1 <!-- $Id: odr.xml,v 1.7 2001-10-26 20:13:44 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 freelist 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>).
264 Examples of encoding/decoding functions:
268 int odr_integer(ODR o, int **p, int optional, const char *name);
270 int z_APDU(ODR o, Z_APDU **p, int optional, const char *name);
274 If the data is absent (or doesn't match the tag corresponding to
275 the type), the return value will be either 0 or 1 depending on the
276 <literal>optional</literal> flag. If <literal>optional</literal>
277 is 0 and the data is absent, an error flag will be raised in the
278 stream, and you'll need to call <function>odr_reset()</function> before
279 you can use the stream again. If <literal>optional</literal> is
280 nonzero, the pointer <emphasis>pointed</emphasis> to/ by
281 <literal>p</literal> will be set to the null value, and the function
283 The <literal>name</literal> argument is used to pretty-print the
284 tag in question. It may be set to <literal>NULL</literal> if
285 pretty-printing is not desired.
289 If the data value is found where it's expected, the pointer
290 <emphasis>pointed to</emphasis> by the <literal>p</literal> argument
291 will be set to point to the decoded type.
292 The space for the type will be allocated and owned by the &odr;
293 stream, and it will live until you call
294 <function>odr_reset()</function> on the stream. You cannot use
295 <function>free(2)</function> to release the memory.
296 You can decode several data elements (by repeated calls to
297 <function>odr_setbuf()</function> and your decoding function), and
298 new memory will be allocated each time. When you do call
299 <function>odr_reset()</function>, everything decoded since the
300 last call to <function>odr_reset()</function> will be released.
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.
313 void do_nothing_useful(int value)
320 /* allocate streams */
321 if (!(encode = odr_createmem(ODR_ENCODE)))
323 if (!(decode = odr_createmem(ODR_DECODE)))
327 if (odr_integer(encode, &valp, 0, 0) == 0)
329 printf("encoding went bad\n");
332 bufferp = odr_getbuf(encode, &len);
333 printf("length of encoded data is %d\n", len);
335 /* now let's decode the thing again */
336 odr_setbuf(decode, bufferp, len);
337 if (odr_integer(decode, &resvalp, 0, 0) == 0)
339 printf("decoding went bad\n");
342 printf("the value is %d\n", *resvalp);
351 This looks like a lot of work, offhand. In practice, the &odr; streams
352 will typically be allocated once, in the beginning of your program
353 (or at the beginning of a new network session), and the encoding
354 and decoding will only take place in a few, isolated places in your
355 program, so the overhead is quite manageable.
360 <sect2><title>Diagnostics</title>
363 The encoding/decoding functions all return 0 when an error occurs.
364 Until you call <function>odr_reset()</function>, you cannot use the
365 stream again, and any function called will immediately return 0.
369 To provide information to the programmer or administrator, the function
373 void odr_perror(ODR o, char *message);
377 is provided, which prints the <literal>message</literal> argument to
378 <literal>stderr</literal> along with an error message from the stream.
382 You can also use the function
386 int odr_geterror(ODR o);
390 to get the current error number from the screen. The number will be
391 one of these constants:
394 <table frame="top"><title>ODR Error codes</title>
399 <entry>Description</entry>
404 <entry>OMEMORY</entry><entry>Memory allocation failed.</entry>
408 <entry>OSYSERR</entry><entry>A system- or library call has failed.
409 The standard diagnostic variable <literal>errno</literal> should be
410 examined to determine the actual error.</entry>
414 <entry>OSPACE</entry><entry>No more space for encoding.
415 This will only occur when the user has explicitly provided a
416 buffer for an encoding stream without allowing the system to
417 allocate more space.</entry>
421 <entry>OREQUIRED</entry><entry>This is a common protocol error; A
422 required data element was missing during encoding or decoding.</entry>
426 <entry>OUNEXPECTED</entry><entry>An unexpected data element was
427 found during decoding.</entry>
430 <row><entry>OOTHER</entry><entry>Other error. This is typically an
431 indication of misuse of the &odr; system by the programmer, and also
432 that the diagnostic system isn't as good as it should be, yet.</entry>
439 The character string array
443 char *odr_errlist[]
447 can be indexed by the error code to obtain a human-readable
448 representation of the problem.
