1 <chapter id="odr"><title>The ODR Module</title>
3 <sect1 id="odr.introduction"><title>Introduction</title>
6 &odr; is the BER-encoding/decoding subsystem of &yaz;. Care as been taken
7 to isolate &odr; from the rest of the package - specifically from the
8 transport interface. &odr; may be used in any context where basic
9 ASN.1/BER representations are used.
13 If you are only interested in writing a Z39.50 implementation based on
14 the PDUs that are already provided with &yaz;, you only need to concern
15 yourself with the section on managing ODR streams
16 (<xref linkend="odr.use"/>). Only if you need to
17 implement ASN.1 beyond that which has been provided, should you
18 worry about the second half of the documentation
19 (<xref linkend="odr.programming"/>).
20 If you use one of the higher-level interfaces, you can skip this
25 This is important, so we'll repeat it for emphasis: <emphasis>You do
26 not need to read <xref linkend="odr.programming"/>
27 to implement Z39.50 with &yaz;.</emphasis>
31 If you need a part of the protocol that isn't already in &yaz;, you
32 should contact the authors before going to work on it yourself: We
33 might already be working on it. Conversely, if you implement a useful
34 part of the protocol before us, we'd be happy to include it in a
39 <sect1 id="odr.use"><title>Using ODR</title>
41 <sect2 id="odr.streams"><title>ODR Streams</title>
44 Conceptually, the ODR stream is the source of encoded data in the
45 decoding mode; when encoding, it is the receptacle for the encoded
46 data. Before you can use an ODR stream it must be allocated. This is
47 done with the function
51 ODR odr_createmem(int direction);
55 The <function>odr_createmem()</function> function takes as argument one
56 of three manifest constants: <literal>ODR_ENCODE</literal>,
57 <literal>ODR_DECODE</literal>, or <literal>ODR_PRINT</literal>.
58 An &odr; stream can be in only one mode - it is not possible to change
59 its mode once it's selected. Typically, your program will allocate
60 at least two ODR streams - one for decoding, and one for encoding.
64 When you're done with the stream, you can use
68 void odr_destroy(ODR o);
72 to release the resources allocated for the stream.
76 <sect2 id="odr.memory.management"><title id="memory">Memory Management</title>
79 Two forms of memory management take place in the &odr; system. The first
80 one, which has to do with allocating little bits of memory (sometimes
81 quite large bits of memory, actually) when a protocol package is
82 decoded, and turned into a complex of interlinked structures. This
83 section deals with this system, and how you can use it for your own
84 purposes. The next section deals with the memory management which is
85 required when encoding data - to make sure that a large enough buffer is
86 available to hold the fully encoded PDU.
90 The &odr; module has its own memory management system, which is
91 used whenever memory is required. Specifically, it is used to allocate
92 space for data when decoding incoming PDUs. You can use the memory
93 system for your own purposes, by using the function
97 void *odr_malloc(ODR o, int size);
101 You can't use the normal <function>free(2)</function> routine to free
102 memory allocated by this function, and &odr; doesn't provide a parallel
103 function. Instead, you can call
107 void odr_reset(ODR o, int size);
111 when you are done with the
112 memory: Everything allocated since the last call to
113 <function>odr_reset()</function> is released.
114 The <function>odr_reset()</function> call is also required to clear
115 up an error condition on a stream.
123 int odr_total(ODR o);
127 returns the number of bytes allocated on the stream since the last call to
128 <function>odr_reset()</function>.
132 The memory subsystem of &odr; is fairly efficient at allocating and
133 releasing little bits of memory. Rather than managing the individual,
134 small bits of space, the system maintains a free-list of larger chunks
135 of memory, which are handed out in small bits. This scheme is
136 generally known as a <emphasis>nibble memory</emphasis> system.
137 It is very useful for maintaining short-lived constructions such
142 If you want to retain a bit of memory beyond the next call to
143 <function>odr_reset()</function>, you can use the function
147 ODR_MEM odr_extract_mem(ODR o);
151 This function will give you control of the memory recently allocated
152 on the ODR stream. The memory will live (past calls to
153 <function>odr_reset()</function>), until you call the function
157 void odr_release_mem(ODR_MEM p);
161 The opaque <literal>ODR_MEM</literal> handle has no other purpose than
162 referencing the memory block for you until you want to release it.
166 You can use <function>odr_extract_mem()</function> repeatedly between
167 allocating data, to retain individual control of separate chunks of data.
