString domain)
    {
        if(_domainMap.contains(domain) == false){
            return QString();
        }
        return _domainMap.value(domain);
    }

    void DnsParseTask::periodicParse()
    {
        QObject *obj = _socket;
        _socket->connectToHost(_hostname,_port);
        if(_socket->waitForConnected(5000)){
             qWarning() << this->thread()->currentThreadId() << ",[connect to host" << _hostname << "] success!";;
             _parseTimer->stop();
             return;
        }
        qWarning() << this->thread()->currentThreadId() << ",[connect to host" << _hostname << "] failed:" << _socket->errorString();
       return;
#if 0
        QStringList hostNames = _domainMap.keys();
        foreach (const QString &hostName, hostNames) {
            qWarning() << ">>>>>>>>>>>>>>>:" << this->thread()->currentThreadId() << "," << hostName;
            QHostInfo hostInfo = QHostInfo::fromName(hostName);
            if (hostInfo.error() == QHostInfo::NoError) {
                QList<QHostAddress> addresses = hostInfo.addresses();
                foreach (const QHostAddress &address, addresses) {
                    qDebug() << "Resolved IP address:" << address.toString();
                }
            } else {
                qDebug() << "DNS lookup failed:" << hostInfo.errorString();
            }
        }
        return;
#endif
#if 0
        QStringList hostNames = _domainMap.keys();
        foreach (const QString &hostName, hostNames) {
            QHostInfo::lookupHost(hostName, [hostName](const QHostInfo &hostInfo) {
                if (hostInfo.error() == QHostInfo::NoError) {
                    QList<QHostAddress> addresses = hostInfo.addresses();
                    foreach (const QHostAddress &address, addresses) {
                        qDebug() << "Resolved IP address for" << hostName << ":" << address.toString();
                    }
                } else {
                    qDebug() << "DNS lookup failed for" << hostName << ":" << hostInfo.errorString();
                }
            });
        }
#endif
    }

    void DnsParseTask::startRun()
    {
        qWarning() << this->thread()->currentThreadId() << "start run period time:" << _periodTime;
        _domainMap.clear();
        _parseTimer = new QTimer(this);
        connect(_parseTimer,SIGNAL(timeout()),this,SLOT(periodicParse()));
        _parseTimer->setInterval(_periodTime);
        _parseTimer->start();
        _socket = new QTcpSocket(this);
    }
}


And the main function like：
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    DnsParseTask *task = nullptr;

    task = new DnsParseTask;
    task->init("mqtts.lfzk.com",1883,500);

    task = new DnsParseTask;
    task->init("mqtt.lfzk.com",1883,500);

    task = new DnsParseTask;
    task->init("101.200.183.188",1883,100);

    task = new DnsParseTask;
    task->init("cb05b435-ba2e-41de-bec9-b9ef2254a1a5.mock.pstmn.io",80,0);
    a.exec();

    return 0;
}

In the code I create four QTcpSocket and try connect to some domains，the "init" function hold three params，the domain,the port and retry connection interval,and the "periodicParse" function try connect to the domain.On my borad, I use wired netcard "eth0" to connect to servers,”eth0“ configed as dhcp. Before startup the program,I shutdown the netcard by “ifdown eth0”,and then startup the program.The program works 1 minute and print ”Host not found“ error，then I open the netcard ,and my board got correct IP address,but the program still print ”Host not found“ error.I use the cmd "ping -i eth0 mqtts.lfzk.com" ,the cmd works well,I am really confused about this