QSharedMemory not the same size - qt

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;