1 <!-- $Id: odr.xml,v 1.10 2003-11-03 10:45:05 adam Exp $ -->
2 <chapter id="odr"><title>The ODR Module</title>
4 <sect1 id="odr.introduction"><title>Introduction</title>
7 &odr; is the BER-encoding/decoding subsystem of &yaz;. Care as been taken
8 to isolate &odr; from the rest of the package - specifically from the
9 transport interface. &odr; may be used in any context where basic
10 ASN.1/BER representations are used.
14 If you are only interested in writing a Z39.50 implementation based on
15 the PDUs that are already provided with &yaz;, you only need to concern
16 yourself with the section on managing ODR streams (section
17 <link linkend="odr-use">Using ODR</link>). Only if you need to
18 implement ASN.1 beyond that which has been provided, should you
19 worry about the second half of the documentation
20 (section <link linkend="odr-prog">Programming with ODR</link>).
21 If you use one of the higher-level interfaces, you can skip this
26 This is important, so we'll repeat it for emphasis: <emphasis>You do
27 not need to read section <link linkend="odr-prog">Programming with
28 ODR</link> to implement Z39.50 with &yaz;.</emphasis>
32 If you need a part of the protocol that isn't already in &yaz;, you
33 should contact the authors before going to work on it yourself: We
34 might already be working on it. Conversely, if you implement a useful
35 part of the protocol before us, we'd be happy to include it in a
40 <sect1 id="odr.use"><title id="odr-use">Using ODR</title>
42 <sect2><title>ODR Streams</title>
45 Conceptually, the ODR stream is the source of encoded data in the
46 decoding mode; when encoding, it is the receptacle for the encoded
47 data. Before you can use an ODR stream it must be allocated. This is
48 done with the function
52 ODR odr_createmem(int direction);
56 The <function>odr_createmem()</function> function takes as argument one
57 of three manifest constants: <literal>ODR_ENCODE</literal>,
58 <literal>ODR_DECODE</literal>, or <literal>ODR_PRINT</literal>.
59 An &odr; stream can be in only one mode - it is not possible to change
60 its mode once it's selected. Typically, your program will allocate
61 at least two ODR streams - one for decoding, and one for encoding.
65 When you're done with the stream, you can use
69 void odr_destroy(ODR o);
73 to release the resources allocated for the stream.
77 <sect2><title id="memory">Memory Management</title>
80 Two forms of memory management take place in the &odr; system. The first
81 one, which has to do with allocating little bits of memory (sometimes
82 quite large bits of memory, actually) when a protocol package is
83 decoded, and turned into a complex of interlinked structures. This
84 section deals with this system, and how you can use it for your own
85 purposes. The next section deals with the memory management which is
86 required when encoding data - to make sure that a large enough buffer is
87 available to hold the fully encoded PDU.
91 The &odr; module has its own memory management system, which is
92 used whenever memory is required. Specifically, it is used to allocate
93 space for data when decoding incoming PDUs. You can use the memory
94 system for your own purposes, by using the function
98 void *odr_malloc(ODR o, int size);
102 You can't use the normal <function>free(2)</function> routine to free
103 memory allocated by this function, and &odr; doesn't provide a parallel
104 function. Instead, you can call
108 void odr_reset(ODR o, int size);
112 when you are done with the
113 memory: Everything allocated since the last call to
114 <function>odr_reset()</function> is released.
115 The <function>odr_reset()</function> call is also required to clear
116 up an error condition on a stream.
124 int odr_total(ODR o);
128 returns the number of bytes allocated on the stream since the last call to
129 <function>odr_reset()</function>.
133 The memory subsystem of &odr; is fairly efficient at allocating and
134 releasing little bits of memory. Rather than managing the individual,
135 small bits of space, the system maintains a free-list of larger chunks
136 of memory, which are handed out in small bits. This scheme is
137 generally known as a <emphasis>nibble memory</emphasis> system.
138 It is very useful for maintaining short-lived constructions such
143 If you want to retain a bit of memory beyond the next call to
144 <function>odr_reset()</function>, you can use the function
148 ODR_MEM odr_extract_mem(ODR o);
152 This function will give you control of the memory recently allocated
153 on the ODR stream. The memory will live (past calls to
154 <function>odr_reset()</function>), until you call the function
158 void odr_release_mem(ODR_MEM p);
162 The opaque <literal>ODR_MEM</literal> handle has no other purpose than
163 referencing the memory block for you until you want to release it.
