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ssssrrrrvvvv((((DDDD2222)))) ssssrrrrvvvv((((DDDD2222)))) NNNNAAAAMMMMEEEE _ssss_rrrr_vvvv - service queued messages SSSSYYYYNNNNOOOOPPPPSSSSIIIISSSS _####_iiii_nnnn_cccc_llll_uuuu_dddd_eeee _<<<<_ssss_yyyy_ssss_////_tttt_yyyy_pppp_eeee_ssss_...._hhhh_>>>> _####_iiii_nnnn_cccc_llll_uuuu_dddd_eeee _<<<<_ssss_yyyy_ssss_////_ssss_tttt_rrrr_eeee_aaaa_mmmm_...._hhhh_>>>> _####_iiii_nnnn_cccc_llll_uuuu_dddd_eeee _<<<<_ssss_yyyy_ssss_////_ssss_tttt_rrrr_oooo_pppp_tttt_ssss_...._hhhh_>>>> _####_iiii_nnnn_cccc_llll_uuuu_dddd_eeee _<<<<_ssss_yyyy_ssss_////_dddd_dddd_iiii_...._hhhh_>>>> _iiii_nnnn_tttt _p_r_e_f_i_x_rrrr_ssss_rrrr_vvvv_((((_qqqq_uuuu_eeee_uuuu_eeee______tttt _****_q_))))_;;;; _////_**** _rrrr_eeee_aaaa_dddd _ssss_iiii_dddd_eeee _****_//// _iiii_nnnn_tttt _p_r_e_f_i_x_wwww_ssss_rrrr_vvvv_((((_qqqq_uuuu_eeee_uuuu_eeee______tttt _****_q_))))_;;;; _////_**** _wwww_rrrr_iiii_tttt_eeee _ssss_iiii_dddd_eeee _****_//// AAAArrrrgggguuuummmmeeeennnnttttssss _q Pointer to the queue. DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN The _ssss_rrrr_vvvv (service) routine may be included in a STREAMS module or driver for a number of reasons. It provides greater control over the flow of messages in a stream by allowing the module or driver to reorder messages, defer the processing of some messages, or fragment and reassemble messages. The service routine also provides a way to recover from resource allocation failures. RRRReeeettttuuuurrrrnnnn VVVVaaaalllluuuueeeessss Ignored UUUUSSSSAAAAGGGGEEEE This entry point is optional, and is valid for STREAMS drivers and modules only. A message is first passed to a module's or driver's _pppp_uuuu_tttt(D2) routine, which may or may not process it. The _pppp_uuuu_tttt routine can place the message on the queue for processing by the service routine. Once a message has been enqueued, the STREAMS scheduler calls the service routine at some later time. Drivers and modules should not depend on the order in which service procedures are run. This is an implementation- dependent characteristic. In particular, applications should not rely on service procedures running before returning to user-level processing. Every STREAMS queue [see _qqqq_uuuu_eeee_uuuu_eeee(D4)] has limit values it uses to implement flow control. High and low water marks are checked to stop and restart the flow of message processing. Flow control limits apply only between two adjacent queues with service routines. Flow control occurs by service routines following certain rules before passing messages along. By convention, high priority messages are not affected by flow control. STREAMS messages can be defined to have up to 256 different priorities to support some networking protocol requirements for multiple bands of data flow. At a minimum, a stream must distinguish between normal (priority band zero) messages and high priority messages (such as _MMMM______IIII_OOOO_CCCC_AAAA_CCCC_KKKK). High priority messages are always placed at the head of the queue, after any other high priority messages already enqueued. Next are messages from PPPPaaaaggggeeee 1111 ssssrrrrvvvv((((DDDD2222)))) ssssrrrrvvvv((((DDDD2222)))) all included priority bands, which are enqueued in decreasing order of priority. Each priority band has its own flow control limits. By convention, if a band is flow-controlled, all lower priority bands are also stopped. Once a service routine is called by the STREAMS scheduler it must provide for processing all messages on its queue, restarting itself if necessary. Message processing must continue until either the queue is empty, the stream is flow-controlled, or an allocation error occurs. Typically, the service routine will switch on the message type, which is contained in _mmmm_pppp_----_>>>>_bbbb______dddd_aaaa_tttt_aaaa_pppp_----_>>>>_dddd_bbbb______tttt_yyyy_pppp_eeee, taking different actions depending on the message type. For singlethreaded modules and drivers, the framework for the canonical service procedure algorithm is as follows: _qqqq_uuuu_eeee_uuuu_eeee______tttt _****_qqqq_;;;; _mmmm_bbbb_llll_kkkk______tttt _****_mmmm_pppp_;;;; _wwww_hhhh_iiii_llll_eeee _((((_((((_mmmm_pppp _==== _gggg_eeee_tttt_qqqq_((((_qqqq_))))_)))) _!!!!