// #define _XOPEN_SOURCE 700 // for M_PI and lots of other stuff

#include <stdio.h>
#include <math.h>

// const double pi = 4*atan(1); // no, static initializer must be a constant expression

volatile const double pi;

void f( void) { printf( "f: pi = %g\n", pi); }

void g( void); // in file g.c

int main( void)
{
  // ++pi; // should not be allowed
  // printf( "pi = %g\n", pi);
  double *p = (double *) &pi;
  *p = 4*atan(1);
  f();
  g();
  // printf( "M_PI = %g\n", M_PI);
}

/* sample run:

$ alias GCC
gcc -std=c11 -pedantic -Wall
$ GCC pi.c g.c
$ ./a.out
f: pi = 3.14159
g: pi = 3.14159
$

*/