1 <!-- $Id: zoom.xml,v 1.26 2003-05-19 20:45:07 adam 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 scheme <literal>http://</literal>
15 for the 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 Inititialize Request, so
242 the connection phase merely establishes a TCP/IP connection
243 with the SOAP service.
245 <para>Most of the ZOOM connection options do not
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.
251 The <literal>charset</literal> is used in the Content-Type header
256 <sect1 id="zoom.query"><title>Queries</title>
258 Query objects represents queries.
261 ZOOM_query ZOOM_query_create(void);
263 void ZOOM_query_destroy(ZOOM_query q);
265 int ZOOM_query_prefix(ZOOM_query q, const char *str);
267 int ZOOM_query_cql(ZOOM_query s, const char *str);
269 int ZOOM_query_sortby(ZOOM_query q, const char *criteria);
272 Create query objects using <function>ZOOM_query_create</function>
273 and destroy them by calling <function>ZOOM_query_destroy</function>.
274 RPN-queries can be specified in <link linkend="PQF">PQF</link>
275 notation by using the
276 function <function>ZOOM_query_prefix</function>.
277 The <function>ZOOM_query_cql</function> specifies a CQL
278 query to be sent to the server/target.
279 More query types will be added in future versions of &yaz;, such as
280 <link linkend="CCL">CCL</link> to RPN-mapping, native CCL query,
281 etc. In addition to a search, a sort criteria may be set. Function
282 <function>ZOOM_query_sortby</function> specifies a
283 sort criteria using the same string notation for sort as offered by
284 the <link linkend="sortspec">YAZ client</link>.
286 <sect2><title>Protocol behavior</title>
288 The query object is just an interface for the member Query
289 in the SearchRequest. The sortby-function is an interface to the
290 sortSequence member of the SortRequest.
294 <sect1 id="zoom.resultsets"><title>Result sets</title>
296 The result set object is a container for records returned from
300 ZOOM_resultset ZOOM_connection_search(ZOOM_connection,
303 ZOOM_resultset ZOOM_connection_search_pqf(ZOOM_connection c,
306 void ZOOM_resultset_destroy(ZOOM_resultset r);
309 Function <function>ZOOM_connection_search</function> creates
310 a result set given a connection and query.
311 Destroy a result set by calling
312 <function>ZOOM_resultset_destroy</function>.
313 Simple clients may using PQF only may use function
314 <function>ZOOM_connection_search_pqf</function> in which case
315 creating query objects is not necessary.
318 void ZOOM_resultset_option_set (ZOOM_resultset r,
322 const char *ZOOM_resultset_option_get (ZOOM_resultset r,
325 size_t ZOOM_resultset_size (ZOOM_resultset r);
328 Functions <function>ZOOM_resultset_options_set</function> and
329 <function>ZOOM_resultset_get</function> sets and gets an option
330 for a result set similar to <function>ZOOM_connection_option_get</function>
331 and <function>ZOOM_connection_option_set</function>.
334 The number of hits also called result-count is returned by
335 function <function>ZOOM_resultset_size</function>.
337 <table frame="top"><title>ZOOM Result set Options</title>
339 <colspec colwidth="4*" colname="name"></colspec>
340 <colspec colwidth="7*" colname="description"></colspec>
341 <colspec colwidth="2*" colname="default"></colspec>
344 <entry>Option</entry>
345 <entry>Description</entry>
346 <entry>Default</entry>
351 piggyback</entry><entry>True (1) if piggyback should be
352 used in searches; false (0) if not.
353 </entry><entry>1</entry></row>
355 start</entry><entry>Offset of first record to be
356 retrieved from target. First record has offset 0 unlike the
357 protocol specifications where first record has position 1.
358 </entry><entry>0</entry></row>
360 count</entry><entry>Number of records to be retrieved.
361 </entry><entry>0</entry></row>
363 elementSetName</entry><entry>Element-Set name of records.
364 Most targets should honor element set name <literal>B</literal>
365 and <literal>F</literal> for brief and full respectively.
366 </entry><entry>none</entry></row>
368 preferredRecordSyntax</entry><entry>Preferred Syntax, such as
369 <literal>USMARC</literal>, <literal>SUTRS</literal>, etc.
370 </entry><entry>none</entry></row>
372 schema</entry><entry>Schema for retrieval, such as
373 <literal>Gils-schema</literal>, <literal>Geo-schema</literal>, etc.
