Gala Gala - 4 months ago 26
C Question

Qt Logging tool multithreading, calling signal and slot with variable number of arguments form another thread, mixing C and C++

This is my Logging class:

#include "log.h"
#include "ui_log.h"

#include <QDebug>
#include <QMutex>

bool Log::logOpen = false;

Log::Log(QWidget *parent) : QDialog(parent), ui(new Ui::Log)
text = new QString;
this->bar = this->ui->logText->verticalScrollBar();
connect(this, SIGNAL(call_write(char*)), this, SLOT(write(char*)));

delete ui;

void Log::on_buttonClose_clicked()
Log::logOpen = false;

void Log::on_buttonClear_clicked()

void Log::write(char *msg)

void write_c(Log *ptr, char *msg) {
emit ptr->call_write(msg);

void r_printf(char *format, ...) {
char *buf = (char*) malloc(4096);
va_list argList;
va_start(argList, format);
vsnprintf(buf, 4096,format, argList);
write_c(logptr, buf);

This is, by far, THE most complicated piece of code I have ever written. This is how it works:
r_printf(char *format, ...)
can be called from either C or C++, it takes the
arguments, and formats them. Then, it calls
, which sends a signal the Logger, which is connected to itself, so the Qt scheduler schedules an update to the window by calling
, so that Qt does not freak out. Crazy, right?

Here is the log header,

#ifndef LOG_H
#define LOG_H

#include <stdarg.h>

#ifdef __cplusplus

#include <QDialog>
#include <QString>
#include <QScrollBar>

namespace Ui {
class Log;

class Log : public QDialog

static bool logOpen;
explicit Log(QWidget *parent = 0);

Ui::Log *ui;
QString *text;
QScrollBar *bar;

private slots:
void on_buttonClose_clicked();
void on_buttonClear_clicked();
void write(char *msg);

void call_write(char *msg);



typedef struct Log Log;

#endif // __cplusplus

#ifdef __cplusplus
#define EXPORT_C extern "C"
#define EXPORT_C

extern Log *logptr;

EXPORT_C void write_c(Log *ptr, char *msg);
EXPORT_C void r_printf(char *format, ...);

#endif // LOG_H

is the pointer to a Log object inside the Main window.

While this does work, and it does not corrupt memory, is there a better solution to this huge mess? Is this a good solution? Can it be improved?

The goal is to have something like
printf(char *format, ...)
which can be called from either thread, from either C or C++. It should always work.


Here are my nitpicks, arranged in the order of how I thought about it.

  1. The declaration and definition of r_printf differ. Both must be EXPORT_C.

  2. EXPORT_C is likely to clash with some errant library code. Prefer a more unique name, like LOG_H_EXPORT_C.

  3. A signal can have zero or more recipients. The message allocated in r_printf can in principle leak or be multiply deleted. There's no reason to do manual memory management here, use a shared pointer or an implicitly shared data structure like QByteArray instead.

  4. For portability, you could use qvsnprintf.

  5. You're allocating a huge buffer. This is a tradeoff between allocation size and performance. You have an option of calling qvsnprintf with zero size to get the needed buffer size, then allocate a correctly-sized buffer, and call qvsnprintf again. You'd need to profile this to make an informed choice. At the very least, don't allocate a page-sized buffer since on some platforms this pessimizes and allocates more than a page, at a 100% overhead. A 0xFE0 size would be a safer bet.

  6. Prefer QString::asprintf, and simply pass a QString through the slots. This guarantees that the string will be converted from 8-bit to UTF-16 encoding only once.

  7. Since you're now emitting a container like QString or QByteArray, you could factor out the log message source into a separate object. It could be connected to zero or more views, then.

  8. Do not reset the log text. Instead, use QPlainText::appendPlainText. This will avoid the need to re-parse the entire log every time you add to it.

  9. The QPlainTextEdit is abysmally slow and unsuitable for logging. You should use a QListView or a custom widget instead.

  10. You may wish to keep the log scrolled to the bottom if it already is so. See this question for details.

Here's an example:

screenshot of the example


#ifndef LOG_H
#define LOG_H

#ifdef __cplusplus
#include <QObject>
class Log : public QObject {
    /// Indicates that a new message is available to be logged.
    Q_SIGNAL void newMessage(const QString &);
    /// Sends a new message signal from the global singleton. This method is thread-safe.
    static void sendMessage(const QString &);
    /// Returns a global singleton. This method is thread-safe.
    static Log * instance();
#define LOG_H_EXPORT_C extern "C"
#define LOG_H_EXPORT_C

LOG_H_EXPORT_C void r_printf(const char * format, ...);

#endif // LOG_H


#include "Log.h"
#include <cstdarg>


Log * Log::instance() { return log; }

void Log::sendMessage(const QString & msg) {
    emit log->newMessage(msg);

LOG_H_EXPORT_C void r_printf(const char * format, ...) {
    va_list argList;
    va_start(argList, format);
    auto msg = QString::vasprintf(format, argList);


#include <QtWidgets>
#include <QtConcurrent>
#include "Log.h"

int main(int argc, char ** argv) {
   using Q = QObject;
   QApplication app{argc, argv};
   QStringListModel model;
   Q::connect(Log::instance(), &Log::newMessage, &model, [&](const QString & msg) {
      auto row = model.rowCount();
      model.setData(model.index(row), msg);
   QWidget w;
   QVBoxLayout layout{&w};
   QListView view;
   bool viewAtBottom = false;
   QPushButton clear{"Clear"};
   Q::connect(&clear, &QPushButton::clicked,
              &model, [&]{ model.setStringList(QStringList{}); });
   Q::connect(view.model(), &QAbstractItemModel::rowsAboutToBeInserted, &view, [&] {
      auto bar = view.verticalScrollBar();
      viewAtBottom = bar ? (bar->value() == bar->maximum()) : false;
   Q::connect(view.model(), &QAbstractItemModel::rowsInserted,
              &view, [&]{ if (viewAtBottom) view.scrollToBottom(); });

      auto delay = 10;
      for (int ms = 0; ms <= 500; ms += delay) {
         r_printf("%d ms", ms);
   return app.exec();