How to fetch the current file name and line number to be written into a file for logging purpose

chandan94

I am creating a log for my application, printing the current date, time and other stuff. But I am struck at writing the filename and line number(the line from where I am calling my write function) into a file. Basically, I am using

in my qml to call a fileio function which does the write operation. As you can see I am printing the line number and filename manually. Is there a way to print it dynamically?
Can somebody help out?
TIA.

GrecKo

You can do it with qInstallMessageHandler
This will intercepts call to console.log , console.debug ...
The example in the documentation outputs the line number like you want.

chandan94

@GrecKo said in How to fetch the current file name and line number to be written into a file for logging purpose:

documentation

Thanks @GrecKo , But I dont want to print the line number on the console. I want to pass the line number that is fetched dynamically which is to be printed in a file. Also, any suggestions for fetching the file name dynamically which also needs to e printed into a file.
TIA.

hskoglund

Hi, I had a similar problem, where I wanted to log filename+linenumer with a message as well. I came up with this (note: this was for widget-based .cpp code, no qml):

// define a class for logging
class LoggingObject
{
    QString sFilename = "unknown";
    int     nLineNo   = 0;

    public:
    LoggingObject(QString sFilename,int nLineNo)
    {
        this->sFilename = sFilename;
        this->nLineNo   = nLineNo;
    }

    void logInvoker(const char *msg,...)
    {
   // build the log message using vasprintf
       va_list ap;
       va_start(ap, msg);
       QString sMsg = QString::vasprintf(msg, ap);

   // prefix with filename and lineno. and write to our file
      fileio.write(QString("File %1 lineno. %2 msg: '%3'").arg(sFilename).arg(nLineNo).arg(sMsg));
    }
};

then #define a macro to call it

#define log LoggingObject(__FILE__,__LINE__).logInvoker

then in your code you can write something like:

...
log("Back button clicked");
...

GrecKo

@chandan94 with qInstallMessageHandler you do what you want with the message. You can output it to the console, write it to a file or both.

hskoglund

@GrecKo Yes I use qInstallMessageHandler in all my programs, it's a handy function, but it can not help me with displaying the filename and linenumber from where I called the log macro to log something. For example:

...
log("First message");
...
log("Second message);
...

(Those 2 calls should show the same filename like 'mainWindow.cpp' but different linenumbers).

chandan94

@hskoglund
Hi, I am making a Qt quick application. Can you suggest something for this? Also, I am confused how to use qInstallMessageHandler.

GrecKo

@hskoglund Indeed, you have to use qDebug and friends for that.

hskoglund

I'm no expert on Qt quick apps, but qInstallMessageHandler works the same, and instead of qDebug(), in a .qml file you can use console.log. So lets try:

First declare your messagehandler, easiest is to include it in your main.cpp, something like this:

void myMessageHandler(QtMsgType type, const QMessageLogContext &context, const QString &msg)
{
// construct a msg
    QString sFilename = context.category;
    QString sFunction = context.function;
    int nLineNo       = context.line;

    QString s = QString("File '%1' function '%2' at line no %3: '%4'\n").arg(sFilename).arg(sFunction).arg(nLineNo).arg(msg);

// write to file
   FILE* f;
#ifdef _MSC_BUILD
    if (0 != fopen_s(&f,"myfile.log","a+"))   // MSVC compiler: fopen() is frowned upon, use fopen_s()
        f = NULL;
#else
    f = fopen("myfile.log","a+");          // all other compilers: use standard fopen() call
#endif
    if (f)
    {
        fprintf(f,"%s",qUtf8Printable(s));
        fclose(f);
    }

// also show if we're running inside Qt Creator (delayed)
    fprintf(stderr,"%s",qUtf8Printable(s));
}

and then inside your main() install the handler, easiest is at the beginning:


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

    QGuiApplication app(argc, argv);
...

Now try using a console.log('Hello from qml') in a qml file. Good luck!

Pradeep Kumar

Hi @chandan94

u can use the below mentioned way for all the debug, based on critical , fatal also.

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    qDebug() << Q_FUNC_INFO << "In Main " << endl;
#ifdef DEBUG
    FILE * fp;
    fp = fopen (LOG_FILE_PATH, "a+");
    if(fp==NULL)
        fp = fopen (LOG_FILE_PATH, "w");
    if(fp==NULL){
        return 0;
    }
    fclose(fp);
    qInstallMessageHandler(myAppLogsToFile);
#endif
    FirstClass firstClassObjecct;
    firstClassObjecct.show();

    return a.exec();
}

void myAppLogsToFile(QtMsgType type, const QMessageLogContext &context, const QString &msg)
{
    QString timeFormat = "dd-MMM-yyyy hh:mm:ss:zzz"; // u can change the format  of datetime

    QByteArray localMsg = msg.toLocal8Bit();
    QByteArray tim = QDateTime::currentDateTime().toString(timeFormat).toLocal8Bit();
    FILE * fp;
    fp = fopen (LOG_FILE_PATH, "a");

    if(fp==NULL){
        return;
    }
    switch (type) {
    case QtDebugMsg:
        fprintf(fp, "Debug: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
        break;
    case QtWarningMsg:
        fprintf(fp, "Warning: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
        break;
    case QtCriticalMsg:
        fprintf(fp, "Critical: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
        break;
    case QtFatalMsg:
        fprintf(fp, "Fatal: %s (%s:%u, %s)\n", localMsg.constData(), context.file, context.line, context.function);
        abort();
    default :
        break;
    }
    fclose(fp);
}

Thanks,

chandan94

@Pradeep-Kumar said in How to fetch the current file name and line number to be written into a file for logging purpose:

u can use the below mentioned way for all the debug, based on critical , fatal also.

Hi @Pradeep-Kumar ,

I am using :

class FileIO : public QObject
{
Q_OBJECT
public slots:
bool write(const QString& source, const QString& data)
{
if (source.isEmpty())
return false;

    QFile file(source);
    if (!file.open(QFile::Append | QFile::Truncate))
        return false;

    QTextStream out(&file);
    out << data;
    file.close();
    return true;
}

for write function.
And in qml:
fileio.write("/tmp/test.log", Qt.formatDateTime(new Date(), "MMM dd hh:mm:ss.zzzzzz") + "|" + "|" + "|" + "Back button clicked" + "|" + "fileName:Menu.qml" + "|" + "lineNumber:30" + "\n");
So is there some property like QT.formatDateTime property, which I can use directly to get the line number and filename?