Redirecting QDebug, QWarning, and QCritical Output

In Qt, debugging information is often printed to the console using qDebug() and other similar statements. It can be useful to redirect this output to a file instead, especially for cross-platform development. This avoids using shell scripts and provides a more consistent solution.

Custom Message Handler

To redirect debug output, Qt provides the qInstallMessageHandler() function. This allows you to install a custom message handler that processes the messages before they are printed. Here's an example handler:

void myMessageOutput(QtMsgType type, const QMessageLogContext &context, const QString &msg)
{
    QByteArray localMsg = msg.toLocal8Bit();
    switch (type) {
    case QtDebugMsg:
        fprintf(stderr, "Debug: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
        break;
    case QtInfoMsg:
        // ...
    case QtWarningMsg:
        // ...
    case QtCriticalMsg:
        // ...
    case QtFatalMsg:
        fprintf(stderr, "Fatal: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
        abort();
    }
}

This handler outputs the messages to stderr, but you can replace stderr with a file stream to redirect the output.

Installation

To install the custom handler, call qInstallMessageHandler() in your main() function:

qInstallMessageHandler(myMessageOutput);

Once installed, all qDebug and similar messages will be redirected to the handler and written to the file or stream you specified.