452 <sect2><title>Summary and Synopsis</title>
457 ODR odr_createmem(int direction);
459 void odr_destroy(ODR o);
461 void odr_reset(ODR o);
463 char *odr_getbuf(ODR o, int *len);
465 void odr_setbuf(ODR o, char *buf, int len);
467 void *odr_malloc(ODR o, int size);
469 ODR_MEM odr_extract_mem(ODR o);
471 void odr_release_mem(ODR_MEM r);
473 int odr_geterror(ODR o);
475 void odr_perror(char *message);
477 extern char *odr_errlist[];
483 <sect1 id="odr.programming"><title id="odr-prog">Programming with ODR</title>
486 The API of &odr; is designed to reflect the structure of ASN.1, rather
487 than BER itself. Future releases may be able to represent data in
488 other external forms.
492 The interface is based loosely on that of the Sun Microsystems XDR
494 Specifically, each function which corresponds to an ASN.1 primitive
495 type has a dual function. Depending on the settings of the ODR
496 stream which is supplied as a parameter, the function may be used
497 either to encode or decode data. The functions that can be built
498 using these primitive functions, to represent more complex data types,
499 share this quality. The result is that you only have to enter the
500 definition for a type once - and you have the functionality of encoding,
501 decoding (and pretty-printing) all in one unit.
502 The resulting C source code is quite compact, and is a pretty
503 straightforward representation of the source ASN.1 specification.
507 In many cases, the model of the XDR functions works quite well in this
509 In others, it is less elegant. Most of the hassle comes from the optional
510 SEQUENCE members which don't exist in XDR.
513 <sect2><title>The Primitive ASN.1 Types</title>
516 ASN.1 defines a number of primitive types (many of which correspond
517 roughly to primitive types in structured programming languages, such as C).
520 <sect3><title>INTEGER</title>
523 The &odr; function for encoding or decoding (or printing) the ASN.1
524 INTEGER type looks like this:
528 int odr_integer(ODR o, int **p, int optional, const char *name);
532 (we don't allow values that can't be contained in a C integer.)
536 This form is typical of the primitive &odr; functions. They are named
537 after the type of data that they encode or decode. They take an &odr;
538 stream, an indirect reference to the type in question, and an
539 <literal>optional</literal> flag (corresponding to the OPTIONAL keyword
540 of ASN.1) as parameters. They all return an integer value of either one
542 When you use the primitive functions to construct encoders for complex
543 types of your own, you should follow this model as well. This
544 ensures that your new types can be reused as elements in yet more
549 The <literal>o</literal> parameter should obviously refer to a properly
550 initialized &odr; stream of the right type (encoding/decoding/printing)
551 for the operation that you wish to perform.
555 When encoding or printing, the function first looks at
556 <literal>* p</literal>. If <literal>* p</literal> (the pointer pointed
557 to by <literal>p</literal>) is a null pointer, this is taken to mean that
558 the data element is absent. If the <literal>optional</literal> parameter
559 is nonzero, the function will return one (signifying success) without
560 any further processing. If the <literal>optional</literal> is zero, an
561 internal error flag is set in the &odr; stream, and the function will
562 return 0. No further operations can be carried out on the stream without
563 a call to the function <function>odr_reset()</function>.
