This article demonstrates the inter-process communication (IPC) mechanism called Shared memory on Qt using the example applications SharedMemoryLoad and ShowSharedMem.
Shared memory is supported by Qt through the QSharedMemory class. Two processes can share the same memory segment for IPC.
The SharedMemoryLoad application loads a string into the shared memory and the ShowSharedMem application shows the string from the shared memory.
- Run the SharedMemoryLoad application and click the Load button.
- Write some text in the input dialog.
- Send this application to the background.
- Run the ShowSharedMem application and click the Show from shared memory button. The text entered in the first application is displayed.
SharedMemoryLoad is a class derived from QWidget. It contains the QSharedMemory object as a member variable:
In the SharedMemoryLoad class constructor, set the key for the shared memory object to "MySharedMemory":
Loading a string into the shared memory:
// First, test whether a shared memory segment is already attached to the process.
// If so, detach it
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."));
// Write into the shared memory
char *to = (char*)sharedMem.data();
const char *from = buffer.data().data();
memcpy( to, from, qMin( sharedMem.size(), size ) );
In the same way as SharedMemoryLoad, ShowSharedMem is derived from QWidget and owns a QSharedMemory object.
Loading from the shared memory:
//If an attempt of reading from the shared memory before data is written
ui.showButton->setText(tr("Enter a text first"));
in >> text;
// As this is the last process attached to the shared memory segment
// the shared memory segment is released, destroying its contents
Note: This example cannot be tested in the emulator because running multiple Qt applications in the emulator is not possible. Starting the second application returns KErrNotSupported(-5).