I'm trying to recreate a const char* filename parameter for code I have that needs it...The function I'm wrapping is the OpenCV C++ function "imread" it is below in the documentation.  It is used to read an image(jpg etc) from a file and return a pointer to it

http://docs.opencv.org/modules/highgui/doc/reading_and_writing_images_and_video.html?highlight=imread#imread

first I need to wrap in C as below calling  the const char* filename parameter as a String*

the below cv_imread does compile on ubuntu trusty using g++. And the function works as intended using g++.

Mat* cv_imread(String* filename, int flags) {
    return new Mat(cv::imread(*filename, flags));
}

I wrap in Lisp like this:

(defcfun ("cv_imread" %imread) (:pointer mat)
  (filename (:pointer string*))
  (flags :int))

When I usually create a (:pointer string*) parameter I do it like this

(window-name (foreign-alloc :string :initial-element "IMREAD Example"))
              

 and it works to supply strings that aren't filenames but when I try to use a filename both absolute and relative paths it fails...it also fails using double forward slashes in the pathname.

Any advice is appreciated and btw Emacs /SBCL is my Lisp IDE