用C语言实现PHP的base64_encode函数

最近写的程序要用到base64_encode，网上一搜“C语言 base64”出来一堆代码，但是我向来是不惮以最坏的恶意来推测这些代码的。于是看了一下PHP源码，base64_encode函数定义在ext\standard\base64.c文件中。我把它改写了一下，需要注意内存是动态分配的，使用完之后记得free掉，否则会造成内存泄露。

unsigned char *base64_encode(const unsigned char *str)
{
    static const char base64_table[] =
    { 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M',
      'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',
      'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm',
      'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',
      '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '+', '/', '\0'
    };
    static const char base64_pad = '=';
    const unsigned char *current = str;
    int length = strlen(str);
    unsigned char *p;
    unsigned char *result;

    if ((length + 2) < 0 || ((length + 2) / 3) >= (1 << (sizeof(int) * 8 - 2))) {
        return NULL;
    }

    result = (unsigned char *)malloc(((length + 2) / 3) * 4 * sizeof(char) + 1);
    p = result;

    while (length > 2) { 
        *p++ = base64_table[current[0] >> 2];
        *p++ = base64_table[((current[0] & 0x03) << 4) + (current[1] >> 4)];
        *p++ = base64_table[((current[1] & 0x0f) << 2) + (current[2] >> 6)];
        *p++ = base64_table[current[2] & 0x3f];

        current += 3;
        length -= 3; 
    }

    if (length != 0) {
        *p++ = base64_table[current[0] >> 2];
        if (length > 1) {
            *p++ = base64_table[((current[0] & 0x03) << 4) + (current[1] >> 4)];
            *p++ = base64_table[(current[1] & 0x0f) << 2];
            *p++ = base64_pad;
        } else {
            *p++ = base64_table[(current[0] & 0x03) << 4];
            *p++ = base64_pad;
            *p++ = base64_pad;
        }
    }
    *p = '\0';
    return result;
}