567 If <literal>*p</literal> is not a null pointer, it is expected to
568 point to an instance of the data type. The data will be subjected to
569 the encoding rules, and the result will be placed in the buffer held
574 The other ASN.1 primitives have similar functions that operate in
578 <sect3><title>BOOLEAN</title>
581 int odr_bool(ODR o, bool_t **p, int optional, const char *name);
585 <sect3><title>REAL</title>
592 <sect3><title>NULL</title>
595 int odr_null(ODR o, bool_t **p, int optional, const char *name);
599 In this case, the value of **p is not important. If <literal>*p</literal>
600 is different from the null pointer, the null value is present, otherwise
605 <sect3><title>OCTET STRING</title>
608 typedef struct odr_oct
615 int odr_octetstring(ODR o, Odr_oct **p, int optional,
620 The <literal>buf</literal> field should point to the character array
621 that holds the octetstring. The <literal>len</literal> field holds the
622 actual length, while the <literal>size</literal> field gives the size
623 of the allocated array (not of interest to you, in most cases).
624 The character array need not be null terminated.
628 To make things a little easier, an alternative is given for string
629 types that are not expected to contain embedded NULL characters (eg.
634 int odr_cstring(ODR o, char **p, int optional, const char *name);
638 Which encoded or decodes between OCTETSTRING representations and
639 null-terminates C strings.
643 Functions are provided for the derived string types, eg:
647 int odr_visiblestring(ODR o, char **p, int optional,
652 <sect3><title>BIT STRING</title>
655 int odr_bitstring(ODR o, Odr_bitmask **p, int optional,
660 The opaque type <literal>Odr_bitmask</literal> is only suitable for
661 holding relatively brief bit strings, eg. for options fields, etc.
662 The constant <literal>ODR_BITMASK_SIZE</literal> multiplied by 8
663 gives the maximum possible number of bits.
667 A set of macros are provided for manipulating the
668 <literal>Odr_bitmask</literal> type:
672 void ODR_MASK_ZERO(Odr_bitmask *b);
674 void ODR_MASK_SET(Odr_bitmask *b, int bitno);
676 void ODR_MASK_CLEAR(Odr_bitmask *b, int bitno);
678 int ODR_MASK_GET(Odr_bitmask *b, int bitno);
682 The functions are modelled after the manipulation functions that
683 accompany the <literal>fd_set</literal> type used by the
684 <function>select(2)</function> call.
685 <literal>ODR_MASK_ZERO</literal> should always be called first on a
686 new bitmask, to initialize the bits to zero.
690 <sect3><title>OBJECT IDENTIFIER</title>
693 int odr_oid(ODR o, Odr_oid **p, int optional, const char *name);
697 The C OID representation is simply an array of integers, terminated by
698 the value -1 (the <literal>Odr_oid</literal> type is synonymous with
699 the <literal>int</literal> type).
700 We suggest that you use the OID database module (see section
701 <link linkend="oid">Object Identifiers</link>) to handle object identifiers
707 <sect2><title id="tag-prim">Tagging Primitive Types</title>
710 The simplest way of tagging a type is to use the
711 <function>odr_implicit_tag()</function> or
712 <function>odr_explicit_tag()</function> macros:
716 int odr_implicit_tag(ODR o, Odr_fun fun, int class, int tag,
717 int optional, const char *name);
719 int odr_explicit_tag(ODR o, Odr_fun fun, int class, int tag,
720 int optional, const char *name);
724 To create a type derived from the integer type by implicit tagging, you
729 MyInt ::= [210] IMPLICIT INTEGER
733 In the &odr; system, this would be written like:
737 int myInt(ODR o, int **p, int optional, const char *name)
739 return odr_implicit_tag(o, odr_integer, p,
740 ODR_CONTEXT, 210, optional, name);
745 The function <function>myInt()</function> can then be used like any of
746 the primitive functions provided by &odr;. Note that the behavior of
747 <function>odr_explicit()</function>
748 and <function>odr_implicit()</function> macros
749 act exactly the same as the functions they are applied to - they
750 respond to error conditions, etc, in the same manner - they
751 simply have three extra parameters. The class parameter may
752 take one of the values: <literal>ODR_CONTEXT</literal>,
753 <literal>ODR_PRIVATE</literal>, <literal>ODR_UNIVERSAL</literal>, or
754 <literal>/ODR_APPLICATION</literal>.
