Question

Recently I’m doing a project which a function is heavily called, so I want to using C code in this part. I’m a newbie to ctypes, please forgive me if my question is very easy.

Here I have a 2d list in python:

L = [[1],[1,2],[1,2,3]]

I want call a function in C module with it as parameter. Since there is no 2d-list in C I want to convert it to an array of *int.

I don’t want an normal 2D C array because each length of entry is different.

What I’ve done in python part is:

L = [[1],[1,2],[1,2,3]]
entrylist = []
for entry in L:
  c_entry = (ctypes.c_int * len(entry))(*entry) # c_entry is the C array version of entry
  entrylist.append(c_entry)

c_L = (ctypes.POINTER(ctypes.c_int) * len(entrylist))(*entrylist) # create an array of integer pointer, then initial it

c_L is an “LP_c_long_Array_14 object”, when len(L) == 14.

also, I can print it out perfectly by using

for i in range(len(L)):
  for j in range(len(L[i])):
    print(L[i][j], end = ' ')
  print()

On the other hand, in C code, I define my function as:

int fun(int** c_L)
int fun( (int * c_L)[])

neither works. ctypes throws a “Don’t know how to convert parameter 1” Error.

So, please tell me how to make it work? Thank you very much.

Answer 1

You are almost there, you just need some tweaks.

import ctypes as C

lib = C.CDLL("libfoo.so")

l = [[1],[1,2],[1,2,3]]
entrylist = []
lengths = []

for sub_l in l:
    entrylist.append((C.c_int*len(sub_l))(*sub_l))
    lengths.append(C.c_int(len(sub_l)))

c_l = (C.POINTER(C.c_int) * len(entrylist))(*entrylist)
c_lengths = ( C.c_int * len(l))(*lengths)

lib.test(c_l, c_lengths, len(l))  #here we also pass the sizes of all the arrays

where the C side of things looks like:

#include <stdlib.h>
#include <stdio.h>

int test(int **ar,int *lens,int n_ar){
    int ii,jj,kk;
    for (ii=0;ii<n_ar;ii++){
        for (jj=0;jj<lens[ii];jj++){
            printf("%d\t",ar[ii][jj]);
        }  
        printf("\n");
        fflush(stdout);
    }
    return 0;
}

If this doesn’t work, then it may be of use to explicitly cast all your arguments to the c library function.