/**
 * Variables must be explicitly allocated and
 * explicitly freed. All dereference must be 
 * explicit.
 * If you want to allocate memory inside a function and return
 * it to the calling function, then you need to allocate that memory
 * on the heap (malloc!).
 */

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

int* createThingBadly( ) {
    int thing = 10;
    return &thing; // Don't do this!!! The memory that was allocated on the call stack will be gone.
}

// Demonstrate that if we have a pointer to a pointer
// We need to malloc and free both.
typedef struct {
    int *items;
    int num_items;
} Container;

// This allocates a container for num_items ints
// It returns a pointer to  the allocated container
Container *make_container( int num_items ) {
    Container *c;
    c = (Container *)malloc(sizeof(Container));
    c->items = (int*)malloc(sizeof(int)*num_items);
    c->num_items = num_items;
    for (int i = 0; i < num_items; i++) {
        c->items[i] = i+1;
    }
}

// This frees the data in the container and the
// container itself
void destroy_container( Container *c ) {
    free( c -> items );
    free( c );
}

// test code for container
void test_container() {
    Container *c = make_container( 5 );
    //free( c );
    destroy_container( c );
}

int main (int argc, char* arvg[]) {

	int *a = (int *)malloc(sizeof(int));
	
	printf( "%d\n", a == NULL ); // shouldn't check value if it hasn't been assigned a value.
	
	*a = 4;
	printf( "a has a value that is an address: %p\n", a );
	printf( "*a =%d\n", *a );
	
	free(a);	
	printf( "a has a value that is an address: %p\n", a );
	printf( "*a =%d\n", *a ); // illegal but runs!

    int* thg = createThingBadly();
    printf( "thing is %d\n", *thg );

//     a = NULL;
// 	printf( "*a =%d\n", *a ); // crashes
    
    return 0;
}