doc/asn1c-usage.lyx

#LyX 1.3 created this file. For more info see http://www.lyx.org/
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\layout Title

ASN.1 Compiler: Usage
\layout Author

Lev Walkin <
\begin_inset LatexCommand \url{vlm@lionet.info}

\end_inset 

>
\layout Standard


\begin_inset LatexCommand \tableofcontents{}

\end_inset 


\layout Chapter

Basics of ASN.1
\layout Standard


\emph on 
This chapter defines some basic ASN.1 concepts and describes several most
 widely used types.
 It is by no means an authoritative or complete reference.
 For more complete ASN.1 description, please refer to Olivier Dubuisson's
 book 
\begin_inset LatexCommand \cite{Dub00}

\end_inset 

 or the ASN.1 standard itself 
\begin_inset LatexCommand \cite{ITU-T/ASN.1}

\end_inset 

.
\layout Standard

The Abstract Syntax Notation One is used to formally describe the semantics
 of data transmitted across the network.
 Two communicating parties may have different formats of their native data
 types (i.e.
 number of bits in the integer type), thus it is important to have a way
 to describe the data in a manner which is independent from the particular
 machine's representation.
 The ASN.1 specifications is used to achieve one or more of the following:
\layout Itemize

The specification expressed in the ASN.1 notation is a formal and precise
 way to communicate the data semantics to human readers;
\layout Itemize

The ASN.1 specifications may be used as input for automatic compilers which
 produce the code for some target language (C, C++, Java, etc) to encode
 and decode the data according to some encoding rules (which are also defined
 by the ASN.1 standard).
\layout Standard

Consider the following example:
\layout LyX-Code

Rectangle ::= SEQUENCE {
\layout LyX-Code

    height  INTEGER,
\layout LyX-Code

    width   INTEGER
\layout LyX-Code

}
\layout Standard

This ASN.1 specification describes a constructed type, 
\emph on 
Rectangle
\emph default 
, containing two integer fields.
 This specification may tell the reader that there is this kind of data
 structure and that some entity may be prepared to send or receive it.
 The question on 
\emph on 
how
\emph default 
 that entity is going to send or receive the 
\emph on 
encoded data
\emph default 
 is outside the scope of ASN.1.
 For example, this data structure may be encoded according to some encoding
 rules and sent to the destination using the TCP protocol.
 The ASN.1 specifies several ways of encoding (or 
\begin_inset Quotes sld
\end_inset 

serializing
\begin_inset Quotes srd
\end_inset 

, or 
\begin_inset Quotes sld
\end_inset 

marshaling
\begin_inset Quotes srd
\end_inset 

) the data: BER, CER, DER and XER, some of them which will be described
 later.
\layout Standard

The complete specification must be wrapped in a module, which looks like
 this:
\layout LyX-Code

UsageExampleModule1
\layout LyX-Code

    { iso org(3) dod(6) internet(1) private(4)
\layout LyX-Code

      enterprise(1) spelio(9363) software(1)
\layout LyX-Code

      asn1c(5) docs(2) usage(1) 1 } 
\layout LyX-Code

    AUTOMATIC TAGS DEFINITIONS ::=
\layout LyX-Code

BEGIN
\layout LyX-Code

 
\layout LyX-Code

-- This is a comment which describes nothing.
\layout LyX-Code

Rectangle ::= SEQUENCE {
\layout LyX-Code

    height  INTEGER,        -- Height of the rectangle
\layout LyX-Code

    width   INTEGER,        -- Width of the rectangle
\layout LyX-Code

}
\layout LyX-Code

 
\layout LyX-Code

END
\layout Standard

The module header consists of module name (UsageExampleModule1), the module
 object identifier ({...}), some flags (AUTOMATIC TAGS) and 
\begin_inset Quotes sld
\end_inset 

DEFINITIONS ::= BEGIN
\begin_inset Quotes srd
\end_inset 

.
 The module ends with an 
\begin_inset Quotes sld
\end_inset 

END
\begin_inset Quotes srd
\end_inset 

 statement.
\layout Section

Some of the ASN.1 Basic Types
\layout Subsection

The BOOLEAN type
\layout Standard

The BOOLEAN type models the simple binary TRUE/FALSE, YES/NO, ON/OFF or
 a similar kind of two-way choice.
\layout Subsection

The INTEGER type
\layout Standard

The INTEGER type is a signed natural number type without any restrictions
 on its size.
 If the automatic checking on INTEGER value bounds are necessary, the subtype
 constraints must be used.
\layout LyX-Code

