the question is about CvArr* in OpenCV it is defined in the source like this /* CvArr* is used to pass arbitrary  * array-like data structures  * into functions where the particular  * array type is recognized at runtime:  */ typedef void CvArr; and documentation like this CvArr struct CvArr This is the “metatype” used only as a function parameter. It denotes that the function accepts arrays of multiple types, such as IplImage*, CvMat* or even CvSeq* sometimes. The particular array type is determined at runtime by analyzing the first 4 bytes of the header. In C++ interface the role of CvArr is played by InputArray and OutputArray. I'm writing my libraries documentation  so i was wondering when i show my function definition as in c below CvMat* cvGetCol(const CvArr* arr, CvMat* submat, int col) How i would do it ? since CvMat is a struct i so far have this but is it the best it could be (get-cols (arr  cv-arr) (:pointer (:struct submat)) (col :int)) but how would someone more skilled in cffi write the int part(col) and the CvArr part.  In my library CvArr* is defined as this (defctype cv-arr :pointer)....I would appreciate any help on this