1 <!-- $Id: zoom.xml,v 1.24 2003-05-12 13:34:00 mike Exp $ -->
2 <chapter id="zoom"><title>ZOOM</title>
4 &zoom; is an acronym for 'Z39.50 Object-Orientation Model' and is
5 an initiative started by Mike Taylor (Mike is from the UK, which
6 explains the peculiar name of the model). The goal of &zoom; is to
7 provide a common Z39.50 client API not bound to a particular
8 programming language or toolkit.
13 A recent addition to &yaz; is SRW support. You can now make
14 SRW ZOOM connections by specifying another scheme for the
15 hostname for a connection.
20 The lack of a simple Z39.50 client API for &yaz; has become more
21 and more apparent over time. So when the first &zoom; specification
23 an implementation for &yaz; was quickly developed. For the first time, it is
24 now as easy (or easier!) to develop clients than servers with &yaz;. This
25 chapter describes the &zoom; C binding. Before going further, please
26 reconsider whether C is the right programming language for the job.
27 There are other language bindings available for &yaz;, and still
29 are in active development. See the
30 <ulink url="http://zoom.z3950.org/">ZOOM web-site</ulink> for
35 In order to fully understand this chapter you should read and
36 try the example programs <literal>zoomtst1.c</literal>,
37 <literal>zoomtst2.c</literal>, .. in the <literal>zoom</literal>
42 The C language misses features found in object oriented languages
43 such as C++, Java, etc. For example, you'll have to manually,
44 destroy all objects you create, even though you may think of them as
45 temporary. Most objects has a <literal>_create</literal> - and a
46 <literal>_destroy</literal> variant.
47 All objects are in fact pointers to internal stuff, but you don't see
48 that because of typedefs. All destroy methods should gracefully ignore a
49 <literal>NULL</literal> pointer.
52 In each of the sections below you'll find a sub section called
53 protocol behavior, that describes how the API maps to the Z39.50
56 <sect1 id="zoom.connections"><title>Connections</title>
58 <para>The Connection object is a session with a target.
61 #include <yaz/zoom.h>
63 ZOOM_connection ZOOM_connection_new (const char *host, int portnum);
65 ZOOM_connection ZOOM_connection_create (ZOOM_options options);
67 void ZOOM_connection_connect(ZOOM_connection c, const char *host,
69 void ZOOM_connection_destroy (ZOOM_connection c);
72 Connection objects are created with either function
73 <function>ZOOM_connection_new</function> or
74 <function>ZOOM_connection_create</function>.
75 The former creates and automatically attempts to establish a network
76 connection with the target. The latter doesn't establish
77 a connection immediately, thus allowing you to specify options
78 before establishing network connection using the function
79 <function>ZOOM_connection_connect</function>.
80 If the port number, <literal>portnum</literal>, is zero, the
81 <literal>host</literal> is consulted for a port specification.
82 If no port is given, 210 is used. A colon denotes the beginning of
83 a port number in the host string. If the host string includes a
84 slash, the following part specifies a database for the connection.
87 You can prefix the host with a scheme followed by colon. The
88 default scheme is <literal>tcp</literal> (Z39.50 protocol).
89 The scheme <literal>http</literal> selects SRW over HTTP.
92 Connection objects should be destroyed using the function
93 <function>ZOOM_connection_destroy</function>.
96 void ZOOM_connection_option_set (ZOOM_connection c,
100 const char *ZOOM_connection_option_get (ZOOM_connection c,
104 The <function>ZOOM_connection_option_set</function> allows you to
105 set an option given by <parameter>key</parameter> to the value
106 <parameter>value</parameter> for the connection.
107 Function <function>ZOOM_connection_option_get</function> returns
108 the value for an option given by <parameter>key</parameter>.
110 <table frame="top"><title>ZOOM Connection Options</title>
112 <colspec colwidth="4*" colname="name"></colspec>
113 <colspec colwidth="7*" colname="description"></colspec>
114 <colspec colwidth="3*" colname="default"></colspec>
117 <entry>Option</entry>
118 <entry>Description</entry>
119 <entry>Default</entry>
124 implementationName</entry><entry>Name of Your client
125 </entry><entry>none</entry></row>
127 user</entry><entry>Authentication user name
128 </entry><entry>none</entry></row>
130 group</entry><entry>Authentication group name
131 </entry><entry>none</entry></row>
133 pass</entry><entry>Authentication password
134 </entry><entry>none</entry></row>
136 host</entry><entry>Target host. This setting is "read-only".
