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QByteArray::data() returns small buffer

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  • _ _ove_

    @_ove_ said in QByteArray::data() returns small buffer:

    Qt Creator 4.11.1

    I'm using Qt Creator 4.11.1, the debugger shows the content of the char *, 100 bytes from the start of the QByteArray. The debugger also shows the content of the QByteArray to be > 9K
    In order to parse the content I want to use a char *pointer that can be increased in steps of 1, in order to point to the consecutive characters in the 'char *buffer' string.

    J.HilkJ Online
    J.HilkJ Online
    J.Hilk
    Moderators
    wrote on last edited by
    #8

    @_ove_ please show an exact copy of your code.


    Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


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    A: It turns blue.

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    • _ Offline
      _ Offline
      _ove_
      wrote on last edited by
      #9

      I have made a short poc, but how do I upload the files? Tried to 'upload image', but forum says I don't have enough privileges.

      I can try past them in her or ?

      ove

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      • Christian EhrlicherC Offline
        Christian EhrlicherC Offline
        Christian Ehrlicher
        Lifetime Qt Champion
        wrote on last edited by
        #10

        We won't images but text.
        Apart from this - please reread what @jsulm wrote about \0 terminated char* arrays!

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        • _ Offline
          _ Offline
          _ove_
          wrote on last edited by
          #11

          Ok, as an old 'c' programmer I know a few things about \0 terminated strings,
          I will try to paste the files here:
          main.cpp:
          #include "mainwindow.h"
          #include <QApplication>
          int main(int argc, char *argv[])
          {
          QApplication a(argc, argv);
          MainWindow w;
          w.show();
          w.GetPage("https://www.google.com"); // or whatever
          return a.exec();
          }

          mainwindow.h:
          #ifndef MAINWINDOW_H
          #define MAINWINDOW_H
          #include "webreader.h"
          #include "webreaderthread.h"
          #include <QObject>
          #include <QMutex>
          #include <QUrl>
          #include <QMainWindow>
          QT_BEGIN_NAMESPACE
          namespace Ui { class MainWindow; }
          QT_END_NAMESPACE
          class MainWindow : public QMainWindow
          {
          Q_OBJECT
          public:
          MainWindow(QWidget *parent = nullptr);
          ~MainWindow();
          void GetPage(QString page);
          signals:
          void signal_GetPage(QUrl);
          void signal_Close();
          private:
          WebReader m_Reader;
          WebReaderThread *m_pWebThread;
          QByteArray m_ReadBuffer;
          QMutex m_Mutex;
          void ParseDocument(char *source);
          private slots:
          void OnSignalReply(QByteArray &reply);
          void OnThreadFinished() { m_Mutex.unlock(); }
          void OnSignalStarted() { m_Mutex.unlock(); }
          };
          #endif // MAINWINDOW_H

          mainwindow.cpp:
          #include "mainwindow.h"
          #include <QCoreApplication>
          MainWindow::MainWindow(QWidget *parent)
          : QMainWindow(parent)
          , m_Reader()
          , m_pWebThread(new WebReaderThread())
          {
          m_Mutex.lock();
          connect(this, SIGNAL(signal_GetPage(QUrl)), &m_Reader, SLOT(OnSignalGetPage(QUrl)));
          connect(this, SIGNAL(signal_Close()), &m_Reader, SLOT(OnSignalClose()));
          connect(m_pWebThread, &WebReaderThread::finished, this, &MainWindow::OnThreadFinished);
          connect(&m_Reader, SIGNAL(signal_Reply(QByteArray &)), this, SLOT(OnSignalReply(QByteArray &)), Qt::DirectConnection);
          connect(&m_Reader, SIGNAL(signal_Started()), this, SLOT(OnSignalStarted()));
          m_Reader.moveToThread(m_pWebThread);
          m_pWebThread->start();
          }
          MainWindow::~MainWindow()
          {
          if ( nullptr != m_pWebThread ) {
          emit signal_Close();
          m_Mutex.lock();
          delete m_pWebThread;
          }
          else
          m_Mutex.tryLock();
          m_Mutex.unlock();
          }
          void MainWindow::GetPage(QString page)
          {
          emit signal_GetPage(QUrl(page));
          m_Mutex.lock();
          char characters = m_ReadBuffer.data();
          // To illustrate the use of the char
          string
          ParseDocument(characters);
          exit(0);
          }
          void MainWindow::OnSignalReply(QByteArray &reply)
          {
          m_ReadBuffer = reply;
          m_Mutex.unlock();
          }
          void MainWindow::ParseDocument(char *source)
          {
          char *ptr = source;
          for ( ; *ptr != '\0'; ptr++ ) {
          if ( '\n' == *ptr ) {
          // Do something
          }
          }
          }

