1 <!-- $Id: odr.xml,v 1.21 2007-10-16 10:45:53 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
17 (<xref linkend="odr.use"/>). 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 (<xref linkend="odr.programming"/>).
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 <xref linkend="odr.programming"/>
28 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>Using ODR</title>
42 <sect2 id="odr.streams"><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 id="odr.memory.management"><title id="memory">Memory Management</title>
80 Two forms of memory management take place in the &odr; system. The first
81 one, which has to do with allocating little bits of memory (sometimes
82 quite large bits of memory, actually) when a protocol package is
83 decoded, and turned into a complex of interlinked structures. This
84 section deals with this system, and how you can use it for your own
85 purposes. The next section deals with the memory management which is
86 required when encoding data - to make sure that a large enough buffer is
87 available to hold the fully encoded PDU.
91 The &odr; module has its own memory management system, which is
92 used whenever memory is required. Specifically, it is used to allocate
93 space for data when decoding incoming PDUs. You can use the memory
94 system for your own purposes, by using the function
98 void *odr_malloc(ODR o, int size);
102 You can't use the normal <function>free(2)</function> routine to free
103 memory allocated by this function, and &odr; doesn't provide a parallel
104 function. Instead, you can call
108 void odr_reset(ODR o, int size);
112 when you are done with the
113 memory: Everything allocated since the last call to
114 <function>odr_reset()</function> is released.
115 The <function>odr_reset()</function> call is also required to clear
116 up an error condition on a stream.
124 int odr_total(ODR o);
128 returns the number of bytes allocated on the stream since the last call to
129 <function>odr_reset()</function>.
133 The memory subsystem of &odr; is fairly efficient at allocating and
134 releasing little bits of memory. Rather than managing the individual,
135 small bits of space, the system maintains a free-list of larger chunks
136 of memory, which are handed out in small bits. This scheme is
137 generally known as a <emphasis>nibble memory</emphasis> system.
138 It is very useful for maintaining short-lived constructions such
143 If you want to retain a bit of memory beyond the next call to
144 <function>odr_reset()</function>, you can use the function
148 ODR_MEM odr_extract_mem(ODR o);
152 This function will give you control of the memory recently allocated
153 on the ODR stream. The memory will live (past calls to
154 <function>odr_reset()</function>), until you call the function
158 void odr_release_mem(ODR_MEM p);
162 The opaque <literal>ODR_MEM</literal> handle has no other purpose than
163 referencing the memory block for you until you want to release it.
167 You can use <function>odr_extract_mem()</function> repeatedly between
168 allocating data, to retain individual control of separate chunks of data.
172 <sect2 id="odr.encoding.and.decoding"><title>Encoding and Decoding Data</title>
175 When encoding data, the ODR stream will write the encoded octet string
176 in an internal buffer. To retrieve the data, use the function
180 char *odr_getbuf(ODR o, int *len, int *size);
184 The integer pointed to by len is set to the length of the encoded
185 data, and a pointer to that data is returned. <literal>*size</literal>
186 is set to the size of the buffer (unless <literal>size</literal> is null,
187 signaling that you are not interested in the size). The next call to
188 a primitive function using the same &odr; stream will overwrite the
189 data, unless a different buffer has been supplied using the call
193 void odr_setbuf(ODR o, char *buf, int len, int can_grow);
197 which sets the encoding (or decoding) buffer used by
198 <literal>o</literal> to <literal>buf</literal>, using the length
199 <literal>len</literal>.
200 Before a call to an encoding function, you can use
201 <function>odr_setbuf()</function> to provide the stream with an encoding
202 buffer of sufficient size (length). The <literal>can_grow</literal>
203 parameter tells the encoding &odr; stream whether it is allowed to use
204 <function>realloc(2)</function> to increase the size of the buffer when
205 necessary. The default condition of a new encoding stream is equivalent
206 to the results of calling
210 odr_setbuf(stream, 0, 0, 1);
214 In this case, the stream will allocate and reallocate memory as
215 necessary. The stream reallocates memory by repeatedly doubling the
216 size of the buffer - the result is that the buffer will typically
217 reach its maximum, working size with only a small number of reallocation
218 operations. The memory is freed by the stream when the latter is destroyed,
219 unless it was assigned by the user with the <literal>can_grow</literal>
220 parameter set to zero (in this case, you are expected to retain
221 control of the memory yourself).
