1 <!-- $Id: odr.xml,v 1.3 2001-07-20 21:34:36 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
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 realise 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 signalling 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><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 routines.
493 Specifically, each function which corresponds to an ASN.1 primitive
494 type has a dual function. Depending on the settings of the ODR
495 stream which is supplied as a parameter, the function may be used
496 either to encode or decode data. The functions that can be built
497 using these primitive functions, to represent more complex datatypes, share
498 this quality. The result is that you only have to enter the definition
499 for a type once - and you have the functionality of encoding, decoding
500 (and pretty-printing) all in one unit. The resulting C source code is
501 quite compact, and is a pretty straightforward representation of the
502 source ASN.1 specification. Although no ASN.1 compiler is supplied
503 with &odr; at this time, it shouldn't be too difficult to write one, or
504 perhaps even to adapt an existing compiler to output &odr; routines
505 (not surprisingly, writing encoders/decoders using &odr; turns out
510 In many cases, the model of the XDR functions works quite well in this
512 In others, it is less elegant. Most of the hassle comes from the optional
513 SEQUENCE memebers which don't exist in XDR.
516 <sect2><title>The Primitive ASN.1 Types</title>
519 ASN.1 defines a number of primitive types (many of which correspond
520 roughly to primitive types in structured programming languages, such as C).
523 <sect3><title>INTEGER</title>
526 The &odr; function for encoding or decoding (or printing) the ASN.1
527 INTEGER type looks like this:
531 int odr_integer(ODR o, int **p, int optional, const char *name);
535 (we don't allow values that can't be contained in a C integer.)
539 This form is typical of the primitive &odr; functions. They are named
540 after the type of data that they encode or decode. They take an &odr;
541 stream, an indirect reference to the type in question, and an
542 <literal>optional</literal> flag (corresponding to the OPTIONAL keyword
543 of ASN.1) as parameters. They all return an integer value of either one
545 When you use the primitive functions to construct encoders for complex
546 types of your own, you should follow this model as well. This
547 ensures that your new types can be reused as elements in yet more
552 The <literal>o</literal> parameter should obviously refer to a properly
553 initialized &odr; stream of the right type (encoding/decoding/printing)
554 for the operation that you wish to perform.
558 When encoding or printing, the function first looks at
559 <literal>* p</literal>. If <literal>* p</literal> (the pointer pointed
560 to by <literal>p</literal>) is a null pointer, this is taken to mean that
561 the data element is absent. If the <literal>optional</literal> parameter
562 is nonzero, the function will return one (signifying success) without
563 any further processing. If the <literal>optional</literal> is zero, an
564 internal error flag is set in the &odr; stream, and the function will
565 return 0. No further operations can be carried out on the stream without
566 a call to the function <function>odr_reset()</function>.
570 If <literal>*p</literal> is not a null pointer, it is expected to
571 point to an instance of the data type. The data will be subjected to
572 the encoding rules, and the result will be placed in the buffer held
577 The other ASN.1 primitives have similar functions that operate in
581 <sect3><title>BOOLEAN</title>
584 int odr_bool(ODR o, bool_t **p, int optional, const char *name);
588 <sect3><title>REAL</title>
595 <sect3><title>NULL</title>
598 int odr_null(ODR o, bool_t **p, int optional, const char *name);
602 In this case, the value of **p is not important. If <literal>*p</literal>
603 is different from the null pointer, the null value is present, otherwise
608 <sect3><title>OCTET STRING</title>
611 typedef struct odr_oct
618 int odr_octetstring(ODR o, Odr_oct **p, int optional,
623 The <literal>buf</literal> field should point to the character array
624 that holds the octetstring. The <literal>len</literal> field holds the
625 actual length, while the <literal>size</literal> field gives the size
626 of the allocated array (not of interest to you, in most cases).
627 The character array need not be null terminated.
631 To make things a little easier, an alternative is given for string
632 types that are not expected to contain embedded NULL characters (eg.
637 int odr_cstring(ODR o, char **p, int optional, const char *name);
641 Which encoded or decodes between OCTETSTRING representations and
642 null-terminates C strings.
646 Functions are provided for the derived string types, eg:
650 int odr_visiblestring(ODR o, char **p, int optional,
655 <sect3><title>BIT STRING</title>
658 int odr_bitstring(ODR o, Odr_bitmask **p, int optional,
663 The opaque type <literal>Odr_bitmask</literal> is only suitable for
664 holding relatively brief bit strings, eg. for options fields, etc.
665 The constant <literal>ODR_BITMASK_SIZE</literal> multiplied by 8
666 gives the maximum possible number of bits.