137 It's automatically set internally when connecting to a target.
138 </entry><entry>none</entry></row>
140 proxy</entry><entry>Proxy host
141 </entry><entry>none</entry></row>
143 async</entry><entry>If true (1) the connection operates in
144 asynchronous operation which means that all calls are non-blocking
146 <link linkend="zoom.events"><function>ZOOM_event</function></link>.
147 </entry><entry>0</entry></row>
149 maximumRecordSize</entry><entry> Maximum size of single record.
150 </entry><entry>1 MB</entry></row>
152 preferredMessageSize</entry><entry> Maximum size of multiple records.
153 </entry><entry>1 MB</entry></row>
155 lang</entry><entry> Language for negotiation.
156 </entry><entry>none</entry></row>
158 charset</entry><entry> Character set for negotiation.
159 </entry><entry>none</entry></row>
161 targetImplementationId</entry><entry> Implementation ID of target.
162 </entry><entry>none</entry></row>
164 targetImplementationName</entry><entry> Implementation Name of target.
165 </entry><entry>none</entry></row>
167 targetImplementationVersion</entry><entry> Implementation Version
169 </entry><entry>none</entry></row>
171 databaseName</entry><entry>One or more database names
172 separated by character plus (<literal>+</literal>), which to
173 be used by subsequent search requests on this Connection.
174 </entry><entry>Default</entry></row>
179 If either option <literal>lang</literal> or <literal>charset</literal>
181 <ulink url="http://lcweb.loc.gov/z3950/agency/defns/charneg-3.html">
182 Character Set and Language Negotiation</ulink> is in effect.
185 int ZOOM_connection_error (ZOOM_connection c, const char **cp,
186 const char **addinfo);
187 int ZOOM_connection_error_x (ZOOM_connection c, const char **cp,
188 const char **addinfo, const char **dset);
191 Function <function>ZOOM_connection_error</function> checks for
192 errors for the last operation(s) performed. The function returns
193 zero if no errors occurred; non-zero otherwise indicating the error.
194 Pointers <parameter>cp</parameter> and <parameter>addinfo</parameter>
195 holds messages for the error and additional-info if passed as
196 non-<literal>NULL</literal>. Function
197 <function>ZOOM_connection_error_x</function> is an extended version
198 of <function>ZOOM_connection_error</function> that is capable of
199 returning name of diagnostic set in <parameter>dset</parameter>.
201 <sect2><title>Z39.50 Protocol behavior</title>
203 The calls <function>ZOOM_connection_new</function> and
204 <function>ZOOM_connection_connect</function> establishes a TCP/IP
205 connection and sends an Initialize Request to the target if
206 possible. In addition, the calls waits for an Initialize Response
207 from the target and the result is inspected (OK or rejected).
210 If <literal>proxy</literal> is set then the client will establish
211 a TCP/IP connection with the peer as specified by the
212 <literal>proxy</literal> host and the hostname as part of the
213 connect calls will be set as part of the Initialize Request.
214 The proxy server will then "forward" the PDU's transparently
215 to the target behind the proxy.
218 For the authentication parameters, if option <literal>user</literal>
219 is set and both options <literal>group</literal> and
220 <literal>pass</literal> are unset, then Open style
221 authentication is used (Version 2/3) in which case the username
222 is usually followed by a slash, then by a password.
223 If either <literal>group</literal>
224 or <literal>pass</literal> is set then idPass authentication
225 (Version 3 only) is used. If none of the options are set, no
226 authentication parameters are set as part of the Initialize Request
230 When option <literal>async</literal> is 1, it really means that
231 all network operations are postponed (and queued) until the
232 function <literal>ZOOM_event</literal> is invoked. When doing so
233 it doesn't make sense to check for errors after
234 <literal>ZOOM_connection_new</literal> is called since that
235 operation "connecting - and init" is still incomplete and the
236 API cannot tell the outcome (yet).
239 <sect2><title>SRW Protocol behavior</title>
241 The SRW protocol doesn't feature an Init Request, so
242 the connection phase merely establishes a TCP/IP connection
243 with the SOAP service.