          webreaderthread.h:
          #ifndef WEBREADERTHREAD_H
          #define WEBREADERTHREAD_H
          #include <QObject>
          #include <QThread>
          class WebReaderThread : public QThread
          {
          Q_OBJECT
          public:
          WebReaderThread(QObject *parent = nullptr);
          private:
          void Run();
          };
          #endif // WEBREADERTHREAD_H

          webreaderthread.cpp:
          #ifndef WEBREADERTHREAD_H
          #define WEBREADERTHREAD_H
          #include <QObject>
          #include <QThread>
          class WebReaderThread : public QThread
          {
          Q_OBJECT
          public:
          WebReaderThread(QObject *parent = nullptr);
          private:
          void Run();
          };
          #endif // WEBREADERTHREAD_H

          webreader.h:
          #ifndef WEBREADER_H
          #define WEBREADER_H
          #include <QNetworkAccessManager>
          #include <QNetworkRequest>
          #include <QNetworkReply>
          #include <QObject>
          #include <QUrl>
          #include <QMutex>
          class WebReader : public QObject
          {
          Q_OBJECT
          public:
          WebReader(QObject *pParent = nullptr);
          ~WebReader();
          signals:
          void signal_Reply(QByteArray &);
          private:
          QNetworkAccessManager *m_pAccessMgr;
          QNetworkReply *m_pReply;
          QByteArray m_szPage;
          bool m_bError;
          bool m_bSslError;
          bool CreateAccessManager();
          private slots:
          void OnSignalClose();
          void OnSignalGetPage(QUrl url);
          // On signals from QNetworkAccessManager
          void OnFinished(QNetworkReply *);
          // On signals from QNetworkReply
          void OnReadyRead();
          void OnError(QNetworkReply::NetworkError code);
          void OnReplySslError(const QList<QSslError> &errors);
          };
          #endif // WEBREADER_H

          webreader.cpp;
          #include "webreader.h"
          WebReader::WebReader(QObject *pParent)
          : QObject(pParent)
          , m_pAccessMgr(nullptr)
          , m_pReply(nullptr)
          , m_bError(false)
          , m_bSslError(false)
          {
          }

          WebReader::~WebReader()
          {
          if ( NULL != m_pAccessMgr ) {
          delete m_pAccessMgr;
          }
          }
          void WebReader::OnSignalGetPage(QUrl url)
          {
          if ( nullptr == m_pAccessMgr ) {
          if ( !CreateAccessManager() )
          return;
          }
          QNetworkRequest request(url);
          m_pReply = m_pAccessMgr->get(request);
          if ( m_bSslError )
          return;
          connect(m_pReply, &QNetworkReply::readyRead, this, &WebReader::OnReadyRead, Qt::DirectConnection);
          connect(m_pReply, SIGNAL(error(QNetworkReply::NetworkError)), this, SLOT(OnError(QNetworkReply::NetworkError)), Qt::DirectConnection);
          connect(m_pReply, &QNetworkReply::sslErrors, this, &WebReader::OnReplySslError, Qt::DirectConnection);
          m_szPage.clear();
          m_pReply->readAll();
          }
          void WebReader::OnError(QNetworkReply::NetworkError code)
          {
          m_bError = true;
          }

          void WebReader::OnReplySslError(const QList<QSslError> &errors)
          {
          m_bSslError = true;
          }
          void WebReader::OnSignalClose()
          {
          if ( NULL != m_pReply ) {
          delete m_pReply;
          m_pReply = nullptr;
          }
          if ( NULL != m_pAccessMgr ) {
          delete m_pAccessMgr;
          m_pAccessMgr = nullptr;
          }
          exit(0);
          }
          void WebReader::OnFinished(QNetworkReply *)
          {
          emit signal_Reply(m_szPage);
          }
          void WebReader::OnReadyRead()
          {
          m_szPage += m_pReply->readAll();
          }
          bool WebReader::CreateAccessManager()
          {
          m_pAccessMgr = new QNetworkAccessManager();
          if ( nullptr == m_pAccessMgr )
          return false;
          m_pAccessMgr->setAutoDeleteReplies(true);
          connect(m_pAccessMgr, &QNetworkAccessManager::finished, this, &WebReader::OnFinished, Qt::DirectConnection);
          return true;
          }

