#include <stdio.h>

void update( double *a ) // 1. Use a reference parameter variable
{
   // Important: 
   //    a = address of the actual parameter variable
   //   *a = an alias of the actual parameter variable !!!

   printf( "Inside update - before changing *a, *a = %lf\n", *a);
   *a = *a + 1;  // We CAN update a in main !!!
   printf( "Inside update - AFTER  changing *a, *a = %lf\n", *a);
}
 
int main( int argc, char* argv[] )
{
   double a = 3.14;
 
   printf( "Inside main - before calling update(a), a = %lf\n", a);
   update ( &a );   // pass the reference to a !!!
   printf( "Inside main - AFTER  calling update(a), a = %lf\n", a);      
}