Passing a C++ pointer between C and Python

Question

I have a python extension module written in C++, which contains multiple functions. One of these generates an instance of a custom structure, which I then want to use with other functions of my module in Python as follows

import MyModule
var = MyModule.genFunc()
MyModule.readFunc(var)

To do this, I've tried using PyCapsule objects to pass a pointer to these objects between Python and C, but this produces errors when attempting to read them in the second C function ("PyCapsule_GetPointer called with invalid PyCapsule object"). Python, however, if asked to print the PyCapsule object (var) correctly identifies it as a "'capsule object "testcapsule"'. My C code appears as follows:

struct MyStruct {
    int value;
};

static PyObject* genFunc(PyObject* self, PyObject *args) {
    MyStruct var;
    PyObject *capsuleTest;

    var.value = 1;
    capsuleTest = PyCapsule_New(&var, "testcapsule", NULL);
    return capsuleTest;
}

static PyObject* readFunc(PyObject* self, PyObject *args) {
    PyCapsule_GetPointer(args, "testcapsule");
    return 0;
}

Thank you for your help.

Your capsule points to a local variable that only exists for the duration of genFunc. I'm surprised you get an error message rather than a crash though. You need to allocate var with malloc or make it a global variable or something similar to ensure that it lives as long as the capsule. — DavidW
– DavidW, Commented Aug 10, 2016 at 14:35
Apologies -- this was just written as demonstration code; the real example has some malloc calls, which is probably why it produces the error. — metm
– metm, Commented Aug 11, 2016 at 8:48

tynn · Accepted Answer · 2016-08-11 00:20:24Z

1

Like stated in a comment to your question, you'll run into an issue when reading data from the local variable MyStruct var. For this you can use the third destructor to PyCapsule_New.

But that's not the reason for your problem just now. You're using PyCapsule_GetPointer(args, "testcapsule") on the args parameter. And since it's not a capsule, even though var is one, you might have defined the signature of the function as METH_VARARGS. Instead you need to unpack the tuple or use METH_O.

answered Aug 11, 2016 at 0:20

tynn

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1 Comment

metm Over a year ago

Apologies for late response -- but this was exactly the issue. Changing the function signature to METH_O fixed the pycapsule errors. Many thanks.

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