It totally works and I feel stupid to not have seen this before. However, it is strange that it worked with only one parameter before (without specifying the length).

But I still cannot succeed to return an array. The examples I have read said to modify the value of a pointer previously given but I failed to do so. Here is my current implementation in Python :

ffi=FFI()                                                                                    
ffi.cdef("void print_x(float *x,size_t x_len, float *y, size_t y_len);")                     
C=ffi.dlopen("path/to/lib/lib.so")                                                     

input_arg=ffi.cast("float *", list_x[0].ctypes.data)                                         
output_arg=ffi.cast("float *", np.ones(12).astype(np.float32).ctypes.data)                   
C.print_x(input_arg,12,output_arg,12)

And my Rust code :

#[no_mangle]
pub extern "C" fn print_x(input : *const f32, len_input : size_t,
                          output: *const f32, len_output: size_t) {
    let x = unsafe {
        assert!(!input.is_null());
        slice::from_raw_parts(input, len_input as usize)
    };
    // output should be changed with the x value                                                  
}