Pass Structure using QSharedMemory

Question

I am new to the QT. I have seen the basic Example provided by QT for Shared Memory, but i want to share structure using Shared Memory. How can I achieve it?

void Dialog::Send()
{
if (sharedMemory.isAttached())
    detach();
QBuffer buffer;
buffer.open(QBuffer::ReadWrite);
QDataStream out(&buffer);
out << structa->a;
out << structa->b;
out << structa->c;
int size = buffer.size();
  if (!sharedMemory.create(size))
{
    ui.label->setText(tr("Unable to create shared memory segment."));
    return;
}
sharedMemory.lock();
char *to = (char*)sharedMemory.data();
const char *from = buffer.data().data();
memcpy(to, from, qMin(sharedMemory.size(), buffersize));
sharedMemory.unlock();
}

In Receive Function I am getting the data right in the Buffer, but I am unable to convert it to Struct veriable back.

void Dialog::Receive()
{   
if (!(sharedMemory.attach()))
{
    ui.label->setText(tr("Unable to attach to shared memory segment.\n" \
                         "Load an image first."));
    return;
}
QByteArray byteArray;
QBuffer buffer(&byteArray);
QDataStream in(&buffer);
TestStruct tmp;                                     //Re-make the struct
sharedMemory.lock();
buffer.setData((char*)sharedMemory.constData(), sizeof(tmp));
buffer.open(QBuffer::ReadOnly);
int nSize = ((buffer.size()));
memcpy(&tmp, buffer.data().data(), nSize);
qDebug()<< " tmp.a = "<<tmp.a;
qDebug()<< " tmp.b = "<<tmp.b;
qDebug()<< " tmp.c = "<< tmp.c;
sharedMemory.unlock();
sharedMemory.detach();
}

Answer 1

You are writing individual fields using a QDataStream, but reading as a struct using memcpy. This will cause problems!

You should read the data the same way as you are writing.

Example (assuming TestStruct has members a, b and c of same type as structa):

QBuffer buffer;
sharedMemory.lock();
buffer.setData(sharedMemory.constData(), sharedMemory.size());
sharedMemory.unlock();

buffer.open(QIODevice::ReadOnly);
QDataStream in(&buffer);
TestStruct tmp;
in >> tmp.a;
in >> tmp.b;
in >> tmp.c;
buffer.close();

Update:
If you have simple chars in your struct, the extraction code will not work, as there is no operator>>(QDataStream&, char&). You can use this:

// Use intermediate qint8 (or quint8)
qint8 t;
in >> t;
char x = t;

Serializing complete structure

You can write the complete structure at once:

// Writing:
TestStruct structa; // Initialized as needed
QBuffer buffer;
buffer.open(QIODevice::WriteOnly);
buffer.write(&structa, sizeof(TestStruct));
buffer.close();

// Reading:
QBuffer buffer;
buffer.setData(sharedMemory.constData(), sharedMemory.size());
buffer.open(QIODevice::ReadOnly);
TestStruct out;
buffer.read(&out, sizeof(TestStruct));
buffer.close();

But beware! This will probably not be protable! See e.g. this question for padding issues. I do not recommend this method

A general Qt serialization approach

There is quite a good guide in this blog. It gives the basics for a general serialization method in Qt.

The basic idea is to serialize objects into QVariants. A QVariant can store many types, including a QVariantMap, which is a map from QString => QVariant. Therefore, you can create hierachical structures.

These QVariants can be searialized using a QDataStream.

The outline is as follows:

// Enables << and >> for all QVariant supported types
template <typename T>
void operator << (QVariant &v, const T &d) {
    v = QVariant::fromValue<T>(d);
}
template <typename T>
void operator >> (const QVariant &v, T &d) {
    d = v.value<T>();
}

// Enable << and >> for your own type
void operator << (QVariant &v, const TestStruct &t) {
   QVariantMap m;
   m["a"] << t.a; // Re-use already defined operator <<
   m["b"] << t.b;
   m["c"] << t.c; 
   // ...
   v << m; // Put mapped custom type into a single QVariant
}
void operator >> (const QVariant &v, TestStruct &t) {
   QVariantMap m;
   // Load mapped type
   v >> m;
   m["a"] >> t.a; // Re-use already defined operator >>
   m["b"] >> t.b;
   m["c"] >> t.c; 
   // ...
}

You use this as follows:

// Writing
TestStruct structa;
QVariant v;
v << structa;
QBuffer buffer;
buffer.open(QIODevice::WriteOnly);
QDataStream str(&buffer);
str << v;

// Reading
QBuffer buffer;
buffer.setData(...);
buffer.open(QIODevice::ReadOnly);
QDataStream str(&buffer);
str >> v;
TestStruct structa;
v >> structa;