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

      int odr_integer(ODR o, Odr_int **p, int optional, const char *name);
     

int odr_bool(ODR o, Odr_bool **p, int optional, const char *name);
     

int odr_null(ODR o, Odr_null **p, int optional, const char *name);
     

typedef struct odr_oct
{
    unsigned char *buf;
    int len;
} Odr_oct;

int odr_octetstring(ODR o, Odr_oct **p, int optional,
                    const char *name);
     

      int odr_cstring(ODR o, char **p, int optional, const char *name);
     

int odr_visiblestring(ODR o, char **p, int optional,
                      const char *name);
     

int odr_bitstring(ODR o, Odr_bitmask **p, int optional,
                  const char *name);
     

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);
     

int odr_oid(ODR o, Odr_oid **p, int optional, const char *name);
     

The simplest way of tagging a type is to use the odr_implicit_tag() or odr_explicit_tag() macros:

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);
    

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

     MyInt ::= [210] IMPLICIT INTEGER
    

In the ODR system, this would be written like:

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

The function myInt() can then be used like any of the primitive functions provided by ODR. Note that the behavior of odr_explicit_tag() and odr_implicit_tag() 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: ODR_CONTEXT, ODR_PRIVATE, ODR_UNIVERSAL, or /ODR_APPLICATION.

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).

For implementing SEQUENCEs, the functions

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

are provided.

The odr_sequence_begin() 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 size of the type (typically a C structure). On encoding, it returns 1 if * p is a null pointer. The size parameter is ignored. On decoding, it returns 1 if the type is found in the data stream. size bytes of memory are allocated, and *p is set to point to this space. The odr_sequence_end() is called at the end of the complex function. Assume that a type is defined like this:

MySequence ::= SEQUENCE {
     intval INTEGER,
     boolval BOOLEAN OPTIONAL
}
    

The corresponding ODR encoder/decoder function and the associated data structures could be written like this:

typedef struct MySequence
{
    Odr_int *intval;
    Odr_bool *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 && odr_ok(o);
    return
        odr_integer(o, &(*p)->intval, 0, "intval") &&
        odr_bool(o, &(*p)->boolval, 1, "boolval") &&
        odr_sequence_end(o);
}
    

Note the 1 in the call to odr_bool(), to mark that the sequence member is optional. If either of the member types had been tagged, the macros odr_implicit_tag() or odr_explicit_tag() 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 MySequence 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. odr_ok is just that - a predicate that returns the state of the stream. It is used to ensure that the behavior of the new type is compatible with the interface of the primitive types.

To handle sequences (arrays) of a specific type, the function

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

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

Assume a type

MyArray ::= SEQUENCE OF INTEGER
    

The C representation might be

typedef struct MyArray
{
    int num_elements;
    Odr_int **elements;
} MyArray;
    

And the function might look like

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, &(*p)->elements,
        &(*p)->num_elements, name))
        return 1;
    *p = 0;
        return optional && odr_ok(o);
}
    

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

CHOICE types are handled by the function:

int odr_choice(ODR o, Odr_arm arm[], void *p, void *whichp,
               const char *name);
    

The arm 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. whichp 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.

The Odr_arm type is defined thus:

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

The interpretation of the fields are:

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

MyChoice ::= CHOICE {
    untagged INTEGER,
    tagged   [99] IMPLICIT INTEGER,
    other    BOOLEAN
}
    

Your C type might look like

typedef struct MyChoice
{
    enum
    {
        MyChoice_untagged,
        MyChoice_tagged,
        MyChoice_other
    } which;
    union
    {
        Odr_int *untagged;
        Odr_int *tagged;
        Odr_bool *other;
    } u;
};
    

And your function could look like this:

int myChoice(ODR o, MyChoice **p, int optional, const char *name)
{
    static Odr_arm arm[] =
    {
      {-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 && odr_ok(o);

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

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.

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.

The ASN.1 specifications naturally require 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

void odr_choice_bias(ODR o, int what);
    

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

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.