758 <sect2><title>Constructed Types</title>
761 Constructed types are created by combining primitive types. The
762 &odr; system only implements the SEQUENCE and SEQUENCE OF constructions
763 (although adding the rest of the container types should be simple
764 enough, if the need arises).
768 For implementing SEQUENCEs, the functions
772 int odr_sequence_begin(ODR o, void *p, int size, const char *name);
773 int odr_sequence_end(ODR o);
781 The <function>odr_sequence_begin()</function> function should be
782 called in the beginning of a function that implements a SEQUENCE type.
783 Its parameters are the &odr; stream, a pointer (to a pointer to the type
784 you're implementing), and the <literal>size</literal> of the type
785 (typically a C structure). On encoding, it returns 1 if
786 <literal>* p</literal> is a null pointer. The <literal>size</literal>
787 parameter is ignored. On decoding, it returns 1 if the type is found in
788 the data stream. <literal>size</literal> bytes of memory are allocated,
789 and <literal>*p</literal> is set to point to this space.
790 <function>odr_sequence_end()</function> is called at the end of the
791 complex function. Assume that a type is defined like this:
795 MySequence ::= SEQUENCE {
797 boolval BOOLEAN OPTIONAL
802 The corresponding &odr; encoder/decoder function and the associated data
803 structures could be written like this:
807 typedef struct MySequence
813 int mySequence(ODR o, MySequence **p, int optional, const char *name)
815 if (odr_sequence_begin(o, p, sizeof(**p), name) == 0)
816 return optional && odr_ok(o);
818 odr_integer(o, &(*p)->intval, 0, "intval") &&
819 odr_bool(o, &(*p)->boolval, 1, "boolval") &&
826 Note the 1 in the call to <function>odr_bool()</function>, to mark
827 that the sequence member is optional.
828 If either of the member types had been tagged, the macros
829 <function>odr_implicit()</function> or <function>odr_explicit()</function>
830 could have been used.
831 The new function can be used exactly like the standard functions provided
832 with &odr;. It will encode, decode or pretty-print a data value of the
833 <literal>MySequence</literal> type. We like to name types with an
834 initial capital, as done in ASN.1 definitions, and to name the
835 corresponding function with the first character of the name in lower case.
836 You could, of course, name your structures, types, and functions any way
837 you please - as long as you're consistent, and your code is easily readable.
838 <literal>odr_ok</literal> is just that - a predicate that returns the
839 state of the stream. It is used to ensure that the behavior of the new
840 type is compatible with the interface of the primitive types.
844 <sect2><title>Tagging Constructed Types</title>
848 See section <link linkend="tag-prim">Tagging Primitive types</link>
849 for information on how to tag the primitive types, as well as types
850 that are already defined.
854 <sect3><title>Implicit Tagging</title>
857 Assume the type above had been defined as
861 MySequence ::= [10] IMPLICIT SEQUENCE {
863 boolval BOOLEAN OPTIONAL
868 You would implement this in &odr; by calling the function
872 int odr_implicit_settag(ODR o, int class, int tag);
876 which overrides the tag of the type immediately following it. The
877 macro <function>odr_implicit()</function> works by calling
878 <function>odr_implicit_settag()</function> immediately
879 before calling the function pointer argument.
880 Your type function could look like this:
884 int mySequence(ODR o, MySequence **p, int optional, const char *name)
886 if (odr_implicit_settag(o, ODR_CONTEXT, 10) == 0 ||
887 odr_sequence_begin(o, p, sizeof(**p), name) == 0)
888 return optional && odr_ok(o);
890 odr_integer(o, &(*p)->intval, 0, "intval") &&
891 odr_bool(o, &(*p)->boolval, 1, "boolval") &&
897 The definition of the structure <literal>MySequence</literal> would be
902 <sect3><title>Explicit Tagging</title>
905 Explicit tagging of constructed types is a little more complicated,
906 since you are in effect adding a level of construction to the data.
