[PyQt] Access to QSharedMemory with buffer protocol

[Yao Ko]

Hi,

I encountered the same problem back when Alexandre asked the same
question.  This is the workaround that I've been using:

CLIB = ctypes.cdll.LoadLibrary(ctypes.util.find_library('c'))

shared_memory.lock()
try:
  CLIB.memcpy(int(shared_memory.data()), data_bytes, data_len)
finally:
  shared_memory.unlock()

It works on both Mac and Windows.

Thanks,
Yao

On Mon, Nov 1, 2010 at 3:00 PM, Hans-Peter Jansen <hpj at urpla.net> wrote:
> Hi Phil,
>
> end of July last year, Alexandre Raczynski asked about accessing
> QSharedMemory, and you replied, that:
>
> mutable_data = buffer(sharedMemory.data())
>
> ...should give _write_ access to the memory.
>
> Hmm, not here:
>
> $ python
> Python 2.6 (r26:66714, Mar 30 2010, 00:30:21)
> [GCC 4.3.2 [gcc-4_3-branch revision 141291]] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
>>>> from PyQt4 import QtCore
>>>>
>>>> shmem = QtCore.QSharedMemory("xxx")
>>>> shmem.create(10)
> True
>>>> shmem.lock()
> True
>>>> b = buffer(shmem.data())
>>>> repr(b)
> '<read-only buffer for 0xb74a37a0, size -1, offset 0 at 0xb6c0f9a0>'
>>>> b = buffer(shmem.data(), 0, 10)
>>>> repr(b)
> '<read-only buffer for 0xb74a3710, size 10, offset 0 at 0xb6c0f9c0>'
>>>>
>
> Both attempts to create a buffer object resulted in read-only buffers,
> that refuse getting modified. I must be doing something wrong, but
> there's not much room left for manoeuvring..
>
> Pete
>
> python: 2.6
> sip: 4.12-snapshot-12acbffd0085
> qt4: 4.7.0
> pyqt4: 4.8.1
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