1 <!-- $Header: /home/cvsroot/yaz/doc/odr.xml,v 1.1 2001-01-04 13:36:24 adam Exp $ -->
2 <chapter><title id="odr">The ODR Module</title>
4 <sect1><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 not
27 need to read section <link linkend="odr-prog">Programming with ODR</link> to
28 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><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 maintaing 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 signalling 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 <literal>o</literal> to
198 <literal>buf</literal>, using the length <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 realise 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 signalling 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>).
263 Examples of encoding/decoding functions:
267 int odr_integer(ODR o, int **p, int optional, const char *name);
269 int z_APDU(ODR o, Z_APDU **p, int optional, const char *name);
273 If the data is absent (or doesn't match the tag corresponding to the type),
274 the return value will be either 0 or 1 depending on the
275 <literal>optional</literal> flag. If <literal>optional</literal>
276 is 0 and the data is absent, an error flag will be raised in the
277 stream, and you'll need to call <function>odr_reset()</function> before
278 you can use the stream again. If <literal>optional</literal> is
279 nonzero, the pointer <emphasis>pointed</emphasis> to/ by <literal>p</literal>
280 will be set to the null value, and the function will return 1.
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; stream, and
291 it will live until you call <function>odr_reset()</function> on the
292 stream. You cannot use <function>free(2)</function> to release the memory.
293 You can decode several data elements (by repeated calls to
294 <function>odr_setbuf()</function> and your decoding function), and
295 new memory will be allocated each time. When you do call
296 <function>odr_reset()</function>, everything decoded since the
297 last call to <function>odr_reset()</function> will be released.
301 The use of the double indirection can be a little confusing at first
302 (its purpose will become clear later on, hopefully),
303 so an example is in order. We'll encode an integer value, and
304 immediately decode it again using a different stream. A useless, but
305 informative operation.
309 void do_nothing_useful(int value)
316 /* allocate streams */
317 if (!(encode = odr_createmem(ODR_ENCODE)))
319 if (!(decode = odr_createmem(ODR_DECODE)))
323 if (odr_integer(encode, &valp, 0, 0) == 0)
325 printf("encoding went bad\n");
328 bufferp = odr_getbuf(encode, &len);
329 printf("length of encoded data is %d\n", len);
331 /* now let's decode the thing again */
332 odr_setbuf(decode, bufferp, len);
333 if (odr_integer(decode, &resvalp, 0, 0) == 0)
335 printf("decoding went bad\n");
338 printf("the value is %d\n", *resvalp);
347 This looks like a lot of work, offhand. In practice, the &odr; streams
348 will typically be allocated once, in the beginning of your program (or at the
349 beginning of a new network session), and the encoding and decoding
350 will only take place in a few, isolated places in your program, so the
351 overhead is quite manageable.
356 <sect2><title>Diagnostics</title>
359 The encoding/decoding functions all return 0 when an error occurs.
360 Until you call <function>odr_reset()</function>, you cannot use the
361 stream again, and any function called will immediately return 0.
365 To provide information to the programmer or administrator, the function
369 void odr_perror(ODR o, char *message);
373 is provided, which prints the <literal>message</literal> argument to
374 <literal>stderr</literal> along with an error message from the stream.
378 You can also use the function
382 int odr_geterror(ODR o);
386 to get the current error number from the screen. The number will be
387 one of these constants:
390 <table frame="top"><title>ODR Error codes</title>
395 <entry>Description</entry>
400 <entry>OMEMORY</entry><entry>Memory allocation failed.</entry>
404 <entry>OSYSERR</entry><entry>A system- or library call has failed.
405 The standard diagnostic variable <literal>errno</literal> should be
406 examined to determine the actual error.</entry>
410 <entry>OSPACE</entry><entry>No more space for encoding.