_==== _NNNN_UUUU_LLLL_LLLL_)))) _{{{{ _iiii_ffff _((((_mmmm_pppp_----_>>>>_bbbb______dddd_aaaa_tttt_aaaa_pppp_----_>>>>_dddd_bbbb______tttt_yyyy_pppp_eeee _>>>> _QQQQ_PPPP_CCCC_TTTT_LLLL _||||_|||| _cccc_aaaa_nnnn_pppp_uuuu_tttt_((((_qqqq_----_>>>>_qqqq______nnnn_eeee_xxxx_tttt_))))_)))) _{{{{ _////_**** _pppp_rrrr_oooo_cccc_eeee_ssss_ssss _tttt_hhhh_eeee _mmmm_eeee_ssss_ssss_aaaa_gggg_eeee _****_//// _pppp_uuuu_tttt_nnnn_eeee_xxxx_tttt_((((_qqqq_,,,, _mmmm_pppp_))))_;;;; _}}}} _eeee_llll_ssss_eeee _{{{{ _pppp_uuuu_tttt_bbbb_qqqq_((((_qqqq_,,,, _mmmm_pppp_))))_;;;; _rrrr_eeee_tttt_uuuu_rrrr_nnnn_;;;; _}}}} _}}}} If the singlethreaded module or driver cares about priority bands, the algorithm becomes: _qqqq_uuuu_eeee_uuuu_eeee______tttt _****_qqqq_;;;; _mmmm_bbbb_llll_kkkk______tttt _****_mmmm_pppp_;;;; _wwww_hhhh_iiii_llll_eeee _((((_((((_mmmm_pppp _==== _gggg_eeee_tttt_qqqq_((((_qqqq_))))_)))) _!!!!_==== _NNNN_UUUU_LLLL_LLLL_)))) _{{{{ _iiii_ffff _((((_mmmm_pppp_----_>>>>_bbbb______dddd_aaaa_tttt_aaaa_pppp_----_>>>>_dddd_bbbb______tttt_yyyy_pppp_eeee _>>>> _QQQQ_PPPP_CCCC_TTTT_LLLL _||||_|||| _bbbb_cccc_aaaa_nnnn_pppp_uuuu_tttt_((((_qqqq_----_>>>>_qqqq______nnnn_eeee_xxxx_tttt_,,,, _mmmm_pppp_----_>>>>_bbbb______bbbb_aaaa_nnnn_dddd_))))_)))) _{{{{ _////_**** _pppp_rrrr_oooo_cccc_eeee_ssss_ssss _tttt_hhhh_eeee _mmmm_eeee_ssss_ssss_aaaa_gggg_eeee _****_//// _pppp_uuuu_tttt_nnnn_eeee_xxxx_tttt_((((_qqqq_,,,, _mmmm_pppp_))))_;;;; _}}}} _eeee_llll_ssss_eeee _{{{{ _pppp_uuuu_tttt_bbbb_qqqq_((((_qqqq_,,,, _mmmm_pppp_))))_;;;; _rrrr_eeee_tttt_uuuu_rrrr_nnnn_;;;; _}}}} _}}}} Each STREAMS module and driver can have a read and write service routine. If a service routine is not needed (because the _pppp_uuuu_tttt routine processes all messages), a _NNNN_UUUU_LLLL_LLLL pointer should be placed in the module's _qqqq_iiii_nnnn_iiii_tttt(D4) structure. If the service routine finishes running because of any reason other than flow control or an empty queue, then it must explicitly arrange for its rescheduling. For example, if an allocation error occurs during the processing of a message, the service routine can put the message back on the queue with _pppp_uuuu_tttt_bbbb_qqqq, and, before returning, arrange to have itself rescheduled [see _qqqq_eeee_nnnn_aaaa_bbbb_llll_eeee(D3)] at some later time [see _bbbb_uuuu_ffff_cccc_aaaa_llll_llll(D3) and PPPPaaaaggggeeee 2222 ssssrrrrvvvv((((DDDD2222)))) ssssrrrrvvvv((((DDDD2222)))) _iiii_tttt_iiii_mmmm_eeee_oooo_uuuu_tttt(D3)]. Service routines can be interrupted by _pppp_uuuu_tttt routines, unless the processor interrupt level is raised. Only one copy of a queue's service routine will run at a time. Drivers and modules should not call service routines directly. _qqqq_eeee_nnnn_aaaa_bbbb_llll_eeee(D3) should be used to schedule service routines to run. Drivers (excepting multiplexors) should free any messages they do not recognize. Modules should pass on any messages they do not recognize. Drivers should fail any unrecognized _MMMM______IIII_OOOO_CCCC_TTTT_LLLL messages by converting them into _MMMM______IIII_OOOO_CCCC_NNNN_AAAA_KKKK messages and sending them upstream. Modules should pass on any unrecognized _MMMM______IIII_OOOO_CCCC_TTTT_LLLL messages. Service routines should never put high priority messages back on their queues. SSSSyyyynnnncccchhhhrrrroooonnnniiiizzzzaaaattttiiiioooonnnn CCCCoooonnnnssssttttrrrraaaaiiiinnnnttttssss Service routines do not have user context and so may not call any function that sleeps. RRRREEEEFFFFEEEERRRREEEENNNNCCCCEEEESSSS _bbbb_cccc_aaaa_nnnn_pppp_uuuu_tttt(D3), _bbbb_uuuu_ffff_cccc_aaaa_llll_llll(D3), _cccc_aaaa_nnnn_pppp_uuuu_tttt(D3), _dddd_aaaa_tttt_aaaa_bbbb(D4), _gggg_eeee_tttt_qqqq(D3), _mmmm_ssss_gggg_bbbb(D4), _pppp_cccc_mmmm_ssss_gggg(D3), _pppp_uuuu_tttt(D2), _pppp_uuuu_tttt_bbbb_qqqq(D3), _pppp_uuuu_tttt_nnnn_eeee_xxxx_tttt(D3), _pppp_uuuu_tttt_qqqq(D3), _qqqq_eeee_nnnn_aaaa_bbbb_llll_eeee(D3), _qqqq_iiii_nnnn_iiii_tttt(D4), _qqqq_uuuu_eeee_uuuu_eeee(D4), _iiii_tttt_iiii_mmmm_eeee_oooo_uuuu_tttt(D3) PPPPaaaaggggeeee 3333 
