Added XML Docbook documentation for YAZ - will eventually replace

[yaz-moved-to-github.git] / doc / odr.xml

<!-- $Header: /home/cvsroot/yaz/doc/odr.xml,v 1.1 2001-01-04 13:36:24 adam Exp $ -->
<chapter><title id="odr">The ODR Module</title>

<sect1><title>Introduction</title>

<para>
&odr; is the BER-encoding/decoding subsystem of &yaz;. Care as been taken
to isolate &odr; from the rest of the package - specifically from the
transport interface. &odr; may be used in any context where basic
ASN.1/BER representations are used.
</para>

<para>
If you are only interested in writing a Z39.50 implementation based on
the PDUs that are already provided with &yaz;, you only need to concern
yourself with the section on managing ODR streams (section
<link linkend="odr-use">Using ODR</link>). Only if you need to
implement ASN.1 beyond that which has been provided, should you
worry about the second half of the documentation
(section <link linkend="odr-prog">Programming with ODR</link>).
If you use one of the higher-level interfaces, you can skip this
section entirely.
</para>

<para>
This is important, so we'll repeat it for emphasis: <emphasis>You do not
need to read section <link linkend="odr-prog">Programming with ODR</link> to
implement Z39.50 with &yaz;.</emphasis>
</para>

<para>
If you need a part of the protocol that isn't already in &yaz;, you
should contact the authors before going to work on it yourself: We
might already be working on it. Conversely, if you implement a useful
part of the protocol before us, we'd be happy to include it in a
future release.
</para>

</sect1>
<sect1><title id="odr-use">Using ODR</title>

<sect2><title>ODR Streams</title>

<para>
Conceptually, the ODR stream is the source of encoded data in the
decoding mode; when encoding, it is the receptacle for the encoded
data. Before you can use an ODR stream it must be allocated. This is
done with the function
</para>

<synopsis>
  ODR odr_createmem(int direction);
</synopsis>

<para>
The <function>odr_createmem()</function> function takes as argument one
of three manifest constants: <literal>ODR_ENCODE</literal>,
<literal>ODR_DECODE</literal>, or <literal>ODR_PRINT</literal>.
An &odr; stream can be in only one mode - it is not possible to change
its mode once it's selected. Typically, your program will allocate
at least two ODR streams - one for decoding, and one for encoding.
</para>

<para>
When you're done with the stream, you can use
</para>

<synopsis>
  void odr_destroy(ODR o);
</synopsis>

<para>
to release the resources allocated for the stream.
</para>
</sect2>

<sect2><title id="memory">Memory Management</title>

<para>
Two forms of memory management take place in the &odr; system. The first
one, which has to do with allocating little bits of memory (sometimes
quite large bits of memory, actually) when a protocol package is
decoded, and turned into a complex of interlinked structures. This
section deals with this system, and how you can use it for your own
purposes. The next section deals with the memory management which is
required when encoding data - to make sure that a large enough buffer is
available to hold the fully encoded PDU.
</para>

<para>
The &odr; module has its own memory management system, which is
used whenever memory is required. Specifically, it is used to allocate
space for data when decoding incoming PDUs. You can use the memory
system for your own purposes, by using the function
</para>

<synopsis>
void *odr_malloc(ODR o, int size);
</synopsis>

<para>
You can't use the normal <function>free(2)</function> routine to free
memory allocated by this function, and &odr; doesn't provide a parallel
function. Instead, you can call
</para>

<synopsis>
  void odr_reset(ODR o, int size);
</synopsis>

<para>
when you are done with the
memory: Everything allocated since the last call to
<function>odr_reset()</function> is released.
The <function>odr_reset()</function> call is also required to clear
up an error condition on a stream.
</para>

<para>
The function
</para>

<synopsis>
  int odr_total(ODR o);
</synopsis>

<para>
returns the number of bytes allocated on the stream since the last call to
<function>odr_reset()</function>.
</para>

<para>
The memory subsystem of &odr; is fairly efficient at allocating and
releasing little bits of memory. Rather than managing the individual,
small bits of space, the system maintains a freelist of larger chunks
of memory, which are handed out in small bits. This scheme is
generally known as a <emphasis>nibble memory</emphasis> system.
It is very useful for maintaing short-lived constructions such
as protocol PDUs.
</para>

<para>
If you want to retain a bit of memory beyond the next call to
<function>odr_reset()</function>, you can use the function
</para>