411 This will only occur when the user has explicitly provided a
412 buffer for an encoding stream without allowing the system to
413 allocate more space.</entry>
417 <entry>OREQUIRED</entry><entry>This is a common protocol error; A
418 required data element was missing during encoding or decoding.</entry>
422 <entry>OUNEXPECTED</entry><entry>An unexpected data element was
423 found during decoding.</entry>
426 <row><entry>OOTHER</entry><entry>Other error. This is typically an
427 indication of misuse of the &odr; system by the programmer, and also
428 that the diagnostic system isn't as good as it should be, yet.</entry>
435 The character string array
439 char *odr_errlist[]
443 can be indexed by the error code to obtain a human-readable
444 representation of the problem.
448 <sect2><title>Summary and Synopsis</title>
453 ODR odr_createmem(int direction);
455 void odr_destroy(ODR o);
457 void odr_reset(ODR o);
459 char *odr_getbuf(ODR o, int *len);
461 void odr_setbuf(ODR o, char *buf, int len);
463 void *odr_malloc(ODR o, int size);
465 ODR_MEM odr_extract_mem(ODR o);
467 void odr_release_mem(ODR_MEM r);
469 int odr_geterror(ODR o);
471 void odr_perror(char *message);
473 extern char *odr_errlist[];
479 <sect1><title id="odr-prog">Programming with ODR</title>
482 The API of &odr; is designed to reflect the structure of ASN.1, rather
483 than BER itself. Future releases may be able to represent data in
484 other external forms.
488 The interface is based loosely on that of the Sun Microsystems XDR routines.
489 Specifically, each function which corresponds to an ASN.1 primitive
490 type has a dual function. Depending on the settings of the ODR
491 stream which is supplied as a parameter, the function may be used
492 either to encode or decode data. The functions that can be built
493 using these primitive functions, to represent more complex datatypes, share
494 this quality. The result is that you only have to enter the definition
495 for a type once - and you have the functionality of encoding, decoding
496 (and pretty-printing) all in one unit. The resulting C source code is
497 quite compact, and is a pretty straightforward representation of the
498 source ASN.1 specification. Although no ASN.1 compiler is supplied
499 with &odr; at this time, it shouldn't be too difficult to write one, or
500 perhaps even to adapt an existing compiler to output &odr; routines
501 (not surprisingly, writing encoders/decoders using &odr; turns out
506 In many cases, the model of the XDR functions works quite well in this
508 In others, it is less elegant. Most of the hassle comes from the optional
509 SEQUENCE memebers which don't exist in XDR.
512 <sect2><title>The Primitive ASN.1 Types</title>
515 ASN.1 defines a number of primitive types (many of which correspond
516 roughly to primitive types in structured programming languages, such as C).
519 <sect3><title>INTEGER</title>
522 The &odr; function for encoding or decoding (or printing) the ASN.1
523 INTEGER type looks like this:
527 int odr_integer(ODR o, int **p, int optional, const char *name);
531 (we don't allow values that can't be contained in a C integer.)
535 This form is typical of the primitive &odr; functions. They are named
536 after the type of data that they encode or decode. They take an &odr;
537 stream, an indirect reference to the type in question, and an
538 <literal>optional</literal> flag (corresponding to the OPTIONAL keyword
539 of ASN.1) as parameters. They all return an integer value of either one
541 When you use the primitive functions to construct encoders for complex
542 types of your own, you should follow this model as well. This
543 ensures that your new types can be reused as elements in yet more
548 The <literal>o</literal> parameter should obviously refer to a properly
549 initialized &odr; stream of the right type (encoding/decoding/printing)
550 for the operation that you wish to perform.
554 When encoding or printing, the function first looks at
555 <literal>* p</literal>. If <literal>* p</literal> (the pointer pointed
556 to by <literal>p</literal>) is a null pointer, this is taken to mean that
557 the data element is absent. If the <literal>optional</literal> parameter
558 is nonzero, the function will return one (signifying success) without
559 any further processing. If the <literal>optional</literal> is zero, an
560 internal error flag is set in the &odr; stream, and the function will
561 return 0. No further operations can be carried out on the stream without
562 a call to the function <function>odr_reset()</function>.
