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<P>
<H1 ALIGN="CENTER">Using the Free ASN.1 Compiler</H1><DIV>

<P ALIGN="CENTER"><STRONG>Lev Walkin &lt;vlm@lionet.info&gt;</STRONG></P>
</DIV>

<P>
<BR>

<H2><A NAME="SECTION01000000000000000000">
Contents</A>
</H2>
<!--Table of Contents-->

<UL>
<LI><A NAME="tex2html45"
  HREF="asn1c-usage.html#SECTION02000000000000000000">ASN.1 Basics</A>
<UL>
<LI><A NAME="tex2html46"
  HREF="asn1c-usage.html#SECTION02100000000000000000">Abstract Syntax Notation: ASN.1</A>
<UL>
<LI><A NAME="tex2html47"
  HREF="asn1c-usage.html#SECTION02110000000000000000">Some of the ASN.1 Basic Types</A>
<UL>
<LI><A NAME="tex2html48"
  HREF="asn1c-usage.html#SECTION02111000000000000000">The BOOLEAN type</A>
<LI><A NAME="tex2html49"
  HREF="asn1c-usage.html#SECTION02112000000000000000">The INTEGER type</A>
<LI><A NAME="tex2html50"
  HREF="asn1c-usage.html#SECTION02113000000000000000">The ENUMERATED type</A>
<LI><A NAME="tex2html51"
  HREF="asn1c-usage.html#SECTION02114000000000000000">The OCTET STRING type</A>
<LI><A NAME="tex2html52"
  HREF="asn1c-usage.html#SECTION02115000000000000000">The OBJECT IDENTIFIER type</A>
<LI><A NAME="tex2html53"
  HREF="asn1c-usage.html#SECTION02116000000000000000">The RELATIVE-OID type</A>
</UL>
<LI><A NAME="tex2html54"
  HREF="asn1c-usage.html#SECTION02120000000000000000">Some of the ASN.1 String Types</A>
<UL>
<LI><A NAME="tex2html55"
  HREF="asn1c-usage.html#SECTION02121000000000000000">The IA5String type</A>
<LI><A NAME="tex2html56"
  HREF="asn1c-usage.html#SECTION02122000000000000000">The UTF8String type</A>
<LI><A NAME="tex2html57"
  HREF="asn1c-usage.html#SECTION02123000000000000000">The NumericString type</A>
<LI><A NAME="tex2html58"
  HREF="asn1c-usage.html#SECTION02124000000000000000">The PrintableString type</A>
<LI><A NAME="tex2html59"
  HREF="asn1c-usage.html#SECTION02125000000000000000">The VisibleString type</A>
</UL>
<LI><A NAME="tex2html60"
  HREF="asn1c-usage.html#SECTION02130000000000000000">ASN.1 Constructed Types</A>
<UL>
<LI><A NAME="tex2html61"
  HREF="asn1c-usage.html#SECTION02131000000000000000">The SEQUENCE type</A>
<LI><A NAME="tex2html62"
  HREF="asn1c-usage.html#SECTION02132000000000000000">The SET type</A>
<LI><A NAME="tex2html63"
  HREF="asn1c-usage.html#SECTION02133000000000000000">The CHOICE type</A>
<LI><A NAME="tex2html64"
  HREF="asn1c-usage.html#SECTION02134000000000000000">The SEQUENCE OF type</A>
<LI><A NAME="tex2html65"
  HREF="asn1c-usage.html#SECTION02135000000000000000">The SET OF type</A>
</UL>
</UL>
</UL><BR>
<LI><A NAME="tex2html66"
  HREF="asn1c-usage.html#SECTION03000000000000000000">Using the ASN.1 Compiler</A>
<UL>
<LI><A NAME="tex2html67"
  HREF="asn1c-usage.html#SECTION03100000000000000000">Introduction to the ASN.1 Compiler</A>
<LI><A NAME="tex2html68"
  HREF="asn1c-usage.html#SECTION03200000000000000000">Quick start</A>
<LI><A NAME="tex2html69"
  HREF="asn1c-usage.html#SECTION03300000000000000000">Using the ASN.1 Compiler</A>
<UL>
<LI><A NAME="tex2html70"
  HREF="asn1c-usage.html#SECTION03310000000000000000">Command-line options</A>
<LI><A NAME="tex2html71"
  HREF="asn1c-usage.html#SECTION03320000000000000000">Recognizing compiler output</A>
<LI><A NAME="tex2html72"
  HREF="asn1c-usage.html#SECTION03330000000000000000">Invoking the ASN.1 helper code from the application</A>
<UL>
<LI><A NAME="tex2html73"
  HREF="asn1c-usage.html#SECTION03331000000000000000">Decoding BER</A>
<LI><A NAME="tex2html74"
  HREF="asn1c-usage.html#SECTION03332000000000000000">Encoding DER</A>
<LI><A NAME="tex2html75"
  HREF="asn1c-usage.html#SECTION03333000000000000000">Validating the target structure</A>
<LI><A NAME="tex2html76"
  HREF="asn1c-usage.html#SECTION03334000000000000000">Printing the target structure</A>
<LI><A NAME="tex2html77"
  HREF="asn1c-usage.html#SECTION03335000000000000000">Freeing the target structure</A>
</UL>
</UL>
</UL><BR>
<LI><A NAME="tex2html78"
  HREF="asn1c-usage.html#SECTION04000000000000000000">Bibliography</A>
</UL>
<!--End of Table of Contents-->

