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

const double pi = 0; // with const and an initialization value, gcc will put it in read-only memory

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

int main( void)
{
  double *p = (double *) &pi;
  *p = 4*atan(1);
  f();
}

/* sample runs:

$ alias GCC
gcc -std=c11 -pedantic -Wall
$ GCC badpi.c
$ ./a.out
Segmentation fault (core dumped)
$ GCC -g badpi.c
$ gdb -q ./a.out
Reading symbols 
(gdb) run
Starting program
Program received signal SIGSEGV, Segmentation fault.
0x0000000000400538 in main () at badpi.c:11
11        *p = 4*atan(1);
(gdb)

*/