<synopsis>
  ODR_MEM odr_extract_mem(ODR o);
</synopsis>

<para>
This function will give you control of the memory recently allocated
on the ODR stream. The memory will live (past calls to
<function>odr_reset()</function>), until you call the function
</para>

<synopsis>
  void odr_release_mem(ODR_MEM p);
</synopsis>

<para>
The opaque <literal>ODR_MEM</literal> handle has no other purpose than
referencing the memory block for you until you want to release it.
</para>

<para>
You can use <function>odr_extract_mem()</function> repeatedly between
allocating data, to retain individual control of separate chunks of data.
</para>

</sect2>
<sect2><title>Encoding and Decoding Data</title>

<para>
When encoding data, the ODR stream will write the encoded octet string
in an internal buffer. To retrieve the data, use the function
</para>

<synopsis>
  char *odr_getbuf(ODR o, int *len, int *size);
</synopsis>

<para>
The integer pointed to by len is set to the length of the encoded
data, and a pointer to that data is returned. <literal>*size</literal>
is set to the size of the buffer (unless <literal>size</literal> is null,
signalling that you are not interested in the size). The next call to
a primitive function using the same &odr; stream will overwrite the
data, unless a different buffer has been supplied using the call
</para>

<synopsis>
  void odr_setbuf(ODR o, char *buf, int len, int can_grow);
</synopsis>

<para>
which sets the encoding (or decoding) buffer used by <literal>o</literal> to
<literal>buf</literal>, using the length <literal>len</literal>.
Before a call to an encoding function, you can use
<function>odr_setbuf()</function> to provide the stream with an encoding
buffer of sufficient size (length). The <literal>can_grow</literal>
parameter tells the encoding &odr; stream whether it is allowed to use
<function>realloc(2)</function> to increase the size of the buffer when
necessary. The default condition of a new encoding stream is equivalent
to the results of calling
</para>

<synopsis>
odr_setbuf(stream, 0, 0, 1);
</synopsis>

<para>
In this case, the stream will allocate and reallocate memory as
necessary. The stream reallocates memory by repeatedly doubling the
size of the buffer - the result is that the buffer will typically
reach its maximum, working size with only a small number of reallocation
operations. The memory is freed by the stream when the latter is destroyed,
unless it was assigned by the user with the <literal>can_grow</literal>
parameter set to zero (in this case, you are expected to retain
control of the memory yourself).
</para>

<para>
To assume full control of an encoded buffer, you must first call
<function>odr_getbuf()</function> to fetch the buffer and its length.
Next, you should call <function>odr_setbuf()</function> to provide a
different buffer (or a null pointer) to the stream. In the simplest
case, you will reuse the same buffer over and over again, and you
will just need to call <function>odr_getbuf()</function> after each
encoding operation to get the length and address of the buffer.
Note that the stream may reallocate the buffer during an encoding
operation, so it is necessary to retrieve the correct address after
each encoding operation.
</para>

<para>
It is important to realise that the ODR stream will not release this
memory when you call <function>odr_reset()</function>: It will
merely update its internal pointers to prepare for the encoding of a
new data value.
When the stream is released by the <function>odr_destroy()</function>
function, the memory given to it by <function>odr_setbuf</function> will
be released <emphasis>only</emphasis> if the <literal>can_grow</literal>
parameter to <function>odr_setbuf()</function> was nonzero. The
<literal>can_grow</literal> parameter, in other words, is a way of
signalling who is to own the buffer, you or the ODR stream. If you never call
<function>odr_setbuf()</function> on your encoding stream, which is
typically the case, the buffer allocated by the stream will belong to
the stream by default.
</para>

<para>
When you wish to decode data, you should first call
<function>odr_setbuf()</function>, to tell the decoding stream
where to find the encoded data, and how long the buffer is
(the <literal>can_grow</literal> parameter is ignored by a decoding
stream). After this, you can call the function corresponding to the
data you wish to decode (eg, <function>odr_integer()</function> odr
<function>z_APDU()</function>).
</para>

<para>
Examples of encoding/decoding functions:
</para>

<synopsis>
  int odr_integer(ODR o, int **p, int optional, const char *name);

  int z_APDU(ODR o, Z_APDU **p, int optional, const char *name);
</synopsis>

<para>
If the data is absent (or doesn't match the tag corresponding to the type),
the return value will be either 0 or 1 depending on the
<literal>optional</literal> flag. If <literal>optional</literal>
is 0 and the data is absent, an error flag will be raised in the
stream, and you'll need to call <function>odr_reset()</function> before
you can use the stream again. If <literal>optional</literal> is
nonzero, the pointer <emphasis>pointed</emphasis> to/ by <literal>p</literal>
will be set to the null value, and the function will return 1.
The <literal>name</literal> argument is used to pretty-print the
tag in question. It may be set to <literal>NULL</literal> if
pretty-printing is not desired.
</para>

