#include <stdio.h>

 
void update( double* x )
{
   printf("Before update: x = %lf\n", *x);
 
   *x = 1001;    // *x is an alias for the ACTUAL parameter
 
   printf("AFTER  update: x = %lf\n", *x);
 
}
 
int main(int argc, char *argv[])
{
   double a;
 
   a = 4;
 
   printf("Before calling update: a = %lf\n", a);
 
   update ( &a );   
	// Pass the address of a, Simulate passed-by-reference
 
   printf("AFTER  calling update: a = %lf\n", a);
}