base64解码函数

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我是靠谱客的博主爱听歌小馒头，最近开发中收集的这篇文章主要介绍base64解码函数，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

int base64_decode(const void *src, size_t src_size,
size_t *src_pos_r, buffer_t *dest)
{
const char *src_c = src;
size_t src_pos;
unsigned char input[4], output[3];
int ret = 1;
char * word = "=";
for (src_pos = 0; src_pos+3 < src_size; ) {
input[0] = b64dec[src_c[src_pos]];
if (input[0] == 0xff) {
if (unlikely(!IS_EMPTY(src_c[src_pos]))) {
ret = -1;
while(input[0] == 0xff){
src_pos++;
input[0] = b64dec[src_c[src_pos]];
}
}else{
src_pos++;
continue;
}
}

input[1] = b64dec[src_c[src_pos+1]];
while (unlikely(input[1] == 0xff) ) {
if (memcmp(&src_c[src_pos+1], word, 1)){
ret = -1;
src_pos++;
input[1] = b64dec[src_c[src_pos+1]];
}
else
{
goto out;
}
}
output[0] = (input[0] << 2) | (input[1] >> 4);

input[2] = b64dec[src_c[src_pos+2]];
while (input[2] == 0xff ) {
if ( memcmp(&src_c[src_pos+2] ,word, 1) && memcmp(&src_c[src_pos+3], word, 1)) {
ret = -1;
src_pos++;
input[2] = b64dec[src_c[src_pos+2]];
continue;
}
buffer_append(dest, output, 1);
ret = 0;
src_pos += 4;
goto out;
}

output[1] = (input[1] << 4) | (input[2] >> 2);
input[3] = b64dec[src_c[src_pos+3]];
while (input[3] == 0xff ) {
if (memcmp(&src_c[src_pos+3],word, 1)) {
ret = -1;
src_pos++;
input[3] = b64dec[src_c[src_pos+3]];
continue;
}
buffer_append(dest, output, 2);
ret = 0;
src_pos += 4;
goto out;
}

output[2] = ((input[2] << 6) & 0xc0) | input[3];
buffer_append(dest, output, 3);
src_pos += 4;
}
out:
for (; src_pos < src_size; src_pos++) {
if (!IS_EMPTY(src_c[src_pos]))
break;
}

if (src_pos_r != NULL)
*src_pos_r = src_pos;

return ret;
}