171 <sect2 id="odr.encoding.and.decoding"><title>Encoding and Decoding Data</title>
174 When encoding data, the ODR stream will write the encoded octet string
175 in an internal buffer. To retrieve the data, use the function
179 char *odr_getbuf(ODR o, int *len, int *size);
183 The integer pointed to by len is set to the length of the encoded
184 data, and a pointer to that data is returned. <literal>*size</literal>
185 is set to the size of the buffer (unless <literal>size</literal> is null,
186 signaling that you are not interested in the size). The next call to
187 a primitive function using the same &odr; stream will overwrite the
188 data, unless a different buffer has been supplied using the call
192 void odr_setbuf(ODR o, char *buf, int len, int can_grow);
196 which sets the encoding (or decoding) buffer used by
197 <literal>o</literal> to <literal>buf</literal>, using the length
198 <literal>len</literal>.
199 Before a call to an encoding function, you can use
200 <function>odr_setbuf()</function> to provide the stream with an encoding
201 buffer of sufficient size (length). The <literal>can_grow</literal>
202 parameter tells the encoding &odr; stream whether it is allowed to use
203 <function>realloc(2)</function> to increase the size of the buffer when
204 necessary. The default condition of a new encoding stream is equivalent
205 to the results of calling
209 odr_setbuf(stream, 0, 0, 1);
213 In this case, the stream will allocate and reallocate memory as
214 necessary. The stream reallocates memory by repeatedly doubling the
215 size of the buffer - the result is that the buffer will typically
216 reach its maximum, working size with only a small number of reallocation
217 operations. The memory is freed by the stream when the latter is destroyed,
218 unless it was assigned by the user with the <literal>can_grow</literal>
219 parameter set to zero (in this case, you are expected to retain
220 control of the memory yourself).
224 To assume full control of an encoded buffer, you must first call
225 <function>odr_getbuf()</function> to fetch the buffer and its length.
226 Next, you should call <function>odr_setbuf()</function> to provide a
227 different buffer (or a null pointer) to the stream. In the simplest
228 case, you will reuse the same buffer over and over again, and you
229 will just need to call <function>odr_getbuf()</function> after each
230 encoding operation to get the length and address of the buffer.
231 Note that the stream may reallocate the buffer during an encoding
232 operation, so it is necessary to retrieve the correct address after
233 each encoding operation.
237 It is important to realize that the ODR stream will not release this
238 memory when you call <function>odr_reset()</function>: It will
239 merely update its internal pointers to prepare for the encoding of a
241 When the stream is released by the <function>odr_destroy()</function>
242 function, the memory given to it by <function>odr_setbuf</function> will
243 be released <emphasis>only</emphasis> if the <literal>can_grow</literal>
244 parameter to <function>odr_setbuf()</function> was nonzero. The
245 <literal>can_grow</literal> parameter, in other words, is a way of
246 signaling who is to own the buffer, you or the ODR stream. If you never call
247 <function>odr_setbuf()</function> on your encoding stream, which is
248 typically the case, the buffer allocated by the stream will belong to
249 the stream by default.
253 When you wish to decode data, you should first call
254 <function>odr_setbuf()</function>, to tell the decoding stream
255 where to find the encoded data, and how long the buffer is
256 (the <literal>can_grow</literal> parameter is ignored by a decoding
257 stream). After this, you can call the function corresponding to the
258 data you wish to decode (eg, <function>odr_integer()</function> odr
259 <function>z_APDU()</function>).
262 <example id="example.odr.encoding.and.decoding.functions">
263 <title>Encoding and decoding functions</title>
265 int odr_integer(ODR o, int **p, int optional, const char *name);
267 int z_APDU(ODR o, Z_APDU **p, int optional, const char *name);
272 If the data is absent (or doesn't match the tag corresponding to
273 the type), the return value will be either 0 or 1 depending on the
274 <literal>optional</literal> flag. If <literal>optional</literal>
275 is 0 and the data is absent, an error flag will be raised in the
276 stream, and you'll need to call <function>odr_reset()</function> before
277 you can use the stream again. If <literal>optional</literal> is
278 nonzero, the pointer <emphasis>pointed</emphasis> to/ by
279 <literal>p</literal> will be set to the null value, and the function
281 The <literal>name</literal> argument is used to pretty-print the
282 tag in question. It may be set to <literal>NULL</literal> if
283 pretty-printing is not desired.
