Redirect QDebug output to a file.

redirectQDebug.cpp

/*
This code redirects the output of qDebug() to a log file.
It also rotates the file base on size.
Uses QMutex for thread safety.
*/

//some constants to parameterize.
const qint64 LOG_FILE_LIMIT = 3000000;
const QString LOG_PATH = "/application/logs/";
const QString LOG_FILENAME = "mylogfile.log";

    //thread safety
    QMutex mutex;
    
void redirectDebugMessages(QtMsgType type, const QMessageLogContext & context, const QString & str)
{
    //thread safety
    mutex.lock();
    QString txt;
  
    //prepend timestamp to every message
    QString datetime = QDateTime::currentDateTime().toString("yyyy.MM.dd hh:mm:ss");
    //prepend a log level label to every message
    switch (type) {
    case QtDebugMsg:
        txt = QString("[Debug*] ");
        break;
    case QtWarningMsg:
        txt = QString("[Warning] ");
    break;
    case QtInfoMsg:
        txt = QString("[Info] ");
    break;
    case QtCriticalMsg:
        txt = QString("[Critical] ");
    break;
    case QtFatalMsg:
        txt = QString("[Fatal] ");
    }
  
    QString filePath = LOG_PATH + LOG_FILENAME;
    QFile outFile(filePath);
  
    //if file reached the limit, rotate to filename.1
    if(outFile.size() > LOG_FILE_LIMIT){
        //roll the log file.
        QFile::remove(filePath + ".1");
        QFile::rename(filePath, filePath + ".1");
        QFile::resize(filePath, 0);
    }
  
    //write message
    outFile.open(QIODevice::WriteOnly | QIODevice::Append);
    QTextStream ts(&outFile);
    ts << datetime << txt << str << endl;
  
    //close fd
    outFile.close();
    mutex.unlock();
}

//in the main function add this code

int main(int argc, char *argv[]){
 
  qInstallMessageHandler(redirectDebugMessages);
  
}

Thanks for this! I don't think it is thread-safe, though. You are instantiating mutex inside the message handler function, which means every thread will have it's own copy of the mutex and calling mutex.lock() will never block. The mutex should be a global variable for this to be thread-safe.

Nice catch! Thanks