Variable Scope Fun

scope.c

#include <stdio.h>
#include <stdlib.h>

/* Topics to review:
definition vs declaration
scope vs storage duration
storage classes- auto, static, extern, register
internal vs external linkage */

static int my_static = 1; /* This is a definition. This current file and THIS 
FILE ONLY can access this variable from any location- all variables with the 
same name refer to the same memory location. */
/* int my_static = 1; Redefinition- illegal */
extern int my_extern; /* This is a declaration. Other files can access this variable- 
all variables with the same name refer to same memory location. */
/* extern int my_extern = 2; This doesn't make sense, but apparently is legal? */
int my_global = 3; /* This is a definition. This variable is accessible from 
other files. But variables DEFINED with the same name in other files will error 
out (multiple definitions). This is what extern DECLARATION is for. It has neither 
static or extern storage class, but rather static duration, external linkage. 
See http://stackoverflow.com/questions/4239834/global-variable-in-c-are-static-or-not
auto and register are illegal here. */

int main()
{
	auto int my_local = 4; /* auto is implied FOR LOCALS if no other storage 
	class is used. */
	register my_reg_var = 5; /* register- try to place in register. Cannot
	take address of register. */
	
	
	printf("We are in main! Here is the value of my_local: %d\n", my_local);
	printf("Here is the value of my_static: %d\n", my_static);
	printf("Here is the value of my_extern: %d\n", my_extern);
	printf("Here is the value of my_global: %d\n", my_global);
	printf("Here is the value of my_reg_var: %d\n", my_reg_var);
	
	{
		int my_local = 6;
		printf("We are now in a new scope!\n");
		printf("This is the value of my_local in the new scope: %d\n", my_local);
			
		{
			extern int my_extern; /* Same my_extern. This is another declaration. */
			/* int my_extern; */
			
			my_extern = 7;
			int yet_another_local = 8;
			printf("We are in yet another new scope!\n");
			printf("Here is the value of yet_another_local: %d\n", yet_another_local);
			printf("This is the value of my_local in the yet another new scope: %d\n", my_local);
			printf("This is the value of my_extern in the yet another new scope: %d\n", my_extern);
			printf("Notice that my_local's value in the previous scope is available!\n");
		}
		
		
	}
	
	printf("We are now back to the original scope!\n");
	printf("We are in main! Here is the value of my_local: %d\n", my_local);
	printf("Here is the value of my_static: %d\n", my_static);
	printf("Here is the value of my_extern: %d\n", my_extern);
	printf("Here is the value of my_global: %d\n", my_global);
	printf("Here is the value of my_reg_var: %d\n", my_reg_var);
	
	return EXIT_SUCCESS;
}

int my_extern = 2; /* Actual DEFINITION of my_extern. Deliberately putting it here. 
Notice the source file compiles successfully! Also see: http://stackoverflow.com/a/2652568 */