245 <para>None of the ZOOM connection options
246 affect SRW and they are ignored. However, future versions
247 of &yaz; might honor <literal>implementationName</literal> and
248 put that as part of User-Agent header for HTTP requests.
249 The <literal>charset</literal>, and <literal>lang</literal>
250 might also affect HTTP headers in future releases.
254 <sect1 id="zoom.query"><title>Queries</title>
256 Query objects represents queries.
259 ZOOM_query ZOOM_query_create(void);
261 void ZOOM_query_destroy(ZOOM_query q);
263 int ZOOM_query_prefix(ZOOM_query q, const char *str);
265 int ZOOM_query_cql(ZOOM_query s, const char *str);
267 int ZOOM_query_sortby(ZOOM_query q, const char *criteria);
270 Create query objects using <function>ZOOM_query_create</function>
271 and destroy them by calling <function>ZOOM_query_destroy</function>.
272 RPN-queries can be specified in <link linkend="PQF">PQF</link>
273 notation by using the
274 function <function>ZOOM_query_prefix</function>.
275 The <function>ZOOM_query_cql</function> specifies a CQL
276 query to be sent to the server/target.
277 More query types will be added in future versions of &yaz;, such as
278 <link linkend="CCL">CCL</link> to RPN-mapping, native CCL query,
279 etc. In addition to a search, a sort criteria may be set. Function
280 <function>ZOOM_query_sortby</function> specifies a
281 sort criteria using the same string notation for sort as offered by
282 the <link linkend="sortspec">YAZ client</link>.
284 <sect2><title>Protocol behavior</title>
286 The query object is just an interface for the member Query
287 in the SearchRequest. The sortby-function is an interface to the
288 sortSequence member of the SortRequest.
292 <sect1 id="zoom.resultsets"><title>Result sets</title>
294 The result set object is a container for records returned from
298 ZOOM_resultset ZOOM_connection_search(ZOOM_connection,
301 ZOOM_resultset ZOOM_connection_search_pqf(ZOOM_connection c,
304 void ZOOM_resultset_destroy(ZOOM_resultset r);
307 Function <function>ZOOM_connection_search</function> creates
308 a result set given a connection and query.
309 Destroy a result set by calling
310 <function>ZOOM_resultset_destroy</function>.
311 Simple clients may using PQF only may use function
312 <function>ZOOM_connection_search_pqf</function> in which case
313 creating query objects is not necessary.
316 void ZOOM_resultset_option_set (ZOOM_resultset r,
320 const char *ZOOM_resultset_option_get (ZOOM_resultset r,
323 size_t ZOOM_resultset_size (ZOOM_resultset r);
326 Functions <function>ZOOM_resultset_options_set</function> and
327 <function>ZOOM_resultset_get</function> sets and gets an option
328 for a result set similar to <function>ZOOM_connection_option_get</function>
329 and <function>ZOOM_connection_option_set</function>.
332 The number of hits also called result-count is returned by
333 function <function>ZOOM_resultset_size</function>.
335 <table frame="top"><title>ZOOM Result set Options</title>
337 <colspec colwidth="4*" colname="name"></colspec>
338 <colspec colwidth="7*" colname="description"></colspec>
339 <colspec colwidth="2*" colname="default"></colspec>
342 <entry>Option</entry>
343 <entry>Description</entry>
344 <entry>Default</entry>
349 piggyback</entry><entry>True (1) if piggyback should be
350 used in searches; false (0) if not.
351 </entry><entry>1</entry></row>
353 start</entry><entry>Offset of first record to be
354 retrieved from target. First record has offset 0 unlike the
355 protocol specifications where first record has position 1.
356 </entry><entry>0</entry></row>
358 count</entry><entry>Number of records to be retrieved.
359 </entry><entry>0</entry></row>
361 elementSetName</entry><entry>Element-Set name of records.
362 Most targets should honor element set name <literal>B</literal>
363 and <literal>F</literal> for brief and full respectively.
364 </entry><entry>none</entry></row>
366 preferredRecordSyntax</entry><entry>Preferred Syntax, such as
367 <literal>USMARC</literal>, <literal>SUTRS</literal>, etc.
368 </entry><entry>none</entry></row>
370 schema</entry><entry>Schema for retrieval, such as
371 <literal>Gils-schema</literal>, <literal>Geo-schema</literal>, etc.