225 To assume full control of an encoded buffer, you must first call
226 <function>odr_getbuf()</function> to fetch the buffer and its length.
227 Next, you should call <function>odr_setbuf()</function> to provide a
228 different buffer (or a null pointer) to the stream. In the simplest
229 case, you will reuse the same buffer over and over again, and you
230 will just need to call <function>odr_getbuf()</function> after each
231 encoding operation to get the length and address of the buffer.
232 Note that the stream may reallocate the buffer during an encoding
233 operation, so it is necessary to retrieve the correct address after
234 each encoding operation.
238 It is important to realize that the ODR stream will not release this
239 memory when you call <function>odr_reset()</function>: It will
240 merely update its internal pointers to prepare for the encoding of a
242 When the stream is released by the <function>odr_destroy()</function>
243 function, the memory given to it by <function>odr_setbuf</function> will
244 be released <emphasis>only</emphasis> if the <literal>can_grow</literal>
245 parameter to <function>odr_setbuf()</function> was nonzero. The
246 <literal>can_grow</literal> parameter, in other words, is a way of
247 signaling who is to own the buffer, you or the ODR stream. If you never call
248 <function>odr_setbuf()</function> on your encoding stream, which is
249 typically the case, the buffer allocated by the stream will belong to
250 the stream by default.
254 When you wish to decode data, you should first call
255 <function>odr_setbuf()</function>, to tell the decoding stream
256 where to find the encoded data, and how long the buffer is
257 (the <literal>can_grow</literal> parameter is ignored by a decoding
258 stream). After this, you can call the function corresponding to the
259 data you wish to decode (eg, <function>odr_integer()</function> odr
260 <function>z_APDU()</function>).
263 <example id="example.odr.encoding.and.decoding.functions">
264 <title>Encoding and decoding functions</title>
266 int odr_integer(ODR o, int **p, int optional, const char *name);
268 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
274 the type), 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
280 <literal>p</literal> will be set to the null value, and the function
282 The <literal>name</literal> argument is used to pretty-print the
283 tag in question. It may be set to <literal>NULL</literal> if
284 pretty-printing is not desired.
288 If the data value is found where it's expected, the pointer
289 <emphasis>pointed to</emphasis> by the <literal>p</literal> argument
290 will be set to point to the decoded type.
291 The space for the type will be allocated and owned by the &odr;
292 stream, and it will live until you call
293 <function>odr_reset()</function> on the stream. You cannot use
294 <function>free(2)</function> to release the memory.
295 You can decode several data elements (by repeated calls to
296 <function>odr_setbuf()</function> and your decoding function), and
297 new memory will be allocated each time. When you do call
298 <function>odr_reset()</function>, everything decoded since the
299 last call to <function>odr_reset()</function> will be released.
302 <example id="example.odr.encoding.of.integer">
303 <title>Encoding and decoding of an integer</title>
305 The use of the double indirection can be a little confusing at first
306 (its purpose will become clear later on, hopefully),
307 so an example is in order. We'll encode an integer value, and
308 immediately decode it again using a different stream. A useless, but
309 informative operation.
311 <programlisting><![CDATA[
312 void do_nothing_useful(int value)
319 /* allocate streams */
320 if (!(encode = odr_createmem(ODR_ENCODE)))
322 if (!(decode = odr_createmem(ODR_DECODE)))
326 if (odr_integer(encode, &valp, 0, 0) == 0)
328 printf("encoding went bad\n");
331 bufferp = odr_getbuf(encode, &len);
332 printf("length of encoded data is %d\n", len);
334 /* now let's decode the thing again */
335 odr_setbuf(decode, bufferp, len);
336 if (odr_integer(decode, &resvalp, 0, 0) == 0)
338 printf("decoding went bad\n");
341 printf("the value is %d\n", *resvalp);
350 This looks like a lot of work, offhand. In practice, the &odr; streams
351 will typically be allocated once, in the beginning of your program
352 (or at the beginning of a new network session), and the encoding
353 and decoding will only take place in a few, isolated places in your
354 program, so the overhead is quite manageable.