          Project file QByteArrayTest.pro:
          QT += core gui
          QT += network
          greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
          CONFIG += c++11
          DEFINES += QT_DEPRECATED_WARNINGS
          SOURCES +=
          main.cpp
          mainwindow.cpp
          webreader.cpp
          webreaderthread.cpp
          HEADERS +=
          mainwindow.h
          webreader.h
          webreaderthread.h
          qnx: target.path = /tmp/$${TARGET}/bin
          else: unix:!android: target.path = /opt/$${TARGET}/bin
          !isEmpty(target.path): INSTALLS += target

          Hope this will work
          ove

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          • Christian EhrlicherC Offline
            Christian EhrlicherC Offline
            Christian Ehrlicher
            Lifetime Qt Champion
            wrote on last edited by Christian Ehrlicher
            #12

            First this is not really minimal and has a lot of unneeded stuff which makes it hard to find out what should go wrong, second where does the error occur now in your code?

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            • _ Offline
              _ Offline
              _ove_
              wrote on last edited by
              #13

              Sorry, messed up one small file:
              webreaderthread.cpp:
              #include "webreaderthread.h"
              #include <QObject>
              WebReaderThread::WebReaderThread(QObject *parent)
              : QThread(parent)
              {

              }
              void WebReaderThread::Run()
              {
              exec();
              }

              ove

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              • Christian EhrlicherC Offline
                Christian EhrlicherC Offline
                Christian Ehrlicher
                Lifetime Qt Champion
                wrote on last edited by
                #14

                Why do you need this class at all to reproduce the issue? It does nothing more than a plain QThread does. Where exactly is now the issue?

                Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                Visit the Qt Academy at https://academy.qt.io/catalog

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                • _ Offline
                  _ Offline
                  _ove_
                  wrote on last edited by
                  #15

                  Here is the short one;
                  #include <QCoreApplication>
                  #include <QString>
                  int main(int argc, char *argv[])
                  {
                  QString s(2000, QChar('c'));
                  // s now contains 2000 occurences of 'c'
                  char *str = s.toUtf8().data();
                  // str now contains 100 occurences of, what?
                  // Why doesnt the debugger show the content of str?
                  // But there still in only 100
                  exit(0);
                  }

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                  • Christian EhrlicherC Offline
                    Christian EhrlicherC Offline
                    Christian Ehrlicher
                    Lifetime Qt Champion
                    wrote on last edited by
                    #16

                    @_ove_ said in QByteArray::data() returns small buffer:

                    char *str = s.toUtf8().data();

                    C++ basics - you're working on a dangling pointer

                    // str now contains 100 occurences of, what?

                    str does not contain anything. It points to a (now deleted due to your other error) memory location. How you interpret this is up to you. Since it's a char* pointer, the debugger interprets it as characters and prints them out. Since a pointer does not contain any length information, functions like e.g. strlen, strcpy need a stop marker which is \0. So when your memory contains a \0, the output stops.
                    And also the debugger only prints a certain amount of memory - so if there is no \0 character anywhere in your memory, it will not print all but stop somewhere. Maybe even at 100 bytes, don't know and is not important. Important is on how you interpret this pointer and what you're doing with them. Your code does nothing with them so there is no error which can be fixed (apart from the dangling pointer).

                    Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                    Visit the Qt Academy at https://academy.qt.io/catalog

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                    • _ Offline
                      _ Offline
                      _ove_
                      wrote on last edited by
                      #17

                      Ok, I thought that str should point to the internal data structure of the QByteArray, containing the data.
                      Anyway, I have figured out the original problem:
                      when i retrieve the char* pointer from a QByteArray by means of the data() method, the debugger only displays the first100 bytes of the content of the pointer, although the rest of the data is there also. On the other hand, the debugger displays the entire content for the QByteArray variable, not only the first 100 bytes.

                      Thanks for your effort
                      ove

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