<para>
If the data value is found where it's expected, the pointer
<emphasis>pointed to</emphasis> by the <literal>p</literal> argument
will be set to point to the decoded type.
The space for the type will be allocated and owned by the &odr; stream, and
it will live until you call <function>odr_reset()</function> on the
stream. You cannot use <function>free(2)</function> to release the memory.
You can decode several data elements (by repeated calls to
<function>odr_setbuf()</function> and your decoding function), and
new memory will be allocated each time. When you do call 
<function>odr_reset()</function>, everything decoded since the
last call to <function>odr_reset()</function> will be released.
</para>

<para>
The use of the double indirection can be a little confusing at first
(its purpose will become clear later on, hopefully),
so an example is in order. We'll encode an integer value, and
immediately decode it again using a different stream. A useless, but
informative operation.
</para>

<programlisting>
void do_nothing_useful(int value)
{
    ODR encode, decode;
    int *valp, *resvalp;
    char *bufferp;
    int len;

    /* allocate streams */
    if (!(encode = odr_createmem(ODR_ENCODE)))
        return;
    if (!(decode = odr_createmem(ODR_DECODE)))
        return;

    valp = &amp;value;
    if (odr_integer(encode, &amp;valp, 0, 0) == 0)
    {
        printf("encoding went bad\n");
        return;
    }
    bufferp = odr_getbuf(encode, &amp;len);
    printf("length of encoded data is &percnt;d\n", len);

    /* now let's decode the thing again */
    odr_setbuf(decode, bufferp, len);
    if (odr_integer(decode, &amp;resvalp, 0, 0) == 0)
    {
        printf("decoding went bad\n");
        return;
    }
    printf("the value is &percnt;d\n", *resvalp);

    /* clean up */
    odr_destroy(encode);
    odr_destroy(decode);
}
</programlisting>

<para>
This looks like a lot of work, offhand. In practice, the &odr; streams
will typically be allocated once, in the beginning of your program (or at the
beginning of a new network session), and the encoding and decoding
will only take place in a few, isolated places in your program, so the
overhead is quite manageable.
</para>

</sect2>

<sect2><title>Diagnostics</title>

<para>
The encoding/decoding functions all return 0 when an error occurs.
Until you call <function>odr_reset()</function>, you cannot use the
stream again, and any function called will immediately return 0.
</para>

<para>
To provide information to the programmer or administrator, the function
</para>

<synopsis>
  void odr_perror(ODR o, char *message);
</synopsis>

<para>
is provided, which prints the <literal>message</literal> argument to
<literal>stderr</literal> along with an error message from the stream.
</para>

<para>
You can also use the function
</para>

<synopsis>
  int odr_geterror(ODR o);
</synopsis>

<para>
to get the current error number from the screen. The number will be
one of these constants:
</para>

<table frame="top"><title>ODR Error codes</title>
<tgroup cols="2">
<thead>
<row>
<entry>code</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>OMEMORY</entry><entry>Memory allocation failed.</entry>
</row>

<row>
<entry>OSYSERR</entry><entry>A system- or library call has failed.
The standard diagnostic variable <literal>errno</literal> should be
examined to determine the actual error.</entry>
</row>

<row>
<entry>OSPACE</entry><entry>No more space for encoding.
This will only occur when the user has explicitly provided a
buffer for an encoding stream without allowing the system to
allocate more space.</entry>
</row>

<row>
<entry>OREQUIRED</entry><entry>This is a common protocol error; A
required data element was missing during encoding or decoding.</entry>
</row>

<row>
<entry>OUNEXPECTED</entry><entry>An unexpected data element was
found during decoding.</entry>
</row>

<row><entry>OOTHER</entry><entry>Other error. This is typically an
indication of misuse of the &odr; system by the programmer, and also
that the diagnostic system isn't as good as it should be, yet.</entry>
</row>
</tbody>
</tgroup>
</table>

<para>
The character string array
</para>

<synopsis>
  char *odr_errlist&lsqb;&rsqb;
</synopsis>

<para>
can be indexed by the error code to obtain a human-readable
representation of the problem.
</para>