360 <sect2 id="odr.printing"><title>Printing</title>
362 When an ODR stream is created of type <literal>ODR_PRINT</literal>
363 the ODR module will print the contents of a PDU in a readable format.
364 By default output is written to the <literal>stderr</literal> stream.
365 This behavior can be changed, however, by calling the function
367 odr_setprint(ODR o, FILE *file);
369 before encoders or decoders are being invoked.
370 It is also possible to direct the output to a buffer (of indeed
371 another file), by using the more generic mechanism:
373 void odr_set_stream(ODR o, void *handle,
374 void (*stream_write)(ODR o, void *handle, int type,
375 const char *buf, int len),
376 void (*stream_close)(void *handle));
378 Here the user provides an opaque handle and two handlers,
379 <replaceable>stream_write</replaceable> for writing,
380 and <replaceable>stream_close</replaceable> which is supposed
381 to close/free resources associated with handle.
382 The <replaceable>stream_close</replaceable> handler is optional and
383 if NULL for the function is provided, it will not be invoked.
384 The <replaceable>stream_write</replaceable> takes the ODR handle
385 as parameter, the user defined handle, a type
386 <literal>ODR_OCTETSTRING</literal>, <literal>ODR_VISIBLESTRING</literal>
387 which indicates the type of contents is being written.
390 Another utility useful for diagnostics (error handling) or as
391 part of the printing facilities is:
393 const char **odr_get_element_path(ODR o);
395 which returns a list of current elements that ODR deals with at the
396 moment. For the returned array, say <literal>ar</literal>,
397 <literal>ar[0]</literal> is the top level element,
398 <literal>ar[n]</literal> is the last. The last element has the
399 property that <literal>ar[n+1] == NULL</literal>.
401 <example id="example.odr.element.path.record">
402 <title>Element Path for record</title>
404 For a database record part of a PresentResponse the
405 array returned by <function>odr_get_element</function>
406 is <literal>presentResponse</literal>, <literal>databaseOrSurDiagnostics</literal>, <literal>?</literal>, <literal>record</literal>, <literal>?</literal>, <literal>databaseRecord</literal> . The question mark appears due to
407 unnamed constructions.
411 <sect2 id="odr.diagnostics"><title>Diagnostics</title>
414 The encoding/decoding functions all return 0 when an error occurs.
415 Until you call <function>odr_reset()</function>, you cannot use the
416 stream again, and any function called will immediately return 0.
420 To provide information to the programmer or administrator, the function
424 void odr_perror(ODR o, char *message);
428 is provided, which prints the <literal>message</literal> argument to
429 <literal>stderr</literal> along with an error message from the stream.
433 You can also use the function
437 int odr_geterror(ODR o);
441 to get the current error number from the screen. The number will be
442 one of these constants:
445 <table frame="top" id="odr.error.codes">
446 <title>ODR Error codes</title>
451 <entry>Description</entry>
456 <entry>OMEMORY</entry><entry>Memory allocation failed.</entry>
460 <entry>OSYSERR</entry><entry>A system- or library call has failed.
461 The standard diagnostic variable <literal>errno</literal> should be
462 examined to determine the actual error.</entry>
466 <entry>OSPACE</entry><entry>No more space for encoding.
467 This will only occur when the user has explicitly provided a
468 buffer for an encoding stream without allowing the system to
469 allocate more space.</entry>
473 <entry>OREQUIRED</entry><entry>This is a common protocol error; A
474 required data element was missing during encoding or decoding.</entry>
478 <entry>OUNEXPECTED</entry><entry>An unexpected data element was
479 found during decoding.</entry>
482 <row><entry>OOTHER</entry><entry>Other error. This is typically an
483 indication of misuse of the &odr; system by the programmer, and also
484 that the diagnostic system isn't as good as it should be, yet.</entry>
491 The character string array
499 can be indexed by the error code to obtain a human-readable
500 representation of the problem.
