OpenSSL read on non-blocking socket unexpected behavior

Question

I'm implementing HTTPS server using OpenSSL and I've faced some strange behavior while reading a data. The following code I guess is illustrating the issue:

char m_readBuffer[READ_BUFFER_SIZE];
std::vector<char> data;
bool readMore = true;
bool isError = false;

do
{
    SSL *ssl = m_sslClient[i];
    retval = SSL_read(ssl, m_readBuffer, READ_BUFFER_SIZE);
    if (retval <= 0)
    {
        int errorCode = SSL_get_error(ssl, retval);
        if (errorCode == SSL_ERROR_WANT_READ)
        {
            readMore = false;
            isError = false;
        }
        else
        {
            readMore = false;
            isError = true;
            CloseClient(i);
        }
    }
    else
    {
        data.insert(data.end(), m_readBuffer, m_readBuffer + retval);
    }
}
while(readMore == true);

if(isError == false)
{
    ProcessData(i, data);
}

The code works as expected, i.e. I read the data that a browser sends to me without any problem. But I've come to this code let's say experimentally. The code that boring me is the SSL_read return value. I can't (or I guess I can't) determine exactly when the data stream ended. The documentation says that SSL_ERROR_WANT_READ means that there is more data and I have to repeat reading. But by experience I came to the fact that if SSL_read return -1 and then SSL_get_error returns SSL_ERROR_WANT_READ this indicates that there in no more data.

Ok, that works but I'm worried that this may be working incorrectly.

Is there a right way to determine that I read out all the data?