167 You can use <function>odr_extract_mem()</function> repeatedly between
168 allocating data, to retain individual control of separate chunks of data.
172 <sect2><title>Encoding and Decoding Data</title>
175 When encoding data, the ODR stream will write the encoded octet string
176 in an internal buffer. To retrieve the data, use the function
180 char *odr_getbuf(ODR o, int *len, int *size);
184 The integer pointed to by len is set to the length of the encoded
185 data, and a pointer to that data is returned. <literal>*size</literal>
186 is set to the size of the buffer (unless <literal>size</literal> is null,
187 signaling that you are not interested in the size). The next call to
188 a primitive function using the same &odr; stream will overwrite the
189 data, unless a different buffer has been supplied using the call
193 void odr_setbuf(ODR o, char *buf, int len, int can_grow);
197 which sets the encoding (or decoding) buffer used by
198 <literal>o</literal> to <literal>buf</literal>, using the length
199 <literal>len</literal>.
200 Before a call to an encoding function, you can use
201 <function>odr_setbuf()</function> to provide the stream with an encoding
202 buffer of sufficient size (length). The <literal>can_grow</literal>
203 parameter tells the encoding &odr; stream whether it is allowed to use
204 <function>realloc(2)</function> to increase the size of the buffer when
205 necessary. The default condition of a new encoding stream is equivalent
206 to the results of calling
210 odr_setbuf(stream, 0, 0, 1);
214 In this case, the stream will allocate and reallocate memory as
215 necessary. The stream reallocates memory by repeatedly doubling the
216 size of the buffer - the result is that the buffer will typically
217 reach its maximum, working size with only a small number of reallocation
218 operations. The memory is freed by the stream when the latter is destroyed,
219 unless it was assigned by the user with the <literal>can_grow</literal>
220 parameter set to zero (in this case, you are expected to retain
221 control of the memory yourself).
225 To assume full control of an encoded buffer, you must first call
226 <function>odr_getbuf()</function> to fetch the buffer and its length.
227 Next, you should call <function>odr_setbuf()</function> to provide a
228 different buffer (or a null pointer) to the stream. In the simplest
229 case, you will reuse the same buffer over and over again, and you
230 will just need to call <function>odr_getbuf()</function> after each
231 encoding operation to get the length and address of the buffer.
232 Note that the stream may reallocate the buffer during an encoding
233 operation, so it is necessary to retrieve the correct address after
234 each encoding operation.
238 It is important to realize that the ODR stream will not release this
239 memory when you call <function>odr_reset()</function>: It will
240 merely update its internal pointers to prepare for the encoding of a
242 When the stream is released by the <function>odr_destroy()</function>
243 function, the memory given to it by <function>odr_setbuf</function> will
244 be released <emphasis>only</emphasis> if the <literal>can_grow</literal>
245 parameter to <function>odr_setbuf()</function> was nonzero. The
246 <literal>can_grow</literal> parameter, in other words, is a way of
247 signaling who is to own the buffer, you or the ODR stream. If you never call
248 <function>odr_setbuf()</function> on your encoding stream, which is
249 typically the case, the buffer allocated by the stream will belong to
250 the stream by default.
254 When you wish to decode data, you should first call
255 <function>odr_setbuf()</function>, to tell the decoding stream
256 where to find the encoded data, and how long the buffer is
257 (the <literal>can_grow</literal> parameter is ignored by a decoding
258 stream). After this, you can call the function corresponding to the
259 data you wish to decode (eg, <function>odr_integer()</function> odr
260 <function>z_APDU()</function>).
263 <example><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><title>Encoding and decoding of an integer</title>
303 The use of the double indirection can be a little confusing at first
304 (its purpose will become clear later on, hopefully),
305 so an example is in order. We'll encode an integer value, and
306 immediately decode it again using a different stream. A useless, but
307 informative operation.
309 <programlisting><![CDATA[
310 void do_nothing_useful(int value)
317 /* allocate streams */
318 if (!(encode = odr_createmem(ODR_ENCODE)))
320 if (!(decode = odr_createmem(ODR_DECODE)))
324 if (odr_integer(encode, &valp, 0, 0) == 0)
326 printf("encoding went bad\n");
329 bufferp = odr_getbuf(encode, &len);
330 printf("length of encoded data is %d\n", len);
332 /* now let's decode the thing again */
333 odr_setbuf(decode, bufferp, len);
334 if (odr_integer(decode, &resvalp, 0, 0) == 0)
336 printf("decoding went bad\n");
339 printf("the value is %d\n", *resvalp);
348 This looks like a lot of work, offhand. In practice, the &odr; streams
349 will typically be allocated once, in the beginning of your program
350 (or at the beginning of a new network session), and the encoding
351 and decoding will only take place in a few, isolated places in your
352 program, so the overhead is quite manageable.