504 <sect2 id="odr.summary.and.synopsis">
505 <title>Summary and Synopsis</title>
510 ODR odr_createmem(int direction);
512 void odr_destroy(ODR o);
514 void odr_reset(ODR o);
516 char *odr_getbuf(ODR o, int *len);
518 void odr_setbuf(ODR o, char *buf, int len);
520 void *odr_malloc(ODR o, int size);
522 ODR_MEM odr_extract_mem(ODR o);
524 void odr_release_mem(ODR_MEM r);
526 int odr_geterror(ODR o);
528 void odr_perror(char *message);
530 extern char *odr_errlist[];
536 <sect1 id="odr.programming"><title>Programming with ODR</title>
539 The API of &odr; is designed to reflect the structure of ASN.1, rather
540 than BER itself. Future releases may be able to represent data in
541 other external forms.
546 There is an ASN.1 tutorial available at
547 <ulink url="&url.asn.1.tutorial;">this site</ulink>.
548 This site also has standards for ASN.1 (X.680) and BER (X.690)
549 <ulink url="&url.asn.1.standards;">online</ulink>.
554 The ODR interface is based loosely on that of the Sun Microsystems
556 Specifically, each function which corresponds to an ASN.1 primitive
557 type has a dual function. Depending on the settings of the ODR
558 stream which is supplied as a parameter, the function may be used
559 either to encode or decode data. The functions that can be built
560 using these primitive functions, to represent more complex data types,
561 share this quality. The result is that you only have to enter the
562 definition for a type once - and you have the functionality of encoding,
563 decoding (and pretty-printing) all in one unit.
564 The resulting C source code is quite compact, and is a pretty
565 straightforward representation of the source ASN.1 specification.
569 In many cases, the model of the XDR functions works quite well in this
571 In others, it is less elegant. Most of the hassle comes from the optional
572 SEQUENCE members which don't exist in XDR.
575 <sect2 id="odr.primitive.asn1.types">
576 <title>The Primitive ASN.1 Types</title>
579 ASN.1 defines a number of primitive types (many of which correspond
580 roughly to primitive types in structured programming languages, such as C).
583 <sect3 id="odr.integer"><title>INTEGER</title>
586 The &odr; function for encoding or decoding (or printing) the ASN.1
587 INTEGER type looks like this:
591 int odr_integer(ODR o, int **p, int optional, const char *name);
595 (we don't allow values that can't be contained in a C integer.)
599 This form is typical of the primitive &odr; functions. They are named
600 after the type of data that they encode or decode. They take an &odr;
601 stream, an indirect reference to the type in question, and an
602 <literal>optional</literal> flag (corresponding to the OPTIONAL keyword
603 of ASN.1) as parameters. They all return an integer value of either one
605 When you use the primitive functions to construct encoders for complex
606 types of your own, you should follow this model as well. This
607 ensures that your new types can be reused as elements in yet more
612 The <literal>o</literal> parameter should obviously refer to a properly
613 initialized &odr; stream of the right type (encoding/decoding/printing)
614 for the operation that you wish to perform.
618 When encoding or printing, the function first looks at
619 <literal>* p</literal>. If <literal>* p</literal> (the pointer pointed
620 to by <literal>p</literal>) is a null pointer, this is taken to mean that
621 the data element is absent. If the <literal>optional</literal> parameter
622 is nonzero, the function will return one (signifying success) without
623 any further processing. If the <literal>optional</literal> is zero, an
624 internal error flag is set in the &odr; stream, and the function will
625 return 0. No further operations can be carried out on the stream without
626 a call to the function <function>odr_reset()</function>.
