This has little to do with "Game Programming" but it's OpenGL/GLUT related and maybe this is the best forum section for such question. If not, please move it appropriately.

Normally this would be something like:

Code:
glutAddMenuEntry("-", <opid>);
But it doesn't work. Fater looking it up on Google I couldn't find much information about this, maybe it's not possible using GLUT? I found a couple of examples like:

Code:
glutAddMenuEntry("-", 0);
or

Code:
glutAddMenuEntry("-", -1);
But neither worked...

Full code example below:

Code:
    #include <math.h>
    
    #include <GL/glut.h>
    
    #define WINDOW_WIDTH  640
    #define WINDOW_HEIGHT 480
    
    void changeSize(int w, int h) {
    	if(h == 0) h = 1;
    
    	float ratio = 1.0 * w / h;
    
    	glMatrixMode(GL_PROJECTION);
    	glLoadIdentity();
    	
    	glViewport(0, 0, w, h);
    
    	gluPerspective(45.0f, ratio, 1.0f, 1000.0f);
    
    	glMatrixMode(GL_MODELVIEW);
    }
    
    void renderScene(void) {
    	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    	glLoadIdentity();
    
    	gluLookAt(
    		0.0, 0.0, 5.0,
    		0.0, 0.0, 0.0,
    		0.0f, 1.0f, 0.0f
    	);
    
    	glutWireTeapot(1.0);
    
    	glutSwapBuffers();
    }
    
    void processMenuEvents(int option) {
    }
    
    void main(int argc, char **argv) {
    	glutInit(&argc, argv);
    	glutInitDisplayMode(GLUT_DEPTH | GLUT_DOUBLE | GLUT_RGBA);
    	glutInitWindowPosition(
    		(glutGet(GLUT_SCREEN_WIDTH) - WINDOW_WIDTH) / 2,
    		(glutGet(GLUT_SCREEN_HEIGHT) - WINDOW_HEIGHT) / 2
    	);
    	glutInitWindowSize(WINDOW_WIDTH, WINDOW_HEIGHT);
    	glutCreateWindow("Camera Demo");
    
    	glutDisplayFunc(renderScene);
    	glutReshapeFunc(changeSize);
    
    	glutCreateMenu(processMenuEvents);
    
    	glutAddMenuEntry("OPTION 1", 1);
    	glutAddMenuEntry("-", -1);
    	glutAddMenuEntry("OPTION 2", 2);
    
    	glutAttachMenu(GLUT_RIGHT_BUTTON);
    
    	glEnable(GL_DEPTH_TEST);
    	glEnable(GL_CULL_FACE);
    
    	glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
    
    	glutMainLoop();
    }
I don't see a separator like Windows does for other items, instead I see the menu item label as a dash.