#include <gtk/gtk.h>
/* gcc -o test test.c `pkg-config --cflags --libs gtk+-2.0` */

int  main (){ 
/* how the headers define G_PI */
/* #define G_PI   3.1415926535897932384626433832795028841971693993751  
*/

 long double PI = G_PI;

g_print("3.1415926535897932384626433832795028841971693993751\n");
g_print("%0.50e\n",G_PI);
g_print("%0.50Lf\n",G_PI);
g_print("%0.50Lf\n",PI);
g_print("%0.50Lg\n",PI);
g_print("%0.50Le\n",PI);
  
return 0; 
}  


##</code><code>##

                                 ^^^^^^^^^^^ ->  unstable after 15 decimal 
3.1415926535897932384626433832795028841971693993751
3.14159265358979311599796346854418516159057617187500e+00
-0.00000000000000000000000000000000000000000000000000
3.14159265358979311599796346854418516159057617187500
3.141592653589793115997963468544185161590576171875
3.14159265358979311599796346854418516159057617187500e+00
                                 ^^^^^^^^^^^^ -> unstable after 15 decimal