287 If the data value is found where it's expected, the pointer
288 <emphasis>pointed to</emphasis> by the <literal>p</literal> argument
289 will be set to point to the decoded type.
290 The space for the type will be allocated and owned by the &odr;
291 stream, and it will live until you call
292 <function>odr_reset()</function> on the stream. You cannot use
293 <function>free(2)</function> to release the memory.
294 You can decode several data elements (by repeated calls to
295 <function>odr_setbuf()</function> and your decoding function), and
296 new memory will be allocated each time. When you do call
297 <function>odr_reset()</function>, everything decoded since the
298 last call to <function>odr_reset()</function> will be released.
301 <example id="example.odr.encoding.of.integer">
302 <title>Encoding and decoding of an integer</title>
304 The use of the double indirection can be a little confusing at first
305 (its purpose will become clear later on, hopefully),
306 so an example is in order. We'll encode an integer value, and
307 immediately decode it again using a different stream. A useless, but
308 informative operation.
310 <programlisting><![CDATA[
311 void do_nothing_useful(int value)
318 /* allocate streams */
319 if (!(encode = odr_createmem(ODR_ENCODE)))
321 if (!(decode = odr_createmem(ODR_DECODE)))
325 if (odr_integer(encode, &valp, 0, 0) == 0)
327 printf("encoding went bad\n");
330 bufferp = odr_getbuf(encode, &len);
331 printf("length of encoded data is %d\n", len);
333 /* now let's decode the thing again */
334 odr_setbuf(decode, bufferp, len);
335 if (odr_integer(decode, &resvalp, 0, 0) == 0)
337 printf("decoding went bad\n");
340 printf("the value is %d\n", *resvalp);
349 This looks like a lot of work, offhand. In practice, the &odr; streams
350 will typically be allocated once, in the beginning of your program
351 (or at the beginning of a new network session), and the encoding
352 and decoding will only take place in a few, isolated places in your
353 program, so the overhead is quite manageable.
359 <sect2 id="odr.printing"><title>Printing</title>
361 When an ODR stream is created of type <literal>ODR_PRINT</literal>
362 the ODR module will print the contents of a PDU in a readable format.
363 By default output is written to the <literal>stderr</literal> stream.
364 This behavior can be changed, however, by calling the function
366 odr_setprint(ODR o, FILE *file);
368 before encoders or decoders are being invoked.
369 It is also possible to direct the output to a buffer (of indeed
370 another file), by using the more generic mechanism:
372 void odr_set_stream(ODR o, void *handle,
373 void (*stream_write)(ODR o, void *handle, int type,
374 const char *buf, int len),
375 void (*stream_close)(void *handle));
377 Here the user provides an opaque handle and two handlers,
378 <replaceable>stream_write</replaceable> for writing,
379 and <replaceable>stream_close</replaceable> which is supposed
380 to close/free resources associated with handle.
381 The <replaceable>stream_close</replaceable> handler is optional and
382 if NULL for the function is provided, it will not be invoked.
383 The <replaceable>stream_write</replaceable> takes the ODR handle
384 as parameter, the user defined handle, a type
385 <literal>ODR_OCTETSTRING</literal>, <literal>ODR_VISIBLESTRING</literal>
386 which indicates the type of contents is being written.
389 Another utility useful for diagnostics (error handling) or as
390 part of the printing facilities is:
392 const char **odr_get_element_path(ODR o);
394 which returns a list of current elements that ODR deals with at the
395 moment. For the returned array, say <literal>ar</literal>,
396 <literal>ar[0]</literal> is the top level element,
397 <literal>ar[n]</literal> is the last. The last element has the
398 property that <literal>ar[n+1] == NULL</literal>.
400 <example id="example.odr.element.path.record">
401 <title>Element Path for record</title>
403 For a database record part of a PresentResponse the
404 array returned by <function>odr_get_element</function>
405 is <literal>presentResponse</literal>, <literal>databaseOrSurDiagnostics</literal>, <literal>?</literal>, <literal>record</literal>, <literal>?</literal>, <literal>databaseRecord</literal> . The question mark appears due to
406 unnamed constructions.
410 <sect2 id="odr.diagnostics"><title>Diagnostics</title>
413 The encoding/decoding functions all return 0 when an error occurs.
414 Until you call <function>odr_reset()</function>, you cannot use the
415 stream again, and any function called will immediately return 0.
