将这个程序加入减法，乘法和除法

好的，以下是一个包含加法、减法、乘法和除法的16进制计算器的C语言代码：

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

#define MAX_LEN 100

int hex_to_dec(char c) {
    if (isdigit(c)) {
        return c - '0';
    } else if (c >= 'a' && c <= 'f') {
        return c - 'a' + 10;
    } else if (c >= 'A' && c <= 'F') {
        return c - 'A' + 10;
    } else {
        return -1; // error
    }
}

char dec_to_hex(int n) {
    if (n >= 0 && n <= 9) {
        return n + '0';
    } else if (n >= 10 && n <= 15) {
        return n - 10 + 'a';
    } else {
        return '?'; // error
    }
}

void reverse_str(char* s) {
    int len = strlen(s);
    for (int i = 0, j = len - 1; i < j; i++, j--) {
        char temp = s[i];
        s[i] = s[j];
        s[j] = temp;
    }
}

char* add_hex(char* a, char* b) {
    int len_a = strlen(a), len_b = strlen(b), len_c = 0;
    char* c = (char*) malloc(sizeof(char) * (MAX_LEN + 1));
    int carry = 0;

    while (len_a > 0 || len_b > 0 || carry > 0) {
        int x = (len_a > 0) ? hex_to_dec(a[--len_a]) : 0;
        int y = (len_b > 0) ? hex_to_dec(b[--len_b]) : 0;
        int z = x + y + carry;
        carry = z / 16;
        c[len_c++] = dec_to_hex(z % 16);
    }
    c[len_c] = '\0';

    reverse_str(c);
    return c;
}

char* subtract_hex(char* a, char* b) {
    int len_a = strlen(a), len_b = strlen(b), len_c = 0;
    char* c = (char*) malloc(sizeof(char) * (MAX_LEN + 1));
    int borrow = 0;

    while (len_a > 0 || len_b > 0) {
        int x = (len_a > 0) ? hex_to_dec(a[--len_a]) : 0;
        int y = (len_b > 0) ? hex_to_dec(b[--len_b]) : 0;
        int z = x - y - borrow;
        if (z < 0) {
            z += 16;
            borrow = 1;
        } else {
            borrow = 0;
        }
        c[len_c++] = dec_to_hex(z);
    }
    c[len_c] = '\0';

    reverse_str(c);
    return c;
}

char* multiply_hex(char* a, char* b) {
    int len_a = strlen(a), len_b = strlen(b);
    char* c = (char*) calloc(MAX_LEN + 1, sizeof(char));

    for (int i = len_a - 1; i >= 0; i--) {
        int carry = 0;
        for (int j = len_b - 1; j >= 0; j--) {
            int x = hex_to_dec(a[i]);
            int y = hex_to_dec(b[j]);
            int z = x * y + carry + hex_to_dec(c[i + j + 1]);
            carry = z / 16;
            c[i + j + 1] = dec_to_hex(z % 16);
        }
        c[i] = dec_to_hex(carry);
    }

    reverse_str(c);
    return c;
}

char* divide_hex(char* a, char* b) {
    int len_a = strlen(a), len_b = strlen(b);
    char* c = (char*) calloc(MAX_LEN + 1, sizeof(char));
    if (strcmp(b, "0") == 0) { // handle division by zero
        strcpy(c, "error");
        return c;
    }

    char* remainder = (char*) calloc(len_b + 1, sizeof(char));
    strcpy(remainder, a);
    for (int i = 0; i <= len_a - len_b; i++) {
        int q = 0;
        while (strcmp(remainder, b) >= 0) {
            char* temp = subtract_hex(remainder, b);
            strcpy(remainder, temp);
            free(temp);
            q++;
        }
        c[i] = dec_to_hex(q);
        if (i < len_a - len_b) {
            char temp[MAX_LEN + 1] = "";
            strcat(temp, remainder);
            strcat(temp, &a[i + len_b]);
            strcpy(remainder, temp);
        }
    }

    reverse_str(c);
    return c;
}

int main() {
    char a[MAX_LEN + 1], b[MAX_LEN + 1];
    printf("Enter two hexadecimal numbers:\n");
    scanf("%s %s", a, b);

    char* c = add_hex(a, b);
    printf("The sum is: %s\n", c);
    free(c);

    c = subtract_hex(a, b);
    printf("The difference is: %s\n", c);
    free(c);

    c = multiply_hex(a, b);
    printf("The product is: %s\n", c);
    free(c);

    c = divide_hex(a, b);
    printf("The quotient is: %s\n", c);
    free(c);

    return 0;
}

这个程序实现了四种16进制计算：加法、减法、乘法和除法。在 `subtract_hex` 函数中，我们采用了借位法实现16进制数的减法。在 `multiply_hex` 函数中，我们采用了竖式乘法的方法实现16进制数的乘法。在 `divide_hex` 函数中，我们采用了长除法的方法实现16进制数的除法。如果除数为0，我们将返回 "error"。

需要注意的是，输入的两个16进制数必须以字符串形式输入，并且长度不能超过 `MAX_LEN`。