1 <html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>4. EXTERNAL Data</title><meta name="generator" content="DocBook XSL Stylesheets V1.73.2"><link rel="start" href="index.html" title="YAZ User's Guide and Reference"><link rel="up" href="asn.html" title="Chapter 7. The Z39.50 ASN.1 Module"><link rel="prev" href="asn.oid.html" title="3. Object Identifiers (YAZ 2)"><link rel="next" href="asn.pdu.html" title="5. PDU Contents Table"></head><body><link rel="stylesheet" type="text/css" href="common/style1.css"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">4. EXTERNAL Data</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="asn.oid.html">Prev</a> </td><th width="60%" align="center">Chapter 7. The Z39.50 ASN.1 Module</th><td width="20%" align="right"> <a accesskey="n" href="asn.pdu.html">Next</a></td></tr></table><hr></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="asn.external"></a>4. EXTERNAL Data</h2></div></div></div><p>
2 In order to achieve extensibility and adaptability to different
3 application domains, the new version of the protocol defines many
4 structures outside of the main ASN.1 specification, referencing them
5 through ASN.1 EXTERNAL constructs. To simplify the construction and
6 access to the externally referenced data, the Z39.50 ASN.1 module defines a
7 specialized version of the EXTERNAL construct, called
8 <code class="literal">Z_External</code>.It is defined thus:
9 </p><pre class="screen">
10 typedef struct Z_External
12 Odr_oid *direct_reference;
13 int *indirect_reference;
18 Z_External_single = 0,
24 Z_External_explainRecord,
25 Z_External_resourceReport1,
26 Z_External_resourceReport2
34 Odr_any *single_ASN1_type;
35 Odr_oct *octet_aligned;
36 Odr_bitmask *arbitrary;
40 Z_ExplainRecord *explainRecord;
41 Z_ResourceReport1 *resourceReport1;
42 Z_ResourceReport2 *resourceReport2;
49 When decoding, the Z39.50 ASN.1 module will attempt to determine which
50 syntax describes the data by looking at the reference fields
51 (currently only the direct-reference). For ASN.1 structured data, you
52 need only consult the <code class="literal">which</code> field to determine the
53 type of data. You can the access the data directly through the union.
54 When constructing data for encoding, you set the union pointer to point
55 to the data, and set the <code class="literal">which</code> field accordingly.
56 Remember also to set the direct (or indirect) reference to the correct
57 OID for the data type.
58 For non-ASN.1 data such as MARC records, use the
59 <code class="literal">octet_aligned</code> arm of the union.
61 Some servers return ASN.1 structured data values (eg. database
62 records) as BER-encoded records placed in the
63 <code class="literal">octet-aligned</code> branch of the EXTERNAL CHOICE.
64 The ASN-module will <span class="emphasis"><em>not</em></span> automatically decode
65 these records. To help you decode the records in the application, the
67 </p><pre class="screen">
68 Z_ext_typeent *z_ext_gettypebyref(const oid *oid);
70 Can be used to retrieve information about the known, external data
71 types. The function return a pointer to a static area, or NULL, if no
72 match for the given direct reference is found. The
73 <code class="literal">Z_ext_typeent</code>
75 </p><pre class="screen">
76 typedef struct Z_ext_typeent
78 int oid[OID_SIZE]; /* the direct-reference OID. */
79 int what; /* discriminator value for the external CHOICE */
80 Odr_fun fun; /* decoder function */
83 The <code class="literal">what</code> member contains the
84 <code class="literal">Z_External</code> union discriminator value for the
85 given type: For the SUTRS record syntax, the value would be
86 <code class="literal">Z_External_sutrs</code>.
87 The <code class="literal">fun</code> member contains a pointer to the
88 function which encodes/decodes the given type. Again, for the SUTRS
89 record syntax, the value of <code class="literal">fun</code> would be
90 <code class="literal">z_SUTRS</code> (a function pointer).
92 If you receive an EXTERNAL which contains an octet-string value that
93 you suspect of being an ASN.1-structured data value, you can use
94 <code class="literal">z_ext_gettypebyref</code> to look for the provided
96 If the return value is different from NULL, you can use the provided
97 function to decode the BER string (see <a class="xref" href="odr.use.html" title="2. Using ODR">Section 2, “Using ODR”</a>
100 If you want to <span class="emphasis"><em>send</em></span> EXTERNALs containing
101 ASN.1-structured values in the occtet-aligned branch of the CHOICE, this
102 is possible too. However, on the encoding phase, it requires a somewhat
103 involved juggling around of the various buffers involved.
105 If you need to add new, externally defined data types, you must update
106 the struct above, in the source file <code class="filename">prt-ext.h</code>, as
107 well as the encoder/decoder in the file <code class="filename">prt-ext.c</code>.
108 When changing the latter, remember to update both the
109 <code class="literal">arm</code> arrary and the list
110 <code class="literal">type_table</code>, which drives the CHOICE biasing that
111 is necessary to tell the different, structured types apart
113 </p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>
114 Eventually, the EXTERNAL processing will most likely
115 automatically insert the correct OIDs or indirect-refs. First,
116 however, we need to determine how application-context management
117 (specifically the presentation-context-list) should fit into the
119 </p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="asn.oid.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="asn.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="asn.pdu.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">3. Object Identifiers (YAZ 2) </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 5. PDU Contents Table</td></tr></table></div></body></html>