</sect2>
<sect2><title>Summary and Synopsis</title>

<synopsis>
#include &lt;odr.h>

ODR odr_createmem(int direction);

void odr_destroy(ODR o);

void odr_reset(ODR o);

char *odr_getbuf(ODR o, int *len);

void odr_setbuf(ODR o, char *buf, int len);

void *odr_malloc(ODR o, int size);

ODR_MEM odr_extract_mem(ODR o);

void odr_release_mem(ODR_MEM r);

int odr_geterror(ODR o);

void odr_perror(char *message);

extern char *odr_errlist[];
</synopsis>

</sect2>
</sect1>

<sect1><title id="odr-prog">Programming with ODR</title>

<para>
The API of &odr; is designed to reflect the structure of ASN.1, rather
than BER itself. Future releases may be able to represent data in
other external forms.
</para>

<para>
The interface is based loosely on that of the Sun Microsystems XDR routines.
Specifically, each function which corresponds to an ASN.1 primitive
type has a dual function. Depending on the settings of the ODR
stream which is supplied as a parameter, the function may be used
either to encode or decode data. The functions that can be built
using these primitive functions, to represent more complex datatypes, share
this quality. The result is that you only have to enter the definition
for a type once - and you have the functionality of encoding, decoding
(and pretty-printing) all in one unit. The resulting C source code is
quite compact, and is a pretty straightforward representation of the
source ASN.1 specification. Although no ASN.1 compiler is supplied
with &odr; at this time, it shouldn't be too difficult to write one, or
perhaps even to adapt an existing compiler to output &odr; routines
(not surprisingly, writing encoders/decoders using &odr; turns out
to be boring work).
</para>

<para>
In many cases, the model of the XDR functions works quite well in this
role.
In others, it is less elegant. Most of the hassle comes from the optional
SEQUENCE memebers which don't exist in XDR.
</para>

<sect2><title>The Primitive ASN.1 Types</title>

<para>
ASN.1 defines a number of primitive types (many of which correspond
roughly to primitive types in structured programming languages, such as C).
</para>

<sect3><title>INTEGER</title>

<para>
The &odr; function for encoding or decoding (or printing) the ASN.1
INTEGER type looks like this:
</para>

<synopsis>
  int odr_integer(ODR o, int **p, int optional, const char *name);
</synopsis>

<para>
(we don't allow values that can't be contained in a C integer.)
</para>

<para>
This form is typical of the primitive &odr; functions. They are named
after the type of data that they encode or decode. They take an &odr;
stream, an indirect reference to the type in question, and an
<literal>optional</literal> flag (corresponding to the OPTIONAL keyword
of ASN.1) as parameters. They all return an integer value of either one
or zero.
When you use the primitive functions to construct encoders for complex
types of your own, you should follow this model as well. This
ensures that your new types can be reused as elements in yet more
complex types.
</para>

<para>
The <literal>o</literal> parameter should obviously refer to a properly
initialized &odr; stream of the right type (encoding/decoding/printing)
for the operation that you wish to perform.
</para>

<para>
When encoding or printing, the function first looks at
<literal>* p</literal>. If <literal>* p</literal> (the pointer pointed
to by <literal>p</literal>) is a null pointer, this is taken to mean that
the data element is absent. If the <literal>optional</literal> parameter
is nonzero, the function will return one (signifying success) without
any further processing. If the <literal>optional</literal> is zero, an
internal error flag is set in the &odr; stream, and the function will
return 0. No further operations can be carried out on the stream without
a call to the function <function>odr_reset()</function>.
</para>

<para>
If <literal>*p</literal> is not a null pointer, it is expected to
point to an instance of the data type. The data will be subjected to
the encoding rules, and the result will be placed in the buffer held
by the &odr; stream.
</para>

<para>
The other ASN.1 primitives have similar functions that operate in
similar manners:
</para>
</sect3>
<sect3><title>BOOLEAN</title>

<synopsis>
  int odr_bool(ODR o, bool_t **p, int optional, const char *name);
</synopsis>

</sect3>
<sect3><title>REAL</title>

<para>
Not defined.
</para>

</sect3>
<sect3><title>NULL</title>

<synopsis>
  int odr_null(ODR o, bool_t **p, int optional, const char *name);
</synopsis>

<para>
In this case, the value of **p is not important. If <literal>*p</literal>
is different from the null pointer, the null value is present, otherwise
it's absent.
</para>

</sect3>
<sect3><title>OCTET STRING</title>

<synopsis>
  typedef struct odr_oct
  {
      unsigned char *buf;
      int len;
      int size;
  } Odr_oct;

  int odr_octetstring(ODR o, Odr_oct **p, int optional, const char *name);
</synopsis>

