QSharedMemory共享内存例子

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文章信息

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代码示例

源文件: Media:QSharedMemoryExample.zip

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测试基于

设备:: Nokia N96

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兼容于

平台: S60 3rd Edition, FP2

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文章

关键词: QSharedMemory

翻译:

翻译自: QSharedMemory example

由 dougcn

最后由 hamishwillee 在 11 Oct 2012 编辑

概述

本文借助两个例子SharedMemoryLoad and ShowSharedMem，在Qt上演示称作共享内存（Shared Memory）的进程间(IPC)通讯机制。

说明

Qt使用QSharedMemory类支持共享内存。两个进程可共享相同的内存段进行IPC。

做法

SharedMemoryLoad应用将一个字符串填入共享内存，ShowSharedMem应用显示来自共享内存的字符串

1. 运行SharedMemoryLoad，然后点击Load按钮；
2. 在输入对话框中写入一些文本；
3. 将该应用程序移植后台；
4. 运行ShowSharedMem应用，然后点击Show from shared memory按钮。在前一个应用输入的文件就显示出来了。

SharedMemoryLoad

SharedMemoryLoad 是一个从QWidget派生的类。它含有QSharedMemory对象作为成员变量：

  class SharedMemoryLoad : public QWidget
  {
      Q_OBJECT
      ...
  private:
 
      QSharedMemory sharedMem;
  };

在SharedMemoryLoad类的构造函数中，设置共享内存对象的键（key）为"MySharedMemory"：

  SharedMemoryLoad::SharedMemoryLoad( QWidget *parent )
  : QWidget( parent ),
    sharedMem( "MySharedMemory" ) {
    ...
  }

将字符串填入共享内存：

  void SharedMemoryLoad::loadIntoSharedMem()
    {
    // First, test whether a shared memory segment is already attached to the process.
    // If so, detach it
    if (sharedMem.isAttached())
      {
      sharedMem.detach();
      }
 
    ...
 
    QBuffer buffer;
    buffer.open( QBuffer::ReadWrite );
    QDataStream out( &buffer );
    out << text;
    int size = buffer.size();
 
    if ( !sharedMem.create( size ) ) 
      {
      ui.LoadButton->setText(tr("Unable to create shared memory segment."));
      return;
      }
 
    // Write into the shared memory
    sharedMem.lock();
    char *to = (char*)sharedMem.data();
    const char *from = buffer.data().data();
    memcpy( to, from, qMin( sharedMem.size(), size ) );
    sharedMem.unlock();
    }

ShowSharedMem

与SharedMemoryLoad一样, ShowSharedMem派生自QWidget且拥有一个QSharedMemory对象。

  ShowSharedMem::ShowSharedMem(QWidget *parent)
  : QWidget(parent),
    sharedMem("MySharedMemory") {
    ...
  }

从共享内存取出：

  void ShowSharedMem::loadFromSharedMem()
    {
    if (!sharedMem.attach()) 
      {
      //If an attempt of reading from the shared memory before data is written
      ui.showButton->setText(tr("Enter a text first"));
      return;
      }
 
    QBuffer buffer;
    QDataStream in(&buffer);
    QString text;
 
    sharedMem.lock();
    buffer.setData((char*)sharedMem.constData(), sharedMem.size());
    buffer.open(QBuffer::ReadOnly);
    in >> text;
    sharedMem.unlock();
 
    // As this is the last process attached to the shared memory segment
    // the shared memory segment is released, destroying its contents
    sharedMem.detach();
    }

样例程序

File:QSharedMemoryExample.zip

注: 本例不能在模拟器上测试，因为在模拟器上运行不了多个Qt应用程序。启动第二个应用时将返回KErrNotSupported(-5).

参阅

Qt Known Issues