374 </entry><entry>none</entry></row>
376 smallSetUpperBound</entry><entry>If hits is less than or equal to this
377 value, then target will return all records using small element set name
378 </entry><entry>0</entry></row>
380 largeSetLowerBound</entry><entry>If hits is greater than this
381 value, the target will return no records.
382 </entry><entry>1</entry></row>
384 mediumSetPresentNumber</entry><entry>This value represents
385 the number of records to be returned as part of a search when when
386 hits is less than or equal to large set lower bound and if hits
387 is greater than small set upper bound.
388 </entry><entry>0</entry></row>
390 smallSetElementSetName</entry><entry>
391 The element set name to be used for small result sets.
392 </entry><entry>none</entry></row>
394 mediumSetElementSetName</entry><entry>
395 The element set name to be for medium-sized result sets.
396 </entry><entry>none</entry></row>
398 setname</entry><entry>Name of Result Set (Result Set ID).
399 If this option isn't set, the ZOOM module will automatically
400 allocate a result set name.
401 </entry><entry>default</entry></row>
406 <title>Z39.50 Protocol behavior</title>
408 The creation of a result set involves at least a SearchRequest
409 - SearchResponse protocol handshake. Following that, if a sort
410 criteria was specified as part of the query, a SortRequest -
411 SortResponse handshake takes place. Note that it is necessary to
412 perform sorting before any retrieval takes place, so no records will
413 be returned from the target as part of the SearchResponse because these
414 would be unsorted. Hence, piggyback is disabled when sort criteria
415 is set. Following Search - and a Possible sort, Retrieval takes
416 place - as one or more Present Requests - Present Response being
420 The API allows for two different modes for retrieval. A high level
421 mode which is somewhat more powerful and a low level one.
422 The low level is "enabled" when the settings
423 <literal>smallSetUpperBound</literal>,
424 <literal>mediumSetPresentNumber</literal> and
425 <literal>largeSetLowerBound</literal> are set. The low level mode
426 thus allows you to precisely set how records are returned as part
427 of a search response as offered by the Z39.50 protocol.
428 Since the client may be retrieving records as part of the
429 search response, this mode doesn't work well if sorting is used.
432 The high-level mode allows you to fetch a range of records from
433 the result set with a given start offset. When you use this mode
434 the client will automatically use piggyback if that is possible
435 with the target and perform one or more present requests as needed.
436 Even if the target returns fewer records as part of a present response
437 because of a record size limit, etc. the client will repeat sending
438 present requests. As an example, if option <literal>start</literal>
439 is 0 (default) and <literal>count</literal> is 4, and
440 <literal>piggyback</literal> is 1 (default) and no sorting criteria
441 is specified, then the client will attempt to retrieve the 4
442 records as part the search response (using piggyback). On the other
443 hand, if either <literal>start</literal> is positive or if
444 a sorting criteria is set, or if <literal>piggyback</literal>
445 is 0, then the client will not perform piggyback but send Present
449 If either of the options <literal>mediumSetElementSetName</literal> and
450 <literal>smallSetElementSetName</literal> are unset, the value
451 of option <literal>elementSetName</literal> is used for piggyback
452 searches. This means that for the high-level mode you only have
453 to specify one elementSetName option rather than three.
457 <title>SRW Protocol behavior</title>
459 Current version of &yaz; does not take advantage of a result set id
460 returned by the SRW server. Future versions might do, however.
461 Since, the ZOOM driver does not save result set IDs any
462 present (retrieval) is transformed to a SRW SearchRetrieveRequest
463 with same query but, possibly, different offsets.
466 Option <literal>schema</literal> specifies SRW schema
467 for retrieval. However, options <literal>elementSetName</literal> and
468 <literal>preferredRecordSyntax</literal> are ignored.
471 Options <literal>start</literal> and <literal>count</literal>
472 are supported by SRW.
473 The remaining options
474 <literal>piggyback</literal>,
475 <literal>smallSetUpperBound</literal>,
476 <literal>largeSetLowerBound</literal>,
477 <literal>mediumSetPresentNumber</literal>,
478 <literal>mediumSetElementSetName</literal>,
479 <literal>smallSetElementSetName</literal> are
483 SRW supports CQL queries, <emphasis>not</emphasis> PQF.
484 If PQF is used, however, the PQF query is transferred anyway
485 using non-standard element <literal>pQuery</literal> in
486 SRW SearchRetrieveRequest.
489 Unfortunately, SRW does not define a database setting. Hence,
490 <literal>databaseName</literal> is unsupported and ignored.
