C언어의 구조체가 소멸하는 문제

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
#define defaultSize 5
 
typedef enum {FALSE, TRUE} Boolean;
 
typedef struct {
	int nElements;
	int arraySize;
	char **element;
} strset_t;
 
strset_t *createSet()
{
	strset_t *s = (strset_t *)malloc(sizeof(strset_t));
	s->element = (char **)malloc(sizeof(char *) * defaultSize);
 
	s->nElements = 0;
	s->arraySize = defaultSize;
 
	return s;
}
 
void deleteSet(strset_t *s)
{
	free(s->element);
	free(s);
}
 
Boolean isEmpty(strset_t *s)
{
	if(s->nElements == 0)
		return TRUE;
	return FALSE;
}
 
int cardinality(strset_t *s)
{
	return s->nElements;
}
 
int isElement(char *x, strset_t *a)
{
	for(int i = 0; i < a->nElements; i++)
		if(strcmp(x, a->element[i]) == 0)
			return i;
	return -1;
}
 
Boolean addElement(char *x, strset_t *a)
{
	if(a->nElements == a->arraySize){
		a->arraySize *= 2;
		a = realloc(a, a->arraySize);
 
		puts("== extended ==");
	}
 
	if(a == NULL){
		puts("=== NULL ===");
		return FALSE;
	}
 
	a->element[a->nElements] = (char *)malloc(strlen(x) + 1);
	strcpy(a->element[a->nElements], x);
	a->nElements++;	//deep copy를 사용하여 값 전달
 
	return TRUE;
}
 
Boolean removeElement(char *x, strset_t *a)
{
	int idx = isElement(x, a);
 
	if(idx == -1){
		puts("=== Not found ===");
		return FALSE;
	}
 
	for(int i = idx; i < a->nElements - 1; i++){
		free(a->element[i]);
		a->element[i] = a->element[i + 1];
	}
 
	free(a->element[a->nElements - 1]);
	a->nElements--;
}
 
 
void showAll(strset_t *a, strset_t *b)
{
	int i;
 
	printf("A[%d] : ", a->arraySize);
	for(i = 0; i < a->nElements; i++)
		printf("%s ", a->element[i]);
 
	printf("\nB[%d] : ", b->arraySize);
	for(i = 0; i < b->nElements; i++)
		printf("%s ", b->element[i]);
 
}
 
int main()
{
	strset_t *S1, *S2;
 
	S1 = createSet();
	S2 = createSet();
 
	Boolean run = TRUE;
 
	char target, selection;
	char buff[100];
 
	while(run){
		puts("====== MENU ======");
		puts("0. Show All");
		puts("1. Add element");
		puts("2. Remove element");
		puts("3. Check the element(valid or not)");
		puts("4. Check the subset(valid or not)");
		puts("5. Compare and match the sets");	//Equal
		puts("6. Check the disjoint");
		puts("7. Make intersection");
		puts("8. Make Union");
		puts("9. Make difference");
 
		printf("%p %p\n", S1, S2);
 
		fflush(stdin);
 
		switch (getchar()) {
		case '1':
			printf("Target(1, 2)) : ");
			scanf("%d", &target);
			fflush(stdin);
			printf("Element : ");
			if(target == 1)
				addElement(gets(buff), S1);
			else if(target == 2)
				addElement(gets(buff), S2);
 
			break;
 
		case '2':
			printf("Target(1, 2)) : ");
			scanf("%d", &target);
			fflush(stdin);
			printf("Element : ");
			if(target == 1)
				removeElement(gets(buff), S1);
			else if(target == 2)
				removeElement(gets(buff), S2);
 
			break;
 
		case '0':
			showAll(S1, S2);
			break;
 
		case 'X':
			run = FALSE;
			break;
 
		default:
			puts("== Invalid menu ==");
			getchar();
			break;
		}
 
		printf("\n");
	}
 
	deleteSet(S1);
	deleteSet(S2);
 
	system("pause");
    return 0;
}