I can't write or read stdin or stdout of a QProcess from a Qt application

Question

I'm using the Qt framework to make a GUI window, and what I do is open another application by creating a QProcess. I want to communicate with this second process, and the documentation says I can do this by writing to the process's stdin or reading from the process's stdout. I've tried many combinations and can't get it to work.

I can redirect the stdin and stdout of the new process to a file using:

process->setStandardOutputFile("d:\\outputfile.txt");
process->setStandardInputFile("d:\\inputfile.txt");

And then all cin, cout, cerr and clog commands read and write to the files. But I can't communicate directly from the Qt main window (the original application) to the new Qprocess that I create. I use:

process->write("HELLO_FROMMAINWINDOW");

and in the child application in the main loop I check stdin for any input:

fseek(stdin, 0, SEEK_END);
int num = ftell(stdin);
if (num > 0) logEntry("We have input");

The if condition is never true. Also I do std::cout in the main loop of the child process and in the Qt main window I try to read with:

 QByteArray str;
 str = process->readAll();
 str = process->readAllStandardOutput();

But they are always empty strings. I have read the documentation at http://doc.qt.io/qt-5/qprocess.html a few times but it's possible I'm not doing things right, as in maybe I'm forgetting to set something before trying to read and write.

Here is my attempt at a verifiable example:

My main GUI window header looks like this:

//#ifndef MAINWINDOW_H
//#define MAINWINDOW_H
#pragma once
//#endif // MAINWINDOW_H
#include <QMainWindow>

class QProcess;

namespace Ui
{
    class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    explicit MainWindow(QWidget* parent = nullptr);
    ~MainWindow();
    Ui::MainWindow* ui;
    QProcess* engineProcess;
private slots:
    void on_pushButton_clicked();
    void on_pushButton_2_clicked();
};      // This is very much a stock setup with a simple main window.

Here is the main window cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QProcess>

MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent), ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}
MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::on_pushButton_clicked() // CALLED WHEN I CLICK THE START BUTTON
{// OPEN ENGINE PROCESS
    engineProcess = new QProcess(this);
    engineProcess->setWorkingDirectory("E:/Development/Programming/Projects/OpenGLEngine");
    int w = 1600 / 2, h = 900 / 2; // JUST GIVING RESOLUTION VALUES AS ARGUMENTS
    QStringList args = { "editor", std::to_string(w).c_str(), std::to_string(h).c_str() };
    //engineProcess->setStandardOutputFile("d:\\outputfile.txt");
    //engineProcess->setStandardInputFile("d:\\input.txt");
    // THE PREVIOUS TWO CALLS WORK FINE, stdout and stdin read and write them fine.

    engineProcess->start("E:/Development/Programming/Projects/OpenGLEngine/x64/Release/OpenGlEngine.exe", args);
    engineProcess->write("HELLO_FROMMAINWINDOW\n"); // DOESN'T GET DETECTED
}

void MainWindow::on_pushButton_2_clicked()
{
    const char* ch = "HEllo";
    quint64 charsWritten = engineProcess->write(ch, 5); // charsWritten equals 5

   auto result = engineProcess->write("Hello from the editor\n"); // 22 chars written, correct
   engineProcess->write("HELLO_FROMMAINWINDOW\n"); // DOESN'T GET DETECTED

   QByteArray str;
   str = engineProcess->readAll();
   str = engineProcess->readAllStandardOutput(); // EMPTY STRINGS RESULT
}