防止 qDebug() 写入标准输出

Question

我正在使用qDebug()，qInfo()等组合qInstallMessageHandler来编写我的logfiles。执行应用程序时，我还会在批处理屏幕上获得输出。

我只找到了QT_NO_DEBUG_OUTPUT，但我想在运行时切换此配置。有没有办法防止Qt写入标准输出？

Answer 1

可悲的是，您只能访问消息，但无法阻止写入 std 输出。

这至少在Qt 5中是错误的。消息打印代码如下所示：您可以清楚地看到只使用了消息处理程序：

if (grabMessageHandler()) {
// prefer new message handler over the old one
if (msgHandler.load() == qDefaultMsgHandler
|| messageHandler.load() != qDefaultMessageHandler) {
(*messageHandler.load())(msgType, context, message);
} else {
(*msgHandler.load())(msgType, message.toLocal8Bit().constData());
}
ungrabMessageHandler();
} else {
fprintf(stderr, "%sn", message.toLocal8Bit().constData());
}

如您所见，仅当从messageHandler本身内部检测到递归时，fprintf(stderr, ...)才是回退。

因此，您只需要防止任何调试输出即可实现和设置自己的messageHandler。

要完全关闭Qt中的所有调试输出，可以执行以下操作：

#include <QtCore>
int main() {
qDebug() << "I'm not quiet at all yet";
qInstallMessageHandler(+[](QtMsgType, const QMessageLogContext &, const QString &){});
qDebug() << "I'm very, very quiet";
}

无论如何，应用程序范围的文件记录器的合理实现可能如下所示 - 它不会不必要地重新创建QTextStream;它改用QString::toUtf8()，并显式写入行尾。

#include <QtCore>
class Logger {
static struct Data {
Logger *instance;
QtMessageHandler chainedHandler;
} d;
bool m_isOpen;
QFile m_logFile;
QtMessageHandler m_oldHandler = {};
static void handler(QtMsgType type, const QMessageLogContext &context, const QString &msg) {
if (d.instance)
d.instance->log(msg);
if (d.chainedHandler)
d.chainedHandler(type, context, msg);
}
public:
enum ChainMode { DontChain, Chain };
Logger() {
Q_ASSERT(!instance());
m_logFile.setFileName("myLog.txt");
m_isOpen = m_logFile.open(QIODevice::WriteOnly | QIODevice::Append | QIODevice::Text);
d.instance = this;
}
~Logger() { uninstallHandler(); }
bool isOpen() const { return m_isOpen; }
void installHandler(ChainMode mode) {
Q_ASSERT(!m_oldHandler);
m_oldHandler = qInstallMessageHandler(handler);
if (mode == Chain)
d.chainedHandler = m_oldHandler;
}
void uninstallHandler() {
if (m_oldHandler) {
m_oldHandler = nullptr;
d.chainedHandler = nullptr;
qInstallMessageHandler(m_oldHandler);
}
}
/// This method is *not* thread-safe. Use with a thread-safe wrapper such as `qDebug`.
void log(const QString & msg) {
if (isOpen()) {
m_logFile.write(msg.toUtf8());
m_logFile.write("n", 1);
}
}
/// Closes the log file early - this is mostly used for testing.
void endLog() {
uninstallHandler();
m_logFile.close();
}
static Logger *instance() { return d.instance; }
};
Logger::Data Logger::d;
template <typename T> QByteArray streamOutputFor(const T &data) {
QBuffer buf;
buf.open(QIODevice::ReadWrite | QIODevice::Text);
QTextStream s(&buf);
s << data << endl;
buf.close();
return buf.data();
}
QByteArray readEnd(const QString &fileName, int count) {
QFile file(fileName);
if (file.open(QIODevice::ReadOnly | QIODevice::Text) && file.size() >= count) {
file.seek(file.size() - count);
return file.readAll();
}
return {};
}
void test() {
auto const entry = QDateTime::currentDateTime().toString().toUtf8();
Q_ASSERT(Logger::instance()->isOpen());
qDebug() << entry.data();
Logger::instance()->endLog();
auto reference = streamOutputFor(entry.data());
auto readback = readEnd("myLog.txt", reference.size());
Q_ASSERT(!reference.isEmpty());
Q_ASSERT(readback == reference);
}
int main() {
Logger logger;
logger.installHandler(Logger::DontChain);
qDebug() << "Something or else";
test();
}