将字符串列表从python / ctypes传递给C函数，期望char **

Question

I have a C function which expects a list \\0 terminated strings as input:

void external_C( int length , const char ** string_list) { 
   // Inspect the content of string_list - but not modify it.
} 

From python (with ctypes) I would like to call this function based on a list of python strings:

def call_c( string_list ):
    lib.external_C( ?? )

call_c( ["String1" , "String2" , "The last string"])

Any tips on how to build up the datastructure on the python side? Observe that I guarantee that the C function will NOT alter content of the strings in string_list.

Regards

joakim

Answer 1

def call_c(L):
    arr = (ctypes.c_char_p * len(L))()
    arr[:] = L
    lib.external_C(len(L), arr)

Answer 2

Thank you very much; that worked like charm. I also did an alternative variation like this:

def call_c( L ):
    arr = (ctypes.c_char_p * (len(L) + 1))()
    arr[:-1] = L
    arr[ len(L) ] = None
    lib.external_C( arr )

And then in C-function I iterated through the (char **) list until I found a NULL.

Answer 3

Using ctypes

create list of expiries( strings)

expiries = ["1M", "2M", "3M", "6M","9M", "1Y", "2Y", "3Y","4Y", "5Y", "6Y", "7Y","8Y", "9Y", "10Y", "11Y","12Y", "15Y", "20Y", "25Y", "30Y"]

Logic to send string array

convert strings array to bytes array by looping in the array

 expiries_bytes = []
    for i in range(len(expiries)):
        expiries_bytes.append(bytes(expiries[i], 'utf-8'))

Logic ctypes to initiate a pointer with a length of array

expiries_array = (ctypes.c_char_p * (len(expiries_bytes)+1))()

assigning the byte array into the pointer

expiries_array[:-1] = expiries_bytes

Answer 4

I just make it using SWIG typemap

1.write customized typemap in demo.i interface file.

%module demo

/* tell SWIG to treat char ** as a list of strings */
%typemap(in) char ** {
    // check if is a list
    if(PyList_Check($input))
    {
        int size = PyList_Size($input);
        int i = 0;
        $1 = (char **)malloc((size + 1)*sizeof(char *));
        for(i = 0; i < size; i++)
        {
            PyObject * o = PyList_GetItem($input, i);
            if(PyString_Check(o))
                $1[i] = PyString_AsString(o);
            else
            {
                PyErr_SetString(PyExc_TypeError, "list must contain strings");
                free($1);
                return NULL;
            }
        }
    }
    else
    {
        PyErr_SetString(PyExc_TypeError, "not a list");
        return NULL;
    }
}

// clean up the char ** array
%typemap(freearg) char ** {
    free((char *) $1);
}

2.generate extension

$ swig -python demo.i  // generate wrap code
$ gcc -c -fpic demo.c demo_wrap.c
$ gcc -shared demo.o demo_wrap.o -o _demo.so

3.import the module in python.

>>> from demo import yourfunction