C语言字符串中的Base64编码与解码

在C语言中，实现Base64编码和解码可以通过以下步骤进行：

1. 为编码和解码创建字符映射表。
2. 编写Base64编码函数。
3. 编写Base64解码函数。

以下是一个简单的示例：

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

// Base64字符映射表
const char *base64_chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

// Base64编码函数
char *base64_encode(const unsigned char *input, int length) {
    int i, j;
    int encoded_length = 4 * ((length + 2) / 3);
    char *encoded = (char *)malloc(encoded_length + 1);
    if (encoded == NULL) return NULL;

    for (i = 0, j = 0; i< length;) {
        int octet_a = i< length ? input[i++] : 0;
        int octet_b = i< length ? input[i++] : 0;
        int octet_c = i< length ? input[i++] : 0;

        int triple = (octet_a << 0x10) + (octet_b << 0x08) + octet_c;

        encoded[j++] = base64_chars[(triple >> 18) & 0x3F];
        encoded[j++] = base64_chars[(triple >> 12) & 0x3F];
        encoded[j++] = base64_chars[(triple >> 6) & 0x3F];
        encoded[j++] = base64_chars[(triple >> 0) & 0x3F];
    }

    for (i = 0; i < (3 - (length % 3)) % 3; i++) {
        encoded[encoded_length - 1 - i] = '=';
    }

    encoded[encoded_length] = '\0';
    return encoded;
}

// Base64解码函数
unsigned char *base64_decode(const char *input, int *output_length) {
    int i, j;
    int decoded_length = strlen(input) * 3 / 4;
    unsigned char *decoded = (unsigned char *)malloc(decoded_length + 1);
    if (decoded == NULL) return NULL;

    for (i = 0, j = 0; i < strlen(input);) {
        int sextet_a = i < strlen(input) ? strchr(base64_chars, input[i++]) - base64_chars : 0;
        int sextet_b = i < strlen(input) ? strchr(base64_chars, input[i++]) - base64_chars : 0;
        int sextet_c = i < strlen(input) ? strchr(base64_chars, input[i++]) - base64_chars : 0;
        int sextet_d = i < strlen(input) ? strchr(base64_chars, input[i++]) - base64_chars : 0;

        int triple = (sextet_a << 18) + (sextet_b << 12) + (sextet_c << 6) + sextet_d;

        if (j < decoded_length) decoded[j++] = (triple >> 16) & 0xFF;
        if (j < decoded_length) decoded[j++] = (triple >> 8) & 0xFF;
        if (j < decoded_length) decoded[j++] = (triple >> 0) & 0xFF;
    }

    while (decoded[--j] == '=') {
        decoded_length--;
    }

    decoded[decoded_length] = '\0';
    *output_length = decoded_length;
    return decoded;
}

int main() {
    const char *input = "Hello, World!";
    int length = strlen(input);

    // Base64编码
    char *encoded = base64_encode((unsigned char *)input, length);
    printf("Base64编码: %s\n", encoded);

    // Base64解码
    int output_length;
    unsigned char *decoded = base64_decode(encoded, &output_length);
    printf("Base64解码: %.*s\n", output_length, decoded);

    free(encoded);
    free(decoded);

    return 0;
}

这个示例中，我们首先定义了一个Base64字符映射表，然后实现了Base64编码和解码函数。在main函数中，我们对一个字符串进行编码和解码操作，并输出结果。