419 To provide information to the programmer or administrator, the function
423 void odr_perror(ODR o, char *message);
427 is provided, which prints the <literal>message</literal> argument to
428 <literal>stderr</literal> along with an error message from the stream.
432 You can also use the function
436 int odr_geterror(ODR o);
440 to get the current error number from the screen. The number will be
441 one of these constants:
444 <table frame="top" id="odr.error.codes">
445 <title>ODR Error codes</title>
450 <entry>Description</entry>
455 <entry>OMEMORY</entry><entry>Memory allocation failed.</entry>
459 <entry>OSYSERR</entry><entry>A system- or library call has failed.
460 The standard diagnostic variable <literal>errno</literal> should be
461 examined to determine the actual error.</entry>
465 <entry>OSPACE</entry><entry>No more space for encoding.
466 This will only occur when the user has explicitly provided a
467 buffer for an encoding stream without allowing the system to
468 allocate more space.</entry>
472 <entry>OREQUIRED</entry><entry>This is a common protocol error; A
473 required data element was missing during encoding or decoding.</entry>
477 <entry>OUNEXPECTED</entry><entry>An unexpected data element was
478 found during decoding.</entry>
481 <row><entry>OOTHER</entry><entry>Other error. This is typically an
482 indication of misuse of the &odr; system by the programmer, and also
483 that the diagnostic system isn't as good as it should be, yet.</entry>
490 The character string array
498 can be indexed by the error code to obtain a human-readable
499 representation of the problem.
503 <sect2 id="odr.summary.and.synopsis">
504 <title>Summary and Synopsis</title>
509 ODR odr_createmem(int direction);
511 void odr_destroy(ODR o);
513 void odr_reset(ODR o);
515 char *odr_getbuf(ODR o, int *len);
517 void odr_setbuf(ODR o, char *buf, int len);
519 void *odr_malloc(ODR o, int size);
521 ODR_MEM odr_extract_mem(ODR o);
523 void odr_release_mem(ODR_MEM r);
525 int odr_geterror(ODR o);
527 void odr_perror(char *message);
529 extern char *odr_errlist[];
535 <sect1 id="odr.programming"><title>Programming with ODR</title>
538 The API of &odr; is designed to reflect the structure of ASN.1, rather
539 than BER itself. Future releases may be able to represent data in
540 other external forms.
545 There is an ASN.1 tutorial available at
546 <ulink url="&url.asn.1.tutorial;">this site</ulink>.
547 This site also has standards for ASN.1 (X.680) and BER (X.690)
548 <ulink url="&url.asn.1.standards;">online</ulink>.
553 The ODR interface is based loosely on that of the Sun Microsystems
555 Specifically, each function which corresponds to an ASN.1 primitive
556 type has a dual function. Depending on the settings of the ODR
557 stream which is supplied as a parameter, the function may be used
558 either to encode or decode data. The functions that can be built
559 using these primitive functions, to represent more complex data types,
560 share this quality. The result is that you only have to enter the
561 definition for a type once - and you have the functionality of encoding,
562 decoding (and pretty-printing) all in one unit.
563 The resulting C source code is quite compact, and is a pretty
564 straightforward representation of the source ASN.1 specification.
568 In many cases, the model of the XDR functions works quite well in this
570 In others, it is less elegant. Most of the hassle comes from the optional
571 SEQUENCE members which don't exist in XDR.
574 <sect2 id="odr.primitive.asn1.types">
575 <title>The Primitive ASN.1 Types</title>
578 ASN.1 defines a number of primitive types (many of which correspond
579 roughly to primitive types in structured programming languages, such as C).
582 <sect3 id="odr.integer"><title>INTEGER</title>
585 The &odr; function for encoding or decoding (or printing) the ASN.1
586 INTEGER type looks like this:
590 int odr_integer(ODR o, int **p, int optional, const char *name);
594 (we don't allow values that can't be contained in a C integer.)
598 This form is typical of the primitive &odr; functions. They are named
599 after the type of data that they encode or decode. They take an &odr;
600 stream, an indirect reference to the type in question, and an
601 <literal>optional</literal> flag (corresponding to the OPTIONAL keyword
602 of ASN.1) as parameters. They all return an integer value of either one
604 When you use the primitive functions to construct encoders for complex
605 types of your own, you should follow this model as well. This
606 ensures that your new types can be reused as elements in yet more
611 The <literal>o</literal> parameter should obviously refer to a properly
612 initialized &odr; stream of the right type (encoding/decoding/printing)
613 for the operation that you wish to perform.