358 <sect2><title>Diagnostics</title>
361 The encoding/decoding functions all return 0 when an error occurs.
362 Until you call <function>odr_reset()</function>, you cannot use the
363 stream again, and any function called will immediately return 0.
367 To provide information to the programmer or administrator, the function
371 void odr_perror(ODR o, char *message);
375 is provided, which prints the <literal>message</literal> argument to
376 <literal>stderr</literal> along with an error message from the stream.
380 You can also use the function
384 int odr_geterror(ODR o);
388 to get the current error number from the screen. The number will be
389 one of these constants:
392 <table frame="top"><title>ODR Error codes</title>
397 <entry>Description</entry>
402 <entry>OMEMORY</entry><entry>Memory allocation failed.</entry>
406 <entry>OSYSERR</entry><entry>A system- or library call has failed.
407 The standard diagnostic variable <literal>errno</literal> should be
408 examined to determine the actual error.</entry>
412 <entry>OSPACE</entry><entry>No more space for encoding.
413 This will only occur when the user has explicitly provided a
414 buffer for an encoding stream without allowing the system to
415 allocate more space.</entry>
419 <entry>OREQUIRED</entry><entry>This is a common protocol error; A
420 required data element was missing during encoding or decoding.</entry>
424 <entry>OUNEXPECTED</entry><entry>An unexpected data element was
425 found during decoding.</entry>
428 <row><entry>OOTHER</entry><entry>Other error. This is typically an
429 indication of misuse of the &odr; system by the programmer, and also
430 that the diagnostic system isn't as good as it should be, yet.</entry>
437 The character string array
441 char *odr_errlist[]
445 can be indexed by the error code to obtain a human-readable
446 representation of the problem.
450 <sect2><title>Summary and Synopsis</title>
455 ODR odr_createmem(int direction);
457 void odr_destroy(ODR o);
459 void odr_reset(ODR o);
461 char *odr_getbuf(ODR o, int *len);
463 void odr_setbuf(ODR o, char *buf, int len);
465 void *odr_malloc(ODR o, int size);
467 ODR_MEM odr_extract_mem(ODR o);
469 void odr_release_mem(ODR_MEM r);
471 int odr_geterror(ODR o);
473 void odr_perror(char *message);
475 extern char *odr_errlist[];
481 <sect1 id="odr.programming"><title id="odr-prog">Programming with ODR</title>
484 The API of &odr; is designed to reflect the structure of ASN.1, rather
485 than BER itself. Future releases may be able to represent data in
486 other external forms.
491 There is an ASN.1 tutorial available at
492 <ulink url="http://asn1.elibel.tm.fr/en/introduction/">this site</ulink>.
493 This site also has standards for ASN.1 (X.680) and BER (X.690)
494 <ulink url="http://asn1.elibel.tm.fr/en/standards/">online</ulink>.
499 The ODR interface is based loosely on that of the Sun Microsystems
501 Specifically, each function which corresponds to an ASN.1 primitive
502 type has a dual function. Depending on the settings of the ODR
503 stream which is supplied as a parameter, the function may be used
504 either to encode or decode data. The functions that can be built
505 using these primitive functions, to represent more complex data types,
506 share this quality. The result is that you only have to enter the
507 definition for a type once - and you have the functionality of encoding,
508 decoding (and pretty-printing) all in one unit.
509 The resulting C source code is quite compact, and is a pretty
510 straightforward representation of the source ASN.1 specification.
514 In many cases, the model of the XDR functions works quite well in this
516 In others, it is less elegant. Most of the hassle comes from the optional
517 SEQUENCE members which don't exist in XDR.
520 <sect2><title>The Primitive ASN.1 Types</title>
523 ASN.1 defines a number of primitive types (many of which correspond
524 roughly to primitive types in structured programming languages, such as C).
