The ASN module provides you with a set of C struct definitions for the
various PDUs of the protocol, as well as for the complex types
appearing within the PDUs. For the primitive data types, the C
representation often takes the form of an ordinary C language type,
such as int. For ASN.1 constructs that have no direct
representation in C, such as general octet strings and bit strings,
the ODR module (see section
ODR) provides auxiliary
definitions.
A structure representing a complex ASN.1 type doesn't in itself contain the members of that type. Instead, the structure contains pointers to the members of the type. This is necessary, in part, to allow a mechanism for specifying which of the optional structure (SEQUENCE) members are present, and which are not. It follows that you will need to somehow provide space for the individual members of the structure, and set the pointers to refer to the members.
The conversion routines don't care how you allocate and maintain your C structures - they just follow the pointers that you provide. Depending on the complexity of your application, and your personal taste, there are at least three different approaches that you may take when you allocate the structures.
malloc(2) function. If you want to ensure that the data is freed when
it is no longer needed, you will have to define a function that
individually releases each member of a structure before freeing the
structure itself.odr_malloc() function (see section
Using ODR for details). When you use odr_malloc(), you can release
all of the
allocated data in a single operation, independent of any pointers and
relations between the data. odr_malloc() is based on a
"nibble-memory"
scheme, in which large portions of memory are allocated, and then
gradually handed out with each call to odr_malloc(). The next time you
call odr_reset(), all of the memory allocated since the last call is
recycled for future use (actually, it is placed on a free-list).odr_malloc() to
allocate an entire structure and some of its elements, while you leave
other elements pointing to global or per-session default variables.The ASN module provides an important aid in creating new PDUs. For
each of the PDU types (say, Z_InitRequest), a function is provided
that allocates and initializes an instance of that PDU type for you.
In the case of the InitRequest, the function is simply named
zget_InitRequest(), and it sets up reasonable default value for all of
the mandatory members. The optional members are generally initialized to null
pointers. This last aspect is very important: it ensures that if the
PDU definitions are extended after you finish your implementation
(to accommodate
new versions of the protocol, say), you won't get into trouble with
uninitialized pointers in your structures. The functions use
odr_malloc() to
allocate the PDUs and its members, so you can free everything again with a
single call to odr_reset(). We strongly recommend that you use the
zget_*
functions whenever you are preparing a PDU (in a C++ API, the
zget_
functions would probably be promoted to constructors for the
individual types).
The prototype for the individual PDU types generally look like this:
Z_<type> *zget_<type>(ODR o);
eg.:
Z_InitRequest *zget_InitRequest(ODR o);
The ODR handle should generally be your encoding stream, but it needn't be.
As well as the individual PDU functions, a function zget_APDU() is
provided, which allocates a toplevel Z-APDU of the type requested:
Z_APDU *zget_APDU(ODR o, int which);
The which parameter is (of course) the discriminator belonging to the
Z_APDU CHOICE type. All of the interface described here is provided by
the ASN module, and you access it through the proto.h header file.
When you refer to object identifiers in your application, you need to be aware that SR and Z39.50 use two different set of OIDs to refer to the same objects. To handle this easily, YAZ provides a utility module to ASN which provides an internal representation of the OIDs used in both protocols. Each oid is described by a structure:
typedef struct oident
{
enum oid_proto proto;
enum oid_class class;
enum oid_value value;
int oidsuffix[OID_SIZE];
char *desc;
} oident;
The proto field can be set to either PROTO_SR or
PROTO_Z3950. The class might be, say, CLASS_RECSYN, and the
value might be VAL_USMARC for the USMARC record format. Functions
int *oid_ent_to_oid(struct oident *ent, int *dst);
struct oident *oid_getentbyoid(int *o);
are provided to map between object identifiers and database entries.
If you store a member of the oid_proto type in your association
state information, it's a simple matter, at runtime, to generate the
correct OID when you need it. For decoding, you can simply ignore the
proto field, or if you're strict, you can verify that your peer is
using the OID family from the correct protocol. The desc field is
a short, human-readable name for the PDU, useful mainly for diagnostic
output.