910 Assume the definition:
914 MySequence ::= [10] IMPLICIT SEQUENCE {
916 boolval BOOLEAN OPTIONAL
921 Since the new type has an extra level of construction, two new functions
922 are needed to encapsulate the base type:
926 int odr_constructed_begin(ODR o, void *p, int class, int tag,
929 int odr_constructed_end(ODR o);
933 Assume that the IMPLICIT in the type definition above were replaced
934 with EXPLICIT (or that the IMPLICIT keyword were simply deleted, which
935 would be equivalent). The structure definition would look the same,
936 but the function would look like this:
940 int mySequence(ODR o, MySequence **p, int optional, const char *name)
942 if (odr_constructed_begin(o, p, ODR_CONTEXT, 10, name) == 0)
943 return optional && odr_ok(o);
944 if (o->direction == ODR_DECODE)
945 *p = odr_malloc(o, sizeof(**p));
946 if (odr_sequence_begin(o, p, sizeof(**p), 0) == 0)
948 *p = 0; /* this is almost certainly a protocol error */
952 odr_integer(o, &(*p)->intval, 0, "intval") &&
953 odr_bool(o, &(*p)->boolval, 1, "boolval") &&
954 odr_sequence_end(o) &&
955 odr_constructed_end(o);
960 Notice that the interface here gets kind of nasty. The reason is
961 simple: Explicitly tagged, constructed types are fairly rare in
962 the protocols that we care about, so the
963 esthetic annoyance (not to mention the dangers of a cluttered
964 interface) is less than the time that would be required to develop a
965 better interface. Nevertheless, it is far from satisfying, and it's a
966 point that will be worked on in the future. One option for you would
967 be to simply apply the <function>odr_explicit()</function> macro to
968 the first function, and not
969 have to worry about <function>odr_constructed_*</function> yourself.
970 Incidentally, as you might have guessed, the
971 <function>odr_sequence_</function> functions are themselves
972 implemented using the <function>/odr_constructed_</function> functions.
977 <sect2><title>SEQUENCE OF</title>
980 To handle sequences (arrays) of a specific type, the function
984 int odr_sequence_of(ODR o, int (*fun)(ODR o, void *p, int optional),
985 void *p, int *num, const char *name);
989 The <literal>fun</literal> parameter is a pointer to the decoder/encoder
990 function of the type. <literal>p</literal> is a pointer to an array of
991 pointers to your type. <literal>num</literal> is the number of elements
1000 MyArray ::= SEQUENCE OF INTEGER
1004 The C representation might be
1008 typedef struct MyArray
1016 And the function might look like
1020 int myArray(ODR o, MyArray **p, int optional, const char *name)
1022 if (o->direction == ODR_DECODE)
1023 *p = odr_malloc(o, sizeof(**p));
1024 if (odr_sequence_of(o, odr_integer, &(*p)->elements,
1025 &(*p)->num_elements, name))
1028 return optional && odr_ok(o);
1033 <sect2><title>CHOICE Types</title>
1036 The choice type is used fairly often in some ASN.1 definitions, so
1037 some work has gone into streamlining its interface.
1041 CHOICE types are handled by the function:
1045 int odr_choice(ODR o, Odr_arm arm[], void *p, void *whichp,
1050 The <literal>arm</literal> array is used to describe each of the possible
1051 types that the CHOICE type may assume. Internally in your application,
1052 the CHOICE type is represented as a discriminated union. That is, a
1053 C union accompanied by an integer (or enum) identifying the active
1055 <literal>whichp</literal> is a pointer to the union discriminator.