527 <sect3><title>INTEGER</title>
530 The &odr; function for encoding or decoding (or printing) the ASN.1
531 INTEGER type looks like this:
535 int odr_integer(ODR o, int **p, int optional, const char *name);
539 (we don't allow values that can't be contained in a C integer.)
543 This form is typical of the primitive &odr; functions. They are named
544 after the type of data that they encode or decode. They take an &odr;
545 stream, an indirect reference to the type in question, and an
546 <literal>optional</literal> flag (corresponding to the OPTIONAL keyword
547 of ASN.1) as parameters. They all return an integer value of either one
549 When you use the primitive functions to construct encoders for complex
550 types of your own, you should follow this model as well. This
551 ensures that your new types can be reused as elements in yet more
556 The <literal>o</literal> parameter should obviously refer to a properly
557 initialized &odr; stream of the right type (encoding/decoding/printing)
558 for the operation that you wish to perform.
562 When encoding or printing, the function first looks at
563 <literal>* p</literal>. If <literal>* p</literal> (the pointer pointed
564 to by <literal>p</literal>) is a null pointer, this is taken to mean that
565 the data element is absent. If the <literal>optional</literal> parameter
566 is nonzero, the function will return one (signifying success) without
567 any further processing. If the <literal>optional</literal> is zero, an
568 internal error flag is set in the &odr; stream, and the function will
569 return 0. No further operations can be carried out on the stream without
570 a call to the function <function>odr_reset()</function>.
574 If <literal>*p</literal> is not a null pointer, it is expected to
575 point to an instance of the data type. The data will be subjected to
576 the encoding rules, and the result will be placed in the buffer held
581 The other ASN.1 primitives have similar functions that operate in
585 <sect3><title>BOOLEAN</title>
588 int odr_bool(ODR o, bool_t **p, int optional, const char *name);
592 <sect3><title>REAL</title>
599 <sect3><title>NULL</title>
602 int odr_null(ODR o, bool_t **p, int optional, const char *name);
606 In this case, the value of **p is not important. If <literal>*p</literal>
607 is different from the null pointer, the null value is present, otherwise
612 <sect3><title>OCTET STRING</title>
615 typedef struct odr_oct
622 int odr_octetstring(ODR o, Odr_oct **p, int optional,
627 The <literal>buf</literal> field should point to the character array
628 that holds the octetstring. The <literal>len</literal> field holds the
629 actual length, while the <literal>size</literal> field gives the size
630 of the allocated array (not of interest to you, in most cases).
631 The character array need not be null terminated.
635 To make things a little easier, an alternative is given for string
636 types that are not expected to contain embedded NULL characters (eg.
641 int odr_cstring(ODR o, char **p, int optional, const char *name);
645 Which encoded or decodes between OCTETSTRING representations and
646 null-terminates C strings.
650 Functions are provided for the derived string types, eg:
654 int odr_visiblestring(ODR o, char **p, int optional,
659 <sect3><title>BIT STRING</title>
662 int odr_bitstring(ODR o, Odr_bitmask **p, int optional,
667 The opaque type <literal>Odr_bitmask</literal> is only suitable for
668 holding relatively brief bit strings, eg. for options fields, etc.
669 The constant <literal>ODR_BITMASK_SIZE</literal> multiplied by 8
670 gives the maximum possible number of bits.
674 A set of macros are provided for manipulating the
675 <literal>Odr_bitmask</literal> type:
679 void ODR_MASK_ZERO(Odr_bitmask *b);
681 void ODR_MASK_SET(Odr_bitmask *b, int bitno);
683 void ODR_MASK_CLEAR(Odr_bitmask *b, int bitno);
685 int ODR_MASK_GET(Odr_bitmask *b, int bitno);
689 The functions are modeled after the manipulation functions that
690 accompany the <literal>fd_set</literal> type used by the
691 <function>select(2)</function> call.
692 <literal>ODR_MASK_ZERO</literal> should always be called first on a
693 new bitmask, to initialize the bits to zero.
697 <sect3><title>OBJECT IDENTIFIER</title>
700 int odr_oid(ODR o, Odr_oid **p, int optional, const char *name);
704 The C OID representation is simply an array of integers, terminated by
705 the value -1 (the <literal>Odr_oid</literal> type is synonymous with
706 the <literal>int</literal> type).
