Qt 5.11 | Qt Core | Shared Memory Example | dialog.cpp Example File

Qt 5.11.3 Reference Documentation

# dialog.cpp Example File

ipc/sharedmemory/dialog.cpp

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      #include "dialog.h"
      #include <QFileDialog>
      #include <QBuffer>
      #include <QtCore/QDebug>

      /*!
        \class Dialog

        \brief This class is a simple example of how to use QSharedMemory.

        It is a simple dialog that presents a few buttons. To compile the
        example, run make in qt/examples/ipc. Then run the executable twice
        to create two processes running the dialog. In one of the processes,
        press the button to load an image into a shared memory segment, and
        then select an image file to load. Once the first process has loaded
        and displayed the image, in the second process, press the button to
        read the same image from shared memory. The second process displays
        the same image loaded from its new loaction in shared memory.
      */

      /*!
        The class contains a data member \l {QSharedMemory} {sharedMemory},
        which is initialized with the key "QSharedMemoryExample" to force
        all instances of Dialog to access the same shared memory segment.
        The constructor also connects the clicked() signal from each of the
        three dialog buttons to the slot function appropriate for handling
        each button.
      */
      Dialog::Dialog(QWidget *parent)
        : QDialog(parent), sharedMemory("QSharedMemoryExample")
      {
          ui.setupUi(this);
          connect(ui.loadFromFileButton, SIGNAL(clicked()), SLOT(loadFromFile()));
          connect(ui.loadFromSharedMemoryButton,
                  SIGNAL(clicked()),
                  SLOT(loadFromMemory()));
          setWindowTitle(tr("SharedMemory Example"));
      }

      /*!
        This slot function is called when the \tt {Load Image From File...}
        button is pressed on the firs Dialog process. First, it tests
        whether the process is already connected to a shared memory segment
        and, if so, detaches from that segment. This ensures that we always
        start the example from the beginning if we run it multiple times
        with the same two Dialog processes. After detaching from an existing
        shared memory segment, the user is prompted to select an image file.
        The selected file is loaded into a QImage. The QImage is displayed
        in the Dialog and streamed into a QBuffer with a QDataStream.

        Next, it gets a new shared memory segment from the system big enough
        to hold the image data in the QBuffer, and it locks the segment to
        prevent the second Dialog process from accessing it. Then it copies
        the image from the QBuffer into the shared memory segment. Finally,
        it unlocks the shared memory segment so the second Dialog process
        can access it.

        After this function runs, the user is expected to press the \tt
        {Load Image from Shared Memory} button on the second Dialog process.

        \sa loadFromMemory()
       */
      void Dialog::loadFromFile()
      {
          if (sharedMemory.isAttached())
              detach();

          ui.label->setText(tr("Select an image file"));
          QString fileName = QFileDialog::getOpenFileName(0, QString(), QString(),
                                              tr("Images (*.png *.xpm *.jpg)"));
          QImage image;
          if (!image.load(fileName)) {
              ui.label->setText(tr("Selected file is not an image, please select another."));
              return;
          }
          ui.label->setPixmap(QPixmap::fromImage(image));

          // load into shared memory
          QBuffer buffer;
          buffer.open(QBuffer::ReadWrite);
          QDataStream out(&buffer);
          out << image;
          int size = buffer.size();

          if (!sharedMemory.create(size)) {
              ui.label->setText(tr("Unable to create shared memory segment."));
              return;
          }
          sharedMemory.lock();
          char *to = (char*)sharedMemory.data();
          const char *from = buffer.data().data();
          memcpy(to, from, qMin(sharedMemory.size(), size));
          sharedMemory.unlock();
      }

      /*!
        This slot function is called in the second Dialog process, when the
        user presses the \tt {Load Image from Shared Memory} button. First,
        it attaches the process to the shared memory segment created by the
        first Dialog process. Then it locks the segment for exclusive
        access, copies the image data from the segment into a QBuffer, and
        streams the QBuffer into a QImage. Then it unlocks the shared memory
        segment, detaches from it, and finally displays the QImage in the
        Dialog.

        \sa loadFromFile()
       */
      void Dialog::loadFromMemory()
      {
          if (!sharedMemory.attach()) {
              ui.label->setText(tr("Unable to attach to shared memory segment.\n" \
                                   "Load an image first."));
              return;
          }

          QBuffer buffer;
          QDataStream in(&buffer);
          QImage image;

          sharedMemory.lock();
          buffer.setData((char*)sharedMemory.constData(), sharedMemory.size());
          buffer.open(QBuffer::ReadOnly);
          in >> image;
          sharedMemory.unlock();

          sharedMemory.detach();
          ui.label->setPixmap(QPixmap::fromImage(image));
      }

      /*!
        This private function is called by the destructor to detach the
        process from its shared memory segment. When the last process
        detaches from a shared memory segment, the system releases the
        shared memory.
       */
      void Dialog::detach()
      {
          if (!sharedMemory.detach())
              ui.label->setText(tr("Unable to detach from shared memory."));
      }