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\subsection{The Network Reader}
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The network readers task is to read incoming messages for the SICS server.
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These may come in four main forms.
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\begin{itemize}
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\item connection requests
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\item commands
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\item interrupts
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through a command channel
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\item interrupt messages on the UDP port.
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\end{itemize}
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A prior
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version of SICS had a select system call for each of these cases. It was
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found, that the code spent most of its time in the select system call
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thus intrdoducing a major performance problem.
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The select system call can handle more then one file descriptor in one call.
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This is exactly what this module handles. It does a global select on
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all open sockets and forwards any pending data to approriate handlers.
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This scheme brought a drastic
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performance improvement.
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Each of the messages to SICS is handled
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differently:
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A connection request will be validated, a new connection object will be
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created and a new task for this connection object will be started.
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A command will be placed in the apropriate command stack for the task
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belonging to this connection to work on in a later stage. The netreader will
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also take care that all commands are complete, this is the terminator
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\verb+\n+ or \verb+\r+ has been sent.
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Both forms of interrupt will be interpreted and a suitable signal
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will be sent to all runing tasks if the interrupt request is valid.
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In order to perform his tasks the network reader needs to maintain a list of
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all open sockets and their types. Additionally it needs to know about the
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SICS tasker.
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The early version of SICS only supported connections on a plain
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socket. This feauture is not used any more in favour of the Telnet
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option described below. However, as this may be used to transport
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binary data, the obsolete code has not been removed.
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Support for the internet TCP/IP standard telnet protocoll has been added in
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January 1998. Telnet things need to be handled differently, therefore two
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more socket types have been defined: A telnet listen socket on which
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requests for telnet connections are listened for, and a telnet command
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socket which actually process the telnet messages.
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Support for user sockets was added in February 1998. User sockets provides a
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primitive interface to the network readers monitoring functions. This is
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part of a solution to one problem which can occur in SICS: The whole server
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is blocked if one of the serial devices is slow in responding. User sockets
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now provide a method for client code to register a socket for monitoring
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with the network reader and find out if data is pending at it. This support
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is provided by the functions: NetReadRegisterUserSocket,
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NetReadRemoveUserSocket, NetReadReadable and NetReadResetUser described
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below. NetReadWait4Data is a special wait function which waits for data to
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come in on a user socket. This function is problematic with dynamically
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creates and deleted objects such as environment device objects. Its use is
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therefore no longer recommended.
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Thus the interface looks like this:
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@d nrint @{
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/*--------------------------------------------------------------------------*/
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typedef struct __netreader *pNetRead;
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typedef enum {naccept, command, udp, user, taccept, tcommand} eNRType;
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/*--------------------------------------------------------------------------*/
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pNetRead CreateNetReader(pServer pServ, int iPasswdTimeout,
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int iReadTimeout);
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void DeleteNetReader(void *pData);
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/*--------------------------------------------------------------------------*/
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int NetReadRegister(pNetRead self, mkChannel *pSock, eNRType eType,
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SConnection *pCon);
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int NetReadRegisterUserSocket(pNetRead self, int iSocket);
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int NetReadRemove(pNetRead self, mkChannel *pSock);
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int NetReadRemoveUserSocket(pNetRead self, int iSocket);
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/*-------------------------------------------------------------------------*/
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int NetReaderTask(void *pReader);
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void NetReaderSignal(void *pUser, int iSignal, void *pSigData);
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int NetReadWait4Data(pNetRead self, int iSocket);
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int NetReadReadable(pNetRead self, int iSocket);
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int NetReadResetUser(pNetRead self, int iSocket);
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@}
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This starts off with the definition of a data type for the net reader and an
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enum which denotes the possible types of connections.
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\begin{description}
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\item[CreateNetReader] sets a new net reader up. Only parameter is a pointer
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to the task manager to use while creating new connections. iPasswdTimeout
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is the period in which a client is expected to send his userid/password
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pair. iReadTimeout is the time to wait on the select statement for a
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channel to become readable. Both time values are specified in milliseconds.
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\item[DeleteNetReader] deletes the net reader and all its data structures.
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Must be called in order to free all memory.
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\item[NetReadRegister] register the channel pSock of type eType with the net
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reader self. The parameter pData will only be interpreted for connections
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of type command. It must point to the command stack belonging to the
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connection which registers this socket.
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\item[NetReadRegisterUserSocket] registers a socket from within SICS foth
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the network reader. The client code to this module may then find out with a
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call to NetReadReadable if data is pending on this socket. This a
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convenience wrapper to NetReadRegister.
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\item[NetReadRemove] removes the socket pSock from the net reader. This must
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be done when closing the socket, otherwise the net reader will continue
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checking for pending data on this socket.
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\item[NetReadRemoveUserSocket] removes a user socket from
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monitoring. Again, this is a vonvenience wrapper around NetReadRemove.
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\item[NetReaderTask] is the net reader task function which will be
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registered with the task manager. This function builds the select
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mask, does the select and forwards incoming data to the appropriate
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handler functions. These are not documented here, as they are file
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static function in nread.c.
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\item[NetReaderSignal] is the signal handler for the net reader. It
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just stops the NetReaderTask when the SICS server is run down.
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\item[NetReadReadable] returns 1 if data is pending at the user socket or 0
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if not.
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\item[NetReadResetUser] resets a user socket to not readable.
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\end{description}
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@o nread.h @{
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/*------------------------------------------------------------------------
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N E T R E A D E R
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This module will check for pending requests to the SICS server and
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initiate apropriate actions.
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Mark Koenencke, September 1997
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copyright: see copyright.h
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----------------------------------------------------------------------------*/
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#ifndef SICSNETREADER
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#define SICSNETREADER
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@<nrint@>
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#endif
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@}
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