707 We suggest that you use the OID database module (see section
708 <link linkend="oid">Object Identifiers</link>) to handle object identifiers
714 <sect2><title id="tag-prim">Tagging Primitive Types</title>
717 The simplest way of tagging a type is to use the
718 <function>odr_implicit_tag()</function> or
719 <function>odr_explicit_tag()</function> macros:
723 int odr_implicit_tag(ODR o, Odr_fun fun, int class, int tag,
724 int optional, const char *name);
726 int odr_explicit_tag(ODR o, Odr_fun fun, int class, int tag,
727 int optional, const char *name);
731 To create a type derived from the integer type by implicit tagging, you
736 MyInt ::= [210] IMPLICIT INTEGER
740 In the &odr; system, this would be written like:
744 int myInt(ODR o, int **p, int optional, const char *name)
746 return odr_implicit_tag(o, odr_integer, p,
747 ODR_CONTEXT, 210, optional, name);
752 The function <function>myInt()</function> can then be used like any of
753 the primitive functions provided by &odr;. Note that the behavior of
754 <function>odr_explicit_tag()</function>
755 and <function>odr_implicit_tag()</function> macros
756 act exactly the same as the functions they are applied to - they
757 respond to error conditions, etc, in the same manner - they
758 simply have three extra parameters. The class parameter may
759 take one of the values: <literal>ODR_CONTEXT</literal>,
760 <literal>ODR_PRIVATE</literal>, <literal>ODR_UNIVERSAL</literal>, or
761 <literal>/ODR_APPLICATION</literal>.
765 <sect2><title>Constructed Types</title>
768 Constructed types are created by combining primitive types. The
769 &odr; system only implements the SEQUENCE and SEQUENCE OF constructions
770 (although adding the rest of the container types should be simple
771 enough, if the need arises).
775 For implementing SEQUENCEs, the functions
779 int odr_sequence_begin(ODR o, void *p, int size, const char *name);
780 int odr_sequence_end(ODR o);
788 The <function>odr_sequence_begin()</function> function should be
789 called in the beginning of a function that implements a SEQUENCE type.
790 Its parameters are the &odr; stream, a pointer (to a pointer to the type
791 you're implementing), and the <literal>size</literal> of the type
792 (typically a C structure). On encoding, it returns 1 if
793 <literal>* p</literal> is a null pointer. The <literal>size</literal>
794 parameter is ignored. On decoding, it returns 1 if the type is found in
795 the data stream. <literal>size</literal> bytes of memory are allocated,
796 and <literal>*p</literal> is set to point to this space.
797 <function>odr_sequence_end()</function> is called at the end of the
798 complex function. Assume that a type is defined like this:
802 MySequence ::= SEQUENCE {
804 boolval BOOLEAN OPTIONAL
809 The corresponding &odr; encoder/decoder function and the associated data
810 structures could be written like this:
814 typedef struct MySequence
820 int mySequence(ODR o, MySequence **p, int optional, const char *name)
822 if (odr_sequence_begin(o, p, sizeof(**p), name) == 0)
823 return optional && odr_ok(o);
825 odr_integer(o, &(*p)->intval, 0, "intval") &&
826 odr_bool(o, &(*p)->boolval, 1, "boolval") &&
833 Note the 1 in the call to <function>odr_bool()</function>, to mark
834 that the sequence member is optional.
835 If either of the member types had been tagged, the macros
836 <function>odr_implicit_tag()</function> or
837 <function>odr_explicit_tag()</function>
838 could have been used.
839 The new function can be used exactly like the standard functions provided
840 with &odr;. It will encode, decode or pretty-print a data value of the
841 <literal>MySequence</literal> type. We like to name types with an
842 initial capital, as done in ASN.1 definitions, and to name the
843 corresponding function with the first character of the name in lower case.
844 You could, of course, name your structures, types, and functions any way
845 you please - as long as you're consistent, and your code is easily readable.
846 <literal>odr_ok</literal> is just that - a predicate that returns the
847 state of the stream. It is used to ensure that the behavior of the new
848 type is compatible with the interface of the primitive types.
852 <sect2><title>Tagging Constructed Types</title>
856 See section <link linkend="tag-prim">Tagging Primitive types</link>
857 for information on how to tag the primitive types, as well as types
858 that are already defined.
862 <sect3><title>Implicit Tagging</title>
865 Assume the type above had been defined as
869 MySequence ::= [10] IMPLICIT SEQUENCE {
871 boolval BOOLEAN OPTIONAL
876 You would implement this in &odr; by calling the function
880 int odr_implicit_settag(ODR o, int class, int tag);
884 which overrides the tag of the type immediately following it. The
885 macro <function>odr_implicit_tag()</function> works by calling
886 <function>odr_implicit_settag()</function> immediately
887 before calling the function pointer argument.
