Cairo text alignment using cairo_text_extents

cairo-text.cpp

#include <cairo.h>

// gcc main.cpp -o test $(pkg-config cairo --cflags) $(pkg-config cairo --libs) $(pkg-config --cflags gtk+-3.0) $(pkg-config --libs gtk+-3.0) 

#include <cairo.h>
#include <gtk/gtk.h>

static void do_drawing(cairo_t *, int align);

static gboolean on_draw_event(GtkWidget *widget, cairo_t *cr, 
    gpointer user_data)
{      
  do_drawing(cr, 1);
  do_drawing(cr, 2);
  do_drawing(cr, 3);

  return FALSE;
}

#define TEXT "Knowledge is power"

static void do_drawing(cairo_t *cr, int align)
{
  cairo_text_extents_t extends;
  cairo_set_source_rgb(cr, 0, 0, 0);
  cairo_select_font_face(cr, "Courier", CAIRO_FONT_SLANT_NORMAL,
      CAIRO_FONT_WEIGHT_NORMAL);
  cairo_set_font_size(cr, 40.0);

  switch (align) {
  case 1:  
    cairo_move_to(cr, 0, 50.0);
    break;
  case 2:  
    cairo_text_extents(cr, TEXT, &extends);
    cairo_move_to(cr, 800/2 - extends.width/2, 110.0);
    break;
  case 3:  
    cairo_text_extents(cr, TEXT, &extends);
    cairo_move_to(cr, 800 - extends.width, 170.0);
    break;
  }
  cairo_show_text(cr, TEXT);    
}


int main(int argc, char *argv[])
{
  GtkWidget *window;
  GtkWidget *darea;

  gtk_init(&argc, &argv);

  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);

  darea = gtk_drawing_area_new();
  gtk_container_add(GTK_CONTAINER(window), darea);

  g_signal_connect(G_OBJECT(darea), "draw", 
      G_CALLBACK(on_draw_event), NULL); 
  g_signal_connect(window, "destroy",
      G_CALLBACK(gtk_main_quit), NULL);

  gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
  gtk_window_set_default_size(GTK_WINDOW(window), 800, 240); 
  gtk_window_set_title(GTK_WINDOW(window), "GTK window");

  gtk_widget_show_all(window);

  gtk_main();

  return 0;
}