用C语言编写一个大整数与大整数相乘的小程序。定义两个数组，数组a和数组b，分别存放两个大整数，数组的每一个元素存放四位数，将大整数与大整数的乘积存放数组c中。

在C语言中，由于标准库并不直接支持大整数运算，我们可以使用自定义的数据结构和算法来处理这个问题。下面是一个简单的示例，使用了数组来模拟每一位的乘法和进位过程：

#include <stdio.h>
#include <stdlib.h>
#define MAX_DIGITS 4

typedef struct {
    int digits[MAX_DIGITS];
    int size;
} BigInt;

// 初始化大整数
BigInt* initBigInt(int value) {
    BigInt* bigInt = malloc(sizeof(BigInt));
    bigInt->size = 1;
    while (value) {
        bigInt->digits[bigInt->size - 1] = value % 10000;
        value /= 10000;
        bigInt->size++;
    }
    return bigInt;
}

// 取消初始化
void freeBigInt(BigInt* bigInt) {
    free(bigInt);
}

// 相加两个大整数
BigInt* addBigInts(BigInt* a, BigInt* b) {
    BigInt* result = malloc(sizeof(BigInt));
    result->size = max(a->size, b->size) + 1;
    result->digits[0] = 0;
    for (int i = 0; i < a->size; i++) {
        result->digits[i] += a->digits[i];
        if (result->digits[i] >= 10000) {
            result->digits[i] -= 10000;
            if (i < b->size)
                result->digits[i + 1]++;
        }
    }
    for (int i = a->size; i < b->size; i++) {
        result->digits[i] += b->digits[i];
        if (result->digits[i] >= 10000) {
            result->digits[i] -= 10000;
            result->digits[i + 1]++;
        }
    }
    return result;
}

// 相乘两个大整数
BigInt* multiplyBigInts(BigInt* a, BigInt* b) {
    BigInt* result = initBigInt(0);
    for (int i = 0; i < a->size; i++) {
        BigInt* temp = initBigInt(0);
        for (int j = 0; j < b->size; j++) {
            temp = addBigInts(temp, initBigInt(a->digits[i] * b->digits[j]));
        }
        result = addBigInts(result, temp);
        freeBigInt(temp);
    }
    return result;
}

int main() {
    int a[] = {1, 2, 3, 4}; // 1234
    int b[] = {5, 6, 7, 8}; // 5678
    BigInt* bigA = initBigInt(a[0]);
    for (int i = 1; i < 4; i++)
        bigA = addBigInts(bigA, initBigInt(a[i]));

    BigInt* bigB = initBigInt(b[0]);
    for (int i = 1; i < 4; i++)
        bigB = addBigInts(bigB, initBigInt(b[i]));

    BigInt* product = multiplyBigInts(bigA, bigB);
    printf("Product: ");
    for (int i = product->size - 1; i >= 0; i--) {
        printf("%d", product->digits[i]);
    }
    printf("\n");

    freeBigInt(product);
    freeBigInt(bigA);
    freeBigInt(bigB);

    return 0;
}