bresenham's midpoint line algorithm

midpoint.cpp

#include <GL/glut.h>

void init()
{
    glClearColor(0.0, 0.0, 0.0, 0.0);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(0.0, 500.0, 0.0, 500.0);
}

void midPoint(int x1, int y1, int x2, int y2)
{
    if (x1 > x2)
    {
        midPoint(x2, y2, x1, y1);
        return;
    }

    int slope;
    int dx, dy, d, x, y;
    
    dx = x2 - x1;
    dy = y2 - y1;
    d = dx - 2 * dy;
    y = y1;

    if (dy < 0) {
        slope = -1;
        dy = -dy;
    }
    else {
        slope = 1;
    }

    for (x = x1; x < x2; x++) {
        glBegin(GL_POINTS);
        glVertex2f(x, y);
        if (d <= 0) {
            d += 2 * dx - 2 * dy;
            y += slope;
        }
        else {
            d += -2 * dy;
        }
        glEnd();
    }

}

void display()
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glColor3f(1.0, 0.0, 0.0);
    midPoint(10, 10, 110, 210);

    glColor3f(0.0, 1.0, 0.0);
    midPoint(10, 10, 210, 110);

    glColor3f(1.0, 1.0, 0.0);
    midPoint(210, 10, 10, 110);

    glFlush();
}

int main(int argc, char *argv[])
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowPosition(50, 50);
    glutInitWindowSize(500, 500);
    glutCreateWindow("Bresenham\'s midpoint line algorithm");
    init();
    glutDisplayFunc(display);
    glutMainLoop();
    return 0;
}

midPoint(100, 200, 100, 350);
will not give any output since you are iterating only over the x coordinate so the for loop will never run.
use the modified midPoint();
void midPoint(int x1, int y1, int x2, int y2)
{
if (x2 < x1)
{
midPoint(x2, y2, x1, y1);
return;
}

int m = 0;
int dx, dy, d, x, y;

dx = x2 - x1;
dy = y2 - y1;
d = dx - 2 * dy;
y = y1;

if (dy < 0) {
    m = -1;
    dy = -dy;
}
else if (dy >=0) {
    m = 1;
}

if(x1==x2){
    if(y1 < y2){
        int temp = y1;
        y1 = y2;
        y2 = temp;
    }
    for(y = y2;y < y1;y++){
        plot(x1,y);
        glEnd();
    }
}
else{

    for (x = x1; x < x2; x++) {

    plot(x, y);

    if (d <= 0) {
        y += m;
        d += 2*(dx -dy);
    }
    else {
        d += -2 * dy;
    }
    glEnd();
}

}

}