C语言实现base64编解码

本文写给需要用到base64编解码的朋友们，可以作为工具文章，在需要时查阅。

关于什么是base64编解码，度娘比我解释的更清楚。这里只说一下，在一些二进制数据或不可见字符操作不便的情况下（例如http请求参数等），可将这类二进制数据通过base64编码转换为可见字符。当然有编码自然也有对等的解码恢复原始数据。

闲话少叙，代码奉上：

/*
 * Author: 码哥比特
 */
#include <stdio.h>
#include <stdlib.h>

static char baseMap[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

int base64_encode(unsigned char *in, size_t inlen, unsigned char **out, size_t *outlen)
{
    size_t i, state, j;

    *outlen = inlen / 3 * 4;
    if (inlen % 3) (*outlen) += 4;

    *out = (unsigned char *)calloc(1, *outlen + 1);
    if (*out == NULL) return -1;

    unsigned char *o = *out;

    for (i = 0, state = 0, j = 0; i < inlen; ++j) {
        if (state == 0) {
            o[j] = (unsigned char)baseMap[(in[i] >> 2) & 0x3f];
            state = 1;
        } else if (state == 1) {
            if (i+1 >= inlen) {
                o[j++] = (unsigned char)baseMap[(in[i] & 0x3) << 4];
                o[j++] = (unsigned char)'=';
                o[j] = (unsigned char)'=';
                break;
            } else {
                o[j] = (unsigned char)baseMap[((in[i] & 0x3) << 4)|((in[i+1] >> 4) & 0xf)];
                ++i;
                state = 2;
            }
        } else {
            if (i+1 >= inlen) {
                o[j++] = (unsigned char)baseMap[((in[i] & 0xf) << 2)];
                o[j] = (unsigned char)'=';
                break;
            } else {
                o[j++] = (unsigned char)baseMap[((in[i] & 0xf) << 2)|((in[i+1] >> 6) & 0x3)];
                o[j] = (unsigned char)baseMap[in[++i] & 0x3f];
                ++i;
                state = 0;
            }
        }
    }

    return 0;
}

int base64_decode(unsigned char *in, size_t inlen, unsigned char **out, size_t *outlen)
{
    if (inlen % 4) return -1;

    unsigned char ascii[256] = {0};
    size_t i, state, j;

    for (i = 0; i < 64; ++i) {
        ascii[(unsigned char)baseMap[i]] = i;
    }

    *outlen = inlen / 4 * 3;
    if (in[inlen-1] == '=') --(*outlen);
    if (in[inlen-2] == '=') --(*outlen);

    *out = (unsigned char *)calloc(1, *outlen + 1);
    if (*out == NULL) return -1;
    unsigned char *o = *out;

    for (i = 0, state = 0, j = 0; i < inlen; ) {
        if (state == 0) {
            o[j] = (ascii[in[i]] << 2);
            state = 1;
            ++i;
        } else if (state == 1) {
            o[j++] |= ((ascii[in[i]] >> 4) & 0x3);
            state = 2;
        } else if (state == 2) {
            o[j] = (ascii[in[i]] & 0xf) << 4;
            state = 3;
            ++i;
        } else if (state == 3) {
            if (in[i] == '=') break;
            o[j++] |= ((ascii[in[i]] >> 2) & 0xf);
            state = 4;
        } else if (state == 4) {
            o[j] = (ascii[in[i]] & 0x3) << 6;
            state = 5;
            ++i;
        } else {
            if (in[i] == '=') break;
            o[j++] |= (ascii[in[i]] & 0x3f);
            state = 0;
            ++i;
        }
    }

    return 0;
}

void base64_free(unsigned char *data)
{
    if (data == NULL) return;
    free(data);
}

encode/decode函数的参数1、2为输入内容，参数3、4为编解码的结果部分。

如果编解码成功返回值为0，否则返回值为-1。

编解码成功时，参数三将返回一个动态分配的内存空间，因此封装了一个base64_free函数用来释放该部分内存。读者使用时，也可以根据需要将分配和释放内存的方式进行修改。