617 When encoding or printing, the function first looks at
618 <literal>* p</literal>. If <literal>* p</literal> (the pointer pointed
619 to by <literal>p</literal>) is a null pointer, this is taken to mean that
620 the data element is absent. If the <literal>optional</literal> parameter
621 is nonzero, the function will return one (signifying success) without
622 any further processing. If the <literal>optional</literal> is zero, an
623 internal error flag is set in the &odr; stream, and the function will
624 return 0. No further operations can be carried out on the stream without
625 a call to the function <function>odr_reset()</function>.
629 If <literal>*p</literal> is not a null pointer, it is expected to
630 point to an instance of the data type. The data will be subjected to
631 the encoding rules, and the result will be placed in the buffer held
636 The other ASN.1 primitives have similar functions that operate in
640 <sect3 id="odr.boolean"><title>BOOLEAN</title>
643 int odr_bool(ODR o, bool_t **p, int optional, const char *name);
647 <sect3 id="odr.real"><title>REAL</title>
654 <sect3 id="odr.null"><title>NULL</title>
657 int odr_null(ODR o, bool_t **p, int optional, const char *name);
661 In this case, the value of **p is not important. If <literal>*p</literal>
662 is different from the null pointer, the null value is present, otherwise
667 <sect3 id="odr.octet.string"><title>OCTET STRING</title>
670 typedef struct odr_oct
677 int odr_octetstring(ODR o, Odr_oct **p, int optional,
682 The <literal>buf</literal> field should point to the character array
683 that holds the octetstring. The <literal>len</literal> field holds the
684 actual length, while the <literal>size</literal> field gives the size
685 of the allocated array (not of interest to you, in most cases).
686 The character array need not be null terminated.
690 To make things a little easier, an alternative is given for string
691 types that are not expected to contain embedded NULL characters (eg.
696 int odr_cstring(ODR o, char **p, int optional, const char *name);
700 Which encoded or decodes between OCTETSTRING representations and
701 null-terminates C strings.
705 Functions are provided for the derived string types, eg:
709 int odr_visiblestring(ODR o, char **p, int optional,
714 <sect3 id="odr.bit.string"><title>BIT STRING</title>
717 int odr_bitstring(ODR o, Odr_bitmask **p, int optional,
722 The opaque type <literal>Odr_bitmask</literal> is only suitable for
723 holding relatively brief bit strings, eg. for options fields, etc.
724 The constant <literal>ODR_BITMASK_SIZE</literal> multiplied by 8
725 gives the maximum possible number of bits.
729 A set of macros are provided for manipulating the
730 <literal>Odr_bitmask</literal> type:
734 void ODR_MASK_ZERO(Odr_bitmask *b);
736 void ODR_MASK_SET(Odr_bitmask *b, int bitno);
738 void ODR_MASK_CLEAR(Odr_bitmask *b, int bitno);
740 int ODR_MASK_GET(Odr_bitmask *b, int bitno);
744 The functions are modeled after the manipulation functions that
745 accompany the <literal>fd_set</literal> type used by the
746 <function>select(2)</function> call.
747 <literal>ODR_MASK_ZERO</literal> should always be called first on a
748 new bitmask, to initialize the bits to zero.
752 <sect3 id="odr.object.identifier"><title>OBJECT IDENTIFIER</title>
755 int odr_oid(ODR o, Odr_oid **p, int optional, const char *name);
759 The C OID representation is simply an array of integers, terminated by
760 the value -1 (the <literal>Odr_oid</literal> type is synonymous with
761 the <literal>short</literal> type).