888 Your type function could look like this:
892 int mySequence(ODR o, MySequence **p, int optional, const char *name)
894 if (odr_implicit_settag(o, ODR_CONTEXT, 10) == 0 ||
895 odr_sequence_begin(o, p, sizeof(**p), name) == 0)
896 return optional && odr_ok(o);
898 odr_integer(o, &(*p)->intval, 0, "intval") &&
899 odr_bool(o, &(*p)->boolval, 1, "boolval") &&
905 The definition of the structure <literal>MySequence</literal> would be
910 <sect3><title>Explicit Tagging</title>
913 Explicit tagging of constructed types is a little more complicated,
914 since you are in effect adding a level of construction to the data.
918 Assume the definition:
922 MySequence ::= [10] IMPLICIT SEQUENCE {
924 boolval BOOLEAN OPTIONAL
929 Since the new type has an extra level of construction, two new functions
930 are needed to encapsulate the base type:
934 int odr_constructed_begin(ODR o, void *p, int class, int tag,
937 int odr_constructed_end(ODR o);
941 Assume that the IMPLICIT in the type definition above were replaced
942 with EXPLICIT (or that the IMPLICIT keyword were simply deleted, which
943 would be equivalent). The structure definition would look the same,
944 but the function would look like this:
948 int mySequence(ODR o, MySequence **p, int optional, const char *name)
950 if (odr_constructed_begin(o, p, ODR_CONTEXT, 10, name) == 0)
951 return optional && odr_ok(o);
952 if (o->direction == ODR_DECODE)
953 *p = odr_malloc(o, sizeof(**p));
954 if (odr_sequence_begin(o, p, sizeof(**p), 0) == 0)
956 *p = 0; /* this is almost certainly a protocol error */
960 odr_integer(o, &(*p)->intval, 0, "intval") &&
961 odr_bool(o, &(*p)->boolval, 1, "boolval") &&
962 odr_sequence_end(o) &&
963 odr_constructed_end(o);
968 Notice that the interface here gets kind of nasty. The reason is
969 simple: Explicitly tagged, constructed types are fairly rare in
970 the protocols that we care about, so the
971 esthetic annoyance (not to mention the dangers of a cluttered
972 interface) is less than the time that would be required to develop a
973 better interface. Nevertheless, it is far from satisfying, and it's a
974 point that will be worked on in the future. One option for you would
975 be to simply apply the <function>odr_explicit_tag()</function> macro to
976 the first function, and not
977 have to worry about <function>odr_constructed_*</function> yourself.
978 Incidentally, as you might have guessed, the
979 <function>odr_sequence_</function> functions are themselves
980 implemented using the <function>/odr_constructed_</function> functions.
985 <sect2><title>SEQUENCE OF</title>
988 To handle sequences (arrays) of a specific type, the function
992 int odr_sequence_of(ODR o, int (*fun)(ODR o, void *p, int optional),
993 void *p, int *num, const char *name);
997 The <literal>fun</literal> parameter is a pointer to the decoder/encoder
998 function of the type. <literal>p</literal> is a pointer to an array of
999 pointers to your type. <literal>num</literal> is the number of elements
1008 MyArray ::= SEQUENCE OF INTEGER
1012 The C representation might be
1016 typedef struct MyArray
1024 And the function might look like
1028 int myArray(ODR o, MyArray **p, int optional, const char *name)
1030 if (o->direction == ODR_DECODE)
1031 *p = odr_malloc(o, sizeof(**p));
1032 if (odr_sequence_of(o, odr_integer, &(*p)->elements,
1033 &(*p)->num_elements, name))
1036 return optional && odr_ok(o);
1041 <sect2><title>CHOICE Types</title>
1044 The choice type is used fairly often in some ASN.1 definitions, so
1045 some work has gone into streamlining its interface.
1049 CHOICE types are handled by the function:
1053 int odr_choice(ODR o, Odr_arm arm[], void *p, void *whichp,
1058 The <literal>arm</literal> array is used to describe each of the possible
1059 types that the CHOICE type may assume. Internally in your application,
1060 the CHOICE type is represented as a discriminated union. That is, a
1061 C union accompanied by an integer (or enum) identifying the active
1063 <literal>whichp</literal> is a pointer to the union discriminator.
