Shared Memory Example#
Demonstrates doing inter-process communication using shared memory with the
The Shared Memory example shows how to use the
QSharedMemory class to implement inter-process communication using shared memory. To build the example, run make. To run the example, start two instances of the executable. The main() function creates an
application and an instance of our example’s Dialog class. The dialog is displayed and then control is passed to the application in the standard way.
if __name__ == "__main__": application = QApplication(argc, argv) dialog = Dialog() dialog.show() return application.exec()
Two instances of class Dialog appear.
Class Dialog inherits
QDialog . It encapsulates the user interface and an instance of
QSharedMemory . It also has two public slots, loadFromFile() and loadFromMemory() that correspond to the two buttons on the dialog.
class Dialog(QDialog): Q_OBJECT # public Dialog(QWidget parent = None) slots: = public() def loadFromFile(): def loadFromMemory(): # private def detach(): # private Ui.Dialog ui sharedMemory = QSharedMemory()
The constructor builds the user interface widgets and connects the clicked() signal of each button to the corresponding slot function.
def __init__(self, parent): super().__init__(parent) self.sharedMemory = "QSharedMemoryExample" ui.setupUi(self) connect(ui.loadFromFileButton.clicked, self.loadFromFile) connect(ui.loadFromSharedMemoryButton.clicked, self.loadFromMemory) setWindowTitle(tr("SharedMemory Example"))
Note that “QSharedMemoryExample” is passed to the
QSharedMemory() constructor to be used as the key. This will be used by the system as the identifier of the underlying shared memory segment.
Load Image From File... button on one of the dialogs. The loadFromFile() slot is invoked. First, it tests whether a shared memory segment is already attached to the process. If so, that segment is detached from the process, so we can be assured of starting off the example correctly.
def loadFromFile(self): if sharedMemory.isAttached(): detach() ui.label.setText(tr("Select an image file")) fileName = QFileDialog.getOpenFileName(0, QString(), QString(),() tr("Images (*.png *.xpm *.jpg)")) image = QImage() if not image.load(fileName): ui.label.setText(tr("Selected file is not an image, please select another.")) return ui.label.setPixmap(QPixmap.fromImage(image))
The user is then asked to select an image file using
getOpenFileName() . The selected file is loaded into a
QImage . Using a
QImage lets us ensure that the selected file is a valid image, and it also allows us to immediately display the image in the dialog using setPixmap().
Next the image is streamed into a
QBuffer using a
QDataStream . This gives us the size, which we then use to
create() our shared memory segment. Creating a shared memory segment automatically
attaches the segment to the process. Using a
QBuffer here lets us get a pointer to the image data, which we then use to do a memcopy() from the
QBuffer into the shared memory segment.
Note that we
lock() the shared memory segment before we copy into it, and we
unlock() it again immediately after the copy. This ensures we have exclusive access to the shared memory segment to do our memcopy(). If some other process has the segment lock, then our process will block until the lock becomes available.
Note also that the function does not
detach() from the shared memory segment after the memcopy() and unlock(). Recall that when the last process detaches from a shared memory segment, the segment is released by the operating system. Since this process only one that is attached to the shared memory segment at the moment, if loadFromFile() detached from the shared memory segment, the segment would be destroyed before we get to the next step.
When the function returns, if the file you selected was qt.png, your first dialog looks like this.
In the second dialog, click the
Display Image From Shared Memory button. The loadFromMemory() slot is invoked. It first
attaches the process to the same shared memory segment created by the first process. Then it
locks the segment for exclusive access and links a
QBuffer to the image data in the shared memory segment. It then streams the data into a
unlocks the segment.
def loadFromMemory(self): if not sharedMemory.attach(): ui.label.setText(tr("Unable to attach to shared memory segment.\n" \ "Load an image first.")) return buffer = QBuffer() in = QDataStream(buffer) image = QImage() 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))
In this case, the function does
detach() from the segment, because now we are effectively finished using it. Finally, the
QImage is displayed. At this point, both dialogs should be showing the same image. When you close the first dialog, the Dialog destructor calls the
QSharedMemory destructor, which detaches from the shared memory segment. Since this is the last process to be detached from the segment, the operating system will now release the shared memory.