Updtaing a QProgressbar from a different Thread - file

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).