<para>
The <literal>buf</literal> field should point to the character array
that holds the octetstring. The <literal>len</literal> field holds the
actual length, while the <literal>size</literal> field gives the size
of the allocated array (not of interest to you, in most cases).
The character array need not be null terminated.
</para>

<para>
To make things a little easier, an alternative is given for string
types that are not expected to contain embedded NULL characters (eg.
VisibleString):
</para>

<synopsis>
  int odr_cstring(ODR o, char **p, int optional, const char *name);
</synopsis>

<para>
Which encoded or decodes between OCTETSTRING representations and
null-terminates C strings.
</para>

<para>
Functions are provided for the derived string types, eg:
</para>

<synopsis>
  int odr_visiblestring(ODR o, char **p, int optional, const char *name);
</synopsis>

</sect3>
<sect3><title>BIT STRING</title>

<synopsis>
  int odr_bitstring(ODR o, Odr_bitmask **p, int optional, const char *name);
</synopsis>

<para>
The opaque type <literal>Odr_bitmask</literal> is only suitable for
holding relatively brief bit strings, eg. for options fields, etc.
The constant <literal>ODR_BITMASK_SIZE</literal> multiplied by 8
gives the maximum possible number of bits.
</para>

<para>
A set of macros are provided for manipulating the
<literal>Odr_bitmask</literal> type:
</para>

<synopsis>
  void ODR_MASK_ZERO(Odr_bitmask *b);

  void ODR_MASK_SET(Odr_bitmask *b, int bitno);

  void ODR_MASK_CLEAR(Odr_bitmask *b, int bitno);

  int ODR_MASK_GET(Odr_bitmask *b, int bitno);
</synopsis>

<para>
The functions are modelled after the manipulation functions that
accompany the <literal>fd_set</literal> type used by the
<function>select(2)</function> call.
<literal>ODR_MASK_ZERO</literal> should always be called first on a
new bitmask, to initialize the bits to zero.
</para>
</sect3>

<sect3><title>OBJECT IDENTIFIER</title>

<synopsis>
int odr_oid(ODR o, Odr_oid **p, int optional, const char *name);
</synopsis>

<para>
The C OID represenation is simply an array of integers, terminated by
the value -1 (the <literal>Odr_oid</literal> type is synonymous with
the <literal>int</literal> type).
We suggest that you use the OID database module (see section
<link linkend="oid">Object Identifiers</link>) to handle object identifiers
in your application.
</para>

</sect3>
</sect2>
<sect2><title id="tag-prim">Tagging Primitive Types</title>

<para>
The simplest way of tagging a type is to use the
<function>odr_implicit_tag()</function> or 
<function>odr_explicit_tag()</function> macros:
</para>

<synopsis>
  int odr_implicit_tag(ODR o, Odr_fun fun, int class, int tag, int
                       optional, const char *name);

  int odr_explicit_tag(ODR o, Odr_fun fun, int class, int tag,
                       int optional, const char *name);
</synopsis>

<para>
To create a type derived from the integer type by implicit tagging, you
might write:
</para>

<screen>
  MyInt ::= &lsqb;210&rsqb; IMPLICIT INTEGER
</screen>

<para>
In the &odr; system, this would be written like:
</para>

<screen>
int myInt(ODR o, int **p, int optional, const char *name)
{
    return odr_implicit_tag(o, odr_integer, p,
                   ODR_CONTEXT, 210, optional, name);
}
</screen>

<para>
The function <function>myInt()</function> can then be used like any of
the primitive functions provided by &odr;. Note that the behavior of
<function>odr_explicit()</function>
and <function>odr_implicit()</function> macros
act exactly the same as the functions they are applied to - they
respond to error conditions, etc, in the same manner - they
simply have three extra parameters. The class parameter may
take one of the values: <literal>ODR_CONTEXT</literal>,
<literal>ODR_PRIVATE</literal>, <literal>ODR_UNIVERSAL</literal>, or
<literal>/ODR_APPLICATION</literal>.
</para>

</sect2>
<sect2><title>Constructed Types</title>

<para>
Constructed types are created by combining primitive types. The
&odr; system only implements the SEQUENCE and SEQUENCE OF constructions
(although adding the rest of the container types should be simple
enough, if the need arises).
</para>