566 If <literal>*p</literal> is not a null pointer, it is expected to
567 point to an instance of the data type. The data will be subjected to
568 the encoding rules, and the result will be placed in the buffer held
573 The other ASN.1 primitives have similar functions that operate in
577 <sect3><title>BOOLEAN</title>
580 int odr_bool(ODR o, bool_t **p, int optional, const char *name);
584 <sect3><title>REAL</title>
591 <sect3><title>NULL</title>
594 int odr_null(ODR o, bool_t **p, int optional, const char *name);
598 In this case, the value of **p is not important. If <literal>*p</literal>
599 is different from the null pointer, the null value is present, otherwise
604 <sect3><title>OCTET STRING</title>
607 typedef struct odr_oct
614 int odr_octetstring(ODR o, Odr_oct **p, int optional, const char *name);
618 The <literal>buf</literal> field should point to the character array
619 that holds the octetstring. The <literal>len</literal> field holds the
620 actual length, while the <literal>size</literal> field gives the size
621 of the allocated array (not of interest to you, in most cases).
622 The character array need not be null terminated.
626 To make things a little easier, an alternative is given for string
627 types that are not expected to contain embedded NULL characters (eg.
632 int odr_cstring(ODR o, char **p, int optional, const char *name);
636 Which encoded or decodes between OCTETSTRING representations and
637 null-terminates C strings.
641 Functions are provided for the derived string types, eg:
645 int odr_visiblestring(ODR o, char **p, int optional, const char *name);
649 <sect3><title>BIT STRING</title>
652 int odr_bitstring(ODR o, Odr_bitmask **p, int optional, const char *name);
656 The opaque type <literal>Odr_bitmask</literal> is only suitable for
657 holding relatively brief bit strings, eg. for options fields, etc.
658 The constant <literal>ODR_BITMASK_SIZE</literal> multiplied by 8
659 gives the maximum possible number of bits.
663 A set of macros are provided for manipulating the
664 <literal>Odr_bitmask</literal> type:
668 void ODR_MASK_ZERO(Odr_bitmask *b);
670 void ODR_MASK_SET(Odr_bitmask *b, int bitno);
672 void ODR_MASK_CLEAR(Odr_bitmask *b, int bitno);
674 int ODR_MASK_GET(Odr_bitmask *b, int bitno);
678 The functions are modelled after the manipulation functions that
679 accompany the <literal>fd_set</literal> type used by the
680 <function>select(2)</function> call.
681 <literal>ODR_MASK_ZERO</literal> should always be called first on a
682 new bitmask, to initialize the bits to zero.
686 <sect3><title>OBJECT IDENTIFIER</title>
689 int odr_oid(ODR o, Odr_oid **p, int optional, const char *name);
693 The C OID represenation is simply an array of integers, terminated by
694 the value -1 (the <literal>Odr_oid</literal> type is synonymous with
695 the <literal>int</literal> type).
696 We suggest that you use the OID database module (see section
697 <link linkend="oid">Object Identifiers</link>) to handle object identifiers
703 <sect2><title id="tag-prim">Tagging Primitive Types</title>
706 The simplest way of tagging a type is to use the
707 <function>odr_implicit_tag()</function> or
708 <function>odr_explicit_tag()</function> macros:
712 int odr_implicit_tag(ODR o, Odr_fun fun, int class, int tag, int
713 optional, const char *name);
715 int odr_explicit_tag(ODR o, Odr_fun fun, int class, int tag,
716 int optional, const char *name);
720 To create a type derived from the integer type by implicit tagging, you
725 MyInt ::= [210] IMPLICIT INTEGER
729 In the &odr; system, this would be written like:
733 int myInt(ODR o, int **p, int optional, const char *name)
735 return odr_implicit_tag(o, odr_integer, p,
736 ODR_CONTEXT, 210, optional, name);
741 The function <function>myInt()</function> can then be used like any of
742 the primitive functions provided by &odr;. Note that the behavior of
743 <function>odr_explicit()</function>
744 and <function>odr_implicit()</function> macros
745 act exactly the same as the functions they are applied to - they
746 respond to error conditions, etc, in the same manner - they
747 simply have three extra parameters. The class parameter may
748 take one of the values: <literal>ODR_CONTEXT</literal>,
749 <literal>ODR_PRIVATE</literal>, <literal>ODR_UNIVERSAL</literal>, or
750 <literal>/ODR_APPLICATION</literal>.
