OpenGL exception at debugging - visual-studio-2010

Question

here is the code to draw fractal geometry of Sierpinski's Triangle there is no error but at the debugging an unhandled exception rises:
Unhandled exception at 0x77e115de in LastFractLast.exe: 0xC0000005: Access violation writing location 0x000000a8.
and it opens a file called glu.h and pointing to this line :
static void APIENTRY glutInit_ATEXIT_HACK(int *argcp, char **argv) { __glutInitWithExit(argcp, argv, exit); }
here is my code :
#include <glut.h>
GLfloat v[3][2]={{-1.0, -0.58},
{1.0, -0.58}, {0.0, 1.15}};
int n;
void triangle(GLfloat *a, GLfloat *b,
GLfloat *c)
/* display one triangle */
{
glVertex2fv(a);
glVertex2fv(b);
glVertex2fv(c);
}
void divide_triangle(GLfloat *a, GLfloat *b, GLfloat *c,
int m)
{
/* triangle subdivision using vertex numbers */
typedef GLfloat point2[2];
point2 v0, v1, v2;
int j;
if(m>0)
{
for(j=0; j<2; j++) v0[j]=(a[j]+b[j])/2;
for(j=0; j<2; j++) v1[j]=(a[j]+c[j])/2;
for(j=0; j<2; j++) v2[j]=(b[j]+c[j])/2;
divide_triangle(a, v0, v1, m-1);
divide_triangle(c, v1, v2, m-1);
divide_triangle(b, v2, v0, m-1);
}
else(triangle(a,b,c));
/* draw triangle at end of recursion */
}
void display()
{
glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_TRIANGLES);
divide_triangle(v[0], v[1], v[2], n);
glEnd();
glFlush();
}
void myinit()
{
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(-2.0, 2.0, -2.0, 2.0);
glMatrixMode(GL_MODELVIEW);
glClearColor (1.0, 1.0, 1.0,1.0);
glColor3f(0.0,0.0,0.0);
}
int main(int argc, char **argv)
{
n=4;
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
glutInitWindowSize(500, 500);
glutDisplayFunc(display);
myinit();
glutMainLoop();
}

Answer 1

You need a current GL context before executing GL commands.
With GLUT you need to create a window via glutCreateWindow() to get a context:
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
glutInitWindowSize(500, 500);
glutCreateWindow( "Window" );
glutDisplayFunc(display);
glutMainLoop();
Complete:
#include <GL/glut.h>
/* display one triangle */
void triangle(GLfloat *a, GLfloat *b, GLfloat *c)
{
glVertex2fv(a);
glVertex2fv(b);
glVertex2fv(c);
}
void divide_triangle(GLfloat *a, GLfloat *b, GLfloat *c, int m)
{
/* triangle subdivision using vertex numbers */
typedef GLfloat point2[2];
point2 v0, v1, v2;
int j;
if(m>0)
{
for(j=0; j<2; j++) v0[j]=(a[j]+b[j])/2;
for(j=0; j<2; j++) v1[j]=(a[j]+c[j])/2;
for(j=0; j<2; j++) v2[j]=(b[j]+c[j])/2;
divide_triangle(a, v0, v1, m-1);
divide_triangle(c, v1, v2, m-1);
divide_triangle(b, v2, v0, m-1);
}
else
{
/* draw triangle at end of recursion */
triangle(a,b,c);
}
}
void display()
{
glClearColor (1.0, 1.0, 1.0,1.0);
glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-2.0, 2.0, -2.0, 2.0, -1, 1);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glColor3f(0.0,0.0,0.0);
glBegin(GL_TRIANGLES);
GLfloat v[3][2]=
{
{-1.0, -0.58},
{1.0, -0.58},
{0.0, 1.15}
};
divide_triangle(v[0], v[1], v[2], 4);
glEnd();
glFlush();
}
int main(int argc, char **argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
glutInitWindowSize(500, 500);
glutCreateWindow( "Window" );
glutDisplayFunc(display);
glutMainLoop();
}