762 We suggest that you use the OID database module (see
763 <xref linkend="tools.oid.database"/>) to handle object identifiers
769 <sect2 id="odr.tagging.primitive.types"><title>Tagging Primitive Types</title> <!-- tag.prim -->
772 The simplest way of tagging a type is to use the
773 <function>odr_implicit_tag()</function> or
774 <function>odr_explicit_tag()</function> macros:
778 int odr_implicit_tag(ODR o, Odr_fun fun, int class, int tag,
779 int optional, const char *name);
781 int odr_explicit_tag(ODR o, Odr_fun fun, int class, int tag,
782 int optional, const char *name);
786 To create a type derived from the integer type by implicit tagging, you
791 MyInt ::= [210] IMPLICIT INTEGER
795 In the &odr; system, this would be written like:
799 int myInt(ODR o, int **p, int optional, const char *name)
801 return odr_implicit_tag(o, odr_integer, p,
802 ODR_CONTEXT, 210, optional, name);
807 The function <function>myInt()</function> can then be used like any of
808 the primitive functions provided by &odr;. Note that the behavior of
809 <function>odr_explicit_tag()</function>
810 and <function>odr_implicit_tag()</function> macros
811 act exactly the same as the functions they are applied to - they
812 respond to error conditions, etc, in the same manner - they
813 simply have three extra parameters. The class parameter may
814 take one of the values: <literal>ODR_CONTEXT</literal>,
815 <literal>ODR_PRIVATE</literal>, <literal>ODR_UNIVERSAL</literal>, or
816 <literal>/ODR_APPLICATION</literal>.
820 <sect2 id="odr.constructed.types"><title>Constructed Types</title>
823 Constructed types are created by combining primitive types. The
824 &odr; system only implements the SEQUENCE and SEQUENCE OF constructions
825 (although adding the rest of the container types should be simple
826 enough, if the need arises).
830 For implementing SEQUENCEs, the functions
834 int odr_sequence_begin(ODR o, void *p, int size, const char *name);
835 int odr_sequence_end(ODR o);
843 The <function>odr_sequence_begin()</function> function should be
844 called in the beginning of a function that implements a SEQUENCE type.
845 Its parameters are the &odr; stream, a pointer (to a pointer to the type
846 you're implementing), and the <literal>size</literal> of the type
847 (typically a C structure). On encoding, it returns 1 if
848 <literal>* p</literal> is a null pointer. The <literal>size</literal>
849 parameter is ignored. On decoding, it returns 1 if the type is found in
850 the data stream. <literal>size</literal> bytes of memory are allocated,
851 and <literal>*p</literal> is set to point to this space.
852 <function>odr_sequence_end()</function> is called at the end of the
853 complex function. Assume that a type is defined like this:
857 MySequence ::= SEQUENCE {
859 boolval BOOLEAN OPTIONAL
864 The corresponding &odr; encoder/decoder function and the associated data
865 structures could be written like this:
869 typedef struct MySequence
875 int mySequence(ODR o, MySequence **p, int optional, const char *name)
877 if (odr_sequence_begin(o, p, sizeof(**p), name) == 0)
878 return optional && odr_ok(o);
880 odr_integer(o, &(*p)->intval, 0, "intval") &&
881 odr_bool(o, &(*p)->boolval, 1, "boolval") &&
888 Note the 1 in the call to <function>odr_bool()</function>, to mark
889 that the sequence member is optional.
890 If either of the member types had been tagged, the macros
891 <function>odr_implicit_tag()</function> or
892 <function>odr_explicit_tag()</function>
893 could have been used.
894 The new function can be used exactly like the standard functions provided
895 with &odr;. It will encode, decode or pretty-print a data value of the
896 <literal>MySequence</literal> type. We like to name types with an
897 initial capital, as done in ASN.1 definitions, and to name the
898 corresponding function with the first character of the name in lower case.
899 You could, of course, name your structures, types, and functions any way
900 you please - as long as you're consistent, and your code is easily readable.
901 <literal>odr_ok</literal> is just that - a predicate that returns the
902 state of the stream. It is used to ensure that the behavior of the new
903 type is compatible with the interface of the primitive types.
907 <sect2 id="odr.tagging.constructed.types">
908 <title>Tagging Constructed Types</title>
912 See <xref linkend="odr.tagging.primitive.types"/> for information on how to tag
913 the primitive types, as well as types that are already defined.
917 <sect3 id="odr.implicit.tagging">
918 <title>Implicit Tagging</title>
921 Assume the type above had been defined as
925 MySequence ::= [10] IMPLICIT SEQUENCE {
927 boolval BOOLEAN OPTIONAL
932 You would implement this in &odr; by calling the function
936 int odr_implicit_settag(ODR o, int class, int tag);
940 which overrides the tag of the type immediately following it. The
941 macro <function>odr_implicit_tag()</function> works by calling
942 <function>odr_implicit_settag()</function> immediately
943 before calling the function pointer argument.