1056 When encoding, it is examined to determine the current type.
1057 When decoding, it is set to reference the type that was found in
1062 The Odr_arm type is defined thus:
1066 typedef struct odr_arm
1078 The interpretation of the fields are:
1082 <varlistentry><term>tagmode</term>
1083 <listitem><para>Either <literal>ODR_IMPLICIT</literal>,
1084 <literal>ODR_EXPLICIT</literal>, or <literal>ODR_NONE</literal> (-1)
1085 to mark no tagging.</para></listitem>
1088 <varlistentry><term>which</term>
1089 <listitem><para>The value of the discriminator that corresponds to
1090 this CHOICE element. Typically, it will be a #defined constant, or
1091 an enum member.</para></listitem>
1094 <varlistentry><term>fun</term>
1095 <listitem><para>A pointer to a function that implements the type of
1096 the CHOICE member. It may be either a standard &odr; type or a type
1097 defined by yourself.</para></listitem>
1100 <varlistentry><term>name</term>
1101 <listitem><para>Name of tag.</para></listitem>
1106 A handy way to prepare the array for use by the
1107 <function>odr_choice()</function> function is to
1108 define it as a static, initialized array in the beginning of your
1109 decoding/encoding function. Assume the type definition:
1113 MyChoice ::= CHOICE {
1115 tagged [99] IMPLICIT INTEGER,
1121 Your C type might look like
1125 typedef struct MyChoice
1143 And your function could look like this:
1147 int myChoice(ODR o, MyChoice **p, int optional, const char *name)
1149 static Odr_arm arm[] =
1151 {-1, -1, -1, MyChoice_untagged, odr_integer, "untagged"},
1152 {ODR_IMPLICIT, ODR_CONTEXT, 99, MyChoice_tagged, odr_integer,
1154 {-1, -1, -1, MyChoice_other, odr_boolean, "other"},
1158 if (o->direction == ODR_DECODE)
1159 *p = odr_malloc(o, sizeof(**p);
1161 return optional && odr_ok(o);
1163 if (odr_choice(o, arm, &(*p)->u, &(*p)->which), name)
1166 return optional && odr_ok(o);
1171 In some cases (say, a non-optional choice which is a member of a
1172 sequence), you can "embed" the union and its discriminator in the
1173 structure belonging to the enclosing type, and you won't need to
1174 fiddle with memory allocation to create a separate structure to
1175 wrap the discriminator and union.
1179 The corresponding function is somewhat nicer in the Sun XDR interface.
1180 Most of the complexity of this interface comes from the possibility of
1181 declaring sequence elements (including CHOICEs) optional.
1185 The ASN.1 specifications naturally requires that each member of a
1186 CHOICE have a distinct tag, so they can be told apart on decoding.
1187 Sometimes it can be useful to define a CHOICE that has multiple types
1188 that share the same tag. You'll need some other mechanism, perhaps
1189 keyed to the context of the CHOICE type. In effect, we would like to
1190 introduce a level of context-sensitiveness to our ASN.1 specification.
1191 When encoding an internal representation, we have no problem, as long
1192 as each CHOICE member has a distinct discriminator value. For
1193 decoding, we need a way to tell the choice function to look for a
1194 specific arm of the table. The function
1198 void odr_choice_bias(ODR o, int what);
1202 provides this functionality. When called, it leaves a notice for the next
1203 call to <function>odr_choice()</function> to be called on the decoding
1204 stream <literal>o</literal> that only the <literal>arm</literal> entry with
1205 a <literal>which</literal> field equal to <literal>what</literal>
1210 The most important application (perhaps the only one, really) is in
1211 the definition of application-specific EXTERNAL encoders/decoders
1212 which will automatically decode an ANY member given the direct or
1219 <sect1 id="odr.debugging"><title>Debugging</title>
1222 The protocol modules are suffering somewhat from a lack of diagnostic
1223 tools at the moment. Specifically ways to pretty-print PDUs that
1224 aren't recognized by the system. We'll include something to this end
1225 in a not-too-distant release. In the meantime, what we do when we get
1226 packages we don't understand is to compile the ODR module with
1227 <literal>ODR_DEBUG</literal> defined. This causes the module to dump tracing
1228 information as it processes data units. With this output and the
1229 protocol specification (Z39.50), it is generally fairly easy to see
1234 <!-- Keep this comment at the end of the file
1239 sgml-minimize-attributes:nil
1240 sgml-always-quote-attributes:t
1243 sgml-parent-document: "yaz.xml"
1244 sgml-local-catalogs: nil
1245 sgml-namecase-general:t