754 <sect2><title>Constructed Types</title>
757 Constructed types are created by combining primitive types. The
758 &odr; system only implements the SEQUENCE and SEQUENCE OF constructions
759 (although adding the rest of the container types should be simple
760 enough, if the need arises).
764 For implementing SEQUENCEs, the functions
768 int odr_sequence_begin(ODR o, void *p, int size, const char *name);
769 int odr_sequence_end(ODR o);
777 The <function>odr_sequence_begin()</function> function should be
778 called in the beginning of a function that implements a SEQUENCE type.
779 Its parameters are the &odr; stream, a pointer (to a pointer to the type
780 you're implementing), and the <literal>size</literal> of the type
781 (typically a C structure). On encoding, it returns 1 if
782 <literal>* p</literal> is a null pointer. The <literal>size</literal>
783 parameter is ignored. On decoding, it returns 1 if the type is found in
784 the data stream. <literal>size</literal> bytes of memory are allocated,
785 and <literal>*p</literal> is set to point to this space.
786 <function>odr_sequence_end()</function> is called at the end of the
787 complex function. Assume that a type is defined like this:
791 MySequence ::= SEQUENCE {
793 boolval BOOLEAN OPTIONAL }
797 The corresponding &odr; encoder/decoder function and the associated data
798 structures could be written like this:
802 typedef struct MySequence
808 int mySequence(ODR o, MySequence **p, int optional, const char *name)
810 if (odr_sequence_begin(o, p, sizeof(**p), name) == 0)
811 return optional && odr_ok(o);
813 odr_integer(o, &(*p)->intval, 0, "intval") &&
814 odr_bool(o, &(*p)->boolval, 1, "boolval") &&
820 Note the 1 in the call to <function>odr_bool()</function>, to mark
821 that the sequence member is optional.
822 If either of the member types had been tagged, the macros
823 <function>odr_implicit()</function> or <function>odr_explicit()</function>
824 could have been used.
825 The new function can be used exactly like the standard functions provided
826 with &odr;. It will encode, decode or pretty-print a data value of the
827 <literal>MySequence</literal> type. We like to name types with an
828 initial capital, as done in ASN.1 definitions, and to name the
829 corresponding function with the first character of the name in lower case.
830 You could, of course, name your structures, types, and functions any way
831 you please - as long as you're consistent, and your code is easily readable.
832 <literal>odr_ok</literal> is just that - a predicate that returns the
833 state of the stream. It is used to ensure that the behaviour of the new
834 type is compatible with the interface of the primitive types.
838 <sect2><title>Tagging Constructed Types</title>
842 See section <link linkend="tag-prim">Tagging Primitive types</link>
843 for information on how to tag the primitive types, as well as types
844 that are already defined.
848 <sect3><title>Implicit Tagging</title>
851 Assume the type above had been defined as
855 MySequence ::= [10] IMPLICIT SEQUENCE {
857 boolval BOOLEAN OPTIONAL }
861 You would implement this in &odr; by calling the function
865 int odr_implicit_settag(ODR o, int class, int tag);
869 which overrides the tag of the type immediately following it. The
870 macro <function>odr_implicit()</function> works by calling
871 <function>odr_implicit_settag()</function> immediately
872 before calling the function pointer argument.
873 Your type function could look like this:
877 int mySequence(ODR o, MySequence **p, int optional, const char *name)
879 if (odr_implicit_settag(o, ODR_CONTEXT, 10) == 0 ||
880 odr_sequence_begin(o, p, sizeof(**p), name) == 0)
881 return optional && odr_ok(o);
883 odr_integer(o, &(*p)->intval, 0, "intval") &&
884 odr_bool(o, &(*p)->boolval, 1, "boolval") &&
890 The definition of the structure <literal>MySequence</literal> would be
895 <sect3><title>Explicit Tagging</title>
898 Explicit tagging of constructed types is a little more complicated,
899 since you are in effect adding a level of construction to the data.
