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1 /**************************************************************************** |
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2 ** |
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3 ** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies). |
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38 ** $QT_END_LICENSE$ |
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39 ** |
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40 ****************************************************************************/ |
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41 |
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42 /*! |
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43 \example ipc/sharedmemory |
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44 \title Shared Memory Example |
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45 |
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46 The Shared Memory example shows how to use the QSharedMemory class |
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47 to implement inter-process communication using shared memory. To |
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48 build the example, run make. To run the example, start two instances |
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49 of the executable. The main() function creates an \l {QApplication} |
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50 {application} and an instance of our example's Dialog class. The |
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51 dialog is displayed and then control is passed to the application in |
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52 the standard way. |
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53 |
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54 \snippet examples/ipc/sharedmemory/main.cpp 0 |
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55 |
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56 Two instances of class Dialog appear. |
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57 |
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58 \image sharedmemory-example_1.png Screenshot of the Shared Memory example |
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59 |
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60 Class Dialog inherits QDialog. It encapsulates the user interface |
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61 and an instance of QSharedMemory. It also has two public slots, |
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62 loadFromFile() and loadFromMemory() that correspond to the two |
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63 buttons on the dialog. |
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64 |
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65 \snippet examples/ipc/sharedmemory/dialog.h 0 |
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66 |
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67 The constructor builds the user interface widgets and connects the |
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68 clicked() signal of each button to the corresponding slot function. |
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69 |
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70 \snippet examples/ipc/sharedmemory/dialog.cpp 0 |
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71 |
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72 Note that "QSharedMemoryExample" is passed to the \l {QSharedMemory} |
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73 {QSharedMemory()} constructor to be used as the key. This will be |
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74 used by the system as the identifier of the underlying shared memory |
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75 segment. |
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76 |
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77 Click the \tt {Load Image From File...} button on one of the |
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78 dialogs. The loadFromFile() slot is invoked. First, it tests whether |
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79 a shared memory segment is already attached to the process. If so, |
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80 that segment is detached from the process, so we can be assured of |
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81 starting off the example correctly. |
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82 |
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83 \snippet examples/ipc/sharedmemory/dialog.cpp 1 |
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84 |
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85 The user is then asked to select an image file using |
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86 QFileDialog::getOpenFileName(). The selected file is loaded into a |
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87 QImage. Using a QImage lets us ensure that the selected file is a |
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88 valid image, and it also allows us to immediately display the image |
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89 in the dialog using setPixmap(). |
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90 |
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91 Next the image is streamed into a QBuffer using a QDataStream. This |
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92 gives us the size, which we then use to \l {QSharedMemory::} |
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93 {create()} our shared memory segment. Creating a shared memory |
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94 segment automatically \l {QSharedMemory::attach()} {attaches} the |
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95 segment to the process. Using a QBuffer here lets us get a pointer |
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96 to the image data, which we then use to do a memcopy() from the |
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97 QBuffer into the shared memory segment. |
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98 |
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99 \snippet examples/ipc/sharedmemory/dialog.cpp 2 |
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100 |
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101 Note that we \l {QSharedMemory::} {lock()} the shared memory segment |
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102 before we copy into it, and we \l {QSharedMemory::} {unlock()} it |
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103 again immediately after the copy. This ensures we have exclusive |
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104 access to the shared memory segment to do our memcopy(). If some |
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105 other process has the segment lock, then our process will block |
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106 until the lock becomes available. |
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107 |
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108 Note also that the function does not \l {QSharedMemory::} {detach()} |
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109 from the shared memory segment after the memcopy() and |
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110 unlock(). Recall that when the last process detaches from a shared |
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111 memory segment, the segment is released by the operating |
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112 system. Since this process only one that is attached to the shared |
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113 memory segment at the moment, if loadFromFile() detached from the |
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114 shared memory segment, the segment would be destroyed before we get |
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115 to the next step. |
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116 |
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117 When the function returns, if the file you selected was qt.png, your |
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118 first dialog looks like this. |
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119 |
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120 \image sharedmemory-example_2.png Screenshot of the Shared Memory example |
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121 |
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122 In the second dialog, click the \tt {Display Image From Shared |
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123 Memory} button. The loadFromMemory() slot is invoked. It first \l |
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124 {QSharedMemory::attach()} {attaches} the process to the same shared |
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125 memory segment created by the first process. Then it \l |
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126 {QSharedMemory::lock()} {locks} the segment for exclusive access and |
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127 links a QBuffer to the image data in the shared memory segment. It |
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128 then streams the data into a QImage and \l {QSharedMemory::unlock()} |
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129 {unlocks} the segment. |
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130 |
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131 \snippet examples/ipc/sharedmemory/dialog.cpp 3 |
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132 |
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133 In this case, the function does \l {QSharedMemory::} {detach()} from |
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134 the segment, because now we are effectively finished using |
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135 it. Finally, the QImage is displayed. At this point, both dialogs |
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136 should be showing the same image. When you close the first dialog, |
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137 the Dialog destructor calls the QSharedMemory destructor, which |
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138 detaches from the shared memory segment. Since this is the last |
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139 process to be detached from the segment, the operating system will |
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140 now release the shared memory. |
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141 |
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142 */ |