1064 When encoding, it is examined to determine the current type.
1065 When decoding, it is set to reference the type that was found in
1070 The Odr_arm type is defined thus:
1074 typedef struct odr_arm
1086 The interpretation of the fields are:
1090 <varlistentry><term>tagmode</term>
1091 <listitem><para>Either <literal>ODR_IMPLICIT</literal>,
1092 <literal>ODR_EXPLICIT</literal>, or <literal>ODR_NONE</literal> (-1)
1093 to mark no tagging.</para></listitem>
1096 <varlistentry><term>which</term>
1097 <listitem><para>The value of the discriminator that corresponds to
1098 this CHOICE element. Typically, it will be a #defined constant, or
1099 an enum member.</para></listitem>
1102 <varlistentry><term>fun</term>
1103 <listitem><para>A pointer to a function that implements the type of
1104 the CHOICE member. It may be either a standard &odr; type or a type
1105 defined by yourself.</para></listitem>
1108 <varlistentry><term>name</term>
1109 <listitem><para>Name of tag.</para></listitem>
1114 A handy way to prepare the array for use by the
1115 <function>odr_choice()</function> function is to
1116 define it as a static, initialized array in the beginning of your
1117 decoding/encoding function. Assume the type definition:
1121 MyChoice ::= CHOICE {
1123 tagged [99] IMPLICIT INTEGER,
1129 Your C type might look like
1133 typedef struct MyChoice
1151 And your function could look like this:
1155 int myChoice(ODR o, MyChoice **p, int optional, const char *name)
1157 static Odr_arm arm[] =
1159 {-1, -1, -1, MyChoice_untagged, odr_integer, "untagged"},
1160 {ODR_IMPLICIT, ODR_CONTEXT, 99, MyChoice_tagged, odr_integer,
1162 {-1, -1, -1, MyChoice_other, odr_boolean, "other"},
1166 if (o->direction == ODR_DECODE)
1167 *p = odr_malloc(o, sizeof(**p);
1169 return optional && odr_ok(o);
1171 if (odr_choice(o, arm, &(*p)->u, &(*p)->which), name)
1174 return optional && odr_ok(o);
1179 In some cases (say, a non-optional choice which is a member of a
1180 sequence), you can "embed" the union and its discriminator in the
1181 structure belonging to the enclosing type, and you won't need to
1182 fiddle with memory allocation to create a separate structure to
1183 wrap the discriminator and union.
1187 The corresponding function is somewhat nicer in the Sun XDR interface.
1188 Most of the complexity of this interface comes from the possibility of
1189 declaring sequence elements (including CHOICEs) optional.
1193 The ASN.1 specifications naturally requires that each member of a
1194 CHOICE have a distinct tag, so they can be told apart on decoding.
1195 Sometimes it can be useful to define a CHOICE that has multiple types
1196 that share the same tag. You'll need some other mechanism, perhaps
1197 keyed to the context of the CHOICE type. In effect, we would like to
1198 introduce a level of context-sensitiveness to our ASN.1 specification.
1199 When encoding an internal representation, we have no problem, as long
1200 as each CHOICE member has a distinct discriminator value. For
1201 decoding, we need a way to tell the choice function to look for a
1202 specific arm of the table. The function
1206 void odr_choice_bias(ODR o, int what);
1210 provides this functionality. When called, it leaves a notice for the next
1211 call to <function>odr_choice()</function> to be called on the decoding
1212 stream <literal>o</literal> that only the <literal>arm</literal> entry with
1213 a <literal>which</literal> field equal to <literal>what</literal>
1218 The most important application (perhaps the only one, really) is in
1219 the definition of application-specific EXTERNAL encoders/decoders
1220 which will automatically decode an ANY member given the direct or
1227 <sect1 id="odr.debugging"><title>Debugging</title>
1230 The protocol modules are suffering somewhat from a lack of diagnostic
1231 tools at the moment. Specifically ways to pretty-print PDUs that
1232 aren't recognized by the system. We'll include something to this end
1233 in a not-too-distant release. In the meantime, what we do when we get
1234 packages we don't understand is to compile the ODR module with
1235 <literal>ODR_DEBUG</literal> defined. This causes the module to dump tracing
1236 information as it processes data units. With this output and the
1237 protocol specification (Z39.50), it is generally fairly easy to see
1242 <!-- Keep this comment at the end of the file
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