SimpleInteger ::= INTEGER
\layout LyX-Code

-- An integer with a very limited range
\layout LyX-Code

SmallInt ::= INTEGER (0..127)
\layout LyX-Code

-- Integer, negative
\layout LyX-Code

NegativeInt ::= INTEGER (MIN..0)
\layout Subsection

The ENUMERATED type
\layout Standard

The ENUMERATED type is semantically equivalent to the INTEGER type with
 some integer values explicitly named.
\layout LyX-Code

FruitId ::= ENUMERATED { apple(1), orange(2) }
\layout LyX-Code

-- The numbers in braces are optional,
\layout LyX-Code

-- the enumeration may be performed
\layout LyX-Code

-- automatically by the compiler
\layout LyX-Code

ComputerOSType ::= ENUMERATED {
\layout LyX-Code

    FreeBSD,          -- will be 0
\layout LyX-Code

    Windows,          -- will be 1
\layout LyX-Code

    Solaris(5),       -- will remain 5
\layout LyX-Code

    Linux,            -- will be 6
\layout LyX-Code

    MacOS             -- will be 7
\layout LyX-Code

}
\layout Subsection

The OCTET STRING type
\layout Standard

This type models the sequence of 8-bit bytes.
 This may be used to transmit some opaque data or data serialized by other
 types of encoders (i.e.
 video file, photo picture, etc).
\layout Subsection

The OBJECT IDENTIFIER type
\layout Standard

The OBJECT IDENTIFIER is used to represent the unique identifier of any
 object, starting from the very root of the registration tree.
 If your organization needs to uniquely identify something (a router, a
 room, a person, a standard, or whatever), you are encouraged to get your
 own identification subtree at 
\begin_inset LatexCommand \htmlurl{http://www.iana.org/protocols/forms.htm}

\end_inset 

.
\layout Standard

For example, the very first ASN.1 module in this document has the following
 OBJECT IDENTIFIER: 1 3 6 1 4 1 9363 1 5 2 1 1.
\layout LyX-Code

ExampleOID ::= OBJECT IDENTIFIER
\layout LyX-Code

usageExampleModule1-oid ExampleOID
\layout LyX-Code

  ::= { 1 3 6 1 4 1 9363 1 5 2 1 1 }
\layout LyX-Code

-- An identifier of the Internet.
\layout LyX-Code

internet-id OBJECT IDENTIFIER
\layout LyX-Code

  ::= { iso(1) identified-organization(3)
\layout LyX-Code

        dod(6) internet(1) }
\layout Standard

As you see, names are optional.
\layout Subsection

The RELATIVE-OID type
\layout Standard

The RELATIVE-OID type has the semantics of a subtree of an OBJECT IDENTIFIER.
 There may be no need to repeat the whole sequence of numbers from the root
 of the registration tree where the only thing of interest is some of the
 tree's subsequence.
\layout LyX-Code

this-document RELATIVE-OID ::= { docs(2) usage(1) }
\layout LyX-Code

this-example RELATIVE-OID ::= {
\layout LyX-Code

    this-document assorted-examples(0) this-example(1) }
\layout Section

Some of the ASN.1 String Types
\layout Subsection

The IA5String type
\layout Standard

This is essentially the ASCII, with 128 character codes available (7 lower
 bits of 8-bit byte).
\layout Subsection

The UTF8String type
\layout Standard

This is the character string which encodes the full Unicode range (4 bytes)
 using multibyte character sequences.
\layout Subsection

The NumericString type
\layout Standard

This type represents the character string with the alphabet consisting of
 numbers (
\begin_inset Quotes sld
\end_inset 

0
\begin_inset Quotes srd
\end_inset 

 to 
\begin_inset Quotes sld
\end_inset 

9
\begin_inset Quotes srd
\end_inset 

) and a space.
\layout Subsection

The PrintableString type
\layout Standard

The character string with the following alphabet: space, 
\begin_inset Quotes sld
\end_inset 


\series bold 
'
\series default 

\begin_inset Quotes srd
\end_inset 

 (single quote), 
\begin_inset Quotes sld
\end_inset 


\series bold 
(
\series default 

\begin_inset Quotes sld
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
)
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
+
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 

,
\begin_inset Quotes srd
\end_inset 

 (comma), 
\begin_inset Quotes sld
\end_inset 


\series bold 
-
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
.
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
/
\series default 

