Assigning a python list to a C char ** using SWIG

Using a SWIG-generated interface it’s straightforward to assign variables of type char * from a Python script to a C variable: the char * datatype is assumed to be a NULL terminated ASCII string and mapped accordingly. The situation is a little different for variables of type char **, commonly used for arrays of pointers to strings.

The SWIG documentation has a very useful typedef for converting a Python list to a char **. However, I found that it was fine for passing in lists of strings as arguments to functions, but not for simple variable assignments. For example I found that the following example didn’t work:

my_charpp = ["foo", "bar"]

…where my_charpp is declared in C as having type char **. After some experimentation with the typemap and a wrapper function, I found that I could achieve what I wanted by modifying the typemap, to only work on a named pointer, and providing a helper function to do the assignment.

Typemaps:

%typemap(in) (char **pylist){
    ...
}

%typemap(freearg) char **pylist {
    ...
}

Helper functions:

%inline %{

    char **set_list(char **pylist){

         int i = 0;
         char **l;
         int temp = 0;

        while(pylist[i] != NULL)
            i++;

        l = ntg_malloc(i * sizeof(char *));

        while(i--){
            temp = strlen(pylist[i]);
            l[i] = ntg_malloc((temp + 1) * sizeof(char));
            strcpy(l[i], pylist[i]);
        }

            return l;
    }


    void print_list(char **mlist, int n_items){

        while(n_items--)
            printf("%s\n", mlist[n_items]);

    }
%}

So from my Python script, I can do the following:

mylist = ["foo", "bar"]

my_charpp = set_list(mylist)

print_list(my_charpp, 2)

…and it works like a charm! Don’t forget that if you are going to use this code, you need to ‘manually’ free the out-of-place memory allocated by set_list().