code:

// These 3 will return false if the socket would block before reading in the max amount
bool GetPacketIDFromSocket(int& nPacketID);
bool GetPacketLenFromSocket(int& nPacketLen);
bool GetPacketFromSocket(char* szPacketBuffer, const int nMaxAmount, int& nActualAmountRead);

...
    while(try_readpacket())
    {
        do_packet();
    }
...
bool ClassName::try_readpacket()
{
    if(!m_bGotPacketId())
    {
        bool bWouldBlock = GetPacketIDFromSocket(m_CurrentPacket);
        if(bWouldBlock) return false;

        m_bGotPacketId = true;
    }

    if(!m_bGotPacketLen)
    {
        bool bWouldBlock = GetPacketLenFromSocket(m_PacketLen);
        if(bWouldBlock) return false;
    }

    int nAmountToRead = m_PacketLen - m_AmountRead;
    int nAmountActuallyRead = 0;
    bool bWouldBlock = GetPacketFromSocket(m_PacketBuffer[m_AmountRead], nAmountToRead, nAmountActuallyRead);
    if(bWouldBlock)
    {
        m_AmountRead += nAmountActuallyRead;
        return false;
    }

    // Got the entire packet
    m_AmountRead = 0;
    m_bGotPacketId = false;
    m_bGotPacketLen = false;
    return true;
}