630 If <literal>*p</literal> is not a null pointer, it is expected to
631 point to an instance of the data type. The data will be subjected to
632 the encoding rules, and the result will be placed in the buffer held
637 The other ASN.1 primitives have similar functions that operate in
641 <sect3 id="odr.boolean"><title>BOOLEAN</title>
644 int odr_bool(ODR o, bool_t **p, int optional, const char *name);
648 <sect3 id="odr.real"><title>REAL</title>
655 <sect3 id="odr.null"><title>NULL</title>
658 int odr_null(ODR o, bool_t **p, int optional, const char *name);
662 In this case, the value of **p is not important. If <literal>*p</literal>
663 is different from the null pointer, the null value is present, otherwise
668 <sect3 id="odr.octet.string"><title>OCTET STRING</title>
671 typedef struct odr_oct
678 int odr_octetstring(ODR o, Odr_oct **p, int optional,
683 The <literal>buf</literal> field should point to the character array
684 that holds the octetstring. The <literal>len</literal> field holds the
685 actual length, while the <literal>size</literal> field gives the size
686 of the allocated array (not of interest to you, in most cases).
687 The character array need not be null terminated.
691 To make things a little easier, an alternative is given for string
692 types that are not expected to contain embedded NULL characters (eg.
697 int odr_cstring(ODR o, char **p, int optional, const char *name);
701 Which encoded or decodes between OCTETSTRING representations and
702 null-terminates C strings.
706 Functions are provided for the derived string types, eg:
710 int odr_visiblestring(ODR o, char **p, int optional,
715 <sect3 id="odr.bit.string"><title>BIT STRING</title>
718 int odr_bitstring(ODR o, Odr_bitmask **p, int optional,
723 The opaque type <literal>Odr_bitmask</literal> is only suitable for
724 holding relatively brief bit strings, eg. for options fields, etc.
725 The constant <literal>ODR_BITMASK_SIZE</literal> multiplied by 8
726 gives the maximum possible number of bits.
730 A set of macros are provided for manipulating the
731 <literal>Odr_bitmask</literal> type:
735 void ODR_MASK_ZERO(Odr_bitmask *b);
737 void ODR_MASK_SET(Odr_bitmask *b, int bitno);
739 void ODR_MASK_CLEAR(Odr_bitmask *b, int bitno);
741 int ODR_MASK_GET(Odr_bitmask *b, int bitno);
745 The functions are modeled after the manipulation functions that
746 accompany the <literal>fd_set</literal> type used by the
747 <function>select(2)</function> call.
748 <literal>ODR_MASK_ZERO</literal> should always be called first on a
749 new bitmask, to initialize the bits to zero.
753 <sect3 id="odr.object.identifier"><title>OBJECT IDENTIFIER</title>
756 int odr_oid(ODR o, Odr_oid **p, int optional, const char *name);
760 The C OID representation is simply an array of integers, terminated by
761 the value -1 (the <literal>Odr_oid</literal> type is synonymous with
762 the <literal>short</literal> type).
763 We suggest that you use the OID database module (see
764 <xref linkend="tools.oid.database"/>) to handle object identifiers
770 <sect2 id="odr.tagging.primitive.types"><title>Tagging Primitive Types</title> <!-- tag.prim -->
773 The simplest way of tagging a type is to use the
774 <function>odr_implicit_tag()</function> or
775 <function>odr_explicit_tag()</function> macros:
779 int odr_implicit_tag(ODR o, Odr_fun fun, int class, int tag,
780 int optional, const char *name);
782 int odr_explicit_tag(ODR o, Odr_fun fun, int class, int tag,
783 int optional, const char *name);
787 To create a type derived from the integer type by implicit tagging, you
792 MyInt ::= [210] IMPLICIT INTEGER
796 In the &odr; system, this would be written like:
800 int myInt(ODR o, int **p, int optional, const char *name)
802 return odr_implicit_tag(o, odr_integer, p,
803 ODR_CONTEXT, 210, optional, name);
808 The function <function>myInt()</function> can then be used like any of
809 the primitive functions provided by &odr;. Note that the behavior of
810 <function>odr_explicit_tag()</function>
811 and <function>odr_implicit_tag()</function> macros
812 act exactly the same as the functions they are applied to - they
813 respond to error conditions, etc, in the same manner - they
814 simply have three extra parameters. The class parameter may
815 take one of the values: <literal>ODR_CONTEXT</literal>,
816 <literal>ODR_PRIVATE</literal>, <literal>ODR_UNIVERSAL</literal>, or
817 <literal>/ODR_APPLICATION</literal>.
