如何在ctype中将无符号字节数组转换为base64字符串

#include <ctype.h>

char* unsignedByteArrayToBase64(const unsigned char* input, size_t length) {
    size_t encodedLength = 4 * ((length + 2) / 3);  // 计算编码后字符串的长度
    char* output = malloc(encodedLength + 1);  // 分配足够的内存来存储编码后的字符串
    int i, j;

    for (i = 0, j = 0; i < length; i += 3, j += 4) {
        unsigned char byte1 = input[i];
        unsigned char byte2 = (i + 1 < length) ? input[i + 1] : 0;
        unsigned char byte3 = (i + 2 < length) ? input[i + 2] : 0;

        output[j] = isalnum(byte1 >> 2) ? byte1 >> 2 : '=';
        output[j + 1] = isalnum(((byte1 & 0x3) << 4) | (byte2 >> 4)) ? ((byte1 & 0x3) << 4) | (byte2 >> 4) : '=';
        output[j + 2] = isalnum(((byte2 & 0xF) << 2) | (byte3 >> 6)) ? ((byte2 & 0xF) << 2) | (byte3 >> 6) : '=';
        output[j + 3] = isalnum(byte3) ? byte3 : '=';
    }

    output[encodedLength] = '\0';  // 在字符串末尾添加终止符

    return output;
}

unsigned char byteArray[] = { 0x54, 0x65, 0x73, 0x74 };  // 输入无符号字节数组
size_t byteArrayLength = sizeof(byteArray) / sizeof(byteArray[0]);  // 计算字节数组长度

char* base64String = unsignedByteArrayToBase64(byteArray, byteArrayLength);

printf("Base64字符串：%s\n", base64String);

free(base64String);  // 释放内存