<P>

<P>

<H1><A NAME="SECTION02000000000000000000">
ASN.1 Basics</A>
</H1>

<P>

<H1><A NAME="SECTION02100000000000000000">
Abstract Syntax Notation: ASN.1</A>
</H1>

<P>
<I>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 [<A
 HREF="asn1c-usage.html#Dub00">Dub00</A>] or the ASN.1 body
of standards itself [<A
 HREF="asn1c-usage.html#ITU-T_ASN.1">ITU-T/ASN.1</A>].</I>

<P>
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:

<P>

<UL>
<LI>The specification expressed in the ASN.1 notation is a formal and
precise way to communicate the data semantics to human readers;
</LI>
<LI>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).
</LI>
</UL>
Consider the following example:

<P>

<BLOCKQUOTE><PRE>
Rectangle&nbsp;::=&nbsp;SEQUENCE&nbsp;{
&nbsp;&nbsp;&nbsp;&nbsp;height&nbsp;&nbsp;INTEGER,
&nbsp;&nbsp;&nbsp;&nbsp;width&nbsp;&nbsp;&nbsp;INTEGER
}
</PRE>
</BLOCKQUOTE>
This ASN.1 specification describes a constructed type, <I>Rectangle</I>,
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 <I>how</I> that
entity is going to send or receive the <I>encoded data</I> 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
''serializing'', or ''marshaling'') the data: BER, CER, DER and
XER, some of them which will be described later.

<P>
The complete specification must be wrapped in a module, which looks
like this:

<P>

<BLOCKQUOTE><PRE>
UsageExampleModule1
&nbsp;&nbsp;&nbsp;&nbsp;{&nbsp;iso&nbsp;org(3)&nbsp;dod(6)&nbsp;internet(1)&nbsp;private(4)
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;enterprise(1)&nbsp;spelio(9363)&nbsp;software(1)
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;asn1c(5)&nbsp;docs(2)&nbsp;usage(1)&nbsp;1&nbsp;}&nbsp;
&nbsp;&nbsp;&nbsp;&nbsp;DEFINITIONS&nbsp;AUTOMATIC&nbsp;TAGS&nbsp;::=
BEGIN
&nbsp;
--&nbsp;This&nbsp;is&nbsp;a&nbsp;comment&nbsp;which&nbsp;describes&nbsp;nothing.
Rectangle&nbsp;::=&nbsp;SEQUENCE&nbsp;{
&nbsp;&nbsp;&nbsp;&nbsp;height&nbsp;&nbsp;INTEGER,&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;--&nbsp;Height&nbsp;of&nbsp;the&nbsp;rectangle
&nbsp;&nbsp;&nbsp;&nbsp;width&nbsp;&nbsp;&nbsp;INTEGER&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;--&nbsp;Width&nbsp;of&nbsp;the&nbsp;rectangle
}
&nbsp;
END
</PRE>
</BLOCKQUOTE>
The module header consists of module name (UsageExampleModule1), the
module object identifier ({...}), a keyword ''DEFINITIONS'', a
set of module flags (AUTOMATIC TAGS) and ''::= BEGIN''. The module
ends with an ''END'' statement.

<P>

<H1><A NAME="SECTION02110000000000000000">
Some of the ASN.1 Basic Types</A>
</H1>

<P>

<H2><A NAME="SECTION02111000000000000000">
The BOOLEAN type</A>
</H2>

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

<P>

<H2><A NAME="SECTION02112000000000000000">
The INTEGER type</A>
</H2>

<P>
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.

<P>

<BLOCKQUOTE><PRE>
SimpleInteger&nbsp;::=&nbsp;INTEGER
&nbsp;
--&nbsp;An&nbsp;integer&nbsp;with&nbsp;a&nbsp;very&nbsp;limited&nbsp;range
SmallInt&nbsp;::=&nbsp;INTEGER&nbsp;(0..127)
&nbsp;
--&nbsp;Integer,&nbsp;negative
NegativeInt&nbsp;::=&nbsp;INTEGER&nbsp;(MIN..0)
</PRE>
</BLOCKQUOTE>

<P>

<H2><A NAME="SECTION02113000000000000000">
The ENUMERATED type</A>
</H2>

<P>
The ENUMERATED type is semantically equivalent to the INTEGER type
with some integer values explicitly named.

