Linker error : undefined reference to symbol 'glOrtho'

Question

I installed OpenGL packages in Ubuntu 13.04 ( *mesa-common-dev freeglut3-dev* ) and tried to run a sample program.

#include "GL/freeglut.h"
#include "GL/gl.h"

/* display function - code from:
     http://fly.cc.fer.hr/~unreal/theredbook/chapter01.html
This is the actual usage of the OpenGL library. 
The following code is the same for any platform */
void renderFunction()
{
    glClearColor(0.0, 0.0, 0.0, 0.0);
    glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(1.0, 1.0, 1.0);
    glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0);
    glBegin(GL_POLYGON);
        glVertex2f(-0.5, -0.5);
        glVertex2f(-0.5, 0.5);
        glVertex2f(0.5, 0.5);
        glVertex2f(0.5, -0.5);
    glEnd();
    glFlush();
}

/* Main method - main entry point of application
the freeglut library does the window creation work for us, 
regardless of the platform. */
int main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE);
    glutInitWindowSize(500,500);
    glutInitWindowPosition(100,100);
    glutCreateWindow("OpenGL - First window demo");
    glutDisplayFunc(renderFunction);
    glutMainLoop();    
    return 0;
}

However, I encountered this error and don't know what to make of it.

ved@vedvals:~/Desktop/p1$ g++ p1.cpp -lglut
/usr/bin/ld: /tmp/ccgGdeR2.o: undefined reference to symbol 'glOrtho'
/usr/bin/ld: note: 'glOrtho' is defined in DSO /usr/lib/x86_64-linux-gnu/mesa/libGL.so.1 so try adding it to the linker command line
/usr/lib/x86_64-linux-gnu/mesa/libGL.so.1: could not read symbols: Invalid operation
collect2: error: ld returned 1 exit status

I had a look at
OpenGL hello.c fails to build using CMake
as the error is similar but I am not using CMake

IS my code wrong or do I need to include/change/tweak some settings?

I referred this site for installation and code :

Setting up an OpenGL development environment in Ubuntu Linux

Answer 1

You need to link the OpenGL library:

g++ p1.cpp -lglut -lGL

Answer 2

You have not linked the OpenGL lib, where glOrtho() is defined. To make it work, compile/link with g++ p1.cpp -lglut -lGL. Mind the order of linking libs, as it is important in ld (linker used by g++). The GLUT library depends on OpenGL, and so -lGL HAS TO go after -glut. This is because ld only makes one cycle through the libraries, and thus if you linked -lGL -lglut, references from lglut to lGL will not be defined, thus making a linking error. Sorry for such a long answer, but I hope you'll learn something from it.