Header.h

#pragma once
#pragma warning(disable: 4996)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define N 100
#define DISP(ary) for(int i=0;i<N;i++)printf("%d",ary[i]);
#define DISPRES(ary) for(int i=0;i<N*N;i++)printf("%c",ary[i]);

struct point {
	int x;
	int y;
	int calc;
	int sign;
};
void calamul(char*,char* ,char*,struct point);
void padding(int*,int);
int  charToInt(char ch);
void dot_erase(char*);
void add(int *arg, int *arg2, int *res);
void multiple_one(int *arg, int arg2, int *res);
void multiple_complex(int*arg, int*arg2, int*res,int biglen);
void dot_insert(int* result, int dotlen);
int length(char*arg);
void reverse(char*arg);

void calamul(char* arg, char* arg2, char* res,struct point xyz) {
	int i;
	int kou1[N] = { 0 };
	int kou2[N] = { 0 };
	int result[N*N] = { 0 };
	int arg_dotflag = 0;
	int arg2_dotflag = 0;
	int arg_intlen = xyz.x;
	int arg2_intlen = xyz.y;
	int arg_dotlen;
	int arg2_dotlen;
	char *p_arg = arg;
	char *p_arg2=arg2;
	/*小数点チェック*/
	if (strchr(arg, '.') != NULL)
		arg_dotflag = 1;
	if (strchr(arg2, '.') != NULL)
		arg2_dotflag = 1;
	/*小数部分の長さを決める*/
	(arg_dotflag == 1) ? arg_dotlen = strlen(arg) - xyz.x - 1 : arg_dotlen = 0;
	(arg2_dotflag == 1) ? arg2_dotlen = strlen(arg2) - xyz.y - 1 : arg2_dotlen = 0;

	/* charをint型配列に格納 (小数点は含めない)*/
	for (i = 0; *p_arg != '\0'; p_arg++) {
		if (*p_arg == '.') 
			continue;
		kou1[i] = charToInt(*p_arg);
		i++;
	}
	for (i = 0; *p_arg2 != '\0'; p_arg2++) {
		if (*p_arg2 == '.')
			continue;
		kou2[i] = charToInt(*p_arg2);
		i++;
	}
	/* 小数点を考慮して長さを決める */
	int arglen = strlen(arg+arg_dotflag);
	int arg2len = strlen(arg2+arg2_dotflag);
	/* 右詰めにする */
	padding(kou1, arglen);
	padding(kou2, arg2len);
	
	multiple_complex(kou1, kou2, result,N);
	
	/*小数点合わせ*/
	printf("小数部分:%d\n", arg_dotlen);
	printf("小数部分:%d\n", arg2_dotlen);
	dot_insert(result,arg_dotlen+arg2_dotlen);
	/* char型に変換し,結果を逆順に格納 */
	for (int tes = 0,i=N*N-1; tes < N*N; tes++,i--) {
		res[tes] = (char)result[i] + 48;
	}
	int dot=0;
	int end = 0;
	/*dotの位置を割り出す*/
	for (int i = 0; *(res + i) != '\0'; i++) {
		if (*(res + i) == '.') {
			dot = i;
			break;
		}
	}
	/*小数点以下がない場合*/
	if (dot == 0){
		for (int i = N*N-1; i>= 0; i--) {
			if (*(res + i) != '0') {
				end = i;
				break;
			}
		}
	}
	else {
		if (*(res + dot+1) == '0')
			end = i;
		else {
			for (int i = dot; *(res + i) != '\0'; i++) {
				if (*(res + i) == '0') {
					end = --i;
					break;
				}
			}
		}
	}
	char*str;
	if (end == 0)
		str = NULL;
	else
		str = (char*)malloc(sizeof(char)*(end+1));
	strncpy(str,res, end+1);
	str[end + 1] = '\0';
	/*クリア*/
	for (int i = 0; i < N*N; i++) {
		res[i] = '0';
	}
	reverse(str);
	sprintf(res, str);
}

void reverse(char*arg) {
	int len = length(arg);
	int i, j;
	for (i = len - 1,j=0; j < len / 2;j++ ,i--) {
		char tmp = *(arg + i);
		*(arg + i) = *(arg + j);
		*(arg + j) = tmp;
	}
}
int length(char*arg) {
	int i = 0;
	while (*(arg+i) != '\0') {
		i++;
	}
	return i;
}
int charToInt(char ch) {
	return ch - 48;
}
void dot_insert(int* result,int dotlen) {
	int i, j=1;
	int flag = 0;
	if (dotlen <= 0)
		return;
	for (i = 1; i <= N*N - dotlen - 1; i++) {
		result[i - 1] = result[i];
		if (i == N*N - dotlen - 1) 
			result[i] = -2;
	}
}
void padding(int* ary,int len) {
	int i,j=1;
	for (i = len - 1; i >= 0; i--,j++) {
		int tmp = ary[N - j];
		ary[N - j] = ary[i];
		ary[i] = tmp;
	}
}
void add(int *arg,int *arg2,int *res) {
	int i, j=N*N-1,carry=0;
	for (i = N*N-1; i >=0 ; i--) {
		int ans = arg[i] + arg2[i] + carry;
		if (ans / 10)
			carry = ans / 10;
		else
			carry = 0;
		res[j--] = ans % 10;
	}
	res[0] = carry;
}
void multiple_one(int *arg, int arg2, int* res) {
	/* padding関数を通した後でないと正常に動作しない */
	/* 配列の右から順に計算する */
	int i,j=N*N-1,carry=0;
	for (i = N - 1; i >= 0; i--) {
		int ans = arg[i] * arg2 + carry;
		if (ans / 10)
			carry = ans / 10;
		else
			carry = 0;
		res[j--] = ans % 10;
	}
}
/*arg1とarg2の大きい方の長さを入れる*/
void multiple_complex(int*arg, int*arg2, int*res,int biglen){
	int i, j,k,mov=0;
	int tmp[N*N] = { 0 };
	for (i = 1; i<=N; i++) {
		multiple_one(arg, *(arg2 + N - i), tmp);
		/*ずらす処理*/
		for (k = 1; k <= mov; k++) {
			for (j = 1; j < N*N ; j++) {
				tmp[j - 1] = tmp[j];
				tmp[j] = 0;
			}
		}
		add(tmp, res, res);
		mov++;
	}
}

multi.c

#include "Header.h"

int main()
{
	char arg1[N] = "1.190";
	char arg2[N] = "1.22";
	char res[N*N] = { 0 };
	struct point xyz = { 1, 1, 0, 0 };
	calamul(arg1,arg2,res,xyz);
	printf("%s", res);
	getchar();
	return 0;
}

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多倍長演算:乗算