How to cast a pointer to a Python cffi struct to System.IntPtr (.NET)?

Question

I need to pass a System.IntPtr to a .NET function (Python with pythonnet). This pointer should refer to a struct created in cffi.

I found this:

from CLR.System import IntPtr, Int32
i = Int32(32)
p = IntPtr.op_Explicit(i)

This is what I tried so far

import clr
from cffi import FFI

ffi = FFI()

custom_t = '''
            typedef struct
            {
                float x;
                 float y;
                 float z;

            } float3;
            '''

ffi.cdef(custom_t)

cfloat3 = ffi.new("float3 *")
cfloat3.x = 1.0
cfloat3.y = 2.0
cfloat3.z = 3.0
print(cfloat3)

from System import IntPtr

p = IntPtr.op_Explicit(id(cfloat3)) #works, sort of...
print(p)

#I would rather see something like
p2 = IntPtr.op_Explicit(ffi.addressof(cfloat3).value)
p3 = IntPtr(ffi.cast("float3*", cfloat3))
p4 = IntPtr(ffi.cast("void3*", cfloat3))

#type(_), ffi.typeof(_), ffi.typeof(_).kind, ffi.addressof()

But I am not sure if using IntPtr.op_Explicit is the best solution. It looks a bit like workaround in combination with id() and I am pretty sure there is a better solution.

This is the p2 in my example, does not work. Or does it for you? — Joe, Aug 24 '17 at 12:19
I found this: IntPtr is just a .NET type for void*. It doesn't imply any meaning by itself. No pointer type is associated with it even though its name sounds like "pointer to int", it is the equivalent of void* in C/C++. — Joe, Aug 24 '17 at 13:05
Also found this in the cffi docs: To cast a pointer to an int, cast it to intptr_t or uintptr_t, which are defined by C to be large enough integer types. But something like `p = IntPtr(int(ffi.cast("intptr_t", ...)))` does also not work. — Joe, Aug 24 '17 at 13:44
does this example with ctypes help? https://mail.python.org/pipermail/pythondotnet/2013-January/001272.html — denfromufa, Aug 24 '17 at 18:59
I found a similar example earlier, too. Will try that tomorrow. https://mail.python.org/pipermail/pythondotnet/2015-July/001672.html — Joe, Aug 24 '17 at 19:11

score 3 · Answer 1 · answered Aug 24 '17 at 16:46

3

There is no direct support for CFFI inside Python.Net or vice-versa, so you need to cast the pointer from one library to an integer and reimport that integer into the other library.

From CFFI, intaddr = ffi.cast("intptr_t", p) will give you an integer. Then you can probably do IntPtr(intaddr).

answered Aug 24 '17 at 16:46

Armin Rigo

12,048
37
48

I already tried that, see the comment to my question. But IntPtr still complained. Even when I imported the `System.Int64` or `System.Int32` data type from pythonnet and fed the casted integer to there and then the `Int64/32` to IntPtr. Don't have the code here at the moment. I can paste it tomorrow. – Joe Aug 24 '17 at 16:54

score 1 · Answer 2 · answered Aug 24 '17 at 12:18

1

FFI has an addressof function built in:

http://cffi.readthedocs.io/en/latest/ref.html#ffi-addressof

answered Aug 24 '17 at 12:18

lancew

780
4
22

This is the p2 in my example, does not work. Or does it for you? – Joe Aug 24 '17 at 12:21
Right, sorry. Looking into it further, it seems the code `ffi.new("float3 *")` actually returns a pointer already. Are you sure you cant just use the variable cfloat3 as a pointer? – lancew Aug 24 '17 at 12:27
like `p4 = IntPtr(cfloat3)`? – Joe Aug 24 '17 at 12:42

Joe · Accepted Answer · 2017-08-26T10:56:09.793

The solution to this is to use the Overloads method of IntPtr

from System import IntPtr, Int32, Int64
my_ptr = ffi.cast("intptr_t", my_c_struct)
cs_handle = IntPtr.Overloads[Int64](Int64(int(my_ptr)))

as mentioned here and here.

Here is a working example:

import clr
from cffi import FFI

ffi = FFI()

custom_t = '''
            typedef struct
            {
                float x;
                 float y;
                 float z;

            } float3;
            '''

ffi.cdef(custom_t)

cfloat3 = ffi.new("float3 *")
cfloat3.x = 1.0
cfloat3.y = 2.0
cfloat3.z = 3.0
print(cfloat3)

from System import IntPtr, Int32, Int64

my_ptr = ffi.cast("intptr_t", cfloat3)
print(my_ptr)
print(ffi.typeof(my_ptr))
print(ffi.typeof(my_ptr).kind)
print(ffi.typeof(my_ptr).cname)
print(int(my_ptr))
print('hex', hex(int(my_ptr)))

#you don't need to cast to System.Int64 first, also works without:

#with casting the Python int to System.Int64
cs_handle = IntPtr.Overloads[Int64](int(my_ptr))
print(cs_handle)

#without casting the Python int to System.Int64
cs_handle = IntPtr.Overloads[Int64](Int64(int(my_ptr)))
print(cs_handle)

# using this workaround mentioned in the question also works
p = IntPtr.op_Explicit(int(my_ptr)) #works, sort of...
print('op_Explicit', p)

#careful: do not use the id() of the C structure, it is different and incorrect
print(cfloat3)
print(ffi.addressof(cfloat3[0]))
print('id(cfloat3)', id(cfloat3), hex(id(cfloat3)))

Some further info on what is happening above (found here and there):

C#'s IntPtr maps exactly to C/C++'s intptr_t.

intptr_t integer type capable of holding a pointer

To cast a pointer to an int, cast it to intptr_t or uintptr_t, which are defined by C to be large enough integer types. cffi doc with examples

IntPtr is just a .NET type for void*.

The equivalent of an unmanaged pointer in the C# language is IntPtr. You can freely convert a pointer back and forth with a cast. No pointer type is associated with it even though its name sounds like "pointer to int", it is the equivalent of void* in C/C++.

IntPtr(5) complains that int/long' value cannot be converted to System.IntPtr. It seems like it is trying to cast or something instead of calling the constructor. (found here)

Methods of CLR objects have an '_ overloads _', which will soon be deprecated in favor of iPy compatible Overloads, attribute that can be used for this purpose. (found here)

Your second link is broken – denfromufa Aug 26 '17 at 08:59 — denfromufa, Aug 26 '17 at 08:59

How to cast a pointer to a Python cffi struct to System.IntPtr (.NET)?

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