903 Assume the definition:
907 MySequence ::= [10] IMPLICIT SEQUENCE {
909 boolval BOOLEAN OPTIONAL }
913 Since the new type has an extra level of construction, two new functions
914 are needed to encapsulate the base type:
918 int odr_constructed_begin(ODR o, void *p, int class, int tag,
921 int odr_constructed_end(ODR o);
925 Assume that the IMPLICIT in the type definition above were replaced
926 with EXPLICIT (or that the IMPLICIT keyword were simply deleted, which
927 would be equivalent). The structure definition would look the same,
928 but the function would look like this:
932 int mySequence(ODR o, MySequence **p, int optional, const char *name)
934 if (odr_constructed_begin(o, p, ODR_CONTEXT, 10, name) == 0)
935 return optional && odr_ok(o);
936 if (o->direction == ODR_DECODE)
937 *p = odr_malloc(o, sizeof(**p));
938 if (odr_sequence_begin(o, p, sizeof(**p), 0) == 0)
940 *p = 0; /* this is almost certainly a protocol error */
944 odr_integer(o, &(*p)->intval, 0, "intval") &&
945 odr_bool(o, &(*p)->boolval, 1, "boolval") &&
946 odr_sequence_end(o) &&
947 odr_constructed_end(o);
952 Notice that the interface here gets kind of nasty. The reason is
953 simple: Explicitly tagged, constructed types are fairly rare in
954 the protocols that we care about, so the
955 aesthetic annoyance (not to mention the dangers of a cluttered
956 interface) is less than the time that would be required to develop a
957 better interface. Nevertheless, it is far from satisfying, and it's a
958 point that will be worked on in the future. One option for you would
959 be to simply apply the <function>odr_explicit()</function> macro to
960 the first function, and not
961 have to worry about <function>odr_constructed_*</function> yourself.
962 Incidentally, as you might have guessed, the
963 <function>odr_sequence_</function> functions are themselves
964 implemented using the <function>/odr_constructed_</function> functions.
969 <sect2><title>SEQUENCE OF</title>
972 To handle sequences (arrays) of a apecific type, the function
976 int odr_sequence_of(ODR o, int (*fun)(ODR o, void *p, int optional),
977 void *p, int *num, const char *name);
981 The <literal>fun</literal> parameter is a pointer to the decoder/encoder
982 function of the type. <literal>p</literal> is a pointer to an array of
983 pointers to your type. <literal>num</literal> is the number of elements
992 MyArray ::= SEQUENCE OF INTEGER
996 The C representation might be
1000 typedef struct MyArray
1008 And the function might look like
1012 int myArray(ODR o, MyArray **p, int optional, const char *name)
1014 if (o->direction == ODR_DECODE)
1015 *p = odr_malloc(o, sizeof(**p));
1016 if (odr_sequence_of(o, odr_integer, &(*p)->elements,
1017 &(*p)->num_elements, name))
1020 return optional && odr_ok(o);
1025 <sect2><title>CHOICE Types</title>
1028 The choice type is used fairly often in some ASN.1 definitions, so
1029 some work has gone into streamlining its interface.