944 Your type function could look like this:
948 int mySequence(ODR o, MySequence **p, int optional, const char *name)
950 if (odr_implicit_settag(o, ODR_CONTEXT, 10) == 0 ||
951 odr_sequence_begin(o, p, sizeof(**p), name) == 0)
952 return optional && odr_ok(o);
954 odr_integer(o, &(*p)->intval, 0, "intval") &&
955 odr_bool(o, &(*p)->boolval, 1, "boolval") &&
961 The definition of the structure <literal>MySequence</literal> would be
966 <sect3 id="odr.explicit.tagging"><title>Explicit Tagging</title>
969 Explicit tagging of constructed types is a little more complicated,
970 since you are in effect adding a level of construction to the data.
974 Assume the definition:
978 MySequence ::= [10] IMPLICIT SEQUENCE {
980 boolval BOOLEAN OPTIONAL
985 Since the new type has an extra level of construction, two new functions
986 are needed to encapsulate the base type:
990 int odr_constructed_begin(ODR o, void *p, int class, int tag,
993 int odr_constructed_end(ODR o);
997 Assume that the IMPLICIT in the type definition above were replaced
998 with EXPLICIT (or that the IMPLICIT keyword were simply deleted, which
999 would be equivalent). The structure definition would look the same,
1000 but the function would look like this:
1004 int mySequence(ODR o, MySequence **p, int optional, const char *name)
1006 if (odr_constructed_begin(o, p, ODR_CONTEXT, 10, name) == 0)
1007 return optional && odr_ok(o);
1008 if (o->direction == ODR_DECODE)
1009 *p = odr_malloc(o, sizeof(**p));
1010 if (odr_sequence_begin(o, p, sizeof(**p), 0) == 0)
1012 *p = 0; /* this is almost certainly a protocol error */
1016 odr_integer(o, &(*p)->intval, 0, "intval") &&
1017 odr_bool(o, &(*p)->boolval, 1, "boolval") &&
1018 odr_sequence_end(o) &&
1019 odr_constructed_end(o);
1024 Notice that the interface here gets kind of nasty. The reason is
1025 simple: Explicitly tagged, constructed types are fairly rare in
1026 the protocols that we care about, so the
1027 esthetic annoyance (not to mention the dangers of a cluttered
1028 interface) is less than the time that would be required to develop a
1029 better interface. Nevertheless, it is far from satisfying, and it's a
1030 point that will be worked on in the future. One option for you would
1031 be to simply apply the <function>odr_explicit_tag()</function> macro to
1032 the first function, and not
1033 have to worry about <function>odr_constructed_*</function> yourself.
1034 Incidentally, as you might have guessed, the
1035 <function>odr_sequence_</function> functions are themselves
1036 implemented using the <function>/odr_constructed_</function> functions.
1041 <sect2 id="odr.sequence.of"><title>SEQUENCE OF</title>
1044 To handle sequences (arrays) of a specific type, the function
1048 int odr_sequence_of(ODR o, int (*fun)(ODR o, void *p, int optional),
1049 void *p, int *num, const char *name);
1053 The <literal>fun</literal> parameter is a pointer to the decoder/encoder
1054 function of the type. <literal>p</literal> is a pointer to an array of
1055 pointers to your type. <literal>num</literal> is the number of elements
1064 MyArray ::= SEQUENCE OF INTEGER
1068 The C representation might be
1072 typedef struct MyArray
1080 And the function might look like
1084 int myArray(ODR o, MyArray **p, int optional, const char *name)
1086 if (o->direction == ODR_DECODE)
1087 *p = odr_malloc(o, sizeof(**p));
1088 if (odr_sequence_of(o, odr_integer, &(*p)->elements,
1089 &(*p)->num_elements, name))
1092 return optional && odr_ok(o);
1097 <sect2 id="odr.choice.types"><title>CHOICE Types</title>
1100 The choice type is used fairly often in some ASN.1 definitions, so
1101 some work has gone into streamlining its interface.
1105 CHOICE types are handled by the function:
1109 int odr_choice(ODR o, Odr_arm arm[], void *p, void *whichp,
1114 The <literal>arm</literal> array is used to describe each of the possible
1115 types that the CHOICE type may assume. Internally in your application,
1116 the CHOICE type is represented as a discriminated union. That is, a
1117 C union accompanied by an integer (or enum) identifying the active
1119 <literal>whichp</literal> is a pointer to the union discriminator.
