Gambit Scheme FFI

I am trying to use ffi to call opencv's cvReleaseCapture,

void cvReleaseCapture( CvCapture** capture );

http://www.ai.rug.nl/vakinformatie/pas/content/Highgui/opencvref_highgui.htm

I have tried,

(define release-capture (c-lambda ((pointer (pointer "CvCapture"))) void "cvReleaseCapture"))

didn开发者_运维技巧't relly worked.

(c-define-type CvCapture (pointer (pointer (struct "CvCapture"))))

(define release-capture (c-lambda (CvCapture) void "cvReleaseCapture"))