C语言base64函数

C语言中没有内置的base64函数，需要自己实现。

以下是一个简单的实现：

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

static const char base64_table[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

int base64_encode(const unsigned char *src, size_t len, char *dst, size_t dst_len)
{
    size_t i, j;
    unsigned char buf[4];
    int padding = 0;

    for (i = 0, j = 0; i < len; i += 3, j += 4) {
        buf[0] = src[i];
        buf[1] = (i + 1 < len) ? src[i + 1] : 0;
        buf[2] = (i + 2 < len) ? src[i + 2] : 0;

        dst[j] = base64_table[buf[0] >> 2];
        dst[j + 1] = base64_table[((buf[0] & 0x03) << 4) | (buf[1] >> 4)];
        dst[j + 2] = (i + 1 < len) ? base64_table[((buf[1] & 0x0f) << 2) | (buf[2] >> 6)] : '=';
        dst[j + 3] = (i + 2 < len) ? base64_table[buf[2] & 0x3f] : '=';

        if (dst[j + 2] == '=')
            padding++;
        if (dst[j + 3] == '=')
            padding++;
    }

    dst[j] = '\0';
    return padding;
}

int base64_decode(const char *src, size_t len, unsigned char *dst, size_t dst_len)
{
    size_t i, j;
    unsigned char buf[4];
    int padding = 0;

    for (i = 0, j = 0; i < len; i += 4, j += 3) {
        buf[0] = (src[i] >= 'A' && src[i] <= 'Z') ? src[i] - 'A' :
                 (src[i] >= 'a' && src[i] <= 'z') ? src[i] - 'a' + 26 :
                 (src[i] >= '0' && src[i] <= '9') ? src[i] - '0' + 52 :
                 (src[i] == '+') ? 62 :
                 (src[i] == '/') ? 63 : -1;

        buf[1] = (src[i + 1] >= 'A' && src[i + 1] <= 'Z') ? src[i + 1] - 'A' :
                 (src[i + 1] >= 'a' && src[i + 1] <= 'z') ? src[i + 1] - 'a' + 26 :
                 (src[i + 1] >= '0' && src[i + 1] <= '9') ? src[i + 1] - '0' + 52 :
                 (src[i + 1] == '+') ? 62 :
                 (src[i + 1] == '/') ? 63 : -1;

        buf[2] = (src[i + 2] >= 'A' && src[i + 2] <= 'Z') ? src[i + 2] - 'A' :
                 (src[i + 2] >= 'a' && src[i + 2] <= 'z') ? src[i + 2] - 'a' + 26 :
                 (src[i + 2] >= '0' && src[i + 2] <= '9') ? src[i + 2] - '0' + 52 :
                 (src[i + 2] == '+') ? 62 :
                 (src[i + 2] == '/') ? 63 : -1;

        buf[3] = (src[i + 3] >= 'A' && src[i + 3] <= 'Z') ? src[i + 3] - 'A' :
                 (src[i + 3] >= 'a' && src[i + 3] <= 'z') ? src[i + 3] - 'a' + 26 :
                 (src[i + 3] >= '0' && src[i + 3] <= '9') ? src[i + 3] - '0' + 52 :
                 (src[i + 3] == '+') ? 62 :
                 (src[i + 3] == '/') ? 63 : -1;

        if (buf[0] == -1 || buf[1] == -1 || buf[2] == -1 || buf[3] == -1)
            return -1;

        dst[j] = (buf[0] << 2) | (buf[1] >> 4);
        dst[j + 1] = (buf[1] << 4) | (buf[2] >> 2);
        dst[j + 2] = (buf[2] << 6) | buf[3];

        if (src[i + 2] == '=')
            padding++;
        if (src[i + 3] == '=')
            padding++;
    }

    return j - padding;
}

其中，`base64_encode`函数用于将二进制数据编码为base64字符串，`base64_decode`函数用于将base64字符串解码为二进制数据。这两个函数都接受指向源数据和目标数据的指针，以及数据长度和目标缓冲区长度。如果目标缓冲区长度不足以容纳编码/解码结果，则返回-1。在编码时，函数还会返回填充字符的数量，以便在解码时正确处理。