1120 When encoding, it is examined to determine the current type.
1121 When decoding, it is set to reference the type that was found in
1126 The Odr_arm type is defined thus:
1130 typedef struct odr_arm
1142 The interpretation of the fields are:
1146 <varlistentry><term>tagmode</term>
1147 <listitem><para>Either <literal>ODR_IMPLICIT</literal>,
1148 <literal>ODR_EXPLICIT</literal>, or <literal>ODR_NONE</literal> (-1)
1149 to mark no tagging.</para></listitem>
1152 <varlistentry><term>which</term>
1153 <listitem><para>The value of the discriminator that corresponds to
1154 this CHOICE element. Typically, it will be a #defined constant, or
1155 an enum member.</para></listitem>
1158 <varlistentry><term>fun</term>
1159 <listitem><para>A pointer to a function that implements the type of
1160 the CHOICE member. It may be either a standard &odr; type or a type
1161 defined by yourself.</para></listitem>
1164 <varlistentry><term>name</term>
1165 <listitem><para>Name of tag.</para></listitem>
1170 A handy way to prepare the array for use by the
1171 <function>odr_choice()</function> function is to
1172 define it as a static, initialized array in the beginning of your
1173 decoding/encoding function. Assume the type definition:
1177 MyChoice ::= CHOICE {
1179 tagged [99] IMPLICIT INTEGER,
1185 Your C type might look like
1189 typedef struct MyChoice
1207 And your function could look like this:
1211 int myChoice(ODR o, MyChoice **p, int optional, const char *name)
1213 static Odr_arm arm[] =
1215 {-1, -1, -1, MyChoice_untagged, odr_integer, "untagged"},
1216 {ODR_IMPLICIT, ODR_CONTEXT, 99, MyChoice_tagged, odr_integer,
1218 {-1, -1, -1, MyChoice_other, odr_boolean, "other"},
1222 if (o->direction == ODR_DECODE)
1223 *p = odr_malloc(o, sizeof(**p);
1225 return optional && odr_ok(o);
1227 if (odr_choice(o, arm, &(*p)->u, &(*p)->which), name)
1230 return optional && odr_ok(o);
1235 In some cases (say, a non-optional choice which is a member of a
1236 sequence), you can "embed" the union and its discriminator in the
1237 structure belonging to the enclosing type, and you won't need to
1238 fiddle with memory allocation to create a separate structure to
1239 wrap the discriminator and union.
1243 The corresponding function is somewhat nicer in the Sun XDR interface.
1244 Most of the complexity of this interface comes from the possibility of
1245 declaring sequence elements (including CHOICEs) optional.
1249 The ASN.1 specifications naturally requires that each member of a
1250 CHOICE have a distinct tag, so they can be told apart on decoding.
1251 Sometimes it can be useful to define a CHOICE that has multiple types
1252 that share the same tag. You'll need some other mechanism, perhaps
1253 keyed to the context of the CHOICE type. In effect, we would like to
1254 introduce a level of context-sensitiveness to our ASN.1 specification.
1255 When encoding an internal representation, we have no problem, as long
1256 as each CHOICE member has a distinct discriminator value. For
1257 decoding, we need a way to tell the choice function to look for a
1258 specific arm of the table. The function
1262 void odr_choice_bias(ODR o, int what);
1266 provides this functionality. When called, it leaves a notice for the next
1267 call to <function>odr_choice()</function> to be called on the decoding
1268 stream <literal>o</literal> that only the <literal>arm</literal> entry with
1269 a <literal>which</literal> field equal to <literal>what</literal>
1274 The most important application (perhaps the only one, really) is in
1275 the definition of application-specific EXTERNAL encoders/decoders
1276 which will automatically decode an ANY member given the direct or
1283 <sect1 id="odr.debugging"><title>Debugging</title>
1286 The protocol modules are suffering somewhat from a lack of diagnostic
1287 tools at the moment. Specifically ways to pretty-print PDUs that
1288 aren't recognized by the system. We'll include something to this end
1289 in a not-too-distant release. In the meantime, what we do when we get
1290 packages we don't understand is to compile the ODR module with
1291 <literal>ODR_DEBUG</literal> defined. This causes the module to dump tracing
1292 information as it processes data units. With this output and the
1293 protocol specification (Z39.50), it is generally fairly easy to see
1298 <!-- Keep this comment at the end of the file
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1304 sgml-always-quote-attributes:t
1307 sgml-parent-document: "yaz.xml"
1308 sgml-local-catalogs: nil
1309 sgml-namecase-general:t