Updtaing a QProgressbar from a different Thread

Question

I have developed my own hybrid stream cipher and for the GUI i am using Qt. Initially i wrote it on a single thread but it being a stream cipher was making GUI dysfunctional when operating on large files. So i shifted the encryption/decryption to a separate Qthread. Also to show the progress i included a standard QProgressbar onto the GUI. But when I run the File I/O the encryption/decryption works perfectly but the progress bar doesn't update properly. After the whole operation completes, the progress bar suddenly goes from 0% to 100% showing that it didn't get the chance to update during the operation. For the code, I emitted the completed percentage from the FileCrypto to the main GUI thread onto the QProgressbar's setValue(int) slot. Since it didn't work I also tried to sent a int poitner over to the FileCrypto thread whilst updating the pointer with the percentage and using a QTimer on the GUI thread to check the value of the int value locally and update the progress bar but still I got the exact same result.

Here is my code:

The FileCrypto class:

#include <QThread>
#include <QFile>
#include <PolyVernam.h>  //my algo header

class FileCrypto : public QThread
{
    Q_OBJECT

public:
    FileCrypto(QString, QString, int);
    bool stopIt;

protected:
    void run();

signals:
    void completed(int);
    void msg(QString);
    void pathMsg1(QString);
    void pathMsg2(QString);
    void keyMsg(QString);

private:
    QFile src, dest;
    QString tag;
    int mode;
    qint64 length;
    PolyVernam pv;
};

The Code:

#include <FileCrypto.h>

FileCrypto::FileCrypto(QString input, QString keyFile, int mode)
{
    stopIt = false;
    this->mode = mode;
    src.setFileName(input);

    if(mode == 1)
    {
        emit msg("Current Encryption/Decryption status: Encrypting file... :D:D");
        tag = "-encrypted";
        pv.setMode("encrypt", "");
    }
    else
    {
        emit msg("Current Encryption/Decryption status: Decrypting file... :D:D");
        tag = "-decrypted";
        pv.setMode("decrypt", keyFile);
    }

    dest.setFileName(QFileInfo(src).absolutePath() + "/" + QFileInfo(src).baseName()
                     + tag + "." + QFileInfo(src).completeSuffix());

    length = src.bytesAvailable();
}

void FileCrypto::run()
{
    qint64 done = 0;
    quint8 r, outChar;
    char ch;

    QDataStream in(&src);
    in.setVersion(QDataStream::Qt_4_7);
    src.open(QIODevice::ReadOnly);

    QDataStream out(&dest);
    out.setVersion(QDataStream::Qt_4_7);
    dest.open(QIODevice::WriteOnly);

    while(!in.atEnd() && !stopIt)
    {
        done++;

        in >> r;
        ch = char(r);

        if(mode == 1)
            outChar = pv.encrypt(QString(ch)).at(0).toAscii();
        else
            outChar = pv.decrypt(QString(ch)).at(0).toAscii();

        out << outChar;

        emit completed(int((done / length) * 100));
    }

    src.close();
    dest.close();

    if(stopIt)
        this->exit(0);

    if(mode == 1)
    {
        emit pathMsg1(QFileInfo(src).absoluteFilePath());
        emit pathMsg2(QFileInfo(dest).absoluteFilePath());
    }
    else
    {
        emit pathMsg1(QFileInfo(dest).absoluteFilePath());
        emit pathMsg2(QFileInfo(src).absoluteFilePath());
    }

    emit keyMsg(pv.keyFilePath);
    emit msg("Current Encryption/Decryption status: Idle... :'(");
}

This is how I am making the thread and connecting it on the main GUI thread:

FileCrypto *fc = new FileCrypto(ui->lineEdit_4->text(), "", 1);

connect(fc, SIGNAL(completed(int)), ui->progressBar, SLOT(setValue(int)));
connect(fc, SIGNAL(msg(QString)), ui->statusBar, SLOT(showMessage(QString)));
connect(fc, SIGNAL(pathMsg1(QString)), ui->lineEdit_4, SLOT(setText(QString)));
connect(fc, SIGNAL(pathMsg2(QString)), ui->lineEdit_5, SLOT(setText(QString)));
connect(fc, SIGNAL(keyMsg(QString)), ui->lineEdit_2, SLOT(setText(QString)));
connect(fc, SIGNAL(keyMsg(QString)), this, SLOT(done()));

If I don't update the progress bar i.e. don't emit the percentage, the process happens much faster. I also tried printing the percentage. It slows it down like hell but the values are fine. Also can you suggest a way to change it to buffered IO....

Any sort of help is much appreciated here.......

Answer 1

The problem does not lie in the fact that you are calling from a different thread. It is located in:

emit completed(int((done / length) * 100));

Since done and length are int types, and done <= length, done/length == 0. So change it to:

emit completed(100 * done / length);

(it can lead to arithmetic overflow).