670 A set of macros are provided for manipulating the
671 <literal>Odr_bitmask</literal> type:
675 void ODR_MASK_ZERO(Odr_bitmask *b);
677 void ODR_MASK_SET(Odr_bitmask *b, int bitno);
679 void ODR_MASK_CLEAR(Odr_bitmask *b, int bitno);
681 int ODR_MASK_GET(Odr_bitmask *b, int bitno);
685 The functions are modelled after the manipulation functions that
686 accompany the <literal>fd_set</literal> type used by the
687 <function>select(2)</function> call.
688 <literal>ODR_MASK_ZERO</literal> should always be called first on a
689 new bitmask, to initialize the bits to zero.
693 <sect3><title>OBJECT IDENTIFIER</title>
696 int odr_oid(ODR o, Odr_oid **p, int optional, const char *name);
700 The C OID represenation is simply an array of integers, terminated by
701 the value -1 (the <literal>Odr_oid</literal> type is synonymous with
702 the <literal>int</literal> type).
703 We suggest that you use the OID database module (see section
704 <link linkend="oid">Object Identifiers</link>) to handle object identifiers
710 <sect2><title id="tag-prim">Tagging Primitive Types</title>
713 The simplest way of tagging a type is to use the
714 <function>odr_implicit_tag()</function> or
715 <function>odr_explicit_tag()</function> macros:
719 int odr_implicit_tag(ODR o, Odr_fun fun, int class, int tag,
720 int optional, const char *name);
722 int odr_explicit_tag(ODR o, Odr_fun fun, int class, int tag,
723 int optional, const char *name);
727 To create a type derived from the integer type by implicit tagging, you
732 MyInt ::= [210] IMPLICIT INTEGER
736 In the &odr; system, this would be written like:
740 int myInt(ODR o, int **p, int optional, const char *name)
742 return odr_implicit_tag(o, odr_integer, p,
743 ODR_CONTEXT, 210, optional, name);
748 The function <function>myInt()</function> can then be used like any of
749 the primitive functions provided by &odr;. Note that the behavior of
750 <function>odr_explicit()</function>
751 and <function>odr_implicit()</function> macros
752 act exactly the same as the functions they are applied to - they
753 respond to error conditions, etc, in the same manner - they
754 simply have three extra parameters. The class parameter may
755 take one of the values: <literal>ODR_CONTEXT</literal>,
756 <literal>ODR_PRIVATE</literal>, <literal>ODR_UNIVERSAL</literal>, or
757 <literal>/ODR_APPLICATION</literal>.
761 <sect2><title>Constructed Types</title>
764 Constructed types are created by combining primitive types. The
765 &odr; system only implements the SEQUENCE and SEQUENCE OF constructions
766 (although adding the rest of the container types should be simple
767 enough, if the need arises).
771 For implementing SEQUENCEs, the functions
775 int odr_sequence_begin(ODR o, void *p, int size, const char *name);
776 int odr_sequence_end(ODR o);
784 The <function>odr_sequence_begin()</function> function should be
785 called in the beginning of a function that implements a SEQUENCE type.
786 Its parameters are the &odr; stream, a pointer (to a pointer to the type
787 you're implementing), and the <literal>size</literal> of the type
788 (typically a C structure). On encoding, it returns 1 if
789 <literal>* p</literal> is a null pointer. The <literal>size</literal>
790 parameter is ignored. On decoding, it returns 1 if the type is found in
791 the data stream. <literal>size</literal> bytes of memory are allocated,
792 and <literal>*p</literal> is set to point to this space.
793 <function>odr_sequence_end()</function> is called at the end of the
794 complex function. Assume that a type is defined like this:
798 MySequence ::= SEQUENCE {
800 boolval BOOLEAN OPTIONAL
805 The corresponding &odr; encoder/decoder function and the associated data
806 structures could be written like this:
810 typedef struct MySequence
816 int mySequence(ODR o, MySequence **p, int optional, const char *name)
818 if (odr_sequence_begin(o, p, sizeof(**p), name) == 0)
819 return optional && odr_ok(o);
821 odr_integer(o, &(*p)->intval, 0, "intval") &&
822 odr_bool(o, &(*p)->boolval, 1, "boolval") &&
829 Note the 1 in the call to <function>odr_bool()</function>, to mark
830 that the sequence member is optional.
831 If either of the member types had been tagged, the macros
832 <function>odr_implicit()</function> or <function>odr_explicit()</function>
833 could have been used.
834 The new function can be used exactly like the standard functions provided
835 with &odr;. It will encode, decode or pretty-print a data value of the
836 <literal>MySequence</literal> type. We like to name types with an
837 initial capital, as done in ASN.1 definitions, and to name the
838 corresponding function with the first character of the name in lower case.
839 You could, of course, name your structures, types, and functions any way
840 you please - as long as you're consistent, and your code is easily readable.