\begin_inset Quotes srd
\end_inset 

, digits (
\begin_inset Quotes sld
\end_inset 

0
\begin_inset Quotes srd
\end_inset 

 to 
\begin_inset Quotes sld
\end_inset 

9
\begin_inset Quotes srd
\end_inset 

), 
\begin_inset Quotes sld
\end_inset 


\series bold 
:
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
=
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
?
\series default 

\begin_inset Quotes srd
\end_inset 

, upper-case and lower-case letters (
\begin_inset Quotes sld
\end_inset 

A
\begin_inset Quotes srd
\end_inset 

 to 
\begin_inset Quotes sld
\end_inset 

Z
\begin_inset Quotes srd
\end_inset 

 and 
\begin_inset Quotes sld
\end_inset 

a
\begin_inset Quotes srd
\end_inset 

 to 
\begin_inset Quotes sld
\end_inset 

z
\begin_inset Quotes srd
\end_inset 

)
\layout Subsection

The VisibleString type
\layout Standard

The character string with the alphabet which is more or less a subset of
 ASCII between space and 
\begin_inset Quotes sld
\end_inset 

~
\begin_inset Quotes srd
\end_inset 

 (tilde).
 Alternatively, the alphabet may be represented as the PrintableString alphabet
 described earlier, plus the following characters: 
\begin_inset Quotes sld
\end_inset 


\series bold 
!
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 

\begin_inset Quotes srd
\end_inset 


\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
#
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
$
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
%
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
&
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
*
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
;
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
<
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
>
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
[
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 

\backslash 

\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
]
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
^
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
_
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
`
\series default 

\begin_inset Quotes srd
\end_inset 

 (single left quote), 
\begin_inset Quotes sld
\end_inset 


\series bold 
{
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
|
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 


\series bold 
}
\series default 

\begin_inset Quotes srd
\end_inset 

, 
\begin_inset Quotes sld
\end_inset 

~
\begin_inset Quotes srd
\end_inset 

.
\layout Section

ASN.1 Constructed Types
\layout Subsection

The SEQUENCE type
\layout Standard

This is an ordered collection of other simple or constructed types.
 The SEQUENCE constructed type resembles the C 
\begin_inset Quotes sld
\end_inset 

struct
\begin_inset Quotes srd
\end_inset 

 statement.
\layout LyX-Code

Address ::= SEQUENCE {
\layout LyX-Code

    -- The apartment number may be omitted
\layout LyX-Code

    apartmentNumber     NumericString OPTIONAL,
\layout LyX-Code

    streetName           PrintableString,
\layout LyX-Code

    cityName             PrintableString,
\layout LyX-Code

    stateName            PrintableString,
\layout LyX-Code

    -- This one may be omitted too
\layout LyX-Code

    zipNo                NumericString OPTIONAL
\layout LyX-Code

}
\layout Subsection

The SET type
\layout Standard

This is a collection of other simple or constructed types.
 Ordering is not important.
 The data may arrive in the order which is different from the order of specifica
tion.
 Data is encoded in the order not necessarily corresponding to the order
 of specification.
\layout Subsection

The CHOICE type
\layout Standard

This type is just a choice between the subtypes specified in it.
 The CHOICE type contains at most one of the subtypes specified, and it
 is always implicitly known which choice is being decoded or encoded.
 This one resembles the C 
\begin_inset Quotes sld
\end_inset 

union
\begin_inset Quotes srd
\end_inset 

 statement.
\layout Standard

The following type defines a response code, which may be either an integer
 code or a boolean 
\begin_inset Quotes sld
\end_inset 

true
\begin_inset Quotes srd
\end_inset 

/
\begin_inset Quotes srd
\end_inset 

false
\begin_inset Quotes srd
\end_inset 

 code.
\layout LyX-Code

ResponseCode ::= CHOICE {
\layout LyX-Code

    intCode    INTEGER,
\layout LyX-Code

    boolCode   BOOLEAN
\layout LyX-Code

}
\layout LyX-Code

\layout Subsection

The SEQUENCE OF type
\layout Standard

This one is the list (array) of simple or constructed types:
\layout LyX-Code

-- Example 1
\layout LyX-Code

ManyIntegers ::= SEQUENCE OF INTEGER
\layout LyX-Code

-- Example 2
\layout LyX-Code

ManyRectangles ::= SEQUENCE OF Rectangle
\layout LyX-Code

-- More complex example:
\layout LyX-Code

-- an array of structures defined in place.
\layout LyX-Code

ManyCircles ::= SEQUENCE OF SEQUENCE {
\layout LyX-Code

                            radius INTEGER
\layout LyX-Code

                            }
\layout Subsection

The SET OF type
\layout Standard

The SET OF type models the bag of structures.
 It resembles the SEQUENCE OF type, but the order is not important: i.e.
 the elements may arrive in the order which is not necessarily the same
 as the in-memory order on the remote machines.
\layout LyX-Code

