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diff --git a/examples/ipc/doc/src/sharedmemory.qdoc b/examples/ipc/doc/src/sharedmemory.qdoc new file mode 100644 index 0000000000..b9f0c86d44 --- /dev/null +++ b/examples/ipc/doc/src/sharedmemory.qdoc @@ -0,0 +1,131 @@ +/**************************************************************************** +** +** Copyright (C) 2013 Digia Plc and/or its subsidiary(-ies). +** Contact: http://www.qt-project.org/legal +** +** This file is part of the documentation of the Qt Toolkit. +** +** $QT_BEGIN_LICENSE:FDL$ +** Commercial License Usage +** Licensees holding valid commercial Qt licenses may use this file in +** accordance with the commercial license agreement provided with the +** Software or, alternatively, in accordance with the terms contained in +** a written agreement between you and Digia. For licensing terms and +** conditions see http://qt.digia.com/licensing. For further information +** use the contact form at http://qt.digia.com/contact-us. +** +** GNU Free Documentation License Usage +** Alternatively, this file may be used under the terms of the GNU Free +** Documentation License version 1.3 as published by the Free Software +** Foundation and appearing in the file included in the packaging of +** this file. Please review the following information to ensure +** the GNU Free Documentation License version 1.3 requirements +** will be met: http://www.gnu.org/copyleft/fdl.html. +** $QT_END_LICENSE$ +** +****************************************************************************/ + +/*! + \example sharedmemory + \title Shared Memory Example + \ingroup examples-ipc + \brief Demonstrates doing inter-process communication using shared memory with + the QSharedMemory class. + + 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 \l {QApplication} + {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. + + \snippet sharedmemory/main.cpp 0 + + Two instances of class Dialog appear. + + \image sharedmemory-example_1.png Screenshot of the Shared Memory example + + 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. + + \snippet sharedmemory/dialog.h 0 + + The constructor builds the user interface widgets and connects the + clicked() signal of each button to the corresponding slot function. + + \snippet sharedmemory/dialog.cpp 0 + + Note that "QSharedMemoryExample" is passed to the \l {QSharedMemory} + {QSharedMemory()} constructor to be used as the key. This will be + used by the system as the identifier of the underlying shared memory + segment. + + Click the \tt {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. + + \snippet sharedmemory/dialog.cpp 1 + + The user is then asked to select an image file using + QFileDialog::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 \l {QSharedMemory::} + {create()} our shared memory segment. Creating a shared memory + segment automatically \l {QSharedMemory::attach()} {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. + + \snippet sharedmemory/dialog.cpp 2 + + Note that we \l {QSharedMemory::} {lock()} the shared memory segment + before we copy into it, and we \l {QSharedMemory::} {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 \l {QSharedMemory::} {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. + + \image sharedmemory-example_2.png Screenshot of the Shared Memory example + + In the second dialog, click the \tt {Display Image From Shared + Memory} button. The loadFromMemory() slot is invoked. It first \l + {QSharedMemory::attach()} {attaches} the process to the same shared + memory segment created by the first process. Then it \l + {QSharedMemory::lock()} {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 QImage and \l {QSharedMemory::unlock()} + {unlocks} the segment. + + \snippet sharedmemory/dialog.cpp 3 + + In this case, the function does \l {QSharedMemory::} {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. + + */ |