1033 CHOICE types are handled by the function:
1037 int odr_choice(ODR o, Odr_arm arm[], void *p, void *whichp,
1042 The <literal>arm</literal> array is used to describe each of the possible
1043 types that the CHOICE type may assume. Internally in your application,
1044 the CHOICE type is represented as a discriminated union. That is, a C union
1045 accompanied by an integer (or enum) identifying the active 'arm' of
1046 the union. <literal>whichp</literal> is a pointer to the union
1047 discriminator. When encoding, it is examined to determine the current
1048 type. When decoding, it is set to reference the type that was found in
1053 The Odr_arm type is defined thus:
1057 typedef struct odr_arm
1069 The interpretation of the fields are:
1073 <varlistentry><term>tagmode</term>
1074 <listitem><para>Either <literal>ODR_IMPLICIT</literal>,
1075 <literal>ODR_EXPLICIT</literal>, or <literal>ODR_NONE</literal> (-1) to mark
1076 no tagging.</para></listitem>
1079 <varlistentry><term>which</term>
1080 <listitem><para>The value of the discriminator that corresponds to
1081 this CHOICE element. Typically, it will be a #defined constant, or
1082 an enum member.</para></listitem>
1085 <varlistentry><term>fun</term>
1086 <listitem><para>A pointer to a function that implements the type of
1087 the CHOICE member. It may be either a standard &odr; type or a type
1088 defined by yourself.</para></listitem>
1091 <varlistentry><term>name</term>
1092 <listitem><para>Name of tag.</para></listitem>
1097 A handy way to prepare the array for use by the
1098 <function>odr_choice()</function> function is to
1099 define it as a static, initialized array in the beginning of your
1100 decoding/encoding function. Assume the type definition:
1104 MyChoice ::= CHOICE {
1106 tagged [99] IMPLICIT INTEGER,
1112 Your C type might look like
1116 typedef struct MyChoice
1134 And your function could look like this:
1138 int myChoice(ODR o, MyChoice **p, int optional, const char *name)
1140 static Odr_arm arm[] =
1142 {-1, -1, -1, MyChoice_untagged, odr_integer, "untagged"},
1143 {ODR_IMPLICIT, ODR_CONTEXT, 99, MyChoice_tagged, odr_integer,
1145 {-1, -1, -1, MyChoice_other, odr_boolean, "other"},
1149 if (o->direction == ODR_DECODE)
1150 *p = odr_malloc(o, sizeof(**p);
1152 return optional && odr_ok(o);
1154 if (odr_choice(o, arm, &(*p)->u, &(*p)->which), name)
1157 return optional && odr_ok(o);
1162 In some cases (say, a non-optional choice which is a member of a sequence),
1163 you can "embed" the union and its discriminator in the structure
1164 belonging to the enclosing type, and you won't need to fiddle with
1165 memory allocation to create a separate structure to wrap the
1166 discriminator and union.
1170 The corresponding function is somewhat nicer in the Sun XDR interface.
1171 Most of the complexity of this interface comes from the possibility of
1172 declaring sequence elements (including CHOICEs) optional.
1176 The ASN.1 specifictions naturally requires that each member of a
1177 CHOICE have a distinct tag, so they can be told apart on decoding.
1178 Sometimes it can be useful to define a CHOICE that has multiple types
1179 that share the same tag. You'll need some other mechanism, perhaps
1180 keyed to the context of the CHOICE type. In effect, we would like to
1181 introduce a level of context-sensitiveness to our ASN.1 specification.
1182 When encoding an internal representation, we have no problem, as long
1183 as each CHOICE member has a distinct discriminator value. For
1184 decoding, we need a way to tell the choice function to look for a
1185 specific arm of the table. The function
1189 void odr_choice_bias(ODR o, int what);
1193 provides this functionality. When called, it leaves a notice for the next
1194 call to <function>odr_choice()</function> to be called on the decoding
1195 stream <literal>o</literal> that only the <literal>arm</literal> entry with
1196 a <literal>which</literal> field equal to <literal>what</literal>
1201 The most important application (perhaps the only one, really) is in
1202 the definition of application-specific EXTERNAL encoders/decoders
1203 which will automatically decode an ANY member given the direct or
1210 <sect1><title>Debugging</title>
1213 The protocol modules are suffering somewhat from a lack of diagnostic
1214 tools at the moment. Specifically ways to pretty-print PDUs that
1215 aren't recognized by the system. We'll include something to this end
1216 in a not-too-distant release. In the meantime, what we do when we get
1217 packages we don't understand is to compile the ODR module with
1218 <literal>ODR_DEBUG</literal> defined. This causes the module to dump tracing
1219 information as it processes data units. With this output and the
1220 protocol specification (Z39.50), it is generally fairly easy to see