display a video from my webcam trying to implement circle detection

I'm trying to display a video from my webcam (which was working grand) and now I'm trying to implement circle detection into this video stream.

Unhandled exception at 0x001a1a4d in test.exe: 0xC0000005: Access violation reading location 0x00000004.)

The error is linked to the line of code:

cvCircle( img, cvPoint(cvRound(p[0]),cvRound(p[1])), 3, CV_RGB(255,0,0), -1, 8, 0 );. 

Can anyone help please?

#include <stdio.h>
#include "cv.h"
#include "highgui.h"
#include <iostream>
#include <math.h>
#include <string.h>
#include <conio.h>

using namespace std;

IplImage* img = 0;
CvMemStorage * cstorage;
CvMemStorage * hstorage;

void detectCircle( IplImage *frame );


int main( int argc, char **argv )
{
    CvCapture *capture = 0;
    IplImage  *frame = 0;
    int       key = 0;

    hstorage = cvCreateMemStorage( 0 );
    cstorage = cvCreateMemStorage( 0 );

    //CvVideoWriter *writer = 0;
    //int colour = 1;
    //int fps = 25;
    //int frameW = 640;
    //int frameH = 480;
    //writer = cvCreateVideoWriter("test.avi",CV_FOURCC('P', 'I', 'M', '1'),fps,cvSize(frameW,frameH),colour);

    //initialise camera
    capture = cvCaptureFromCAM( 0 );

    //check if camera present
    if ( !capture )
    {
        fprintf( stderr, "cannot open webcam\n");
        return 1;
    }

    //create a window
    cvNamedWindow( "Snooker", CV_WINDOW_AUTOSIZE );

    while(key !='q') 
    {

    //get frame
        frame = cvQueryFrame(capture);
        //int nFrames = 50;
        //for (int i=0; i<nFrames;i++){
            //cvGrabFrame(capture);
            //frame = cvRetrieveFrame(capture);
            //cvWriteFrame(writer, frame);
        //}

    //check for frame
        if( !frame ) break;

        detectCircle(开发者_开发技巧frame);


    //display current frame
        //cvShowImage ("Snooker", frame );

        //exit if Q pressed
        key = cvWaitKey( 20 );

    }
    // free memory
    cvDestroyWindow( "Snooker" );
    cvReleaseCapture( &capture );
    cvReleaseMemStorage( &cstorage);
    cvReleaseMemStorage( &hstorage);
    //cvReleaseVideoWriter(&writer);

    return 0;
}

void detectCircle( IplImage * img )
{
    int px;
    int py;
    int edge_thresh = 1;
    IplImage *gray = cvCreateImage( cvSize(img->width,img->height), 8, 1);
    IplImage *edge = cvCreateImage( cvSize(img->width,img->height), 8, 1);

    cvCvtColor(img, gray, CV_BGR2GRAY);

    gray->origin = 1;

    // color threshold
cvThreshold(gray,gray,100,255,CV_THRESH_BINARY);    

// smooths out image
cvSmooth(gray, gray, CV_GAUSSIAN, 11, 11);

// get edges
cvCanny(gray, edge, (float)edge_thresh, (float)edge_thresh*3, 5); 

// detects circle
CvSeq* circle =  cvHoughCircles(gray, cstorage, CV_HOUGH_GRADIENT, 1, gray->height/50, 5, 35);

// draws circle and its centerpoint
float* p = (float*)cvGetSeqElem( circle, 0 );
cvCircle( img, cvPoint(cvRound(p[0]),cvRound(p[1])), 3, CV_RGB(255,0,0), -1, 8, 0 );
cvCircle( img, cvPoint(cvRound(p[0]),cvRound(p[1])), cvRound(p[2]), CV_RGB(200,0,0), 1, 8, 0 );
px=cvRound(p[0]); 
py=cvRound(p[1]);



cvShowImage ("Snooker", img );
}