<P>

<BLOCKQUOTE><PRE>
FruitId&nbsp;::=&nbsp;ENUMERATED&nbsp;{&nbsp;apple(1),&nbsp;orange(2)&nbsp;}
&nbsp;
--&nbsp;The&nbsp;numbers&nbsp;in&nbsp;braces&nbsp;are&nbsp;optional,
--&nbsp;the&nbsp;enumeration&nbsp;can&nbsp;be&nbsp;performed
--&nbsp;automatically&nbsp;by&nbsp;the&nbsp;compiler
ComputerOSType&nbsp;::=&nbsp;ENUMERATED&nbsp;{
&nbsp;&nbsp;&nbsp;&nbsp;FreeBSD,&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;--&nbsp;will&nbsp;be&nbsp;0
&nbsp;&nbsp;&nbsp;&nbsp;Windows,&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;--&nbsp;will&nbsp;be&nbsp;1
&nbsp;&nbsp;&nbsp;&nbsp;Solaris(5),&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;--&nbsp;will&nbsp;remain&nbsp;5
&nbsp;&nbsp;&nbsp;&nbsp;Linux,&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;--&nbsp;will&nbsp;be&nbsp;6
&nbsp;&nbsp;&nbsp;&nbsp;MacOS&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;--&nbsp;will&nbsp;be&nbsp;7
}
</PRE>
</BLOCKQUOTE>

<P>

<H2><A NAME="SECTION02114000000000000000">
The OCTET STRING type</A>
</H2>

<P>
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).

<P>

<H2><A NAME="SECTION02115000000000000000">
The OBJECT IDENTIFIER type</A>
</H2>

<P>
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 http://www.iana.org/protocols/forms.htm.

<P>
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.

<P>

<BLOCKQUOTE><PRE>
ExampleOID&nbsp;::=&nbsp;OBJECT&nbsp;IDENTIFIER
&nbsp;
usageExampleModule1-oid&nbsp;ExampleOID
&nbsp;&nbsp;::=&nbsp;{&nbsp;1&nbsp;3&nbsp;6&nbsp;1&nbsp;4&nbsp;1&nbsp;9363&nbsp;1&nbsp;5&nbsp;2&nbsp;1&nbsp;1&nbsp;}
&nbsp;
--&nbsp;An&nbsp;identifier&nbsp;of&nbsp;the&nbsp;Internet.
internet-id&nbsp;OBJECT&nbsp;IDENTIFIER
&nbsp;&nbsp;::=&nbsp;{&nbsp;iso(1)&nbsp;identified-organization(3)
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;dod(6)&nbsp;internet(1)&nbsp;}
</PRE>
</BLOCKQUOTE>
As you see, names are optional.

<P>

<H2><A NAME="SECTION02116000000000000000">
The RELATIVE-OID type</A>
</H2>

<P>
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.

<P>

<BLOCKQUOTE><PRE>
this-document&nbsp;RELATIVE-OID&nbsp;::=&nbsp;{&nbsp;docs(2)&nbsp;usage(1)&nbsp;}
&nbsp;
this-example&nbsp;RELATIVE-OID&nbsp;::=&nbsp;{
&nbsp;&nbsp;&nbsp;&nbsp;this-document&nbsp;assorted-examples(0)&nbsp;this-example(1)&nbsp;}
</PRE>
</BLOCKQUOTE>

<P>

<H1><A NAME="SECTION02120000000000000000">
Some of the ASN.1 String Types</A>
</H1>

<P>

<H2><A NAME="SECTION02121000000000000000">
The IA5String type</A>
</H2>

<P>
This is essentially the ASCII, with 128 character codes available
(7 lower bits of an 8-bit byte).

<P>

<H2><A NAME="SECTION02122000000000000000">
The UTF8String type</A>
</H2>

<P>
This is the character string which encodes the full Unicode range
(4 bytes) using multibyte character sequences.

<P>

<H2><A NAME="SECTION02123000000000000000">
The NumericString type</A>
</H2>

<P>
This type represents the character string with the alphabet consisting
of numbers (''0'' to ''9'') and a space.