-- A set of structures defined elsewhere
\layout LyX-Code

SetOfApples :: SET OF Apple
\layout LyX-Code

-- Set of integers encoding the kind of a fruit
\layout LyX-Code

FruitBag ::= SET OF ENUMERATED { apple, orange }
\layout Chapter

ASN.1 Compiler Usage
\layout Standard

The purpose of the ASN.1 compiler, of which this document is part, is to
 convert the ASN.1 specifications to some other target language (currently,
 only C is supported
\begin_inset Foot
collapsed false

\layout Standard

C++ is 
\begin_inset Quotes sld
\end_inset 

supported
\begin_inset Quotes srd
\end_inset 

 too, as long as an object-oriented approach is not a definitive factor.
\end_inset 

).
 The compiler reads the specification and emits a series of target language
 structures and surrounding maintenance code.
 For example, the C structure which may be created by compiler to represent
 the simple 
\emph on 
Rectangle
\emph default 
 specification defined earlier in this document, may look like this
\begin_inset Foot
collapsed false

\layout Standard


\emph on 
-fnative-integers
\emph default 
 compiler option is used to produce basic C 
\emph on 
int
\emph default 
 types instead of generic INTEGER_t.
\end_inset 

:
\layout LyX-Code

typedef struct Rectangle_s {
\layout LyX-Code

    int height;
\layout LyX-Code

    int width;
\layout LyX-Code

} Rectangle_t;
\layout Standard

This would not be of much value for such a simple specification, so the
 compiler goes further and actually produces the code which fills in this
 structure by parsing the binary
\begin_inset Foot
collapsed false

\layout Standard

BER, CER and DER encodings are binary.
 However, the XER encoding is text (XML) based.
\end_inset 

 data provided in some buffer.
 It also produces the code that takes this structure as an argument and
 performs structure serialization by emitting a series of bytes.
\layout Section

Quick start
\layout Standard

After building and installing the compiler, the asn1c command may be used
 to compile the ASN.1 specification
\begin_inset Foot
collapsed false

\layout Standard

This is probably 
\series bold 
not
\series default 
 what you want to try out right now -- read through the rest of this chapter
 to find out about -P option.
\end_inset 

:
\layout LyX-Code

asn1c 
\emph on 
<spec.asn1>
\layout Standard

If several specifications contain interdependencies, all of them must be
 specified:
\layout LyX-Code

asn1c 
\emph on 
<spec1.asn1> <spec2.asn1> ...
\layout Standard

The compiler -E and -EF options are used for testing the parser and the
 semantic fixer, respectively.
 These options will instruct the compiler to dump out the parsed (and fixed)
 ASN.1 specification as it was "understood" by the compiler.
 It might might be useful to check whether a particular syntactic construction
 is properly supported by the compiler.
\layout LyX-Code

asn1c -EF 
\emph on 
<spec-to-test.asn1>
\layout Standard

The -P option is used to dump the compiled output on the screen instead
 of creating a bunch of .c and .h files on disk in the current directory.
 You would probably want to start with -P option instead of creating a mess
 in your current directory.
\layout Section

Slow start
\layout Subsection

Recognizing compiler output
\layout Standard

After compiling, the following entities will be created in your current
 directory:
\layout Itemize

A set of .c and .h files, generally a single pair for each type defined in
 the ASN.1 specifications.
 These files will be named similarly to the ASN.1 types (
\emph on 
Rectangle.c
\emph default 
 and 
\emph on 
Rectangle.h
\emph default 
 for the specification defined in the beginning of this document).
\layout Itemize

A set of helper .c and .h files which contain generic encoders, decoders and
 other useful routines.
 There will be many of them, some of them even not necessary
\begin_inset Foot
collapsed false

\layout Standard

Soon the compiler will be modified to emit the smallest subset of necessary
 files.
\end_inset 

, but the overall amount of code after compiling will be rather small anyway.
\layout Standard

It is your responsibility to create .c file with the
\emph on 
 int main()
\emph default 
 routine and the Makefile (if needed).
 Compiler helps you with the latter by creating the Makefile.am.sample, containing
 the skeleton definition for the automake, should you want to use autotools.
\layout Standard

In other words, after compiling the Rectangle module, you have the following
 set of files: { Makefile.am.sample, Rectangle.c, Rectangle.h, 
\series bold 
\SpecialChar \ldots{}

