MQTT 单片机端讲解

/**
 * Decodes the message length according to the MQTT algorithm
 * @param getcharfn pointer to function to read the next character from the data source
 * @param value the decoded length returned
 * @return the number of bytes read from the socket
 */
int MQTTPacket_decode(int (*getcharfn)(unsigned char*, int), int* value)
{
    unsigned char c;
    int multiplier = 1;
    int len = 0;
#define MAX_NO_OF_REMAINING_LENGTH_BYTES 4 //读取更多数据

    FUNC_ENTRY;
    *value = 0;
    do
    {
        int rc = MQTTPACKET_READ_ERROR;

        if (++len > MAX_NO_OF_REMAINING_LENGTH_BYTES)
        {
            rc = MQTTPACKET_READ_ERROR;    /* bad data */
            goto exit;
        }
//        rc = (*getcharfn)(&c, 1);
//        if (rc != 1)
//            goto exit;
        *value += (c & 127) * multiplier;
        multiplier *= 128;
    } while ((c & 128) != 0);
exit:
    FUNC_EXIT_RC(len);
    return len;
}

/**
 * Encodes the message length according to the MQTT algorithm
 * @param buf the buffer into which the encoded data is written
 * @param length the length to be encoded
 * @return the number of bytes written to buffer
 */
int MQTTPacket_encode(unsigned char* buf, int length)
{
    int rc = 0;

    FUNC_ENTRY;
    do
    {
        char d = length % 128;
        length /= 128;
        /* if there are more digits to encode, set the top bit of this digit */
        if (length > 0)
            d |= 0x80;
        buf[rc++] = d;
    } while (length > 0);
    FUNC_EXIT_RC(rc);
    return rc;
}