<P>

<H2><A NAME="SECTION02124000000000000000">
The PrintableString type</A>
</H2>

<P>
The character string with the following alphabet: space, ''<B>'</B>''
(single quote), ''<B>(</B>'', ''<B>)</B>'', ''<B>+</B>'',
'','' (comma), ''<B>-</B>'', ''<B>.</B>'', ''<B>/</B>'',
digits (''0'' to ''9''), ''<B>:</B>'', ''<B>=</B>'', ''<B>?</B>'',
upper-case and lower-case letters (''A'' to ''Z'' and ''a''
to ''z'')

<P>

<H2><A NAME="SECTION02125000000000000000">
The VisibleString type</A>
</H2>

<P>
The character string with the alphabet which is more or less a subset
of ASCII between space and ''<B>~</B>'' (tilde).
Alternatively, the alphabet may be described as the PrintableString
alphabet presented earlier, plus the following characters: ''<B>!</B>'',
''<B>''</B>'', ''<B>#</B>'', ''<B>$</B>'', ''<B>%</B>'',
''<B>&amp;</B>'', ''<B>*</B>'', ''<B>;</B>'', ''<B>&lt;</B>'',
''<B>&gt;</B>'', ''<B>[</B>'', ''<B>&#92;</B>'',
''<B>]</B>'', ''<B>&#94;</B>'', ''<B>_</B>'',
''<B>`</B>'' (single left quote), ''<B>{</B>'', ''<B>|</B>'',
''<B>}</B>'', ''<B>~</B>''.

<P>

<H1><A NAME="SECTION02130000000000000000">
ASN.1 Constructed Types</A>
</H1>

<P>

<H2><A NAME="SECTION02131000000000000000">
The SEQUENCE type</A>
</H2>

<P>
This is an ordered collection of other simple or constructed types.
The SEQUENCE constructed type resembles the C ''struct'' statement.

<P>

<BLOCKQUOTE><PRE>
Address&nbsp;::=&nbsp;SEQUENCE&nbsp;{
&nbsp;&nbsp;&nbsp;&nbsp;--&nbsp;The&nbsp;apartment&nbsp;number&nbsp;may&nbsp;be&nbsp;omitted
&nbsp;&nbsp;&nbsp;&nbsp;apartmentNumber&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;NumericString&nbsp;OPTIONAL,
&nbsp;&nbsp;&nbsp;&nbsp;streetName&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;PrintableString,
&nbsp;&nbsp;&nbsp;&nbsp;cityName&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;PrintableString,
&nbsp;&nbsp;&nbsp;&nbsp;stateName&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;PrintableString,
&nbsp;&nbsp;&nbsp;&nbsp;--&nbsp;This&nbsp;one&nbsp;may&nbsp;be&nbsp;omitted&nbsp;too
&nbsp;&nbsp;&nbsp;&nbsp;zipNo&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;NumericString&nbsp;OPTIONAL
}
</PRE>
</BLOCKQUOTE>

<P>

<H2><A NAME="SECTION02132000000000000000">
The SET type</A>
</H2>

<P>
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 specification. Data is encoded in the order not
necessarily corresponding to the order of specification.

<P>

<H2><A NAME="SECTION02133000000000000000">
The CHOICE type</A>
</H2>

<P>
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 ''union'' statement.

<P>
The following type defines a response code, which may be either an
integer code or a boolean ''true''/''false'' code.

<P>

<BLOCKQUOTE><PRE>
ResponseCode&nbsp;::=&nbsp;CHOICE&nbsp;{
&nbsp;&nbsp;&nbsp;&nbsp;intCode&nbsp;&nbsp;&nbsp;&nbsp;INTEGER,
&nbsp;&nbsp;&nbsp;&nbsp;boolCode&nbsp;&nbsp;&nbsp;BOOLEAN
}
</PRE>
</BLOCKQUOTE>

<P>

<H2><A NAME="SECTION02134000000000000000">
The SEQUENCE OF type</A>
</H2>

<P>
This one is the list (array) of simple or constructed types:

<P>

<BLOCKQUOTE><PRE>
--&nbsp;Example&nbsp;1
ManyIntegers&nbsp;::=&nbsp;SEQUENCE&nbsp;OF&nbsp;INTEGER
&nbsp;
--&nbsp;Example&nbsp;2
ManyRectangles&nbsp;::=&nbsp;SEQUENCE&nbsp;OF&nbsp;Rectangle
&nbsp;
--&nbsp;More&nbsp;complex&nbsp;example:
--&nbsp;an&nbsp;array&nbsp;of&nbsp;structures&nbsp;defined&nbsp;in&nbsp;place.
ManyCircles&nbsp;::=&nbsp;SEQUENCE&nbsp;OF&nbsp;SEQUENCE&nbsp;{
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;radius&nbsp;INTEGER
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}
</PRE>
</BLOCKQUOTE>

<P>

<H2><A NAME="SECTION02135000000000000000">
The SET OF type</A>
</H2>

<P>
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.