<para>
For implementing SEQUENCEs, the functions
</para>

<synopsis>
  int odr_sequence_begin(ODR o, void *p, int size, const char *name);
  int odr_sequence_end(ODR o);
</synopsis>

<para>
are provided.
</para>

<para>
The <function>odr_sequence_begin()</function> function should be
called in the beginning of a function that implements a SEQUENCE type.
Its parameters are the &odr; stream, a pointer (to a pointer to the type
you're implementing), and the <literal>size</literal> of the type
(typically a C structure). On encoding, it returns 1 if
<literal>* p</literal> is a null pointer. The <literal>size</literal>
parameter is ignored. On decoding, it returns 1 if the type is found in
the data stream. <literal>size</literal> bytes of memory are allocated,
and <literal>*p</literal> is set to point to this space.
<function>odr_sequence_end()</function> is called at the end of the
complex function. Assume that a type is defined like this:
</para>

<screen>
  MySequence ::= SEQUENCE {
      intval INTEGER,
      boolval BOOLEAN OPTIONAL }
</screen>

<para>
The corresponding &odr; encoder/decoder function and the associated data
structures could be written like this:
</para>

<screen>
  typedef struct MySequence
  {
    int *intval;
    bool_t *boolval;
  } MySequence;
    
  int mySequence(ODR o, MySequence **p, int optional, const char *name)
  {
    if (odr_sequence_begin(o, p, sizeof(**p), name) == 0)
        return optional &amp;&amp; odr_ok(o);
    return
        odr_integer(o, &amp;(*p)->intval, 0, "intval") &amp;&amp;
        odr_bool(o, &amp;(*p)->boolval, 1, "boolval") &amp;&amp;
        odr_sequence_end(o);
  }
</screen>

<para>
Note the 1 in the call to <function>odr_bool()</function>, to mark
that the sequence member is optional.
If either of the member types had been tagged, the macros
<function>odr_implicit()</function> or <function>odr_explicit()</function>
could have been used.
The new function can be used exactly like the standard functions provided
with &odr;. It will encode, decode or pretty-print a data value of the
<literal>MySequence</literal> type. We like to name types with an
initial capital, as done in ASN.1 definitions, and to name the
corresponding function with the first character of the name in lower case.
You could, of course, name your structures, types, and functions any way
you please - as long as you're consistent, and your code is easily readable.
<literal>odr_ok</literal> is just that - a predicate that returns the
state of the stream. It is used to ensure that the behaviour of the new
type is compatible with the interface of the primitive types.
</para>

</sect2>
<sect2><title>Tagging Constructed Types</title>

<note>
<para>
See section <link linkend="tag-prim">Tagging Primitive types</link>
for information on how to tag the primitive types, as well as types
that are already defined.
</para>
</note>

<sect3><title>Implicit Tagging</title>

<para>
Assume the type above had been defined as
</para>

<screen>
  MySequence ::= &lsqb;10&rsqb; IMPLICIT SEQUENCE {
     intval INTEGER,
     boolval BOOLEAN OPTIONAL }
</screen>

<para>
You would implement this in &odr; by calling the function
</para>

<synopsis>
  int odr_implicit_settag(ODR o, int class, int tag);
</synopsis>

<para>
which overrides the tag of the type immediately following it. The
macro <function>odr_implicit()</function> works by calling
<function>odr_implicit_settag()</function> immediately
before calling the function pointer argument.
Your type function could look like this:
</para>

<screen>
  int mySequence(ODR o, MySequence **p, int optional, const char *name)
  {
    if (odr_implicit_settag(o, ODR_CONTEXT, 10) == 0 ||
        odr_sequence_begin(o, p, sizeof(**p), name) == 0)
        return optional &amp;&amp; odr_ok(o);
    return
        odr_integer(o, &amp;(*p)->intval, 0, "intval") &amp;&amp;
        odr_bool(o, &amp;(*p)->boolval, 1, "boolval") &amp;&amp;
       odr_sequence_end(o);
}
</screen>

<para>
The definition of the structure <literal>MySequence</literal> would be
the same.
</para>
</sect3>

<sect3><title>Explicit Tagging</title>

<para>
Explicit tagging of constructed types is a little more complicated,
since you are in effect adding a level of construction to the data.
</para>

<para>
Assume the definition:
</para>

<screen>
  MySequence ::= &lsqb;10&rsqb; IMPLICIT SEQUENCE {
      intval INTEGER,
      boolval BOOLEAN OPTIONAL }
</screen>

<para>
Since the new type has an extra level of construction, two new functions
are needed to encapsulate the base type:
</para>

