Core dump when outputting log into QTextEdit in qt5

[WorstCodeWay]

I want to implement a log viewer in qt with spdlog. But the output log in QTextEdit is messed up in format while is well-formed in a log file. My codes are:

MainWindow.h

#include <QMainWindow>
#include <QTextEdit>
#include <spdlog/spdlog.h>
#include <spdlog/sinks/daily_file_sink.h>
#include <spdlog/sinks/base_sink.h>
#include <spdlog/details/null_mutex.h>
#include "log.h"

template<typename Mutex>
  class qtextedit_sink : public spdlog::sinks::base_sink<Mutex>
  {
  public:
    explicit qtextedit_sink(QSharedPointer<QTextEdit> qtTextEdit) : _qtTextEdit(qtTextEdit) {}
    qtextedit_sink(const qtextedit_sink &) = delete;
    qtextedit_sink &operator=(const qtextedit_sink &) = delete;

  protected:
    void sink_it_(const spdlog::details::log_msg &msg) override
    {
      spdlog::memory_buf_t formatted;
      spdlog::sinks::base_sink<Mutex>::formatter_->format(msg, formatted);

      if (_qtTextEdit)
      {
        std::string tempMsg = fmt::to_string(formatted);

        _qtTextEdit->insertPlainText(fmt::to_string(formatted).c_str());
      }
    }

    void flush_() override {}

  private:
    QSharedPointer<QTextEdit>_qtTextEdit{ nullptr };
  };

  using qtextedit_sink_mt = qtextedit_sink<std::mutex>;
  using qtextedit_sink_st = qtextedit_sink<spdlog::details::null_mutex>;

  typedef std::shared_ptr<spdlog::sinks::daily_file_sink_mt> DailyFileSinkMtPtr;
  typedef std::shared_ptr<qtextedit_sink_mt> QTextEditSinkMtPtr;
  typedef std::shared_ptr<spdlog::logger> LoggerPtr;

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
  Q_OBJECT

public:
  MainWindow(QWidget *parent = nullptr);
  ~MainWindow();

private:
  Ui::MainWindow *ui;
  QSharedPointer<QTextEdit> txt;
  std::shared_ptr<spdlog::logger> projectLogger;
  std::shared_ptr<spdlog::sinks::daily_file_sink_mt> projectFileSink;
  std::shared_ptr<vistrack::qtextedit_sink_mt> projectWidgetSink;
};

MainWindow.cpp

MainWindow::MainWindow(QWidget *parent)
  : QMainWindow(parent)
  , ui(new Ui::MainWindow)
{
  ui->setupUi(this);

  spdlog::sinks_init_list sinks;
  projectLogger = std::make_shared<spdlog::logger>("Project",  sinks);

  projectFileSink = std::make_shared<spdlog::sinks::daily_file_sink_mt>(
    "./logs", 23, 59);
    projectLogger->sinks().push_back(projectFileSink);

  txt = QSharedPointer<QTextEdit>(ui->textEdit);
  projectWidgetSink = std::make_shared<qtextedit_sink_mt>(txt);
  projectLogger->sinks().push_back(projectWidgetSink);

  projectLogger->set_pattern("[%L] [%n] [%Y/%m/%d %H:%M:%S:%e] %v");
  projectLogger->flush_on(spdlog::level::info);

  std::thread([&](){
    int i = 0;
    while(true)
    {
      i++;
      projectLogger->info("This is information {}", i);
      projectLogger->warn("This is warning {}", i);
      projectLogger->error("This is error {}", i);
      usleep(1000000);
    }
  }).detach();

  std::cout << "infromation output " << std::endl;
}

MainWindow::~MainWindow()
{
  delete ui;
}

Here is the core dump stack:

1   QFontEngineFT::shouldUseDesignMetrics        qfontengine_ft.cpp      1595 0x7ffff08ee451 
2   QFontEngineFT::recalcAdvances                qfontengine_ft.cpp      1634 0x7ffff08ee451 
3   _hb_qt_font_get_glyph_h_advance              qharfbuzzng.cpp         517  0x7ffff6f281c9 
4   hb_font_t::get_glyph_h_advance               hb-font-private.hh      221  0x7ffff72121d9 
5   hb_ot_position_default                       hb-ot-shape.cc          671  0x7ffff72121d9 
6   hb_ot_position                               hb-ot-shape.cc          769  0x7ffff72121d9 
7   hb_ot_shape_internal                         hb-ot-shape.cc          855  0x7ffff72121d9 
8   _hb_ot_shape                                 hb-ot-shape.cc          882  0x7ffff72121d9 
9   hb_shape_plan_execute                        hb-shaper-list.hh       43   0x7ffff71f517a 
10  hb_shape_full                                hb-shape.cc             132  0x7ffff71f4adc 
11  QTextEngine::shapeTextWithHarfbuzzNG         qtextengine.cpp         1706 0x7ffff6e8ccd3 
12  QTextEngine::shapeText                       qtextengine.cpp         1545 0x7ffff6e8f83d 
13  QTextEngine::shape                           qtextengine.cpp         2028 0x7ffff6e906e9 
14  QTextEngine::shapeLine                       qtextengine.cpp         1342 0x7ffff6e95cf3 
15  QTextLineItemIterator::QTextLineItemIterator qtextengine.cpp         4052 0x7ffff6e982ea 
16  QTextLine::draw                              qtextlayout.cpp         2539 0x7ffff6e9c0f0 
17  QTextLayout::draw                            qtextlayout.cpp         1262 0x7ffff6e9da2d 
18  QTextDocumentLayoutPrivate::drawBlock        qtextdocumentlayout.cpp 2090 0x7ffff6ee1e52 
19  QTextDocumentLayoutPrivate::drawFlow         qtextdocumentlayout.cpp 1988 0x7ffff6ee3dcd 
20  QTextDocumentLayoutPrivate::drawFrame        qtextdocumentlayout.cpp 1220 0x7ffff6ee3755 
... <More>                                                                                   

[tt4g]

Qt Widget is not guaranteed to work when operated from other than the GUI thread (usually the main thread). Logging from a different thread will crash it.

A workaround for the problem is provided by using the Qt event thread. See: https://doc.qt.io/qt-6/threads-synchronizing.html

Since spdlog::sinks::qt_sink has been added to spdlog (#2018), why don't you try using it?

---

By the way. If I remember correctly, the destructor of the parent Widget will also delete the child Widget, depending on the Qt design. There is a bug in QSharedPtr and delete ui; which does a double delete.

[WorstCodeWay]

@tt4g Thanks for quick reply. I will look into and try spdlog::sinks::qt_sink, but I wonder if this qt_sink can support colorful message?

And I will read thread you introduced. I think it will help a lot.

"If I remember correctly, the destructor of the parent Widget will also delete the child Widget, depending on the Qt design. There is a bug in QSharedPtr and delete ui; which does a double delete."

This is a problem indeed.

I will be back soon after I have some progress.

And you make such great a work!

[tt4g]

Change the color of QTextEdit when the log is received by Qt slot function. You should be able to add the log level as an argument with reference to 'spdlog::sinks::qt_sink'.

https://github.com/gabime/spdlog/blob/eb3220622e73a4889eee355ffa37972b3cac3df5/include/spdlog/sinks/qt_sinks.h#L34-L40

[WorstCodeWay]

That qt_sinks solved my problem. Learnt a lot! Thank you @tt4g.