841 <literal>odr_ok</literal> is just that - a predicate that returns the
842 state of the stream. It is used to ensure that the behaviour of the new
843 type is compatible with the interface of the primitive types.
847 <sect2><title>Tagging Constructed Types</title>
851 See section <link linkend="tag-prim">Tagging Primitive types</link>
852 for information on how to tag the primitive types, as well as types
853 that are already defined.
857 <sect3><title>Implicit Tagging</title>
860 Assume the type above had been defined as
864 MySequence ::= [10] IMPLICIT SEQUENCE {
866 boolval BOOLEAN OPTIONAL
871 You would implement this in &odr; by calling the function
875 int odr_implicit_settag(ODR o, int class, int tag);
879 which overrides the tag of the type immediately following it. The
880 macro <function>odr_implicit()</function> works by calling
881 <function>odr_implicit_settag()</function> immediately
882 before calling the function pointer argument.
883 Your type function could look like this:
887 int mySequence(ODR o, MySequence **p, int optional, const char *name)
889 if (odr_implicit_settag(o, ODR_CONTEXT, 10) == 0 ||
890 odr_sequence_begin(o, p, sizeof(**p), name) == 0)
891 return optional && odr_ok(o);
893 odr_integer(o, &(*p)->intval, 0, "intval") &&
894 odr_bool(o, &(*p)->boolval, 1, "boolval") &&
900 The definition of the structure <literal>MySequence</literal> would be
905 <sect3><title>Explicit Tagging</title>
908 Explicit tagging of constructed types is a little more complicated,
909 since you are in effect adding a level of construction to the data.
913 Assume the definition:
917 MySequence ::= [10] IMPLICIT SEQUENCE {
919 boolval BOOLEAN OPTIONAL
924 Since the new type has an extra level of construction, two new functions
925 are needed to encapsulate the base type:
929 int odr_constructed_begin(ODR o, void *p, int class, int tag,
932 int odr_constructed_end(ODR o);
936 Assume that the IMPLICIT in the type definition above were replaced
937 with EXPLICIT (or that the IMPLICIT keyword were simply deleted, which
938 would be equivalent). The structure definition would look the same,
939 but the function would look like this:
943 int mySequence(ODR o, MySequence **p, int optional, const char *name)
945 if (odr_constructed_begin(o, p, ODR_CONTEXT, 10, name) == 0)
946 return optional && odr_ok(o);
947 if (o->direction == ODR_DECODE)
948 *p = odr_malloc(o, sizeof(**p));
949 if (odr_sequence_begin(o, p, sizeof(**p), 0) == 0)
951 *p = 0; /* this is almost certainly a protocol error */
955 odr_integer(o, &(*p)->intval, 0, "intval") &&
956 odr_bool(o, &(*p)->boolval, 1, "boolval") &&
957 odr_sequence_end(o) &&
958 odr_constructed_end(o);
963 Notice that the interface here gets kind of nasty. The reason is
964 simple: Explicitly tagged, constructed types are fairly rare in
965 the protocols that we care about, so the
966 aesthetic annoyance (not to mention the dangers of a cluttered
967 interface) is less than the time that would be required to develop a
968 better interface. Nevertheless, it is far from satisfying, and it's a
969 point that will be worked on in the future. One option for you would
970 be to simply apply the <function>odr_explicit()</function> macro to
971 the first function, and not
972 have to worry about <function>odr_constructed_*</function> yourself.
973 Incidentally, as you might have guessed, the
974 <function>odr_sequence_</function> functions are themselves
975 implemented using the <function>/odr_constructed_</function> functions.
980 <sect2><title>SEQUENCE OF</title>
983 To handle sequences (arrays) of a apecific type, the function
987 int odr_sequence_of(ODR o, int (*fun)(ODR o, void *p, int optional),
988 void *p, int *num, const char *name);
992 The <literal>fun</literal> parameter is a pointer to the decoder/encoder
993 function of the type. <literal>p</literal> is a pointer to an array of
994 pointers to your type. <literal>num</literal> is the number of elements
1003 MyArray ::= SEQUENCE OF INTEGER
1007 The C representation might be
1011 typedef struct MyArray
1019 And the function might look like
1023 int myArray(ODR o, MyArray **p, int optional, const char *name)
1025 if (o->direction == ODR_DECODE)
1026 *p = odr_malloc(o, sizeof(**p));
1027 if (odr_sequence_of(o, odr_integer, &(*p)->elements,
1028 &(*p)->num_elements, name))
1031 return optional && odr_ok(o);
1036 <sect2><title>CHOICE Types</title>
1039 The choice type is used fairly often in some ASN.1 definitions, so
1040 some work has gone into streamlining its interface.