491 However, the path part in host parameter for functions
492 <function>ZOOM_connecton_new</function> and
493 <function>ZOOM_connection_connect</function> acts as a
494 database (at least for the &yaz; SRW server).
498 <sect1 id="zoom.records"><title>Records</title>
500 A record object is a retrieval record on the client side -
501 created from result sets.
504 void ZOOM_resultset_records (ZOOM_resultset r,
506 size_t start, size_t count);
507 ZOOM_record ZOOM_resultset_record (ZOOM_resultset s, size_t pos);
509 const char *ZOOM_record_get (ZOOM_record rec, const char *type,
512 ZOOM_record ZOOM_record_clone (ZOOM_record rec);
514 void ZOOM_record_destroy (ZOOM_record rec);
517 References to temporary records are returned by functions
518 <function>ZOOM_resultset_records</function> or
519 <function>ZOOM_resultset_record</function>.
522 If a persistent reference to a record is desired
523 <function>ZOOM_record_clone</function> should be used.
524 It returns a record reference that should be destroyed
525 by a call to <function>ZOOM_record_destroy</function>.
528 A single record is returned by function
529 <function>ZOOM_resultset_record</function> that takes a
530 position as argument. First record has position zero.
531 If no record could be obtained <literal>NULL</literal> is returned.
534 Function <function>ZOOM_resultset_records</function> retrieves
535 a number of records from a result set. Parameter <literal>start</literal>
536 and <literal>count</literal> specifies the range of records to
537 be returned. Upon completion array
538 <literal>recs[0], ..recs[count-1]</literal>
539 holds record objects for the records. The array of records
540 <literal>recs</literal> should be allocated prior the call
541 <function>ZOOM_resultset_records</function>. Note that for those
542 records that couldn't be retrieved from the target
543 <literal>recs[ ..]</literal> is set to <literal>NULL</literal>.
545 <para id="zoom.record.get">
546 In order to extract information about a single record,
547 <function>ZOOM_record_get</function> is provided. The
548 function returns a pointer to certain record information. The
549 nature (type) of the pointer depends on the parameter,
550 <function>type</function>.
551 In addition, for certain types, the length
552 <literal>len</literal> passed will be set to the size in bytes of
553 the returned information.
555 <varlistentry><term><literal>database</literal></term>
556 <listitem><para>Database of record is returned
557 as a C null-terminated string. Return type
558 <literal>const char *</literal>.
561 <varlistentry><term><literal>syntax</literal></term>
562 <listitem><para>The transfer syntax (OID) of the record is returned
563 as a C null-terminated string. Return type is
564 <literal>const char *</literal>.
567 <varlistentry><term><literal>render</literal></term>
568 <listitem><para>The record is returned in a display friendly
569 format. Upon completion buffer is returned
570 (type <literal>const char *</literal>) and length is stored in
571 <literal>*len</literal>.
574 <varlistentry><term><literal>raw</literal></term>
575 <listitem><para>The record is returned in the internal
576 YAZ specific format. For GRS-1, Explain, and others, the
577 raw data is returned as type
578 <literal>Z_External *</literal> which is just the type for
579 the member <literal>retrievalRecord</literal> in
580 type <literal>NamePlusRecord</literal>.
581 For SUTRS and octet aligned record (including all MARCs) the
582 octet buffer is returned and the length of the buffer.
587 <sect2><title>Z39.50 Protocol behavior</title>
589 The functions <function>ZOOM_resultset_record</function> and
590 <function>ZOOM_resultset_records</function> inspects the client-side
591 record cache. Records not found in cache are fetched using
593 The functions may block (and perform network I/O) - even though option
594 <literal>async</literal> is 1, because they return records objects.
595 (and there's no way to return records objects without retrieving them!).
598 There is a trick, however, in the usage of function
599 <function>ZOOM_resultset_records</function> that allows for
600 delayed retrieval (and makes it non-blocking). By passing
601 a null pointer for <parameter>recs</parameter> you're indicating
602 you're not interested in getting records objects
603 <emphasis>now</emphasis>.
606 <sect2><title>SRW Protocol behavior</title>
608 The ZOOM driver for SRW treats records returned by a SRW server
609 as if they where Z39.50 records with transfer syntax XML and
610 no element set name or database name.
