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<html><head><meta charset="ISO-8859-1"><title>3.EXTERNAL Data</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot"><link rel="home" href="index.html" title="YAZ User's Guide and Reference"><link rel="up" href="asn.html" title="Chapter5.The Z39.50 ASN.1 Module"><link rel="prev" href="asn.preparing.html" title="2.Preparing PDUs"><link rel="next" href="asn.pdu.html" title="4.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">3.EXTERNAL Data</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="asn.preparing.html">Prev</a></td><th width="60%" align="center">Chapter5.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"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="asn.external"></a>3.EXTERNAL Data</h2></div></div></div><p>
In order to achieve extensibility and adaptability to different
application domains, the new version of the protocol defines many
structures outside of the main ASN.1 specification, referencing them
through ASN.1 EXTERNAL constructs. To simplify the construction and
access to the externally referenced data, the Z39.50 ASN.1 module defines a
specialized version of the EXTERNAL construct, called
<code class="literal">Z_External</code>.It is defined thus:
</p><pre class="screen">
typedef struct Z_External
{
Odr_oid *direct_reference;
int *indirect_reference;
char *descriptor;
enum
{
/* Generic types */
Z_External_single = 0,
Z_External_octet,
Z_External_arbitrary,
/* Specific types */
Z_External_SUTRS,
Z_External_explainRecord,
Z_External_resourceReport1,
Z_External_resourceReport2
...
} which;
union
{
/* Generic types */
Odr_any *single_ASN1_type;
Odr_oct *octet_aligned;
Odr_bitmask *arbitrary;
/* Specific types */
Z_SUTRS *sutrs;
Z_ExplainRecord *explainRecord;
Z_ResourceReport1 *resourceReport1;
Z_ResourceReport2 *resourceReport2;
...
} u;
} Z_External;
</pre><p>
When decoding, the Z39.50 ASN.1 module will attempt to determine which
syntax describes the data by looking at the reference fields
(currently only the direct-reference). For ASN.1 structured data, you
need only consult the <code class="literal">which</code> field to determine the
type of data. You can the access the data directly through the union.
When constructing data for encoding, you set the union pointer to point
to the data, and set the <code class="literal">which</code> field accordingly.
Remember also to set the direct (or indirect) reference to the correct
OID for the data type.
For non-ASN.1 data such as MARC records, use the
<code class="literal">octet_aligned</code> arm of the union.
</p><p>
Some servers return ASN.1 structured data values (e.g. database
records) as BER-encoded records placed in the
<code class="literal">octet-aligned</code> branch of the EXTERNAL CHOICE.
The ASN-module will <span class="emphasis"><em>not</em></span> automatically decode
these records. To help you decode the records in the application, the
function
</p><pre class="screen">
Z_ext_typeent *z_ext_gettypebyref(const oid *oid);
</pre><p>
can be used to retrieve information about the known, external data
types. The function returns a pointer to a static area, or NULL, if no
match for the given direct reference is found. The
<code class="literal">Z_ext_typeent</code>
is defined as:
</p><pre class="screen">
typedef struct Z_ext_typeent
{
int oid[OID_SIZE]; /* the direct-reference OID. */
int what; /* discriminator value for the external CHOICE */
Odr_fun fun; /* decoder function */
} Z_ext_typeent;
</pre><p>
The <code class="literal">what</code> member contains the
<code class="literal">Z_External</code> union discriminator value for the
given type: For the SUTRS record syntax, the value would be
<code class="literal">Z_External_sutrs</code>.
The <code class="literal">fun</code> member contains a pointer to the
function which encodes/decodes the given type. Again, for the SUTRS
record syntax, the value of <code class="literal">fun</code> would be
<code class="literal">z_SUTRS</code> (a function pointer).
</p><p>
If you receive an EXTERNAL which contains an octet-string value that
you suspect of being an ASN.1-structured data value, you can use
<code class="literal">z_ext_gettypebyref</code> to look for the provided
direct-reference.
If the return value is different from NULL, you can use the provided
function to decode the BER string (see <a class="xref" href="odr.use.html" title="2.Using ODR">Section2, “Using ODR”</a>
).
</p><p>
If you want to <span class="emphasis"><em>send</em></span> EXTERNALs containing
ASN.1-structured values in the octet-aligned branch of the CHOICE, this
is possible too. However, on the encoding phase, it requires a somewhat
involved juggling around of the various buffers involved.
</p><p>
If you need to add new, externally defined data types, you must update
the struct above, in the source file <code class="filename">prt-ext.h</code>, as
well as the encoder/decoder in the file <code class="filename">prt-ext.c</code>.
When changing the latter, remember to update both the
<code class="literal">arm</code> array and the list
<code class="literal">type_table</code>, which drives the CHOICE biasing that
is necessary to tell the different, structured types apart
on decoding.
</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>
Eventually, the EXTERNAL processing will most likely
automatically insert the correct OIDs or indirect-refs. First,
however, we need to determine how application-context management
(specifically the presentation-context-list) should fit into the
various modules.
</p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="asn.preparing.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">2.Preparing PDUs</td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top">4.PDU Contents Table</td></tr></table></div></body></html>
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