<P>

<BLOCKQUOTE><PRE>
--&nbsp;A&nbsp;set&nbsp;of&nbsp;structures&nbsp;defined&nbsp;elsewhere
SetOfApples&nbsp;::&nbsp;SET&nbsp;OF&nbsp;Apple
&nbsp;
--&nbsp;Set&nbsp;of&nbsp;integers&nbsp;encoding&nbsp;the&nbsp;kind&nbsp;of&nbsp;a&nbsp;fruit
FruitBag&nbsp;::=&nbsp;SET&nbsp;OF&nbsp;ENUMERATED&nbsp;{&nbsp;apple,&nbsp;orange&nbsp;}
</PRE>
</BLOCKQUOTE>

<P>

<H1><A NAME="SECTION03000000000000000000">
Using the ASN.1 Compiler</A>
</H1>

<P>

<H1><A NAME="SECTION03100000000000000000">
Introduction to the ASN.1 Compiler</A>
</H1>

<P>
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<A NAME="tex2html1"
  HREF="#foot144"><SUP>2.1</SUP></A>). 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 <I>Rectangle</I> specification defined earlier in this document,
may look like this<A NAME="tex2html2"
  HREF="#foot379"><SUP>2.2</SUP></A>:

<P>

<BLOCKQUOTE><PRE>
typedef&nbsp;struct&nbsp;Rectangle_s&nbsp;{
&nbsp;&nbsp;&nbsp;&nbsp;int&nbsp;height;
&nbsp;&nbsp;&nbsp;&nbsp;int&nbsp;width;
}&nbsp;Rectangle_t;
</PRE>
</BLOCKQUOTE>
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 opaque binary<A NAME="tex2html3"
  HREF="#foot151"><SUP>2.3</SUP></A> 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.

<P>

<H1><A NAME="SECTION03200000000000000000">
Quick start</A>
</H1>

<P>
After building and installing the compiler, the <I>asn1c</I><A NAME="tex2html4"
  HREF="#foot380"><SUP>3.1</SUP></A> command may be used to compile the ASN.1 specification<A NAME="tex2html5"
  HREF="#foot381"><SUP>3.2</SUP></A>:

<P>

<BLOCKQUOTE><PRE>
asn1c&nbsp;<I>&lt;spec.asn1&gt;</I>
</PRE>
</BLOCKQUOTE>
If several specifications contain interdependencies, all of the files
must be specified altogether:

<P>

<BLOCKQUOTE><PRE>
asn1c&nbsp;<I>&lt;spec1.asn1&gt;&nbsp;&lt;spec2.asn1&gt;&nbsp;...</I>
</PRE>
</BLOCKQUOTE>
The compiler <B>-E</B> and <B>-EF</B> 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, if <B>-F</B>
is involved) ASN.1 specification as it was &#34;understood&#34;
by the compiler. It might be useful to check whether a particular
syntactic construction is properly supported by the compiler.

<P>

<BLOCKQUOTE><PRE>
asn1c&nbsp;<B>-EF</B>&nbsp;<I>&lt;spec-to-test.asn1&gt;</I>
</PRE>
</BLOCKQUOTE>
The <B>-P</B> 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 <B>-P</B>
option instead of creating a mess in your current directory. Another
option, <B>-R</B>, asks compiler to only generate the files which
need to be generated, and supress linking in the numerous support
files.

<P>
Print the compiled output instead of creating multiple source files:

<P>

<BLOCKQUOTE><PRE>
asn1c&nbsp;<B>-P</B>&nbsp;<I>&lt;spec-to-compile-and-print.asn1&gt;</I>
</PRE>
</BLOCKQUOTE>

<P>

<H1><A NAME="SECTION03300000000000000000">
Using the ASN.1 Compiler</A>
</H1>

<P>

<H1><A NAME="SECTION03310000000000000000">
Command-line options</A>
</H1>

<P>
The <A HREF=#Table1>Table 1</A> summarizes various options affecting
the compiler's behavior.

