Segmentation Fault 11 Redux

我正在编写一个程序，该程序应在640x480窗口中的任意位置创建随机大小的绿色框​​。运行以下代码时出现分段错误。问题出在两个“ for”循环中。段错误通常发生在带有startx的嵌套“ for”循环中。我怀疑缓冲区溢出，但是不知道如何减少笨重的代码。

//Globals
int width, height;
int endx, endy, starty, startx, randEnd, randStartX, randStartY;
unsigned char *pixmap;

void setPixels(){

for (int j = 1; j<100; j++) { // j == j-1 # of boxes

    randStartX = rand() % width; // random # btw 0 and width
    randStartY = rand() % height; // random # btw 0 and height
    randEnd = 1 + (rand() % 100); // random # btw 0 - 100, not allowing box > 100.

    startx = randStartX;
    starty = randStartY;
    endx = startx + randEnd;
    endy = starty + randEnd;

    for(int y = starty; y < endy; y++) { // first y coordinate of box
        for(int x = startx; x < endx; x++) { // first x coordinate of box
            cout << "endx = " << endx << endl;
            int i = (y * width + x) * 3; // movement upwards for each pixel
            pixmap[i++] = 0x00; //Increments i by one to move to the next part of pixel.
            pixmap[i++] = 0xFF; 
            pixmap[i] = 0x00; 
            }
        }
    }
}

int main(int argc, char *argv[])
{
    //initialize the global variables
    srand (time(0));
    width = 640;
    height = 480;
    pixmap = new unsigned char[width * height * 3];  

    setPixels(); // write code like ./pr01 red, etc.

    glutInit(&argc, argv);
    glutInitWindowPosition(100, 100); // Where the window will display on-screen.
    glutInitWindowSize(width, height);
    glutInitDisplayMode(GLUT_RGB | GLUT_SINGLE);
    glutCreateWindow("Assignment 01");
    init();
    glutReshapeFunc(windowResize);
    glutDisplayFunc(windowDisplay);
    glutMouseFunc(handleButton);
    glutMainLoop();

    return 0; 
}

知道是什么原因造成的吗？这里有任何公然的逻辑问题吗？提前致谢。