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?

Answer 1

The following code works on arbitrary lists:

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

Answer 2

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.