[PyQt] Access to QSharedMemory with buffer protocol

[Hans-Peter Jansen]

On Tuesday 02 November 2010, 11:16:35 Phil Thompson wrote:
> On Tue, 2 Nov 2010 00:00:06 +0200, "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.
>
> ...bad advice. I didn't realise that you can't create mutable buffers
> from Python. (Although there is no reason why you shouldn't be able
> to.)

That's a pity.

> > 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..
>
> Looks like I need to add methods to sip.voidptr so that you can do...
>
> shmem.data()[:] = "xyz"
>
> Question...
>
> Should the length of the slice (ie. 10) match the length of the data
> (ie. 3) (and so the above would raise an exception)? Or should the
> last 7 bytes be left untouched, with no exception?

Raising an exception in case the slice size doesn't _match_ the shared 
memory size sounds like a major headache to me. While this opens a 
chance to access uninitialized memory, mmap seems to allow similar 
silliness. As long as the silliness is kept in bounds, that sounds 
acceptable to me ;-).

Pete