821 <sect2 id="odr.constructed.types"><title>Constructed Types</title>
824 Constructed types are created by combining primitive types. The
825 &odr; system only implements the SEQUENCE and SEQUENCE OF constructions
826 (although adding the rest of the container types should be simple
827 enough, if the need arises).
831 For implementing SEQUENCEs, the functions
835 int odr_sequence_begin(ODR o, void *p, int size, const char *name);
836 int odr_sequence_end(ODR o);
844 The <function>odr_sequence_begin()</function> function should be
845 called in the beginning of a function that implements a SEQUENCE type.
846 Its parameters are the &odr; stream, a pointer (to a pointer to the type
847 you're implementing), and the <literal>size</literal> of the type
848 (typically a C structure). On encoding, it returns 1 if
849 <literal>* p</literal> is a null pointer. The <literal>size</literal>
850 parameter is ignored. On decoding, it returns 1 if the type is found in
851 the data stream. <literal>size</literal> bytes of memory are allocated,
852 and <literal>*p</literal> is set to point to this space.
853 <function>odr_sequence_end()</function> is called at the end of the
854 complex function. Assume that a type is defined like this:
858 MySequence ::= SEQUENCE {
860 boolval BOOLEAN OPTIONAL
865 The corresponding &odr; encoder/decoder function and the associated data
866 structures could be written like this:
870 typedef struct MySequence
876 int mySequence(ODR o, MySequence **p, int optional, const char *name)
878 if (odr_sequence_begin(o, p, sizeof(**p), name) == 0)
879 return optional && odr_ok(o);
881 odr_integer(o, &(*p)->intval, 0, "intval") &&
882 odr_bool(o, &(*p)->boolval, 1, "boolval") &&
889 Note the 1 in the call to <function>odr_bool()</function>, to mark
890 that the sequence member is optional.
891 If either of the member types had been tagged, the macros
892 <function>odr_implicit_tag()</function> or
893 <function>odr_explicit_tag()</function>
894 could have been used.
895 The new function can be used exactly like the standard functions provided
896 with &odr;. It will encode, decode or pretty-print a data value of the
897 <literal>MySequence</literal> type. We like to name types with an
898 initial capital, as done in ASN.1 definitions, and to name the
899 corresponding function with the first character of the name in lower case.
900 You could, of course, name your structures, types, and functions any way
901 you please - as long as you're consistent, and your code is easily readable.
902 <literal>odr_ok</literal> is just that - a predicate that returns the
903 state of the stream. It is used to ensure that the behavior of the new
904 type is compatible with the interface of the primitive types.
908 <sect2 id="odr.tagging.constructed.types">
909 <title>Tagging Constructed Types</title>
913 See <xref linkend="odr.tagging.primitive.types"/> for information on how to tag
914 the primitive types, as well as types that are already defined.
918 <sect3 id="odr.implicit.tagging">
919 <title>Implicit Tagging</title>
922 Assume the type above had been defined as
926 MySequence ::= [10] IMPLICIT SEQUENCE {
928 boolval BOOLEAN OPTIONAL
933 You would implement this in &odr; by calling the function
937 int odr_implicit_settag(ODR o, int class, int tag);
941 which overrides the tag of the type immediately following it. The
942 macro <function>odr_implicit_tag()</function> works by calling
943 <function>odr_implicit_settag()</function> immediately
944 before calling the function pointer argument.
945 Your type function could look like this:
949 int mySequence(ODR o, MySequence **p, int optional, const char *name)
951 if (odr_implicit_settag(o, ODR_CONTEXT, 10) == 0 ||
952 odr_sequence_begin(o, p, sizeof(**p), name) == 0)
953 return optional && odr_ok(o);
955 odr_integer(o, &(*p)->intval, 0, "intval") &&
956 odr_bool(o, &(*p)->boolval, 1, "boolval") &&
962 The definition of the structure <literal>MySequence</literal> would be
967 <sect3 id="odr.explicit.tagging"><title>Explicit Tagging</title>
970 Explicit tagging of constructed types is a little more complicated,
971 since you are in effect adding a level of construction to the data.