\series default 
 }, where 
\series bold 

\begin_inset Quotes sld
\end_inset 

\SpecialChar \ldots{}

\begin_inset Quotes srd
\end_inset 


\series default 
 stands for the set of additional 
\begin_inset Quotes sld
\end_inset 

helper
\begin_inset Quotes srd
\end_inset 

 files created by the compiler.
 If you add the simple file with the 
\emph on 
int main()
\emph default 
 routine, it would even be possible to compile everything with the single
 instruction:
\layout LyX-Code

cc -o rectangle *.c   # It could be 
\emph on 
that
\emph default 
 simple
\begin_inset Foot
collapsed false

\layout Standard

Provided that you've also created a .c file with the 
\emph on 
int main()
\emph default 
 routine.
\end_inset 


\layout Subsection

Invoking the ASN.1 helper code from the application
\layout Standard

First of all, you would want to include one or more header files into your
 application.
 For the Rectangle module, including the Rectangle.h file is enough:
\layout LyX-Code

#include <Rectangle.h>
\layout Standard

The header files defines the C structure corresponding to the ASN.1 definition
 of a rectangle and the declaration of the ASN.1 type descriptor, which is
 used as an argument to most of the functions provided by the ASN.1 module.
 For example, here is the code which frees the Rectangle_t structure:
\layout LyX-Code

Rectangle_t *rect = ;
\layout LyX-Code

asn1_DEF_Rectangle->free_struct(&asn1_DEF_Rectangle,
\layout LyX-Code

    rect, 0);
\layout Standard

This code defines a 
\emph on 
rect
\emph default 
 pointer which points to the Rectangle_t structure which needs to be freed.
 The second line invokes the generic free_struct routine created specifically
 for this Rectangle_t structure.
 The 
\emph on 
asn1_DEF_Rectangle
\emph default 
 is the type descriptor, which holds a collection of generic routines to
 deal with the Rectangle_t structure.
\layout Standard

There are several generic functions available:
\layout Description

check_constraints Check that the contents of the target structure are semantical
ly valid and constrained to appropriate implicit or explicit subtype constraints.
 Please refer to Section 
\begin_inset LatexCommand \vref{sub:Validating-the-target}

\end_inset 

.
\layout Description

ber_decoder This is the generic 
\emph on 
restartable
\begin_inset Foot
collapsed false

\layout Standard

Restartable means that if the decoder encounters the end of the buffer,
 it will fail, but may later be invoked again with the rest of the buffer
 to continue decoding.
\end_inset 

 
\emph default 
BER decoder (Basic Encoding Rules).
 This decoder would create and/or fill the target structure for you.
 Please refer to Section 
\begin_inset LatexCommand \ref{sub:Decoding-BER}

\end_inset 

.
\layout Description

der_encoder This is the generic DER encoder (Distinguished Encoding Rules).
 This decoder will take the target structure and encode it into a series
 of bytes.
 Please refer to Section 
\begin_inset LatexCommand \ref{sub:Encoding-DER}

\end_inset 

.
\layout Description

print_struct This function convert the contents of the passed target structure
 into human readable form.
 This form is not formal and cannot be converted back into the structure,
 but it may turn out to be useful for debugging or quick-n-dirty printing.
 Please refer to Section 
\begin_inset LatexCommand \ref{sub:Printing-the-target}

\end_inset 

.
\layout Description

free_struct This is a generic disposal which frees the target structure.
 Please refer to Section 
\begin_inset LatexCommand \ref{sub:Freeing-the-target}

\end_inset 

.
\layout Standard

Each of the above function takes the type descriptor (
\emph on 
asn1_DEF_\SpecialChar \ldots{}

\emph default 
) and the target structure (
\emph on 
rect
\emph default 
, in the above example).
 The target structure is typically created by the generic BER decoder or
 by the application itself.
\layout Standard