<P>
<BR><P></P>
<DIV ALIGN="CENTER"><A NAME="384"></A>
<TABLE>
<CAPTION><STRONG><A NAME=Table1>Table 1:</A></STRONG>
The list of asn1c command line options</CAPTION>
<TR><TD><TABLE COLS=2 BORDER FRAME=BOX RULES=GROUPS>
<COLGROUP><COL ALIGN=LEFT><COLGROUP><COL ALIGN=JUSTIFY WIDTH="3in">
<TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP> 
<B>Overall Options</B></TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
<B>Description</B></TD></TR>
</TBODY><TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP>-E</TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
<FONT SIZE="-1">Stop after the parsing stage and print the reconstructed ASN.1
specification code to the standard output.</FONT></TD></TR>
</TBODY><TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP>-F</TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
<FONT SIZE="-1">Used together with -E, instructs the compiler to stop after
the ASN.1 syntax tree fixing stage and dump the reconstructed ASN.1
specification to the standard output.</FONT></TD></TR>
</TBODY><TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP>-P</TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
<FONT SIZE="-1">Dump the compiled output to the standard output instead of
cre- ating the target language files on disk.</FONT></TD></TR>
</TBODY><TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP>-R</TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
<FONT SIZE="-1">Restrict the compiler to generate only the ASN.1 tables, omit-
ting the usual support code.</FONT></TD></TR>
</TBODY><TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP>-S <I>&lt;directory&gt;</I></TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
<FONT SIZE="-1">Use the specified directory with ASN.1 skeleton files.</FONT></TD></TR>
</TBODY><TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP>-t <I>&lt;data-string&gt;</I></TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
<FONT SIZE="-1">Interpret the data-string as a sequence of hexadecimal values
representing the start of BER TLV encoding. Print the human readable
explanation.</FONT></TD></TR>
</TBODY><TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP><B>Warning Options</B></TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
<B>Description</B></TD></TR>
</TBODY><TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP>-Werror</TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
<FONT SIZE="-1">Treat warnings as errors; abort if any warning is produced.</FONT></TD></TR>
</TBODY><TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP>-Wdebug-lexer</TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
<FONT SIZE="-1">Enable lexer debugging during the ASN.1 parsing stage.</FONT></TD></TR>
</TBODY><TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP>-Wdebug-fixer</TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
 <FONT SIZE="-1">Enable ASN.1 syntax  tree  fixer  debugging  during  the
 fixing stage.</FONT></TD></TR>
</TBODY><TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP>-Wdebug-compiler</TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
<FONT SIZE="-1">Enable debugging during the actual compile time.</FONT></TD></TR>
</TBODY><TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP><B>Language Options</B></TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
<B>Description</B></TD></TR>
</TBODY><TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP>-fbless-SIZE</TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
<FONT SIZE="-1">Allow SIZE() constraint for INTEGER, ENUMERATED, and other
types for which this constraint is normally prohibited by the standard.
This is a violation of an ASN.1 standard and compiler may fail to
produce the meaningful code.</FONT></TD></TR>
</TBODY><TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP>-fnative-integers</TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
<FONT SIZE="-1">Use native machine's integer types whenever possible, instead
of the complex ASN.1 INTEGER and ENUMERATED types. </FONT></TD></TR>
</TBODY><TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP>-funnamed-unions</TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
<FONT SIZE="-1">Enable  unnamed  unions  in the definitions of target language's
structures.</FONT></TD></TR>
</TBODY><TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP>-ftypes88</TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
<FONT SIZE="-1">Use only ASN.1:1988 embedded types.</FONT></TD></TR>
</TBODY><TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP><B>Output Options</B></TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
<B>Description</B></TD></TR>
</TBODY><TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP>-print-constraints</TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
<FONT SIZE="-1">When -EF are also specified, this option forces the compiler
to explain its internal understanding of subtype constraints.</FONT></TD></TR>
</TBODY><TBODY>
<TR><TD VALIGN=BASELINE ALIGN=LEFT NOWRAP>-print-lines</TD><TD VALIGN=BASELINE ALIGN=LEFT WIDTH="216">
<FONT SIZE="-1">Generate &#34;- #line&#34; comments in -E output.</FONT></TD></TR>
</TBODY>
</TABLE>

<P>
</TD></TR>
</TABLE>
</DIV><P></P><BR>

<P>

<H1><A NAME="SECTION03320000000000000000">
Recognizing compiler output</A>
</H1>

<P>
After compiling, the following entities will be created in your current
directory:

<P>

<UL>
<LI>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 (<I>Rectangle.c</I> and <I>Rectangle.h</I> for the
specification defined in the beginning of this document).
</LI>
<LI>A set of helper .c and .h files which contain generic encoders, decoders
and other useful routines. There will be quite a few of them, some
of them even are not always necessary, but the overall amount of code
after compiling will be rather small anyway.
</LI>
</UL>
It is your responsibility to create .c file with the <I>int main()</I>
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.

<P>
In other words, after compiling the Rectangle module, you have the
following set of files: { Makefile.am.sample, Rectangle.c, Rectangle.h,
<B>...</B> }, where <B>''...''</B> stands for the
set of additional ''helper'' files created by the compiler. If you
add the simple file with the <I>int main()</I> routine, it would even
be possible to compile everything with the single instruction:

<P>

<BLOCKQUOTE><PRE>
cc&nbsp;-o&nbsp;rectangle&nbsp;*.c&nbsp;&nbsp;&nbsp;#&nbsp;It&nbsp;could&nbsp;be&nbsp;<I>that</I>&nbsp;simple<A NAME="tex2html7"
  HREF="#foot387"><SUP>4.1</SUP></A>
</PRE>
</BLOCKQUOTE>

<P>

<H1><A NAME="SECTION03330000000000000000">
Invoking the ASN.1 helper code from the application</A>
</H1>

<P>
First of all, you should to include one or more header files into
your application. For our Rectangle module, including the Rectangle.h
file is enough:

