QSharedMemory not the same size

Question

QSharedMemory is changing sizes on me.

create:

    ...
int size = buffer->size();
    qDebug() << "buffer->size()" << size << "points" << points->size() << "share name" << sharedMemoryName;
    if (!m_sharedMemory->create(size)) {
        qCritical() << tr("Unable to create shared memory segment.");
        return;
    }
    m_sharedMemory->lock();
    char *to = (char*)m_sharedMemory->data();
    memcpy(to, buffer->data(), qMin(m_sharedMemory->size(), size));
    m_sharedMemory->unlock();

read:

QSharedMemory sharedMemory(sharedMemoryName);
    if (!sharedMemory.attach(QSharedMemory::AccessMode::ReadOnly)) {
        qCritical() << "Unable to attach to shared memory segment.";
        return nullptr;
    }
    qDebug() << sharedMemoryName << sharedMemory.size();

Not the same size. when create the size is 658824 when read the size is 659456

ok - sounds crazy now, but I run the read multiple times and all the sudden the size was correct. Then I restarted everything (same size on create) and the error came back.

edit:

I just realized that the size of the QSharedMemory is not necessarily then same as the QBuffer

memcpy(to, buffer->data(), qMin(m_sharedMemory->size(), size));

why is that and how can I know "on the other side" the correct size (without making an ugly workaround)

edit 2: I may found it. looks like QSharedMemory reserves the memory in 4096 blocks.

solution for me is to check the QByteArray for empty

if(in.at(i) == QChar(0))
            break;