975 Assume the definition:
979 MySequence ::= [10] IMPLICIT SEQUENCE {
981 boolval BOOLEAN OPTIONAL
986 Since the new type has an extra level of construction, two new functions
987 are needed to encapsulate the base type:
991 int odr_constructed_begin(ODR o, void *p, int class, int tag,
994 int odr_constructed_end(ODR o);
998 Assume that the IMPLICIT in the type definition above were replaced
999 with EXPLICIT (or that the IMPLICIT keyword were simply deleted, which
1000 would be equivalent). The structure definition would look the same,
1001 but the function would look like this:
1005 int mySequence(ODR o, MySequence **p, int optional, const char *name)
1007 if (odr_constructed_begin(o, p, ODR_CONTEXT, 10, name) == 0)
1008 return optional && odr_ok(o);
1009 if (o->direction == ODR_DECODE)
1010 *p = odr_malloc(o, sizeof(**p));
1011 if (odr_sequence_begin(o, p, sizeof(**p), 0) == 0)
1013 *p = 0; /* this is almost certainly a protocol error */
1017 odr_integer(o, &(*p)->intval, 0, "intval") &&
1018 odr_bool(o, &(*p)->boolval, 1, "boolval") &&
1019 odr_sequence_end(o) &&
1020 odr_constructed_end(o);
1025 Notice that the interface here gets kind of nasty. The reason is
1026 simple: Explicitly tagged, constructed types are fairly rare in
1027 the protocols that we care about, so the
1028 esthetic annoyance (not to mention the dangers of a cluttered
1029 interface) is less than the time that would be required to develop a
1030 better interface. Nevertheless, it is far from satisfying, and it's a
1031 point that will be worked on in the future. One option for you would
1032 be to simply apply the <function>odr_explicit_tag()</function> macro to
1033 the first function, and not
1034 have to worry about <function>odr_constructed_*</function> yourself.
1035 Incidentally, as you might have guessed, the
1036 <function>odr_sequence_</function> functions are themselves
1037 implemented using the <function>/odr_constructed_</function> functions.
1042 <sect2 id="odr.sequence.of"><title>SEQUENCE OF</title>
1045 To handle sequences (arrays) of a specific type, the function
1049 int odr_sequence_of(ODR o, int (*fun)(ODR o, void *p, int optional),
1050 void *p, int *num, const char *name);
1054 The <literal>fun</literal> parameter is a pointer to the decoder/encoder
1055 function of the type. <literal>p</literal> is a pointer to an array of
1056 pointers to your type. <literal>num</literal> is the number of elements
1065 MyArray ::= SEQUENCE OF INTEGER
1069 The C representation might be
1073 typedef struct MyArray
1081 And the function might look like
1085 int myArray(ODR o, MyArray **p, int optional, const char *name)
1087 if (o->direction == ODR_DECODE)
1088 *p = odr_malloc(o, sizeof(**p));
1089 if (odr_sequence_of(o, odr_integer, &(*p)->elements,
1090 &(*p)->num_elements, name))
1093 return optional && odr_ok(o);
1098 <sect2 id="odr.choice.types"><title>CHOICE Types</title>
1101 The choice type is used fairly often in some ASN.1 definitions, so
1102 some work has gone into streamlining its interface.
1106 CHOICE types are handled by the function:
1110 int odr_choice(ODR o, Odr_arm arm[], void *p, void *whichp,
1115 The <literal>arm</literal> array is used to describe each of the possible
1116 types that the CHOICE type may assume. Internally in your application,
1117 the CHOICE type is represented as a discriminated union. That is, a
1118 C union accompanied by an integer (or enum) identifying the active
1120 <literal>whichp</literal> is a pointer to the union discriminator.