<P>

<BLOCKQUOTE><PRE>
#include&nbsp;&lt;Rectangle.h&gt;
</PRE>
</BLOCKQUOTE>
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:

<P>

<BLOCKQUOTE><PRE>
Rectangle_t&nbsp;*rect&nbsp;=&nbsp;...;
&nbsp;
asn1_DEF_Rectangle-&gt;free_struct(&amp;asn1_DEF_Rectangle,
&nbsp;&nbsp;&nbsp;&nbsp;rect,&nbsp;0);
</PRE>
</BLOCKQUOTE>
This code defines a <I>rect</I> 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 <I>asn1_DEF_Rectangle</I> is the type descriptor, which holds
a collection of generic routines to deal with the Rectangle_t structure.

<P>
There are several generic functions available:

<P>
<DL>
<DT><STRONG>check_constraints</STRONG></DT>
<DD>Check that the contents of the target structure
are semantically valid and constrained to appropriate implicit or
explicit subtype constraints. Please refer to Section sub:Validating-the-target.
</DD>
<DT><STRONG>ber_decoder</STRONG></DT>
<DD>This is the generic <I>restartable</I><A NAME="tex2html8"
  HREF="#foot241"><SUP>4.2</SUP></A> BER decoder (Basic Encoding Rules). This decoder would create
and/or fill the target structure for you. Please refer to Section
[<A HREF="#sub:Decoding-BER">Decoding-BER</A>].
</DD>
<DT><STRONG>der_encoder</STRONG></DT>
<DD>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 [<A HREF="#sub:Encoding-DER">Encoding-DER</A>].
</DD>
<DT><STRONG>print_struct</STRONG></DT>
<DD>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 [<A HREF="#sub:Printing-the-target">Printing-the-target</A>].
</DD>
<DT><STRONG>free_struct</STRONG></DT>
<DD>This is a generic disposal which frees the target structure.
Please refer to Section [<A HREF="#sub:Freeing-the-target">Freeing-the-target</A>].
</DD>
</DL>
Each of the above function takes the type descriptor (<I>asn1_DEF_...</I>)
and the target structure (<I>rect</I>, in the above example). The
target structure is typically created by the generic BER decoder or
by the application itself.

<P>
Here is how the buffer can be deserialized into the structure:

<P>

<BLOCKQUOTE><PRE>
Rectangle_t&nbsp;*
simple_deserializer(const&nbsp;void&nbsp;*buffer,&nbsp;size_t&nbsp;buf_size)&nbsp;{
&nbsp;&nbsp;&nbsp;&nbsp;Rectangle_t&nbsp;*rect&nbsp;=&nbsp;0;&nbsp;&nbsp;&nbsp;&nbsp;/*&nbsp;Note&nbsp;this&nbsp;0!&nbsp;*/
&nbsp;&nbsp;&nbsp;&nbsp;ber_dec_rval_t&nbsp;rval;
&nbsp;
&nbsp;&nbsp;&nbsp;&nbsp;rval&nbsp;=&nbsp;asn1_DEF_Rectangle-&gt;ber_decoder(
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&amp;asn1_DEF_Rectangle,
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(void&nbsp;**)&amp;rect,
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;buffer,&nbsp;buf_size,
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;0);
&nbsp;
&nbsp;&nbsp;&nbsp;&nbsp;if(rval<B>.code</B>&nbsp;==&nbsp;RC_OK)&nbsp;{
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return&nbsp;rect;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;/*&nbsp;Decoding&nbsp;succeeded&nbsp;*/
&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;else&nbsp;{
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;/*&nbsp;Free&nbsp;partially&nbsp;decoded&nbsp;rect&nbsp;*/
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;asn1_DEF_Rectangle-&gt;free_struct(
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&amp;asn1_DEF_Rectangle,&nbsp;rect,&nbsp;0);
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return&nbsp;0;
&nbsp;&nbsp;&nbsp;&nbsp;}
}
</PRE>
</BLOCKQUOTE>
The code above defines a function, <I>simple_deserializer</I>, 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 <I>ber_decoder</I> routine and returns NULL. <B>This
freeing is necessary</B> 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 <I>ber_decoder</I> 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.

<P>
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 sub:Decoding-BER

<P>

<H2><A NAME="SECTION03331000000000000000"></A><A NAME="sub:Decoding-BER"></A><BR>
Decoding BER
</H2>

<P>
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.

<P>
The ASN.1 compiler provides the generic BER decoder which is implicitly
capable of decoding BER, CER and DER encoded data.

<P>
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.

<P>
Suppose, you have two buffers of encoded data: 100 bytes and 200 bytes.

<P>

<UL>
<LI>You may concatenate these buffers and feed the BER decoder with 300
bytes of data, or
</LI>
<LI>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.
</LI>
</UL>
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.

