How does the recv of TCP socket receive messages of specified length?

occasionally ,tcp socket You must receive the specified number of bytes to meet the expectation , Then how to achieve ？

It is introduced in many books of network programming , Let's see ( Always accept , Up to standard )：

int recvLength(char *sRecvBuffer, size_t iRecvLen)
{
    int iRet = checkSocket();
    if(iRet < 0)
    {
        return iRet;
    }

    size_t iRecvLeft = iRecvLen;
    iRecvLen = 0;

    TC_Epoller epoller(false);
    epoller.create(1);
    epoller.add(_socket.getfd(), 0, EPOLLIN);

    while(iRecvLeft != 0)
    {
        int iRetCode = epoller.wait(_timeout);
        if (iRetCode < 0)
        {
            _socket.close();
            return EM_SELECT;
        }
        else if (iRetCode == 0)
        {
            _socket.close();
            return EM_TIMEOUT;
        }

        epoll_event ev  = epoller.get(0);
        if(ev.events & EPOLLIN)
        {
            int len = _socket.recv((void*)(sRecvBuffer + iRecvLen), iRecvLeft);
            if (len < 0)
            {
                _socket.close();
                return EM_RECV;
            }
            else if (len == 0)
            {
                _socket.close();
                return EM_CLOSE;
            }

            iRecvLeft -= len;
            iRecvLen += len;
        }
        else
        {
            _socket.close();
            return EM_SELECT;
        }
    }

    return EM_SUCCESS;
}