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Access violation, cannot read memory xxxxxxx

applebibo
applebibo asked
on
Medium Priority
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Last Modified: 2013-11-20
Hi, I recently encountered an access violation problem. In VC++, if I run it, the error code is "cannot read memory xxxxxxx"; in the debug mode, it will show "xxxxxxxx: access violation".

I found out that this exception happened when a function was called to a number of times -- I am not sure if it happened at the same number. But the function works and this only happens after a number of calls, which frustrates me. Please help me!

Here is the functions. drawSurface() is to generate a set of x and y coordinates, and for each set, use surfacePoint() to calculate the z value of that point, and draw it in drawSurface().

void CBSplineView::drawSurface()
{
      MyPoint* s=new MyPoint();
      glColor3f(1.0,1.0,0.0);
      std::vector<MyPoint*> stor,cal;
      int i,j;
      double u,v;
      u=0.0;
      for (i=0;i<cal.size();i++) delete cal[i];
      cal.clear();
      for (j=0;j<=NUMBER; j++)
      {
            v=(double)j/fNUMBER*n;
            s=new MyPoint();
            surfacePoint(u,v,1,s);
            cal.push_back(s);
      }
      for (i=1; i<=NUMBER; i++)  
      {
            u=(double)i/fNUMBER*m;
            for (int k=0;k<stor.size();k++) delete stor[k];
            stor.clear();
            stor=cal;
            v=0.0;
            cal.clear();
            s=new MyPoint();
            surfacePoint(u,v,1,s);
            cal.push_back(s);
            for (j=1; j<=NUMBER; j++)
            {
                  v=(double)j/fNUMBER*n;
                  
                  glBegin(GL_POLYGON);
                        glVertex3f(stor[j-1]->x,stor[j-1]->y,stor[j-1]->z);
                        glVertex3f(stor[j]->x,stor[j]->y,stor[j]->z);
                        s=new MyPoint();
                        surfacePoint(u,v,1,s);
                        cal.push_back(s);
                        glVertex3f(cal[j]->x,cal[j]->y,cal[j]->z);
                        glVertex3f(cal[j-1]->x,cal[j-1]->y,cal[j-1]->z);
                  glEnd();
            }
      }
      
      for (i=0;i<cal.size();i++) delete cal[i];
      for (int k=0;k<stor.size();k++) delete stor[k];

}

void CBSplineView::surfacePoint(double u, double v, int h, MyPoint* S)
{
      int i,j,k,l;
      double s,t,temp;

      i=floor(u*h);
      j=floor(v*h);
      s=(u-floor(u))*h;
      t=(v-floor(v))*h;

      temp=0.0;

      for (k=0;k<=m_p;k++)
      {
            for (l=0;l<=m_q;l++)
            {
                  temp=temp+B(k,m_p,s)*B(l,m_q,t)*points[(i+k)*(n*h+m_q)+j+l]->z; // WHEN AN ERROR OCCURS, THE PROGRAM STOPS AT THIS LINE.
            }
      }

      S->x=u;
      S->y=v;
      S->z=temp;
      
}
Comment
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Commented:
I guess some index is out of range. Replace this line with number of lines by the following way:

int i1 = B(k,m_p,s);
int i2 = B(l,m_q,t);
int i3= (i+k)*(n*h+m_q)+j+l;   // this is index, possibly out of range
MyPoint* p1 = mypoints[i3];
...
temp = i1 * i2 ...;

This can give more information what happens. When exception occurs, enter debugger and see variable values.


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mahesh1402IT Professional
Top Expert 2005

Commented:
temp=temp+B(k,m_p,s)*B(l,m_q,t)*points[(i+k)*(n*h+m_q)+j+l]->z; <== whats values of m_p and m_q ? have you crosschecked them at run time ?

How you want to evaluate above expression ? Do close it with proper parenthesis.

MAHESH

Author

Commented:
Thanks! i3 is out of range at a very special point. Other variables are OK.
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