<synopsis>
  int odr_constructed_begin(ODR o, void *p, int class, int tag,
                            const char *name);

  int odr_constructed_end(ODR o);
</synopsis>

<para>
Assume that the IMPLICIT in the type definition above were replaced
with EXPLICIT (or that the IMPLICIT keyword were simply deleted, which
would be equivalent). The structure definition would look the same,
but the function would look like this:
</para>

<screen>
  int mySequence(ODR o, MySequence **p, int optional, const char *name)
  {
    if (odr_constructed_begin(o, p, ODR_CONTEXT, 10, name) == 0)
        return optional &amp;&amp; odr_ok(o);
    if (o->direction == ODR_DECODE)
        *p = odr_malloc(o, sizeof(**p));
    if (odr_sequence_begin(o, p, sizeof(**p), 0) == 0)
    {
      *p = 0; /* this is almost certainly a protocol error */
      return 0;
    }
    return
        odr_integer(o, &amp;(*p)->intval, 0, "intval") &amp;&amp;
        odr_bool(o, &amp;(*p)->boolval, 1, "boolval") &amp;&amp;
        odr_sequence_end(o) &amp;&amp;
        odr_constructed_end(o);
  }
</screen>

<para>
Notice that the interface here gets kind of nasty. The reason is
simple: Explicitly tagged, constructed types are fairly rare in
the protocols that we care about, so the
aesthetic annoyance (not to mention the dangers of a cluttered
interface) is less than the time that would be required to develop a
better interface. Nevertheless, it is far from satisfying, and it's a
point that will be worked on in the future. One option for you would
be to simply apply the <function>odr_explicit()</function> macro to
the first function, and not
have to worry about <function>odr_constructed_*</function> yourself.
Incidentally, as you might have guessed, the
<function>odr_sequence_</function> functions are themselves
implemented using the <function>/odr_constructed_</function> functions.
</para>

</sect3>
</sect2>
<sect2><title>SEQUENCE OF</title>

<para>
To handle sequences (arrays) of a apecific type, the function
</para>

<synopsis>
  int odr_sequence_of(ODR o, int (*fun)(ODR o, void *p, int optional),
                      void *p, int *num, const char *name);
</synopsis>

<para>
The <literal>fun</literal> parameter is a pointer to the decoder/encoder
function of the type. <literal>p</literal> is a pointer to an array of
pointers to your type. <literal>num</literal> is the number of elements
in the array.
</para>

<para>
Assume a type
</para>

<screen>
  MyArray ::= SEQUENCE OF INTEGER
</screen>

<para>
The C representation might be
</para>

<screen>
  typedef struct MyArray
  {
    int num_elements;
    int **elements;
  } MyArray;
</screen>

<para>
And the function might look like
</para>

<screen>
  int myArray(ODR o, MyArray **p, int optional, const char *name)
  {
    if (o->direction == ODR_DECODE)
        *p = odr_malloc(o, sizeof(**p));
    if (odr_sequence_of(o, odr_integer, &amp;(*p)->elements,
        &amp;(*p)->num_elements, name))
        return 1;
    *p = 0;
    return optional &amp;&amp; odr_ok(o);
  }
</screen>

</sect2>
<sect2><title>CHOICE Types</title>

<para>
The choice type is used fairly often in some ASN.1 definitions, so
some work has gone into streamlining its interface.
</para>

<para>
CHOICE types are handled by the function:
</para>

<synopsis>
  int odr_choice(ODR o, Odr_arm arm&lsqb;&rsqb;, void *p, void *whichp,
                 const char *name);
</synopsis>

<para>
The <literal>arm</literal> array is used to describe each of the possible
types that the CHOICE type may assume. Internally in your application,
the CHOICE type is represented as a discriminated union. That is, a C union
accompanied by an integer (or enum) identifying the active 'arm' of
the union. <literal>whichp</literal> is a pointer to the union
discriminator. When encoding, it is examined to determine the current
type. When decoding, it is set to reference the type that was found in
the input stream.
</para>

<para>
The Odr_arm type is defined thus:
</para>

<screen>
  typedef struct odr_arm
  {
    int tagmode;
    int class;
    int tag;
    int which;
    Odr_fun fun;
    char *name;
  } Odr_arm;
</screen>

<para>
The interpretation of the fields are:
</para>

<variablelist>
<varlistentry><term>tagmode</term>
 <listitem><para>Either <literal>ODR_IMPLICIT</literal>,
<literal>ODR_EXPLICIT</literal>, or <literal>ODR_NONE</literal> (-1) to mark
no tagging.</para></listitem>
</varlistentry>

