Qt QSharedMemory How to Update Shared Memory Location

Question

I have a library ( testlib-lib.so ) that is linked against by 2 different Qt Apps. My problem is that I cannot update the shared memory after initializing it.

void TestLib::createInitialSharedMemory() {

    // Create shared memory
    qDebug() << "Creating new shared memory...";
    sharedAudioMemory.setKey("globalAudioBufferSharedMemory");

    // First, test whether a shared memory segment is already attached to the process.
    // If so, detach it
    if (sharedAudioMemory.isAttached())
    {
        sharedAudioMemory.detach();
    }

    // Try to create memory of our required size
    int sizeOfSharedData = 60 * 8000 * 2; // 60 Seconds x 8 KHz * 2 Bytes ( signed short )
    if ( !sharedAudioMemory.create( sizeOfSharedData ) )
    {
      qDebug() << "ERROR: Failed to Allocate Shared Memory of size: " << sizeOfSharedData;
    }

}

void TestLib::writeToSharedMemory( QString text ) {

    // Create a buffer and data stream and some text to shove in there
    QBuffer buffer;
    buffer.open( QBuffer::ReadWrite );
    QDataStream out( &buffer );
    out << text;
    int size = buffer.size();

    if (!sharedAudioMemory.attach()) {

        // If an attempt of reading from the shared memory before data is written
        qDebug() << "Cannot attach to shared memory to update!";

    }

    // Write into the shared memory
    sharedAudioMemory.lock();
    qDebug() << "Writing data to buffer: " << text;
    char *to = (char*)sharedAudioMemory.data();
    const char *from = buffer.data().data();
    memcpy( to, from, qMin( sharedAudioMemory.size(), size ) );
    sharedAudioMemory.unlock();

}

Qt App A has the following in it's main():

// Create the shared memory and load some data
TestLib loadData;
loadData.createInitialSharedMemory();
loadData.writeToSharedMemory("12345 init shared memory");

// This is the key
QSharedMemory sharedAudioMemory("globalAudioBufferSharedMemory");

// Attempt to attach to shared memory for the audio buffer
if (!sharedAudioMemory.attach()) {

    //If an attempt of reading from the shared memory before data is written
    qDebug() << "ERROR: Failed to attach to shared memory...";
    return -1;

}

// Define a buffer to read the data in
QBuffer buffer;
QDataStream in(&buffer);
QString text;

// Lock the memory and read the data
sharedAudioMemory.lock();
char* pointer = (char*)sharedAudioMemory.constData();
buffer.setData((char*)sharedAudioMemory.constData(), sharedAudioMemory.size()); // Sets the contents of the internal buffer to be the first size bytes of data.
buffer.open(QBuffer::ReadOnly);
in >> text;
sharedAudioMemory.unlock();
sharedAudioMemory.detach();

Qt App B, attempts to update the shared memory by doing:

void Control::updateText() {

    // Create seed for the random
    // That is needed only once on application startup
    QTime time = QTime::currentTime();
    qsrand((uint)time.msec());

    // Get random value between 0-100
    int randomValue = qrand() % ((1000 + 1) - 2) + 2;

    // Create the shared memory and load some data
    TestLib loadData;
    QString randomText = QString("%1 test asdf %2 test asdf %3 test asdf %4").arg(randomValue).arg(randomValue).arg(randomValue).arg(randomValue);
    loadData.writeToSharedMemory(randomText);

}

After initializing the shared memory to "12345 init shared memory", I cannot seem to update the text store there. When I call the updateText() method in App B, the library method writeToSharedMemory() is called and the qDebug() in the method prints the text I wish to update in the shared memory, but when I call the following code after attempting to update the shared memory, it always prints the origin inital text "12345 init shared memory":

// Attempt to attach to shared memory for the audio buffer
if (!sharedAudioMemory.attach()) {

    // If an attempt of reading from the shared memory before data is written
    qDebug() << "ERROR: Failed to attach to shared memory...";

}

// Define a buffer to read the data in
QBuffer buffer;
QDataStream in(&buffer);
QString text;

// Lock the memory and read the data
sharedAudioMemory.lock();
char* pointer = (char*)sharedAudioMemory.constData();
buffer.setData((char*)sharedAudioMemory.constData(), sharedAudioMemory.size()); // Sets the contents of the internal buffer to be the first size bytes of data.
buffer.open(QBuffer::ReadOnly);
in >> text;
sharedAudioMemory.unlock();
sharedAudioMemory.detach();

// Print the data
qDebug() << "SHARED TEXT IS: " << text;

Why can'y I seem to update the shared memory? What am I doing wrong? Thanks -

Answer 1

I found the error after printing out the 'errorString()' here:

if (!sharedAudioMemory.attach()) {

        // If an attempt of reading from the shared memory before data is written
        qDebug() << "Cannot attach to shared memory to update! ERROR: " << sharedAudioMemory.errorString();

}

There was no key associated so it was not detaching. So I changed this function to:

// Assign the key so we can update the data
sharedAudioMemory.setKey("globalAudioBufferSharedMemory");
if (!sharedAudioMemory.attach()) {

    // If an attempt of reading from the shared memory before data is written
    qDebug() << "Cannot attach to shared memory to update! ERROR: " << sharedAudioMemory.errorString();

}

And it works - I can modify the shared memory just fine.