Passing a memory address (pointer) to an extension?

Robert Kern robert.kern at
Thu Oct 23 02:33:23 CEST 2008

Philip Semanchuk wrote:
> I'm writing a Python extension in C that wraps a function which takes a 
> void * as a parameter. (The function is shmat() which attaches a chunk 
> of shared memory to the process at the address supplied by the caller.) 
> I would like to expose this function to Python, but I don't know how to 
> define the interface.
> Specifically, when calling PyArg_ParseTuple(), what letter should I use 
> to represent the pointer in the format string? The best idea I can come 
> up with is to use a long and then cast it to a void *, but assuming that 
> a long is big enough to store a void * is a shaky assumption. I could 
> use a long long (technically still risky, but practically probably OK) 
> but I'm not sure how widespread long longs are.

I recommend not giving the user access to that argument. Just use NULL and let 
shmat() pick a starting address. I don't think it's really safe to let the user 
pick, even in C. Perhaps if you're doing *really* low level stuff. Of course, 
being that low level seems to be the point of posix_ipc, so maybe I should let 
you get on with it. Anyways, the format "n" in Python >= 2.5 will correspond to 
a Py_ssize_t integer, which will always be the size of a pointer.

You can return the void* that you get from shmat() with a PyCPointer object or 
make a new, small type that encapsulates a pointer attached via shmat(). The 
benefit of a real type is that you can type-check the input to shmdt() for 
safety. I strongly recommend either of those approaches over returning an integer.

Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco

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