How to pass numpy array inside of struct to ctypes function?

Question

I'm trying to wrap some third-party code from a library in my Python application. Essentially, the function I would like to call takes a struct as input that houses (among other things) pointers to double arrays. A simplified example would be: myfunc.h: ``` typedef struct mystruct_t { int n; double *x; } mystruct_t; double myfunc(mystruct_t* data); ``` myfunc.c: ``` #include "myfunc.h" double myfunc(mystruct_t* data) { double result = 0.; for(int i = 0; i < data->n; i++) { result += data->x[i]; } return result; } ``` Makefile: ``` CC = gcc CFLAGS = -g -Wall -fPIC -lm -std=gnu99 all: libmyfunc.so m.PHONY : clean libmyfunc.so: myfunc.o gcc -shared -Wl,-soname,$@ -o $@ $^ %.o: %.c $(CC) -c $(CFLAGS) $< clean: rm -vf libmyfunc.so *.o ``` I would like to wrap `myfunc` using NumPy, `ctypes.Structure` and `numpy.ctypeslib` so that I can pass NumPy arrays to `myfunc` as attributes of `mystruct_t`. So far I've been trying the following: myfunc.py: ``` #!/usr/bin/env python import numpy as np i…

Accepted Answer

You can remove `array_1d_double`. It is not needed.

The struct should be declared like this:

```
class MyStruct(ctypes.Structure):
    _fields_ = [
        ('n', ctypes.c_int),
        ('x', ctypes.POINTER(ctypes.c_double))
    ]
```

I've changed both types. You had `c_int16`, but the type you used in the C code is `int`. That maps to `c_int`. And likewise for the array, that is `double*`. In ctypes that is `POINTER(ctypes.c_double)`.

The struct should be initialised like this:

```
mystruct = MyStruct()
mystruct.n = len(x)
mystruct.x = npct.as_ctypes(x)
```

With these changes, your code works as intended.

How to pass numpy array inside of struct to ctypes function?

Solution

Problem