NOTE: The old function oid_getoidbyent still exists but is not thread safe. Use oid_ent_to_oid instead and pass an array of size OID_SIZE.
NOTE: Plans are underway to merge the two protocols into a single definition, with one set of object identifiers. When this happens, the oid module will no longer be required to support protocol independence, but it should still be useful as a simple OID database.
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 ASN module defines a
specialized version of the EXTERNAL construct, called Z_External.
It is defined thus:
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;
When decoding, the ASN 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 which 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 which 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
octet_aligned arm of the union.
Some servers return ASN.1 structured data values (eg. database
records) as BER-encoded records placed in the octet-aligned
branch of the EXTERNAL CHOICE. The ASN-module will not
automatically decode these records. To help you decode the records in
the application, the function
Z_ext_typeent *z_ext_gettypebyref(oid_value ref);
Can be used to retrieve information about the known, external data
types. The function return a pointer to a static area, or NULL, if no
match for the given direct reference is found. The Z_ext_typeent
is defined as:
typedef struct Z_ext_typeent
{
oid_value dref; /* the direct-reference OID value. */
int what; /* discriminator value for the external CHOICE */
Odr_fun fun; /* decoder function */
} Z_ext_typeent;
The what member contains the Z_External union discriminator value
for the given type: For the SUTRS record syntax, the value would be
Z_External_sutrs. The fun member contains a pointer to the
function which encodes/decodes the given type. Again, for the SUTRS
record syntax, the value of fun would be z_SUTRS (a function
pointer).
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
z_ext_gettypebyref 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 section
Using ODR).
If you want to send EXTERNALs containing ASN.1-structured values in the occtet-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.
If you need to add new, externally defined data types, you must update
the struct above, in the source file prt-ext.h, as well as the
encoder/decoder in the file prt-ext.c. When changing the latter,
remember to update both the arm arrary and the list type_table,
which drives the CHOICE biasing that is necessary to tell the
different, structured types apart on decoding.
NOTE: 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.
We include, for reference, a listing of the fields of each top-level PDU, as well as their default settings.
Z_InitRequest ------------- Field Type Default value referenceId Z_ReferenceId NULL protocolVersion Odr_bitmask Empty bitmask options Odr_bitmask Empty bitmask preferredMessageSize int 30*1024 maximumRecordSize int 30*1024 idAuthentication Z_IdAuthentication NULL implementationId char* "YAZ (id=81)" implementationName char* "Index Data/YAZ" implementationVersion char* YAZ_VERSION userInformationField Z_UserInformation NULL otherInfo Z_OtherInformation NULL Z_InitResponse -------------- Field Type Default value referenceId Z_ReferenceId NULL protocolVersion Odr_bitmask Empty bitmask options Odr_bitmask Empty bitmask preferredMessageSize int 30*1024 maximumRecordSize int 30*1024 result bool_t TRUE implementationId char* "YAZ (id=81)" implementationName char* "Index Data/YAZ" implementationVersion char* YAZ_VERSION userInformationField Z_UserInformat.. NULL otherInfo Z_OtherInformation NULL Z_SearchRequest --------------- Field Type Default value referenceId Z_ReferenceId NULL smallSetUpperBound int 0 largeSetLowerBound int 1 mediumSetPresentNumber int 0 replaceIndicator bool_t TRUE resultSetName char* "default" num_databaseNames int 0 databaseNames char** NULL smallSetElementSetNames Z_ElementSetNames