1044 CHOICE types are handled by the function:
1048 int odr_choice(ODR o, Odr_arm arm[], void *p, void *whichp,
1053 The <literal>arm</literal> array is used to describe each of the possible
1054 types that the CHOICE type may assume. Internally in your application,
1055 the CHOICE type is represented as a discriminated union. That is, a
1056 C union accompanied by an integer (or enum) identifying the active
1058 <literal>whichp</literal> is a pointer to the union discriminator.
1059 When encoding, it is examined to determine the current type.
1060 When decoding, it is set to reference the type that was found in
1065 The Odr_arm type is defined thus:
1069 typedef struct odr_arm
1081 The interpretation of the fields are:
1085 <varlistentry><term>tagmode</term>
1086 <listitem><para>Either <literal>ODR_IMPLICIT</literal>,
1087 <literal>ODR_EXPLICIT</literal>, or <literal>ODR_NONE</literal> (-1)
1088 to mark no tagging.</para></listitem>
1091 <varlistentry><term>which</term>
1092 <listitem><para>The value of the discriminator that corresponds to
1093 this CHOICE element. Typically, it will be a #defined constant, or
1094 an enum member.</para></listitem>
1097 <varlistentry><term>fun</term>
1098 <listitem><para>A pointer to a function that implements the type of
1099 the CHOICE member. It may be either a standard &odr; type or a type
1100 defined by yourself.</para></listitem>
1103 <varlistentry><term>name</term>
1104 <listitem><para>Name of tag.</para></listitem>
1109 A handy way to prepare the array for use by the
1110 <function>odr_choice()</function> function is to
1111 define it as a static, initialized array in the beginning of your
1112 decoding/encoding function. Assume the type definition:
1116 MyChoice ::= CHOICE {
1118 tagged [99] IMPLICIT INTEGER,
1124 Your C type might look like
1128 typedef struct MyChoice
1146 And your function could look like this:
1150 int myChoice(ODR o, MyChoice **p, int optional, const char *name)
1152 static Odr_arm arm[] =
1154 {-1, -1, -1, MyChoice_untagged, odr_integer, "untagged"},
1155 {ODR_IMPLICIT, ODR_CONTEXT, 99, MyChoice_tagged, odr_integer,
1157 {-1, -1, -1, MyChoice_other, odr_boolean, "other"},
1161 if (o->direction == ODR_DECODE)
1162 *p = odr_malloc(o, sizeof(**p);
1164 return optional && odr_ok(o);
1166 if (odr_choice(o, arm, &(*p)->u, &(*p)->which), name)
1169 return optional && odr_ok(o);
1174 In some cases (say, a non-optional choice which is a member of a
1175 sequence), you can "embed" the union and its discriminator in the
1176 structure belonging to the enclosing type, and you won't need to
1177 fiddle with memory allocation to create a separate structure to
1178 wrap the discriminator and union.
1182 The corresponding function is somewhat nicer in the Sun XDR interface.
1183 Most of the complexity of this interface comes from the possibility of
1184 declaring sequence elements (including CHOICEs) optional.
1188 The ASN.1 specifictions naturally requires that each member of a
1189 CHOICE have a distinct tag, so they can be told apart on decoding.
1190 Sometimes it can be useful to define a CHOICE that has multiple types
1191 that share the same tag. You'll need some other mechanism, perhaps
1192 keyed to the context of the CHOICE type. In effect, we would like to
1193 introduce a level of context-sensitiveness to our ASN.1 specification.
1194 When encoding an internal representation, we have no problem, as long
1195 as each CHOICE member has a distinct discriminator value. For
1196 decoding, we need a way to tell the choice function to look for a
1197 specific arm of the table. The function
1201 void odr_choice_bias(ODR o, int what);
1205 provides this functionality. When called, it leaves a notice for the next
1206 call to <function>odr_choice()</function> to be called on the decoding
1207 stream <literal>o</literal> that only the <literal>arm</literal> entry with
1208 a <literal>which</literal> field equal to <literal>what</literal>
1213 The most important application (perhaps the only one, really) is in
1214 the definition of application-specific EXTERNAL encoders/decoders
1215 which will automatically decode an ANY member given the direct or
1222 <sect1><title>Debugging</title>
1225 The protocol modules are suffering somewhat from a lack of diagnostic
1226 tools at the moment. Specifically ways to pretty-print PDUs that
1227 aren't recognized by the system. We'll include something to this end
1228 in a not-too-distant release. In the meantime, what we do when we get
1229 packages we don't understand is to compile the ODR module with
1230 <literal>ODR_DEBUG</literal> defined. This causes the module to dump tracing
1231 information as it processes data units. With this output and the
1232 protocol specification (Z39.50), it is generally fairly easy to see
1237 <!-- Keep this comment at the end of the file
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