Question about the example "\Qt\4.6.2\examples\ipc\sharedmemory"

[to_guliang]

I have some question to the following code:

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(), size));
    sharedMemory.unlock();

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(), size));
    sharedMemory.unlock();

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1.

sharedMemory.create(size)

sharedMemory.create(size)

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Creates a shared memory segment of size bytes,Why sharedMemory.size() != size?
2.In addition, it seems like not sure about which one is big between sharedMemory.size() and

size(qMin(sharedMemory.size(), size)

size(qMin(sharedMemory.size(), size)

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) ?


thanks very much for any advice!

[CeeKey]

Hey,

sharedMemory.create(size)

sharedMemory.create(size)

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creates a sharedMemory with a size rounded up to a multiple of pagesize.

I think that

size(qMin(sharedMemory.size(), size)

size(qMin(sharedMemory.size(), size)

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is important to check, because it is possible that another Thread has changed the sharedmemory before it was locked.