Here is how the buffer can be deserialized into the structure:
\layout LyX-Code

Rectangle_t *
\layout LyX-Code

simple_deserializer(void *buffer, size_t buf_size) {
\layout LyX-Code

    Rectangle_t *rect = 0;    /* Note this 0! */
\layout LyX-Code

    ber_dec_rval_t rval;
\layout LyX-Code

 
\layout LyX-Code

    rval = asn1_DEF_Rectangle->ber_decoder(
\layout LyX-Code

          &asn1_DEF_Rectangle,
\layout LyX-Code

          (void **)&rect,
\layout LyX-Code

          buffer, buf_size,
\layout LyX-Code

          0);
\layout LyX-Code

 
\layout LyX-Code

    if(rval
\series bold 
.code
\series default 
 == RC_OK) {
\layout LyX-Code

        return rect;          /* Decoding succeeded */
\layout LyX-Code

    } else {
\layout LyX-Code

        asn1_DEF_Rectangle->free_struct(
\layout LyX-Code

            &asn1_DEF_Rectangle, rect, 0);
\layout LyX-Code

        return 0;
\layout LyX-Code

    }
\layout LyX-Code

}
\layout Standard

The code above defines a function, 
\emph on 
simple_deserializer
\emph default 
, which takes a buffer and its length and expected to return a pointer to
 the Rectangle_t structure.
 Inside, it tries to convert the bytes passed into the target structure
 (rect) using the generic BER decoder and returns the rect pointer afterwards.
 If the structure cannot be deserialized, it frees the memory which might
 be left allocated by the unfinished 
\emph on 
ber_decoder
\emph default 
 routine and returns NULL.
 
\series bold 
This freeing is necessary
\series default 
 because the ber_decoder is a restartable procedure, and may fail just because
 there is more data needs to be provided before decoding could be finalized.
 The code above obviously does not take into account the way the 
\emph on 
ber_decoder
\emph default 
 failed, so the freeing is necessary because the part of the buffer may
 already be decoded into the structure by the time something goes wrong.
\layout Standard

Restartable decoding is a little bit trickier: you need to provide the old
 target structure pointer (which might be already half-decoded) and react
 on RC_WMORE return code.
 This will be explained later in Section 
\begin_inset LatexCommand \vref{sub:Decoding-BER}

\end_inset 


\layout Subsubsection


\begin_inset LatexCommand \label{sub:Decoding-BER}

\end_inset 

Decoding BER
\layout Standard

The Basic Encoding Rules describe the basic way how the structure can be
 encoded and decoded.
 Several other encoding rules (CER, DER) define a more restrictive versions
 of BER, so the generic BER parser is also capable of decoding the data
 encoded by CER and DER encoders.
 The opposite is not true.
\layout Standard

The ASN.1 compiler provides the generic BER decoder which is implicitly capable
 of decoding BER, CER and DER encoded data.
\layout Standard

The decoder is restartable (stream-oriented), which means that in case the
 buffer has less data than it is expected, the decoder will process whatever
 it is available and ask for more data to be provided.
 Please note that the decoder may actually process less data than it is
 given in the buffer, which means that you should be able to make the next
 buffer contain the unprocessed part of the previous buffer.
\layout Standard

Suppose, you have two buffers of encoded data: 100 bytes and 200 bytes.
\layout Itemize

You may concatenate these buffers and feed the BER decoder with 300 bytes
 of data, or
\layout Itemize

You may feed it the first buffer of 100 bytes of data, realize that the
 ber_decoder consumed only 95 bytes from it and later feed the decoder with
 205 bytes buffer which consists of 5 unprocessed bytes from the first buffer
 and the latter 200 bytes from the second buffer.
\layout Standard

This is not as convenient as it could be (like, the BER encoder would consume
 the whole 100 bytes and keep these 5 bytes in some temporary storage),
 but in case of stream-based processing it might actually be OK.
 Suggestions are welcome.
\layout Standard

There are two ways to invoke a BER decoder.
 The first one is a direct reference of the type-specific decoder.
 This way was shown in the previous example of 
\emph on 
simple_deserializer
\emph default 
 function.
 The second way is to invoke a 
\emph on 
ber_decode
\emph default 
 function, which is just a simple wrapper of the former approach into a
 less wordy notation:
\layout LyX-Code

rval = ber_decode(&asn1_DEF_Rectangle, (void **)&rect,
\layout LyX-Code

    buffer, buf_size);
\layout Standard

Note that the initial (asn1_DEF_Rectangle->ber_decoder) reference is gone,
 and also the last argument (0) is no longer necessary.
\layout Standard

These two ways of invocations are fully equivalent.
\layout Standard

The BER de
\emph on 
coder
\emph default 
 may fail because (
\emph on 
the following RC_\SpecialChar \ldots{}
 codes are defined in ber_decoder.h
\emph default 
):
\layout Itemize

RC_WMORE: There is more data expected than it is provided (stream mode continuat
ion feature);
\layout Itemize

