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It's very strange because your binary image is very good! Try with the two last parameters:

CvSeq *circles = cvHoughCircles(hough_in, storage,
    CV_HOUGH_GRADIENT, 4, size.height/10, 100, 40, 1, size.height);

If it's not working, try to analyse storage value (I don't use C version, so I'm not sure what the function returns). But as suggested by thdrksdfthmn, use the C++ version of opencv ;-) Look for this exemple in c++, it's easier and well tested!