Save openGL output using DevIL or SOIL

I have tried using both soil and devil image libraries but the screenshots created are completely black images. For devil I used the function found here Take screenshot with openGL and save it as png but the image is still black screen.

Any ideas about saving screenshot or exporting opengl output?

Try this:

#include <vector>
#include <GL/glut.h>
#include <SOIL/SOIL.h>

using namespace std;

bool save = false;
void display()
{
    glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );

    glMatrixMode( GL_PROJECTION );
    glLoadIdentity();
    glOrtho( -2, 2, -2, 2, -1, 1);

    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity();

    glColor3ub( 255, 0, 0 );
    glBegin( GL_QUADS );
    glVertex2i( -1, -1 );
    glVertex2i(  1, -1 );
    glVertex2i(  1,  1 );
    glVertex2i( -1,  1 );
    glEnd();

    if( save )
    {
        int w = glutGet( GLUT_WINDOW_WIDTH );
        int h = glutGet( GLUT_WINDOW_HEIGHT );
        vector< unsigned char > buf( w * h * 3 );

        glPixelStorei( GL_PACK_ALIGNMENT, 1 );
        glReadPixels( 0, 0, w, h, GL_RGB, GL_UNSIGNED_BYTE, &buf[0] );

        int err = SOIL_save_image
            (
            "img.bmp",
            SOIL_SAVE_TYPE_BMP,
            w, h, 3,
            &buf[0]
            );

        save = false;
    }

    glutSwapBuffers();
}

void keyboard( unsigned char key, int x, int y )
{
    if( key == 's' ) 
    {
        save = true;
        glutPostRedisplay();
    }
}

int main( int argc, char **argv )
{
    glutInit( &argc, argv );
    glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );
    glutInitWindowSize( 640, 480 );
    glutCreateWindow( "GLUT" );
    glutDisplayFunc( display );
    glutKeyboardFunc( keyboard );
    glutMainLoop();
    return 0;
}