How do I convert a Python list into a C array by using ctypes?

Question

If I have the follow 2 sets of code, how do I glue them together?

void
c_function(void *ptr) {
    int i;

    for (i = 0; i < 10; i++) {
        printf("%p", ptr[i]);
    }

    return;
}


def python_routine(y):
    x = []
    for e in y:
        x.append(e)

How can I call the c_function with a contiguous list of elements in x? I tried to cast x to a c_void_p, but that didn't work.

I also tried to use something like

x = c_void_p * 10 
for e in y:
    x[i] = e

but this gets a syntax error.

The C code clearly wants the address of an array. How do I get this to happen?

score 94 · Accepted Answer · edited Feb 20 '17 at 13:23

94

The following code works on arbitrary lists:

import ctypes
pyarr = [1, 2, 3, 4]
arr = (ctypes.c_int * len(pyarr))(*pyarr)

edited Feb 20 '17 at 13:23

Delgan

14,714
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answered Nov 10 '10 at 15:08

Gabi Purcaru

28,478
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"*pyarr" in python... What does it mean? – AaronYC Dec 21 '12 at 08:34
5

@AaronYC I'm sorry for the confusion; `pyarr` is an ordinary python list, like `pyar = [1,2,3,4]`. If you're wondering about the star before the name, check this out: http://stackoverflow.com/questions/400739/what-does-mean-in-python – Gabi Purcaru Dec 22 '12 at 05:07
4

Good answer, but a bit glib for us unwashed ones. – Jiminion Oct 18 '16 at 20:58
1

@Jiminion See my answer below for an explanation of the syntax. – akhan Jun 30 '18 at 19:16
1

Very well, now how would one free that `arr` variable ? Surely we don't have to use `free()` from C here do we ? – Sergiy Kolodyazhnyy Mar 24 '19 at 09:15
What is the extension of this to produce a 2dim array for a given list like `[[1, 2], [3, 4]]`? – Vishal Aug 03 '19 at 06:31

score 16 · Answer 2 · answered Jun 30 '18 at 19:14

This is an explanation of the accepted answer.

ctypes.c_int * len(pyarr) creates an array (sequence) of type c_int of length 4 (python3, python 2). Since c_int is an object whose constructor takes one argument, (ctypes.c_int * len(pyarr)(*pyarr) does a one shot init of each c_int instance from pyarr. An easier to read form is:

pyarr = [1, 2, 3, 4]
seq = ctypes.c_int * len(pyarr)
arr = seq(*pyarr)

Use type function to see the difference between seq and arr.

score 11 · Answer 3 · answered Nov 10 '10 at 15:07

11

From the ctypes tutorial:

>>> IntArray5 = c_int * 5
>>> ia = IntArray5(5, 1, 7, 33, 99)

answered Nov 10 '10 at 15:07

Ryan Ginstrom

13,275
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1

Try to create arrays more than with 255 items. – Pavel Patrin Jun 19 '15 at 13:14
3

Works with more than 255 items: `IntArray300 = c_int * 300; arrayWith300Elements=IntArray300(*list([i for i in range(300)]))` – Guillaume Jacquenot Dec 20 '16 at 10:33

score 2 · Answer 4 · answered Jan 03 '19 at 22:29

2

import ctypes
import typing

def foo(aqs : typing.List[int]) -> ctypes.Array:
    array_type = ctypes.c_int64 * len(aqs)
    ans = array_type(*aqs)
    return ans

for el in foo([1,2,3]):
    print(el)

this will give:

1
2
3

answered Jan 03 '19 at 22:29

Jörg Beyer

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How do I convert a Python list into a C array by using ctypes?

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