Qt QNetworkAccessManager post method works only in main thread - c++

Question

I have a problem with Qt QNetworkAccessManager. I want to use it to interact with my web server which propose an POST api.
In my main function, I can use QNetworkAccessManager::post(). My server receive the data from the client.
But if I move the post function in another thread, my server receive anything.
Here an example of my code :
Worked code :
#include <iostream>
#include <QNetworkRequest>
#include <QUrl>
#include <QNetworkAccessManager>
#include <QCoreApplication>
#include <thread>
#include <QThread>
#include <QObject>
#include <QNetworkReply>
class PostClass: public QObject {
Q_OBJECT
public:
PostClass() {
manager = new QNetworkAccessManager(this);
connect(manager, SIGNAL(finished(QNetworkReply*)), this, SLOT(onfinish(QNetworkReply*)));
};
QNetworkAccessManager* manager = nullptr;
public slots:
void post() {
const QUrl url("http://[ip:port]/users");
QNetworkRequest req(url);
manager->get(req);
QNetworkRequest req1(QUrl("[ip:port]/event"));
req1.setHeader(QNetworkRequest::ContentTypeHeader, "application/json");
qDebug() << "Send new request";
QByteArray data(R"({"data": "QtGroupHEllo"})");
manager->post(req1, data);
};
void onfinish(QNetworkReply* rep) {
qDebug() << "reply delete!";
rep->deleteLater();
qDebug() << "https post_request done!";
};
};
class ThreadTmp: public QThread {
Q_OBJECT
public:
ThreadTmp() = default;
public slots:
void onfinish(QNetworkReply* rep) {
rep->deleteLater();
qDebug() << "reply delete!";
qDebug() << "https post_request done!";
};
signals:
void postRequest();
protected:
void run() override {
while (1) {
emit postRequest();
sleep(1);
}
}
};
int main(int argc, char* argv[]) {
QCoreApplication app (argc, argv);
std::cout << "Hello, World!" << std::endl;
PostClass postObj;
ThreadTmp tmp();
QObject::connect(&tmp, SIGNAL(postRequest()), &postObj, SLOT(post()));
tmp.start();
return app.exec();
}
#include "main.moc"
But if I move the PostClass instance in my ThreadTmp class, in the run function, only get works and post doesn't work because in my server, I only received the get request and not the post request :
#include <iostream>
#include <QNetworkRequest>
#include <QUrl>
#include <QNetworkAccessManager>
#include <QCoreApplication>
#include <thread>
#include <QThread>
#include <QObject>
#include <QNetworkReply>
class PostClass: public QObject {
Q_OBJECT
public:
PostClass() {
manager = new QNetworkAccessManager(this);
connect(manager, SIGNAL(finished(QNetworkReply*)), this, SLOT(onfinish(QNetworkReply*)));
// connect(manager, SIGNAL(finished(QNetworkReply*)), manager, SLOT(deleteLater()));
};
QNetworkAccessManager* manager = nullptr;
public slots:
void post() {
const QUrl url("http://[ip:port]/users");
QNetworkRequest req(url);
manager->get(req);
QNetworkRequest req1(QUrl("http://[ip:port]/event"));
req1.setHeader(QNetworkRequest::ContentTypeHeader, "application/json");
qDebug() << "Send new request";
QByteArray data(R"({"data": "QtGroupHEllo"})");
manager->post(req1, data);
};
void onfinish(QNetworkReply* rep) {
qDebug() << "reply delete!";
rep->deleteLater();
qDebug() << "https post_request done!";
};
};
class ThreadTmp: public QThread {
Q_OBJECT
public:
ThreadTmp() = default;
public slots:
void onfinish(QNetworkReply* rep) {
rep->deleteLater();
qDebug() << "reply delete!";
qDebug() << "https post_request done!";
};
signals:
void postRequest();
protected:
void run() override {
PostClass postObj;
connect(this, SIGNAL(postRequest()), &postObj, SLOT(post()));
while (1) {
emit postRequest();
sleep(1);
}
}
};
int main(int argc, char* argv[]) {
QCoreApplication app (argc, argv);
std::cout << "Hello, World!" << std::endl;
ThreadTmp tmp();
tmp.start();
return app.exec();
}
#include "main.moc"
I don't know why I have this behavior ...

Answer 1

You need to use a blocking .exec() in order to have an event loop in a thread. Just overriding run(), causes the thread to fire off that code and then terminate. When you post via the QNetworkAccessManager, that actually occurs asynchronously, in the background, driven by the QEventLoop out of the box. It should not typically be necessary to use another thread, if you are interacting with a simple REST api, or even if using a get() to download a large file. Qt handles for you, what you are probably trying to avoid with another thread, i.e. clogging up the main thread and hindering performance with network traffic. The best reason to use another thread would be to process the response from the server in the event that's going to take a long time. To do that, just hook up the signals/slots across threads, while leaving the post operation / network manager in your main one. (Generally speaking of course...)