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"PyLong_FromVoidPtr works for uintptr_t, but not intptr_t."
Ok correct. You should use something like:
PyObject*
PyLong_FromIntptr_t(intptr_t value)
{
 if (sizeof(intptr_t) == sizeof(long))
 return PyLong_FromLong(value);
 else {
 assert(sizeof(intptr_t) <= sizeof(PY_LONG_LONG));
 return PyLong_FromLongLong((PY_LONG_LONG)value);
 }
}
The "if (sizeof(intptr_t) == sizeof(long))" should be optimized by your compiler.
I don't know if Python should provide such function. What is your use case for intptr_t?
It looks like intptr_t is only really used in the implementation of os.spawnv() and os.spawnve(). Extract:
#if SIZEOF_LONG == SIZEOF_VOID_P
 return Py_BuildValue("l", (long) spawnval);
#else
 return Py_BuildValue("L", (PY_LONG_LONG) spawnval);
#endif