372 </entry><entry>none</entry></row>
374 smallSetUpperBound</entry><entry>If hits is less than or equal to this
375 value, then target will return all records using small element set name
376 </entry><entry>0</entry></row>
378 largeSetLowerBound</entry><entry>If hits is greater than this
379 value, the target will return no records.
380 </entry><entry>1</entry></row>
382 mediumSetPresentNumber</entry><entry>This value represents
383 the number of records to be returned as part of a search when when
384 hits is less than or equal to large set lower bound and if hits
385 is greater than small set upper bound.
386 </entry><entry>0</entry></row>
388 smallSetElementSetName</entry><entry>
389 The element set name to be used for small result sets.
390 </entry><entry>none</entry></row>
392 mediumSetElementSetName</entry><entry>
393 The element set name to be for medium-sized result sets.
394 </entry><entry>none</entry></row>
396 setname</entry><entry>Name of Result Set (Result Set ID).
397 If this option isn't set, the ZOOM module will automatically
398 allocate a result set name.
399 </entry><entry>default</entry></row>
404 <title>Z39.50 Protocol behavior</title>
406 The creation of a result set involves at least a SearchRequest
407 - SearchResponse protocol handshake. Following that, if a sort
408 criteria was specified as part of the query, a SortRequest -
409 SortResponse handshake takes place. Note that it is necessary to
410 perform sorting before any retrieval takes place, so no records will
411 be returned from the target as part of the SearchResponse because these
412 would be unsorted. Hence, piggyback is disabled when sort criteria
413 is set. Following Search - and a Possible sort, Retrieval takes
414 place - as one or more Present Requests - Present Response being
418 The API allows for two different modes for retrieval. A high level
419 mode which is somewhat more powerful and a low level one.
420 The low level is "enabled" when the settings
421 <literal>smallSetUpperBound</literal>,
422 <literal>mediumSetPresentNumber</literal> and
423 <literal>largeSetLowerBound</literal> are set. The low level mode
424 thus allows you to precisely set how records are returned as part
425 of a search response as offered by the Z39.50 protocol.
426 Since the client may be retrieving records as part of the
427 search response, this mode doesn't work well if sorting is used.
430 The high-level mode allows you to fetch a range of records from
431 the result set with a given start offset. When you use this mode
432 the client will automatically use piggyback if that is possible
433 with the target and perform one or more present requests as needed.
434 Even if the target returns fewer records as part of a present response
435 because of a record size limit, etc. the client will repeat sending
436 present requests. As an example, if option <literal>start</literal>
437 is 0 (default) and <literal>count</literal> is 4, and
438 <literal>piggyback</literal> is 1 (default) and no sorting criteria
439 is specified, then the client will attempt to retrieve the 4
440 records as part the search response (using piggyback). On the other
441 hand, if either <literal>start</literal> is positive or if
442 a sorting criteria is set, or if <literal>piggyback</literal>
443 is 0, then the client will not perform piggyback but send Present
447 If either of the options <literal>mediumSetElementSetName</literal> and
448 <literal>smallSetElementSetName</literal> are unset, the value
449 of option <literal>elementSetName</literal> is used for piggyback
450 searches. This means that for the high-level mode you only have
451 to specify one elementSetName option rather than three.
455 <title>SRW Protocol behavior</title>
457 Current version of &yaz; does not take advantage of a result set id
458 returned by the SRW server. Future versions might do, however.
459 Since, the ZOOM driver does not save result set IDs any
460 present (retrieval) is transformed to a SRW SearchRetrieveRequest
461 with same query but, possibly, different offsets.
464 Option <literal>schema</literal> specifies SRW schema
465 for retrieval. However, options <literal>elementSetName</literal> and
466 <literal>preferredRecordSyntax</literal> are ignored.
469 Options <literal>start</literal> and <literal>count</literal>
470 are supported by SRW.
471 The remaining options
472 <literal>piggyback</literal>,
473 <literal>smallSetUpperBound</literal>,
474 <literal>largeSetLowerBound</literal>,
475 <literal>mediumSetPresentNumber</literal>,
476 <literal>mediumSetElementSetName</literal>,
477 <literal>smallSetElementSetName</literal> are
481 SRW supports CQL queries, <emphasis>not</emphasis> PQF.
482 If PQF is used, however, the PQF query is transferred anyway
483 using non-standard element <literal>pQuery</literal> in
484 SRW SearchRetrieveRequest.
487 Unfortunately, SRW does not define a database setting. Hence,
488 <literal>databaseName</literal> is unsupported and ignored.
