输入glBegin()后OpenGL分割错误

我对OpenGL编程非常陌生，一切都很好，直到我收到了无处不在的分割错误，现在无法摆脱它。谁能发现这个分割错误并解释给我听?

注:虽然这是一项作业，但我不是来这里寻找答案的。事实上，我甚至不会提及分配的细节，甚至不会发送完整的代码(尽管，下面的代码也会导致段错误):

#  include <stdio.h>
#  include <stdlib.h>
#  ifdef __APPLE__
#  include <OpenGL/gl.h>
#  include <OpenGL/glu.h>
#  include <GLUT/glut.h>
#  else
#  include <GL/gl.h>
#  include <GL/glu.h>
#  include <GL/freeglut.h>
#  endif
#  include <iostream>
#  include <ctime>
#  include <cmath>
using namespace std;
#define GRID 50 
/* Eye location */
float eye[]={5,5,5};
void initDefaults(void){
    glClearColor(0, 0, 0, 0);
    glColor3f(1, 1, 1);
    glMatrixMode(GL_PROJECTION);
    glFrustum(-10,10,-10,10,0,100);
    gluPerspective(45,1,1,100);
}
/* TEMPORARY */
void createGrid(){  
//After checking, I narrowed down the segmentation fault to here.   
    glColor3f(0, 0, 0);
    printf("help");
    glBegin(GL_POINTS);
    glVertex3f(25,25,25);
    glEnd();
}
/* displays UI to user */
void display(void){
    //Clears the board upon (re)display
    glClearColor(1.0,1.0,1.0,0.0);
    glClear(GL_COLOR_BUFFER_BIT);
    //Enables backface culling
    glEnable(GL_CULL_FACE);
    glCullFace(GL_BACK);
    //Sets up the perspective matrix (viewing/camera)
    glMatrixMode(GL_MODELVIEW); 
    glLoadIdentity();
    gluLookAt(eye[0], eye[1], eye[2], GRID/2, GRID/2, 0, 0, 1, 0); 
    //I believe the segmentation fault occurs in here.
    //************************************
    createGrid();
    //************************************
    //Used for double-buffering
    glutSwapBuffers();
}
/* main function - program entry point */
int main(int argc, char** argv){
    glutInit(&argc, argv);                                      
    glutInitWindowSize(600, 600);
    glutInitWindowPosition(200,200);
    glutCreateWindow("Terrain");                                
    glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGBA);
    glutDisplayFunc(display);                         
    gluOrtho2D(-1, GRID + 1, -1, GRID + 1);
    initDefaults();
    glutMainLoop();                                             
    return(0);                                                  
}