程序代码：

#include <stdio.h>
#include <string.h>

#define SIZE 100

void Inverse(char *a, int len)
{    //将字符串逆序;len = strlrn(a)
    char temp;int i;
    for (i = 0;i < --len;++i)
        temp = a[i], a[i] = a[len], a[len] = temp;
}
int compare(char *a, char *b)
{    //判断 a < b ?
    int a_len = strlen(a);
    int b_len = strlen(b);
    if (a_len == b_len)
        return strcmp(a, b) < 0;
    else
        return a_len < b_len;
}
void change(char *a)
{    //去掉字符串前的 '0'
    int m = 0, n = 0;
    while (a[m] && a[m++] == '0');--m;             //求出串前 '0' 个数，如果是 全'0'，m++会短路
    while (a[n] = a[m+n])    ++n;                  //移动
}
void Add(char *a, char *b, char *c)
{    //c = a + b;
    int Result, i = 0;
    int a_len = strlen(a);
    int b_len = strlen(b);

    memset(c, 0, a_len > b_len ? a_len + 2 : b_len + 2);     //将 c置空
    for (--b_len;--a_len >= 0 && b_len >= 0;--b_len)
    {
        Result = a[a_len]-'0' + b[b_len]-'0' + c[i];
        c[i++] = Result % 10 + '0';
        c[i] = (Result /= 10);
    }
    while (a_len >= 0)       
    {   //说明 a > b, 把 a剩余部分加进去
        Result = a[a_len--]-'0' + c[i];
        c[i++] = Result % 10 + '0';
        c[i] = (Result /= 10);
    }
    while (b_len >= 0)
    {   //说明 b > a
        Result = b[b_len--]-'0' + c[i];
        c[i++] = Result % 10 + '0';
        c[i] = (Result /= 10);
    }
    if (Result)    c[i++] += '0'; //若有进位，特殊处理
    Inverse(c, i);
}
void Sub(char *a, char *b, char *c)
{    //c = a - b;前提是 a > b
    int a_len, n = 0, b_len = strlen(b);
    for (a_len = 0;c[a_len] = a[a_len];++a_len);     //strcpy（c, a）&& a_len = strlen(a);
    while (--a_len, --b_len >= 0)
    {
        c[n = a_len] -= b[b_len] - '0';
        while (c[n] < '0')    c[n] += 10, --c[--n];
    }
    change(c);
}
void Subtract(char *a, char *b, char *c)
{    //c = a - b;
    int len;
    if (compare(a, b))
    {
        Sub(b, a, c);
        len = strlen(c);
        Inverse(c, len);
        c[len] = '-', c[len+1] = '\0';
        Inverse(c, len+1);
    }
    else    Sub(a, b, c);
}
void Multiply(char *a, char *b, char *c)
{    //c = a * b;模拟列竖式乘法运算
    int Result, i, j;
    int a_len = strlen(a);
    int b_len = strlen(b);
    int len = a_len + b_len;     // c最多有len位

    memset(c, 0, len + 1);
    for (i = a_len-1;i >= 0;--i)
    for (j = b_len-1;j >= 0;--j)
    {
        Result = (a[i]-'0') * (b[j]-'0') + c[len-2-i-j];
        c[len-2-i-j] = Result % 10;
        c[len-1-i-j] += Result / 10;
    }
    for (i = 0;i < a_len+b_len;c[i++] += '0');
    if (i && c[i-1] != '0')    c[i] = '\0';
    else    c[--i] = '\0';

    Inverse(c, i);
}
void Divide(char *a, char *b, char *c)
{    //c = a / b;模拟列竖式除法运算
    int i, j = 0, m, n, o;
    int a_len = strlen(a);
    int b_len = strlen(b);
    char aa[SIZE], bb[9][SIZE];

    memset(c, 0, a_len+1);     //c最多有a_len位
    aa[0] = bb[0][0] = '0';aa[1] = bb[0][1] = '\0';         //在a前添加字符‘0’，存入aa
    for (strcat(aa, a), strcat(bb[0], b), i = 1;i < 9;++i)
        Add(bb[i-1], b, bb[i]);                             //bb[i] = (i+1) * b

    for (i = 0;i < a_len-b_len+1;++i)
    {
        for (j = 0;strncmp(&aa[i], bb[j], b_len+1) >= 0 && j < 9;++j);
        if (c[i] = j + '0', !j)    continue;             //j = 0时，不用减
        for (m = i+b_len, n = b_len;n >= 0;--m, --n)
        {
            aa[o = m] -= bb[j-1][n] - '0';
            while (aa[o] < '0')    aa[o] += 10, --aa[--o];
        }
    }

    if (i)    change(c);     //i不为 0时， 将 c前的‘0’去掉
    else    strcpy(c, "0");  //i 为 0时，必有a < b, 结果为 0
}
void Remainder(char *a, char *b, char *c)
{    //c = a % b;直接从 Divide 那复制过来的，额
    int i, j = 0, m, n, o;
    int a_len = strlen(a);
    int b_len = strlen(b);
    char bb[9][SIZE];

    memset(c, 0, a_len+1);
    c[0] = bb[0][0] = '0';bb[0][1] = '\0';     //上面的 aa最后就是余数了，这里直接把aa改成了 c
    for (strcat(c, a), strcat(bb[0], b), i = 1;i < 9;++i)
        Add(bb[i-1], b, bb[i]);

    for (i = 0;i < a_len-b_len+1;++i)
    {
        for (j = 0;strncmp(&c[i], bb[j], b_len+1) >= 0 && j < 9;++j);
        if (!j)    continue;
        for (m = i+b_len, n = b_len;n >= 0;--m, --n)
        {
            c[o = m] -= bb[j-1][n] - '0';
            while (c[o] < '0')    c[o] += 10, --c[--o];
        }
    }
    change(c);
}
int main()
{
    char a[SIZE], b[SIZE], c[SIZE*2-1];
    puts("Input a:");gets(a);
    puts("Input b:");gets(b);

    printf("\nCompare:\n%s %s %s\n", a, compare(a, b) ? "<" : ">=", b);
    Add(a, b, c);        printf("\nAdd:\n%s + %s = %s\n", a, b, c);
    Subtract(a, b, c);    printf("\nSubtract:\n%s - %s = %s\n", a, b, c);
    Multiply(a, b, c);    printf("\nMultiply:\n%s * %s = %s\n", a, b, c);
    Divide(a, b, c);    printf("\nDivide:\n%s / %s = %s\n", a, b, c);
    Remainder(a, b, c);    printf("\nRemainder:\n%s %% %s = %s\n", a, b, c);
    puts("");

    return 0;
}