614 <sect1 id="zoom.scan"><title>Scan</title>
616 This section describes an interface for Scan. Scan is not an
617 official part of the ZOOM model yet. The result of a scan operation
618 is the <literal>ZOOM_scanset</literal> which is a set of terms
619 returned by a target.
623 The Scan interface is Z39.50 only. SRW version 1.0 does not
628 ZOOM_scanset ZOOM_connection_scan (ZOOM_connection c,
629 const char *startterm);
631 size_t ZOOM_scanset_size(ZOOM_scanset scan);
633 const char * ZOOM_scanset_term(ZOOM_scanset scan, size_t pos,
634 int *occ, size_t *len);
637 void ZOOM_scanset_destroy (ZOOM_scanset scan);
639 const char *ZOOM_scanset_option_get (ZOOM_scanset scan,
642 void ZOOM_scanset_option_set (ZOOM_scanset scan, const char *key,
646 The scan set is created by function
647 <function>ZOOM_connection_scan</function> which performs a scan
648 operation on the connection and start term given.
649 If the operation was successful, the size of the scan set can be
650 retrieved by a call to <function>ZOOM_scanset_size</function>.
651 Like result sets, the items are numbered 0,..size-1.
652 To obtain information about a particular scan term, call function
653 <function>ZOOM_scanset_term</function>. This function takes
654 a scan set offset <literal>pos</literal> and returns a pointer
655 to an actual term or <literal>NULL</literal> if non-present.
656 If present, the <literal>occ</literal> and <literal>len</literal>
657 are set to the number of occurrences and the length
658 of the actual term respectively.
659 A scan set may be freed by a call to function
660 <function>ZOOM_scanset_destroy</function>.
661 Functions <function>ZOOM_scanset_option_get</function> and
662 <function>ZOOM_scanset_option_set</function> retrieves and sets
663 an option respectively.
666 <table frame="top"><title>ZOOM Scan Set Options</title>
668 <colspec colwidth="4*" colname="name"></colspec>
669 <colspec colwidth="7*" colname="description"></colspec>
670 <colspec colwidth="2*" colname="default"></colspec>
673 <entry>Option</entry>
674 <entry>Description</entry>
675 <entry>Default</entry>
680 number</entry><entry>Number of Scan Terms requested in next scan.
681 After scan it holds the actual number of terms returned.
682 </entry><entry>10</entry></row>
684 position</entry><entry>Preferred Position of term in response
685 in next scan; actual position after completion of scan.
686 </entry><entry>1</entry></row>
688 stepSize</entry><entry>Step Size
689 </entry><entry>0</entry></row>
691 scanStatus</entry><entry>An integer indicating the Scan Status
693 </entry><entry>0</entry></row>
699 <sect1 id="zoom.options"><title>Options</title>
701 Most &zoom; objects provide a way to specify options to change behavior.
702 From an implementation point of view a set of options is just like
703 an associative array / hash array, etc.
706 ZOOM_options ZOOM_options_create (void);
708 ZOOM_options ZOOM_options_create_with_parent (ZOOM_options parent);
710 void ZOOM_options_destroy (ZOOM_options opt);
713 const char *ZOOM_options_get (ZOOM_options opt, const char *name);
715 void ZOOM_options_set (ZOOM_options opt, const char *name,
719 typedef const char *(*ZOOM_options_callback)
720 (void *handle, const char *name);
722 ZOOM_options_callback
723 ZOOM_options_set_callback (ZOOM_options opt,
724 ZOOM_options_callback c,
728 <sect1 id="zoom.events"><title>Events</title>
730 If you're developing non-blocking applications, you have to deal
734 int ZOOM_event (int no, ZOOM_connection *cs);
737 The <function>ZOOM_event</function> executes pending events for
738 a number of connections. Supply the number of connections in
739 <literal>no</literal> and an array of connections in
740 <literal>cs</literal> (<literal>cs[0] ... cs[no-1]</literal>).
741 A pending event could be a sending a search, receiving a response,
743 When an event has occurred for one of the connections, this function
744 returns a positive integer <literal>n</literal> denoting that an event
745 occurred for connection <literal>cs[n-1]</literal>.
746 When no events are pending for the connections, a value of zero is
748 To ensure that all outstanding requests are performed call this function
749 repeatedly until zero is returned.
754 <!-- Keep this comment at the end of the file
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