How to share data between processes using QSharedMemory?

Christian Ehrlicher

You can not share objects snce most of them have pointers to other memory in their private implementations - only plain data, or serialized objects

Roberrt

@Christian-Ehrlicher how to serialize the object? qt has implementation for this task?

Christian Ehrlicher

@Roberrt said in How to share data between processes using QSharedMemory?:

qt has implementation for this task?

You can use QDataStream for it.

JonB

@Roberrt
And Serializing Qt Data Types lists which Qt classes come with support. Note that this does not include any kind of QObject!

Roberrt

I got it working now, something that i should change or take note?
process1

struct Data {
    QMap<int, QString> map;
};

filemap1::filemap1(QWidget* parent)
    : QMainWindow(parent)
    , ui(new Ui::filemap1Class())
{
    ui->setupUi(this);

    // Create an instance of QSharedMemory and attach it to a unique key
    sharedMemory.setKey("MyKey");

    Data data;
    data.map[1] = "Hello";
    data.map[2] = "World";

    QByteArray serializedData;
    QDataStream stream(&serializedData, QIODevice::WriteOnly);
    stream << data.map;

    if (!sharedMemory.create(serializedData.size())) 
    {
        if (!sharedMemory.attach()) {
            qWarning() << "Unable to attach to shared memory: " << sharedMemory.errorString();;
            return;
        }
    }

    sharedMemory.lock();
    char* sharedMemoryData = static_cast<char*>(sharedMemory.data());
    std::memcpy(sharedMemoryData, serializedData.constData(), serializedData.size());
    sharedMemory.unlock();
}

process2

struct Data {
    QMap<int, QString> map;
};

filemap2::filemap2(QWidget *parent) : QMainWindow(parent) , ui(new Ui::filemap2Class())
{
    ui->setupUi(this);

    // Create an instance of QSharedMemory and attach it 
    // to the same key as process 1
    QSharedMemory sharedMemory("MyKey");
    if (!sharedMemory.attach()) {
        qWarning() << "Unable to attach to shared memory: " << sharedMemory.errorString();;
        return;
    }

    sharedMemory.lock();
    // Read the serialized data from shared memory
    char* sharedMemoryData = static_cast<char*>(sharedMemory.data());
    QByteArray serializedData(sharedMemoryData, sharedMemory.size());

    // Deserialize the object
    QDataStream stream(&serializedData, QIODevice::ReadOnly);
    Data data;
    stream >> data.map;

    sharedMemory.unlock();
}

How do i avoid the process getting stuck when trying to attach to a sharedMemory which is locked for too much time?

For example, if process2 lock the sharedMemory and crash, when process1 try to read it, it will stuck, as it wait forever to the memory be unlocked.

JonB

@Roberrt
I think the attach() is OK, you mean when the second process tries a lock().
I think the answer is that Qt does not provide any kind of tryLock(), see https://bugreports.qt.io/browse/QTBUG-4073 and https://bugreports.qt.io/browse/QTBUG-2443.
I don't know what happens of one side actually "crashes", OS-dependent behaviour as to whether it gets released.
There may be other ways to share data with IPC which don't risk this QSharedMemory behaviour if that is important to you.

Roberrt

@JonB How to keep the sharedMemory even after process1 exit?

JonB

@Roberrt
Have you read the platform description in https://doc.qt.io/qt-6/qsharedmemory.html#details ?

It also says

QSharedMemory automatically destroys the shared memory segment when the last instance of QSharedMemory is detached from the segment, and no references to the segment remain.

I think if process1 exits but process 2 has attached it persists. Until all processes have released it.

Roberrt

@JonB I see, whats the best way to inform process2 that new data has been written to the QSharedMemory?

JonB

@Roberrt
You might have to have a semaphore for that. There isn't a "notification".
Before you go any further you should read

and consider your alternatives, and what you are using the shared memory for.

Kent-Dorfman

Another huge mistake people make when using shared memory is they think in terms of peer-to-peer, which is bad juju. You really have to manage shared memory as a master/slave relationship. ie, it is set-up/initialized by a controller and then some number of other processes can use it...and there is no implicit "notify of changes" mechanism. you have to poll for changes or implement some sort of cross-process signalling mechanism in addition to correct use of semaphores/mutexes.

Roberrt

Do i still need to call .lock into the QSharedMemory when i'm just reading it without modifying the data?

JonB

@Roberrt
void *QSharedMemory::data()

Remember to lock the shared memory with lock() before reading from or writing to the shared memory

If you don't lock() for read and someone else has lock()ed and writes, I simply don't know/maybe not defined what you see in the read.

Kent-Dorfman

@Roberrt

Read characteristics are undefined if you don't guarantee exclusive access, or at least use data types that are guaranteed to be atomic. Generally the read of a native word sized int is atomic across architectures because the CPU microcode likes to deal with a whole word at a time, and preemption wont occur in the middle of a machine code instruction.

But it is safer to lock for reads as well.