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

python c ctypes
No One in Particular picture No One in Particular · Nov 10, 2010 · Viewed 49.7k times · Source

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?

Answer

Gabi Purcaru picture Gabi Purcaru · Nov 10, 2010

The following code works on arbitrary lists:

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

Related questions

Wrapping a C library in Python: C, Cython or ctypes?

I want to call a C library from a Python application. I don't want to wrap the whole API, only the functions and datatypes that are relevant to my case. As I see it, I have three choices: Create an …

Read More
Calling C functions in Python

I have a bunch of functions that I've written in C and I'd like some code I've written in Python to be able to access those functions. I've read several questions on here that deal with a similar problem (here …

Read More
ctypes and string

I have a simple C-file: char* initializetest() { char * test = malloc(1000); return test; } int searchtest( char* test ) { strcpy(test,"test"); return 0; } main() { char *test = initializetest(); searchtest(test); printf("%s\n", test ); } and python file: from ctypes import * class Test(object): def __…

Read More