<QtLogging> - Qt Logging Types

 if (!driver()->isOpen() || driver()->isOpenError()) {
     qWarning("QSqlQuery::exec: database not open");
     return false;
 }

 void myMessageHandler(QtMsgType, const QMessageLogContext &, const QString &);

 #include <QApplication>
 #include <stdio.h>
 #include <stdlib.h>

 QtMessageHandler originalHandler = nullptr;

 void logToFile(QtMsgType type, const QMessageLogContext &context, const QString &msg)
 {
     QString message = qFormatLogMessage(type, context, msg);
     static FILE *f = fopen("log.txt", "a");
     fprintf(f, "%s\n", qPrintable(message));
     fflush(f);

     if (originalHandler)
         *originalHandler(type, context, msg);
 }

 int main(int argc, char **argv)
 {
     originalHandler = qInstallMessageHandler(logToFile);
     QApplication app(argc, argv);
     ...
     return app.exec();
 }

     QT_MESSAGE_PATTERN="[%{time yyyyMMdd h:mm:ss.zzz t} %{if-debug}D%{endif}%{if-info}I%{endif}%{if-warning}W%{endif}%{if-critical}C%{endif}%{if-fatal}F%{endif}] %{file}:%{line} - %{message}"

Macro Documentation

qCritical(const char *message, ...)

Calls the message handler with the critical message message. If no message handler has been installed, the message is printed to stderr. Under Windows, the message is sent to the debugger. On QNX the message is sent to slogger2.

This function takes a format string and a list of arguments, similar to the C printf() function. The format should be a Latin-1 string.

Example:

 void load(const QString &fileName)
 {
     QFile file(fileName);
     if (!file.exists())
         qCritical("File '%s' does not exist!", qUtf8Printable(fileName));
 }

If you include <QtDebug>, a more convenient syntax is also available:

 qCritical() << "Brush:" << myQBrush << "Other value:" << i;

A space is inserted between the items, and a newline is appended at the end.

To suppress the output at runtime, you can define logging rules or register a custom filter.

For debugging purposes, it is sometimes convenient to let the program abort for critical messages. This allows you then to inspect the core dump, or attach a debugger - see also qFatal(). To enable this, set the environment variable QT_FATAL_CRITICALS to a number n. The program terminates then for the n-th critical message. That is, if the environment variable is set to 1, it will terminate on the first call; if it contains the value 10, it will exit on the 10th call. Any non-numeric value in the environment variable is equivalent to 1.

Note: This macro is thread-safe.

See also qCCritical(), qDebug(), qInfo(), qWarning(), qFatal(), qInstallMessageHandler(), and Debugging Techniques.

qDebug(const char *message, ...)

Calls the message handler with the debug message message. If no message handler has been installed, the message is printed to stderr. Under Windows the message is sent to the console, if it is a console application; otherwise, it is sent to the debugger. On QNX, the message is sent to slogger2. This function does nothing if QT_NO_DEBUG_OUTPUT was defined during compilation.

If you pass the function a format string and a list of arguments, it works in similar way to the C printf() function. The format should be a Latin-1 string.

Example:

 qDebug("Items in list: %d", myList.size());

If you include <QtDebug>, a more convenient syntax is also available:

 qDebug() << "Brush:" << myQBrush << "Other value:" << i;

With this syntax, the function returns a QDebug object that is configured to use the QtDebugMsg message type. It automatically puts a single space between each item, and outputs a newline at the end. It supports many C++ and Qt types.

To suppress the output at runtime, install your own message handler with qInstallMessageHandler().

Note: This macro is thread-safe.

See also qCDebug(), qInfo(), qWarning(), qCritical(), qFatal(), qInstallMessageHandler(), and Debugging Techniques.

qFatal(const char *message, ...)

Calls the message handler with the fatal message message. If no message handler has been installed, the message is printed to stderr. Under Windows, the message is sent to the debugger. On QNX the message is sent to slogger2.

If you are using the default message handler this function will abort to create a core dump. On Windows, for debug builds, this function will report a _CRT_ERROR enabling you to connect a debugger to the application.

This function takes a format string and a list of arguments, similar to the C printf() function.

Example:

 int divide(int a, int b)
 {
     if (b == 0)                                // program error
         qFatal("divide: cannot divide by zero");
     return a / b;
 }

To suppress the output at runtime, install your own message handler with qInstallMessageHandler().

See also qCFatal(), qDebug(), qInfo(), qWarning(), qCritical(), qInstallMessageHandler(), and Debugging Techniques.

qInfo(const char *message, ...)

Calls the message handler with the informational message message. If no message handler has been installed, the message is printed to stderr. Under Windows, the message is sent to the console, if it is a console application; otherwise, it is sent to the debugger. On QNX the message is sent to slogger2. This function does nothing if QT_NO_INFO_OUTPUT was defined during compilation.

If you pass the function a format string and a list of arguments, it works in similar way to the C printf() function. The format should be a Latin-1 string.

Example:

 qInfo("Items in list: %d", myList.size());

If you include <QtDebug>, a more convenient syntax is also available:

 qInfo() << "Brush:" << myQBrush << "Other value:" << i;

With this syntax, the function returns a QDebug object that is configured to use the QtInfoMsg message type. It automatically puts a single space between each item, and outputs a newline at the end. It supports many C++ and Qt types.

To suppress the output at runtime, install your own message handler using qInstallMessageHandler().

Note: This macro is thread-safe.

See also qCInfo(), qDebug(), qWarning(), qCritical(), qFatal(), qInstallMessageHandler(), and Debugging Techniques.

qWarning(const char *message, ...)

Calls the message handler with the warning message message. If no message handler has been installed, the message is printed to stderr. Under Windows, the message is sent to the debugger. On QNX the message is sent to slogger2.

This function takes a format string and a list of arguments, similar to the C printf() function. The format should be a Latin-1 string.

Example:

 void f(int c)
 {
     if (c > 200)
         qWarning("f: bad argument, c == %d", c);
 }

If you include <QtDebug>, a more convenient syntax is also available:

 qWarning() << "Brush:" << myQBrush << "Other value:" << i;

This syntax inserts a space between each item, and appends a newline at the end.

This function does nothing if QT_NO_WARNING_OUTPUT was defined during compilation. To suppress the output at runtime, you can set logging rules or register a custom filter.

For debugging purposes, it is sometimes convenient to let the program abort for warning messages. This allows you then to inspect the core dump, or attach a debugger - see also qFatal(). To enable this, set the environment variable QT_FATAL_WARNINGS to a number n. The program terminates then for the n-th warning. That is, if the environment variable is set to 1, it will terminate on the first call; if it contains the value 10, it will exit on the 10th call. Any non-numeric value in the environment variable is equivalent to 1.

Note: This macro is thread-safe.

See also qCWarning(), qDebug(), qInfo(), qCritical(), qFatal(), qInstallMessageHandler(), and Debugging Techniques.