NULL mediumSetElementSetNames Z_ElementSetNames NULL preferredRecordSyntax Odr_oid NULL query Z_Query NULL additionalSearchInfo Z_OtherInformation NULL otherInfo Z_OtherInformation NULL Z_SearchResponse ---------------- Field Type Default value referenceId Z_ReferenceId NULL resultCount int 0 numberOfRecordsReturned int 0 nextResultSetPosition int 0 searchStatus bool_t TRUE resultSetStatus int NULL presentStatus int NULL records Z_Records NULL additionalSearchInfo Z_OtherInformation NULL otherInfo Z_OtherInformation NULL Z_PresentRequest ---------------- Field Type Default value referenceId Z_ReferenceId NULL resultSetId char* "default" resultSetStartPoint int 1 numberOfRecordsRequested int 10 num_ranges int 0 additionalRanges Z_Range NULL recordComposition Z_RecordComposition NULL preferredRecordSyntax Odr_oid NULL maxSegmentCount int NULL maxRecordSize int NULL maxSegmentSize int NULL otherInfo Z_OtherInformation NULL Z_PresentResponse ----------------- Field Type Default value referenceId Z_ReferenceId NULL numberOfRecordsReturned int 0 nextResultSetPosition int 0 presentStatus int Z_PRES_SUCCESS records Z_Records NULL otherInfo Z_OtherInformation NULL Z_DeleteResultSetRequest ------------------------ Field Type Default value referenceId Z_ReferenceId NULL deleteFunction int Z_DeleteRequest_list num_ids int 0 resultSetList char** NULL otherInfo Z_OtherInformation NULL Z_DeleteResultSetResponse ------------------------- Field Type Default value referenceId Z_ReferenceId NULL deleteOperationStatus int Z_DeleteStatus_success num_statuses int 0 deleteListStatuses Z_ListStatus** NULL numberNotDeleted int NULL num_bulkStatuses int 0 bulkStatuses Z_ListStatus NULL deleteMessage char* NULL otherInfo Z_OtherInformation NULL Z_ScanRequest ------------- Field Type Default value referenceId Z_ReferenceId NULL num_databaseNames int 0 databaseNames char** NULL attributeSet Odr_oid NULL termListAndStartPoint Z_AttributesPlus... NULL stepSize int NULL numberOfTermsRequested int 20 preferredPositionInResponse int NULL otherInfo Z_OtherInformation NULL Z_ScanResponse -------------- Field Type Default value referenceId Z_ReferenceId NULL stepSize int NULL scanStatus int Z_Scan_success numberOfEntriesReturned int 0 positionOfTerm int NULL entries Z_ListEntris NULL attributeSet Odr_oid NULL otherInfo Z_OtherInformation NULL Z_TriggerResourceControlRequest ------------------------------- Field Type Default value referenceId Z_ReferenceId NULL requestedAction int Z_TriggerResourceCtrl_resou.. prefResourceReportFormat Odr_oid NULL resultSetWanted bool_t NULL otherInfo Z_OtherInformation NULL Z_ResourceControlRequest ------------------------ Field Type Default value referenceId Z_ReferenceId NULL suspendedFlag bool_t NULL resourceReport Z_External NULL partialResultsAvailable int NULL responseRequired bool_t FALSE triggeredRequestFlag bool_t NULL otherInfo Z_OtherInformation NULL Z_ResourceControlResponse ------------------------- Field Type Default value referenceId Z_ReferenceId NULL continueFlag bool_t TRUE resultSetWanted bool_t NULL otherInfo Z_OtherInformation NULL Z_AccessControlRequest ---------------------- Field Type Default value referenceId Z_ReferenceId NULL which enum Z_AccessRequest_simpleForm; u union NULL otherInfo Z_OtherInformation NULL Z_AccessControlResponse ----------------------- Field Type Default value referenceId Z_ReferenceId NULL which enum Z_AccessResponse_simpleForm u union NULL diagnostic Z_DiagRec NULL otherInfo Z_OtherInformation NULL Z_Segment --------- Field Type Default value referenceId Z_ReferenceId NULL numberOfRecordsReturned int value=0 num_segmentRecords int 0 segmentRecords Z_NamePlusRecord NULL otherInfo Z_OtherInformation NULL Z_Close ------- Field Type Default value referenceId Z_ReferenceId NULL closeReason int Z_Close_finished diagnosticInformation char* NULL resourceReportFormat Odr_oid NULL resourceFormat Z_External NULL otherInfo Z_OtherInformation NULL