489 However, the path part in host parameter for functions
490 <function>ZOOM_connecton_new</function> and
491 <function>ZOOM_connection_connect</function> acts as a
492 database (at least for the &yaz; SRW server).
496 <sect1 id="zoom.records"><title>Records</title>
498 A record object is a retrieval record on the client side -
499 created from result sets.
502 void ZOOM_resultset_records (ZOOM_resultset r,
504 size_t start, size_t count);
505 ZOOM_record ZOOM_resultset_record (ZOOM_resultset s, size_t pos);
507 const char *ZOOM_record_get (ZOOM_record rec, const char *type,
510 ZOOM_record ZOOM_record_clone (ZOOM_record rec);
512 void ZOOM_record_destroy (ZOOM_record rec);
515 References to temporary records are returned by functions
516 <function>ZOOM_resultset_records</function> or
517 <function>ZOOM_resultset_record</function>.
520 If a persistent reference to a record is desired
521 <function>ZOOM_record_clone</function> should be used.
522 It returns a record reference that should be destroyed
523 by a call to <function>ZOOM_record_destroy</function>.
526 A single record is returned by function
527 <function>ZOOM_resultset_record</function> that takes a
528 position as argument. First record has position zero.
529 If no record could be obtained <literal>NULL</literal> is returned.
532 Function <function>ZOOM_resultset_records</function> retrieves
533 a number of records from a result set. Parameter <literal>start</literal>
534 and <literal>count</literal> specifies the range of records to
535 be returned. Upon completion array
536 <literal>recs[0], ..recs[count-1]</literal>
537 holds record objects for the records. The array of records
538 <literal>recs</literal> should be allocated prior the call
539 <function>ZOOM_resultset_records</function>. Note that for those
540 records that couldn't be retrieved from the target
541 <literal>recs[ ..]</literal> is set to <literal>NULL</literal>.
543 <para id="zoom.record.get">
544 In order to extract information about a single record,
545 <function>ZOOM_record_get</function> is provided. The
546 function returns a pointer to certain record information. The
547 nature (type) of the pointer depends on the parameter,
548 <function>type</function>.
549 In addition, for certain types, the length
550 <literal>len</literal> passed will be set to the size in bytes of
551 the returned information.
553 <varlistentry><term><literal>database</literal></term>
554 <listitem><para>Database of record is returned
555 as a C null-terminated string. Return type
556 <literal>const char *</literal>.
559 <varlistentry><term><literal>syntax</literal></term>
560 <listitem><para>The transfer syntax (OID) of the record is returned
561 as a C null-terminated string. Return type is
562 <literal>const char *</literal>.
565 <varlistentry><term><literal>render</literal></term>
566 <listitem><para>The record is returned in a display friendly
567 format. Upon completion buffer is returned
568 (type <literal>const char *</literal>) and length is stored in
569 <literal>*len</literal>.
572 <varlistentry><term><literal>raw</literal></term>
573 <listitem><para>The record is returned in the internal
574 YAZ specific format. For GRS-1, Explain, and others, the
575 raw data is returned as type
576 <literal>Z_External *</literal> which is just the type for
577 the member <literal>retrievalRecord</literal> in
578 type <literal>NamePlusRecord</literal>.
579 For SUTRS and octet aligned record (including all MARCs) the
580 octet buffer is returned and the length of the buffer.
585 <sect2><title>Z39.50 Protocol behavior</title>
587 The functions <function>ZOOM_resultset_record</function> and
588 <function>ZOOM_resultset_records</function> inspects the client-side
589 record cache. Records not found in cache are fetched using
591 The functions may block (and perform network I/O) - even though option
592 <literal>async</literal> is 1, because they return records objects.
593 (and there's no way to return records objects without retrieving them!).
596 There is a trick, however, in the usage of function
597 <function>ZOOM_resultset_records</function> that allows for
598 delayed retrieval (and makes it non-blocking). By passing
599 a null pointer for <parameter>recs</parameter> you're indicating
600 you're not interested in getting records objects
601 <emphasis>now</emphasis>.
604 <sect2><title>SRW Protocol behavior</title>
606 The ZOOM driver for SRW treats records returned by a SRW server
607 as if they where Z39.50 records with transfer syntax XML and
608 no element set name or database name.
