[SOLVED] Python passing read-only char ** as argument to a function

Python passing read-only char ** as argument to a function

I have a C library function that expects a char const ** argument, such as:

void set_values(void* object, char const ** values, int dim1, int dim2);

Where values is an array of dim1*dim2 length pointing to null terminated string values.

When I try to call this function using cffi/ctypes, the Python code below results in error:

LP_c_char  = ctypes.POINTER(ctypes.c_char)

dim1 = 2
dim2 = 3
values = ('1.0', '2.0', '1.2', '2.2', '1.5', '2.5')

values_ptr = (LP_c_char * (dim1 * dim2))()
for i, value in enumerate(values):
    values_ptr[i] = ctypes.create_string_buffer(value.encode('utf-8'))

mylib.set_values(object, values_ptr, dim1, dim2)

The result is the error below at mylib.set_values(...) line:

TypeError: initializer for ctype 'char * *' must be a cdata pointer, not LP_c_char_Array_6

Sadly values cannot be a double* array because the library accepts variable names and expressions.

I was following this old thread. I'm using Python 3.7.2 and cffi 1.14.0.

Can someone point out what I'm doing wrong here?

Solution

You can't mix ctypes and cffi like that. They are two different projects. In this case, you're trying to call a function exposed by cffi but passing arguments that are ctypes objects. Try building cffi objects instead, e.g. with ffi.new().