<varlistentry><term>which</term>
 <listitem><para>The value of the discriminator that corresponds to
this CHOICE element. Typically, it will be a &num;defined constant, or
an enum member.</para></listitem>
</varlistentry>

<varlistentry><term>fun</term>
 <listitem><para>A pointer to a function that implements the type of
the CHOICE member. It may be either a standard &odr; type or a type
defined by yourself.</para></listitem>
</varlistentry>

<varlistentry><term>name</term>
 <listitem><para>Name of tag.</para></listitem>
</varlistentry>
</variablelist>

<para>
A handy way to prepare the array for use by the
<function>odr_choice()</function> function is to
define it as a static, initialized array in the beginning of your
decoding/encoding function. Assume the type definition:
</para>

<screen>
  MyChoice ::= CHOICE {
    untagged INTEGER,
    tagged   &lsqb;99&rsqb; IMPLICIT INTEGER,
    other    BOOLEAN
  }
</screen>

<para>
Your C type might look like
</para>

<screen>
  typedef struct MyChoice
  {
      enum
      {
          MyChoice_untagged,
          MyChoice_tagged,
          MyChoice_other
      } which;
      union
      {
          int *untagged;
          int *tagged;
          bool_t *other;
      } u;
  };
</screen>

<para>
And your function could look like this:
</para>

<screen>
int myChoice(ODR o, MyChoice **p, int optional, const char *name)
{
    static Odr_arm arm&lsqb;&rsqb; =
    {
        {-1, -1, -1, MyChoice_untagged, odr_integer, "untagged"},
        {ODR_IMPLICIT, ODR_CONTEXT, 99, MyChoice_tagged, odr_integer,
                                                            "tagged"},
        {-1, -1, -1, MyChoice_other, odr_boolean, "other"},
        {-1, -1, -1, -1, 0}
    };

    if (o->direction == ODR_DECODE)
        *p = odr_malloc(o, sizeof(**p);
    else if (!*p)
        return optional &amp;&amp; odr_ok(o);

    if (odr_choice(o, arm, &amp;(*p)->u, &amp;(*p)->which), name)
        return 1;
    *p = 0;
    return optional &amp;&amp; odr_ok(o);
}
</screen>

<para>
In some cases (say, a non-optional choice which is a member of a sequence),
you can "embed" the union and its discriminator in the structure
belonging to the enclosing type, and you won't need to fiddle with
memory allocation to create a separate structure to wrap the
discriminator and union.
</para>

<para>
The corresponding function is somewhat nicer in the Sun XDR interface.
Most of the complexity of this interface comes from the possibility of
declaring sequence elements (including CHOICEs) optional.
</para>

<para>
The ASN.1 specifictions naturally requires that each member of a
CHOICE have a distinct tag, so they can be told apart on decoding.
Sometimes it can be useful to define a CHOICE that has multiple types
that share the same tag. You'll need some other mechanism, perhaps
keyed to the context of the CHOICE type. In effect, we would like to
introduce a level of context-sensitiveness to our ASN.1 specification.
When encoding an internal representation, we have no problem, as long
as each CHOICE member has a distinct discriminator value. For
decoding, we need a way to tell the choice function to look for a
specific arm of the table. The function
</para>

<synopsis>
  void odr_choice_bias(ODR o, int what);
</synopsis>

<para>
provides this functionality. When called, it leaves a notice for the next
call to <function>odr_choice()</function> to be called on the decoding
stream <literal>o</literal> that only the <literal>arm</literal> entry with
a <literal>which</literal> field equal to <literal>what</literal>
should be tried.
</para>

<para>
The most important application (perhaps the only one, really) is in
the definition of application-specific EXTERNAL encoders/decoders
which will automatically decode an ANY member given the direct or
indirect reference.
</para>

</sect2>
</sect1>

<sect1><title>Debugging</title>

<para>
The protocol modules are suffering somewhat from a lack of diagnostic
tools at the moment. Specifically ways to pretty-print PDUs that
aren't recognized by the system. We'll include something to this end
in a not-too-distant release. In the meantime, what we do when we get
packages we don't understand is to compile the ODR module with
<literal>ODR_DEBUG</literal> defined. This causes the module to dump tracing
information as it processes data units. With this output and the
protocol specification (Z39.50), it is generally fairly easy to see
what goes wrong.
</para>
</sect1>
</chapter>