1121 When encoding, it is examined to determine the current type.
1122 When decoding, it is set to reference the type that was found in
1127 The Odr_arm type is defined thus:
1131 typedef struct odr_arm
1143 The interpretation of the fields are:
1147 <varlistentry><term>tagmode</term>
1148 <listitem><para>Either <literal>ODR_IMPLICIT</literal>,
1149 <literal>ODR_EXPLICIT</literal>, or <literal>ODR_NONE</literal> (-1)
1150 to mark no tagging.</para></listitem>
1153 <varlistentry><term>which</term>
1154 <listitem><para>The value of the discriminator that corresponds to
1155 this CHOICE element. Typically, it will be a #defined constant, or
1156 an enum member.</para></listitem>
1159 <varlistentry><term>fun</term>
1160 <listitem><para>A pointer to a function that implements the type of
1161 the CHOICE member. It may be either a standard &odr; type or a type
1162 defined by yourself.</para></listitem>
1165 <varlistentry><term>name</term>
1166 <listitem><para>Name of tag.</para></listitem>
1171 A handy way to prepare the array for use by the
1172 <function>odr_choice()</function> function is to
1173 define it as a static, initialized array in the beginning of your
1174 decoding/encoding function. Assume the type definition:
1178 MyChoice ::= CHOICE {
1180 tagged [99] IMPLICIT INTEGER,
1186 Your C type might look like
1190 typedef struct MyChoice
1208 And your function could look like this:
1212 int myChoice(ODR o, MyChoice **p, int optional, const char *name)
1214 static Odr_arm arm[] =
1216 {-1, -1, -1, MyChoice_untagged, odr_integer, "untagged"},
1217 {ODR_IMPLICIT, ODR_CONTEXT, 99, MyChoice_tagged, odr_integer,
1219 {-1, -1, -1, MyChoice_other, odr_boolean, "other"},
1223 if (o->direction == ODR_DECODE)
1224 *p = odr_malloc(o, sizeof(**p);
1226 return optional && odr_ok(o);
1228 if (odr_choice(o, arm, &(*p)->u, &(*p)->which), name)
1231 return optional && odr_ok(o);
1236 In some cases (say, a non-optional choice which is a member of a
1237 sequence), you can "embed" the union and its discriminator in the
1238 structure belonging to the enclosing type, and you won't need to
1239 fiddle with memory allocation to create a separate structure to
1240 wrap the discriminator and union.
1244 The corresponding function is somewhat nicer in the Sun XDR interface.
1245 Most of the complexity of this interface comes from the possibility of
1246 declaring sequence elements (including CHOICEs) optional.
1250 The ASN.1 specifications naturally requires that each member of a
1251 CHOICE have a distinct tag, so they can be told apart on decoding.
1252 Sometimes it can be useful to define a CHOICE that has multiple types
1253 that share the same tag. You'll need some other mechanism, perhaps
1254 keyed to the context of the CHOICE type. In effect, we would like to
1255 introduce a level of context-sensitiveness to our ASN.1 specification.
1256 When encoding an internal representation, we have no problem, as long
1257 as each CHOICE member has a distinct discriminator value. For
1258 decoding, we need a way to tell the choice function to look for a
1259 specific arm of the table. The function
1263 void odr_choice_bias(ODR o, int what);
1267 provides this functionality. When called, it leaves a notice for the next
1268 call to <function>odr_choice()</function> to be called on the decoding
1269 stream <literal>o</literal> that only the <literal>arm</literal> entry with
1270 a <literal>which</literal> field equal to <literal>what</literal>
1275 The most important application (perhaps the only one, really) is in
1276 the definition of application-specific EXTERNAL encoders/decoders
1277 which will automatically decode an ANY member given the direct or
1284 <sect1 id="odr.debugging"><title>Debugging</title>
1287 The protocol modules are suffering somewhat from a lack of diagnostic
1288 tools at the moment. Specifically ways to pretty-print PDUs that
1289 aren't recognized by the system. We'll include something to this end
1290 in a not-too-distant release. In the meantime, what we do when we get
1291 packages we don't understand is to compile the ODR module with
1292 <literal>ODR_DEBUG</literal> defined. This causes the module to dump tracing
1293 information as it processes data units. With this output and the
1294 protocol specification (Z39.50), it is generally fairly easy to see
1299 <!-- Keep this comment at the end of the file
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