612 <sect1 id="zoom.scan"><title>Scan</title>
614 This section describes an interface for Scan. Scan is not an
615 official part of the ZOOM model yet. The result of a scan operation
616 is the <literal>ZOOM_scanset</literal> which is a set of terms
617 returned by a target.
621 The Scan interface is Z39.50 only. SRW version 1.0 does not
626 ZOOM_scanset ZOOM_connection_scan (ZOOM_connection c,
627 const char *startterm);
629 size_t ZOOM_scanset_size(ZOOM_scanset scan);
631 const char * ZOOM_scanset_term(ZOOM_scanset scan, size_t pos,
632 int *occ, size_t *len);
635 void ZOOM_scanset_destroy (ZOOM_scanset scan);
637 const char *ZOOM_scanset_option_get (ZOOM_scanset scan,
640 void ZOOM_scanset_option_set (ZOOM_scanset scan, const char *key,
644 The scan set is created by function
645 <function>ZOOM_connection_scan</function> which performs a scan
646 operation on the connection and start term given.
647 If the operation was successful, the size of the scan set can be
648 retrieved by a call to <function>ZOOM_scanset_size</function>.
649 Like result sets, the items are numbered 0,..size-1.
650 To obtain information about a particular scan term, call function
651 <function>ZOOM_scanset_term</function>. This function takes
652 a scan set offset <literal>pos</literal> and returns a pointer
653 to an actual term or <literal>NULL</literal> if non-present.
654 If present, the <literal>occ</literal> and <literal>len</literal>
655 are set to the number of occurrences and the length
656 of the actual term respectively.
657 A scan set may be freed by a call to function
658 <function>ZOOM_scanset_destroy</function>.
659 Functions <function>ZOOM_scanset_option_get</function> and
660 <function>ZOOM_scanset_option_set</function> retrieves and sets
661 an option respectively.
664 <table frame="top"><title>ZOOM Scan Set Options</title>
666 <colspec colwidth="4*" colname="name"></colspec>
667 <colspec colwidth="7*" colname="description"></colspec>
668 <colspec colwidth="2*" colname="default"></colspec>
671 <entry>Option</entry>
672 <entry>Description</entry>
673 <entry>Default</entry>
678 number</entry><entry>Number of Scan Terms requested in next scan.
679 After scan it holds the actual number of terms returned.
680 </entry><entry>10</entry></row>
682 position</entry><entry>Preferred Position of term in response
683 in next scan; actual position after completion of scan.
684 </entry><entry>1</entry></row>
686 stepSize</entry><entry>Step Size
687 </entry><entry>0</entry></row>
689 scanStatus</entry><entry>An integer indicating the Scan Status
691 </entry><entry>0</entry></row>
697 <sect1 id="zoom.options"><title>Options</title>
699 Most &zoom; objects provide a way to specify options to change behavior.
700 From an implementation point of view a set of options is just like
701 an associative array / hash array, etc.
704 ZOOM_options ZOOM_options_create (void);
706 ZOOM_options ZOOM_options_create_with_parent (ZOOM_options parent);
708 void ZOOM_options_destroy (ZOOM_options opt);
711 const char *ZOOM_options_get (ZOOM_options opt, const char *name);
713 void ZOOM_options_set (ZOOM_options opt, const char *name,
717 typedef const char *(*ZOOM_options_callback)
718 (void *handle, const char *name);
720 ZOOM_options_callback
721 ZOOM_options_set_callback (ZOOM_options opt,
722 ZOOM_options_callback c,
726 <sect1 id="zoom.events"><title>Events</title>
728 If you're developing non-blocking applications, you have to deal
732 int ZOOM_event (int no, ZOOM_connection *cs);
735 The <function>ZOOM_event</function> executes pending events for
736 a number of connections. Supply the number of connections in
737 <literal>no</literal> and an array of connections in
738 <literal>cs</literal> (<literal>cs[0] ... cs[no-1]</literal>).
739 A pending event could be a sending a search, receiving a response,
741 When an event has occurred for one of the connections, this function
742 returns a positive integer <literal>n</literal> denoting that an event
743 occurred for connection <literal>cs[n-1]</literal>.
744 When no events are pending for the connections, a value of zero is
746 To ensure that all outstanding requests are performed call this function
747 repeatedly until zero is returned.
752 <!-- Keep this comment at the end of the file
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