RC_FAIL: General failure to decode the buffer;
\layout Itemize

\SpecialChar \ldots{}
 other codes may be defined as well.
\layout Standard

Together with the return code (.code) the ber_dec_rval_t type contains the
 number of bytes which is consumed from the buffer.
 In the previous hypothetical example of two buffers (of 100 and 200 bytes),
 the first call to ber_decode() would return with .code = RC_WMORE and .consumed
 = 95.
 The .consumed field of the BER decoder return value is 
\series bold 
always
\series default 
 valid, even if the decoder succeeds or fails with any other return code.
\layout Standard

Please look into ber_decoder.h for the precise definition of ber_decode()
 and related types.
\layout Subsubsection


\begin_inset LatexCommand \label{sub:Encoding-DER}

\end_inset 

Encoding DER
\layout Standard

The Distinguished Encoding Rules is the variant of BER encoding rules which
 is oriented on representing the structures with length known beforehand.
 This is probably exactly how you want to encode: either after a BER decoding
 or after a manual fill-up, the target structure contains the data which
 size is implicitly known before encoding.
 The DER encoding is used, for example, to encode X.509 certificates.
\layout Standard

As with BER decoder, the DER encoder may be invoked either directly from
 the ASN.1 type descriptor (asn1_DEF_Rectangle) or from the stand-alone function,
 which is somewhat simpler:
\layout LyX-Code

/*
\layout LyX-Code

 * This is a custom function which writes the
\layout LyX-Code

 * encoded output into some FILE stream.
\layout LyX-Code

 */
\layout LyX-Code

int _write_stream(void *buffer, size_t size, void *app_key) {
\layout LyX-Code

    FILE *ostream = app_key;
\layout LyX-Code

    size_t wrote;
\layout LyX-Code

 
\layout LyX-Code

    wrote = fwrite(buffer, 1, size, ostream);
\layout LyX-Code

 
\layout LyX-Code

    return (wrote == size) ? 0 : -1;
\layout LyX-Code

}
\layout LyX-Code

 
\layout LyX-Code

/*
\layout LyX-Code

 * This is the serializer itself,
\layout LyX-Code

 * it supplies the DER encoder with the
\layout LyX-Code

 * pointer to the custom output function.
\layout LyX-Code

 */
\layout LyX-Code

ssize_t
\layout LyX-Code

simple_serializer(FILE *ostream, Rectangle_t *rect) {
\layout LyX-Code

    der_enc_rval_t rval;  /* Return value */
\layout LyX-Code

 
\layout LyX-Code

    rval = der_encode(&asn1_DEF_Rect, rect,
\layout LyX-Code

        _write_stream, ostream);
\layout LyX-Code

    if(rval
\series bold 
.encoded
\series default 
 == -1) {
\layout LyX-Code

        /*
\layout LyX-Code

         * Failure to encode the rectangle data.
\layout LyX-Code

         */
\layout LyX-Code

        fprintf(stderr, 
\begin_inset Quotes sld
\end_inset 

Cannot encode %s: %s
\backslash 
n
\begin_inset Quotes srd
\end_inset 

,
\layout LyX-Code

            rval
\series bold 
.failed_type
\series default 
->name,
\layout LyX-Code

            strerror(errno));
\layout LyX-Code

        return -1;
\layout LyX-Code

    } else {
\layout LyX-Code

        /* Return the number of bytes */
\layout LyX-Code

        return rval.encoded;
\layout LyX-Code

    }
\layout LyX-Code

}
\layout Standard

As you see, the DER encoder does not write into some sort of buffer or something.
 It just invokes the custom function (possible, multiple times) which would
 save the data into appropriate storage.
 The optional argument 
\emph on 
app_key
\emph default 
 is opaque for the DER encoder code and just used by 
\emph on 
_write_stream()
\emph default 
 as the pointer to the appropriate output stream to be used.
\layout Standard

If the custom write function is not given (passed as 0), then the DER encoder
 will essentially do the same thing (i.e., encode the data) but no callbacks
 will be invoked (so the data goes nowhere).
 It may prove useful to determine the size of the structure's encoding before
 actually doing the encoding
\begin_inset Foot
collapsed false

\layout Standard

It is actually faster too: the encoder might skip over some computations
 which aren't important for the size determination.
\end_inset 

.
\layout Standard

Please look into der_encoder.h for the precise definition of der_encode()
 and related types.
\layout Subsubsection


\begin_inset LatexCommand \label{sub:Validating-the-target}

\end_inset 

Validating the target structure
\layout Standard

Sometimes the target structure needs to be validated.
 For example, if the structure was created by the application (as opposed
 to being decoded from some external source), some important information
 required by the ASN.1 specification might be missing.
 On the other hand, the successful decoding of the data from some external
 source does not necessarily mean that the data is fully valid either.
 It might well be the case that the specification describes some subtype
 constraints that were not taken into account during decoding, and it would
 actually be useful to perform the last check when the data is ready to
 be encoded or when the data has just been decoded to ensure its validity
 according to some stricter rules.
\layout Standard

