How can I pass an "array of integer pointers" to a Function in c library from python

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.

ebarr · Accepted Answer · 2012-11-18 01:45:44Z

4

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.

edited Nov 18, 2012 at 1:45

answered Nov 18, 2012 at 1:39

ebarr

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1 Comment

Weichu Liu Over a year ago

Amazing solution. Thank you so much.

HYRY · Accepted Answer · 2012-11-18 03:15:52Z

0

I think you can avoid int** pointer by packing all you data into one list:

[1, 1, 2, 1, 2, 3]

and pass another list that represents the start of every sub list such as:

[0, 1, 3, 6]

so the first sub list starts at 0, ends at 1, the second sub list starts at 1, ends at 3, and the third sub list starts at 3 ends at 6.

convert this two list to int array and pass it to the c function with the length of the second array.

Here is an example:

#include <stdio.h>
void sum_list(int * data, int * positions, int length)
{
    int i, j, s;
    for(i=0;i<length-1;i++)
    {
        s = 0;
        for(j=positions[i];j<positions[i+1];j++)
        {
            s += data[j];
        }
        printf("%d\n", s);
    }
}

and the python code:

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

data = []
positions = [0]
for sub in L:
    data.extend(sub)
    positions.append(len(data))

from ctypes import *

lib = CDLL("./sum_list.out")

c_data = (c_int * len(data))(*data)
c_positions = (c_int * len(positions))(*positions)

lib.sum_list(c_data, c_positions, len(c_positions))

edited Nov 18, 2012 at 3:15

answered Nov 18, 2012 at 3:01

HYRY

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1 Comment

Weichu Liu Over a year ago

Actually thank you for inspiring answer. I used to write my c code with a len(L) times malloc in a for loop (excuse me for my amateur c level), after I mod it to an 1d array with an pointer array pointing to offset, the speed actually increased dramatically, which give me a lecture not to use malloc in loop :-)

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