(C 언어 허프만) ffulsh(stdin) 에러 질문

#include<stdio.h>

#include<string.h>

#include<stdlib.h>

struct huffTable{

	char ch;

	int count;

	char code[20];

	double frequency;

}HT[128];

typedef struct Tree{

	double val;

	char ch;

	char dad_val;

	struct Tree *left, *right, *dad;

}Tree;

typedef struct min{

	int index;

	float val;

}min;

int exception[100], exception_count = 0;

int HT_SIZE = 0;

void init(){

	for(int i = 0; i < 128; i ++){

		HT[i].ch ='\0';

		HT[i].count = 0;

		for(int a = 0; a < 20; a ++)

			HT[i].code[a] = '\0';

}

}

int chk_HT(char c){

	for(int i = 0; i < HT_SIZE; i ++){

		if(HT[i].ch == c)

			return i;

}

	return -1;

}

void sort_(){

	struct huffTable tmp ;

	for(int i = 0; i < HT_SIZE-1; i++){

		for(int j = i+1; j < HT_SIZE; j++)

			if(HT[i].count > HT[j].count){

				tmp = HT[j];

				HT[j] = HT[i];

				HT[i] = tmp;

}

}

}

int chk_exception(int index){

	for(int z = 0; z < exception_count; z++){

		if(index == exception[z])

			return -1;

}

	return 0;

}

void mk(){

	int chk_rt = -1;

	int rt = HT_SIZE;

	min min[2];

	int size = ((HT_SIZE - 1) * 2);

	int break_chk = -1;

	char code_tmp[20];

	Tree node[size];

	for(int i = 0; i < size; i++){

		node[i].val = HT[i].frequency;

		node[i].ch = HT[i].ch;

		node[i].left = NULL;

		node[i].right = NULL;

		node[i].dad = NULL;

		node[i].dad_val = '\0';

}

	fflush(stdin);

	if(node[rt].left == NULL && node[rt].right == NULL){

		node[rt].left = &node[0];

		node[rt].right = &node[1];

		node[rt].val = node[0].val + node[1].val;

		node[0].dad = &node[rt];

		node[1].dad = &node[rt];

		node[0].dad_val = '0';

		node[1].dad_val = '1';

		printf("\nFIRST RT ! \nch = %c val = %f  left char :  %c  %f code : %c right char :  %c  %f code : %c\n",node[rt].ch,node[rt].val,node[rt].left->ch,node[rt].left->val, node[rt].left->dad_val,node[rt].right->ch,node[rt].right->val,node[rt].right->dad_val);

		exception[exception_count++] = 0;

		exception[exception_count++] = 1;

		rt++;

}

	for(int i = 1; i < (size- HT_SIZE)+1  ; i++){

		int min_count = 0;

		fflush(stdin);

		for(int i = 0; i <2; i++){

			min[i].val = 1;

			min[i].index = -1;

}

		for(int x = 0; x < 2; x++){

			for(int z = 0; z < rt; z++){

				if(chk_exception(z) == -1)

					continue;

				if(min[0].index == z)

					continue;

				if(min[min_count].val > node[z].val){

					min[min_count].index = z;

					min[min_count].val = node[z].val;

}

			}min_count = 1;

}

		exception[exception_count++] = min[0].index;

		exception[exception_count++] = min[1].index;

		node[rt].val = node[min[0].index].val + node[min[1].index].val;

		node[rt].left = &node[min[0].index];

		node[rt].right = &node[min[1].index];

		node[min[0].index].dad = &node[rt];

		node[min[1].index].dad = &node[rt];

		node[min[0].index].dad_val = '0';

		node[min[1].index].dad_val = '1';

		printf("\n%d RT ! \nch = %c val = %f  left  char : %c  %f code : %c right  char : %c  %f code : %c\n",i+1,node[rt].ch,node[rt].val,node[rt].left->ch,node[rt].left->val, node[rt].left->dad_val,node[rt].right->ch,node[rt].right->val,node[rt].right->dad_val);

		rt++;

		fflush(stdin);

}

	fflush(stdin);

	fflush(stdin);

	fflush(stdin);

	for(int g = 0; g < HT_SIZE; g++){

		for(int z = 0; z < 20; z ++){

			code_tmp[z] = '\0';

}

		for(int index = 0; node[g].dad_val != '\0'; index++){

			code_tmp[index] = node[g].dad_val;

			node[g] = *node[g].dad;

}

		for(int z = 0; z <= strlen(code_tmp); z++){

			HT[g].code[z] = code_tmp[strlen(code_tmp) -1 - z];

}

  		HT[g].code[strlen(HT[g].code)-1] = '\0';

		fflush(stdin);

}

}

int main(int ar, char *argv[]){

	int len, totla_len = 0;

	float fre = 0;

	char text[100];

	Tree *Tre = NULL;

	init();

	if(ar != 2){

		fprintf(stderr,"how to using huffman....\n(huffman_exeFile txtFile)");

		return -1;

}

	FILE *f = fopen(argv[1],"r");

	while(fgets(text,100,f) != NULL){

		len = strlen(text);

		totla_len += len;

		if(HT[0].ch == '\0'){

			HT[0].ch = text[0];

			HT[0].count++;

			HT_SIZE++;

}

		for(int i = 1; i < len; i ++){

			int chk = 0;

			chk = chk_HT(text[i]);

			if(chk != -1){

				HT[chk].count++;

			}else{

				HT[HT_SIZE].ch = text[i];

				HT[HT_SIZE].count++;

				HT_SIZE++;

}

}

}

	printf("TOTAL LEN : %d\n",totla_len);

	for(int i = 0; i <HT_SIZE; i++){

		fre += HT[i].frequency = ((float)HT[i].count / (float)(totla_len-1));

}

	sort_();

	mk();

	printf("\n\n");

	for(int i = 0; i < HT_SIZE; i++){

		printf("%c %d %f code :%s\n",HT[i].ch,HT[i].count,HT[i].frequency,HT[i].code);

}

	printf("frequence : %f \n",fre);

	printf("\nHT_SIZE  : %d\n",HT_SIZE);

	fclose(f);

	return 0;

}

위에 fflush(stdin)함수를 하나라도 빼버리면 세그먼테이션 폴트 11이 나오고 다 넣고 컴파일하면 정상적인 값이 나옵니다...

fflush자체가 버퍼를 비워주는 함수인데 왜 하나라도 빼면 에러가 나는거죠..?