The asn_check_constraints() function checks the type for various implicit
 and explicit constraints.
 It is recommended to use asn_check_constraints() function after each decoding
 and before each encoding.
\layout Standard

Please look into constraints.h for the precise definition of asn_check_constraint
s() and related types.
\layout Subsubsection


\begin_inset LatexCommand \label{sub:Printing-the-target}

\end_inset 

Printing the target structure
\layout Standard

There are two ways to print the target structure: either invoke the print_struct
 member of the ASN.1 type descriptor, or using the asn_fprint() function,
 which is a simpler wrapper of the former:
\layout LyX-Code

asn_fprint(stdout, &asn1_DEF_Rectangle, rect);
\layout Standard

Please look into constr_TYPE.h for the precise definition of asn_fprint()
 and related types.
\layout Subsubsection


\begin_inset LatexCommand \label{sub:Freeing-the-target}

\end_inset 

Freeing the target structure
\layout Standard

Freeing the structure is slightly more complex than it may seem to.
 When the ASN.1 structure is freed, all the members of the structure and
 their submembers etc etc are recursively freed too.
 But it might not be feasible to free the structure itself.
 Consider the following case:
\layout LyX-Code

struct my_figure {       /* The custom structure */
\layout LyX-Code

    int flags;           /* <some custom member> */
\layout LyX-Code

    /* The type is generated by the ASN.1 compiler */
\layout LyX-Code

    
\emph on 
Rectangle_t rect;
\layout LyX-Code

    /* other members of the structure */
\layout LyX-Code

};
\layout Standard

In this example, the application programmer defined a custom structure with
 one ASN.1-derived member (rect).
 This member is not a reference to the Rectangle_t, but an in-place inclusion
 of the Rectangle_t structure.
 If the freeing is necessary, the usual procedure of freeing everything
 must not be applied to the &rect pointer itself, because it does not point
 to the memory block directly allocated by memory allocation routine, but
 instead lies within such a block allocated for my_figure structure.
\layout Standard

To solve this problem, the free_struct routine has the additional argument
 (besides the intuitive type descriptor and target structure pointers),
 which is the flag specifying whether the outer pointer itself must be freed
 (0, default) or it should be left intact (non-zero value).
\layout LyX-Code

/* Rectangle_t is defined within my_figure */
\layout LyX-Code

struct my_figure *mf = 
\series bold 
...
\series default 
;
\layout LyX-Code

/*
\layout LyX-Code

 * Freeing the Rectangle_td
\layout LyX-Code

 * without freeing the mf->rect pointer
\layout LyX-Code

 */
\layout LyX-Code

asn1_DEF_Rectangle->free_struct(
\layout LyX-Code

    &asn1_DEF_Rectangle, &mf->rect, 
\emph on 
1
\emph default 
 /* !free */);
\layout LyX-Code

 
\layout LyX-Code

/* Rectangle_t is a stand-alone pointer */
\layout LyX-Code

Rectangle_t *rect = 
\series bold 
...
\series default 
;
\layout LyX-Code

/*
\layout LyX-Code

 * Freeing the Rectangle_t
\layout LyX-Code

 * and freeing the rect pointer
\layout LyX-Code

 */
\layout LyX-Code

asn1_DEF_Rectangle->free_struct(
\layout LyX-Code

    &asn1_DEF_Rectangle, rect, 
\emph on 
0
\emph default 
 /* free the pointer too */);
\layout Standard

It is safe to invoke the 
\emph on 
free_struct
\emph default 
 function with the target structure pointer set to 0 (NULL), the function
 will do nothing.
\layout Bibliography
\bibitem [Dub00]{Dub00}

Olivier Dubuisson -- 
\emph on 
ASN.1 Communication between heterogeneous systems
\emph default 
 -- Morgan Kaufmann Publishers, 2000.
 
\begin_inset LatexCommand \htmlurl{http://asn1.elibel.tm.fr/en/book/}

\end_inset 

.
 ISBN:0-12-6333361-0.
\layout Bibliography
\bibitem [ITU-T/ASN.1]{ITU-T/ASN.1}

ITU-T Study Group 17 -- Languages for Telecommunication Systems 
\begin_inset LatexCommand \url{http://www.itu.int/ITU-T/studygroups/com17/languages/}

\end_inset 


\the_end