<P>
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 <I>simple_deserializer</I> function. The second
way is to invoke a <I>ber_decode</I> function, which is just a simple
wrapper of the former approach into a less wordy notation:

<P>

<BLOCKQUOTE><PRE>
rval&nbsp;=&nbsp;ber_decode(&amp;asn1_DEF_Rectangle,&nbsp;(void&nbsp;**)&amp;rect,
&nbsp;&nbsp;&nbsp;&nbsp;buffer,&nbsp;buf_size);
</PRE>
</BLOCKQUOTE>
Note that the initial (asn1_DEF_Rectangle-&gt;ber_decoder) reference
is gone, and also the last argument (0) is no longer necessary.

<P>
These two ways of invocations are fully equivalent.

<P>
The BER de<I>coder</I> may fail because (<I>the following RC_...
codes are defined in ber_decoder.h</I>):

<P>

<UL>
<LI>RC_WMORE: There is more data expected than it is provided (stream
mode continuation feature);
</LI>
<LI>RC_FAIL: General failure to decode the buffer;
</LI>
<LI>... other codes may be defined as well.
</LI>
</UL>
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 <B>always</B>
valid, even if the decoder succeeds or fails with any other return
code.

<P>
Please look into ber_decoder.h for the precise definition of ber_decode()
and related types.

<P>

<H2><A NAME="SECTION03332000000000000000"></A><A NAME="sub:Encoding-DER"></A><BR>
Encoding DER
</H2>

<P>
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.

<P>
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:

<P>

<BLOCKQUOTE><PRE>
/*
&nbsp;*&nbsp;This&nbsp;is&nbsp;a&nbsp;custom&nbsp;function&nbsp;which&nbsp;writes&nbsp;the
&nbsp;*&nbsp;encoded&nbsp;output&nbsp;into&nbsp;some&nbsp;FILE&nbsp;stream.
&nbsp;*/
static&nbsp;int
write_stream(const&nbsp;void&nbsp;*buffer,&nbsp;size_t&nbsp;size,&nbsp;void&nbsp;*app_key)&nbsp;{
&nbsp;&nbsp;&nbsp;&nbsp;FILE&nbsp;*ostream&nbsp;=&nbsp;app_key;
&nbsp;&nbsp;&nbsp;&nbsp;size_t&nbsp;wrote;
&nbsp;
&nbsp;&nbsp;&nbsp;&nbsp;wrote&nbsp;=&nbsp;fwrite(buffer,&nbsp;1,&nbsp;size,&nbsp;ostream);
&nbsp;
&nbsp;&nbsp;&nbsp;&nbsp;return&nbsp;(wrote&nbsp;==&nbsp;size)&nbsp;?&nbsp;0&nbsp;:&nbsp;-1;
}
&nbsp;
/*
&nbsp;*&nbsp;This&nbsp;is&nbsp;the&nbsp;serializer&nbsp;itself,
&nbsp;*&nbsp;it&nbsp;supplies&nbsp;the&nbsp;DER&nbsp;encoder&nbsp;with&nbsp;the
&nbsp;*&nbsp;pointer&nbsp;to&nbsp;the&nbsp;custom&nbsp;output&nbsp;function.
&nbsp;*/
ssize_t
simple_serializer(FILE&nbsp;*ostream,&nbsp;Rectangle_t&nbsp;*rect)&nbsp;{
&nbsp;&nbsp;&nbsp;&nbsp;der_enc_rval_t&nbsp;rval;&nbsp;&nbsp;/*&nbsp;Return&nbsp;value&nbsp;*/
&nbsp;
&nbsp;&nbsp;&nbsp;&nbsp;rval&nbsp;=&nbsp;der_encode(&amp;asn1_DEF_Rect,&nbsp;rect,
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;write_stream,&nbsp;ostream);
&nbsp;&nbsp;&nbsp;&nbsp;if(rval<B>.encoded</B>&nbsp;==&nbsp;-1)&nbsp;{
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;/*
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;*&nbsp;Failure&nbsp;to&nbsp;encode&nbsp;the&nbsp;rectangle&nbsp;data.
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;*/
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;fprintf(stderr,&nbsp;''Cannot&nbsp;encode&nbsp;%s:&nbsp;%s&#92;n'',
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;rval<B>.failed_type</B>-&gt;name,
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;strerror(errno));
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return&nbsp;-1;
&nbsp;&nbsp;&nbsp;&nbsp;}&nbsp;else&nbsp;{
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;/*&nbsp;Return&nbsp;the&nbsp;number&nbsp;of&nbsp;bytes&nbsp;*/
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return&nbsp;rval.encoded;
&nbsp;&nbsp;&nbsp;&nbsp;}
}
</PRE>
</BLOCKQUOTE>
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 <I>app_key</I> is opaque for the DER encoder code and just
used by <I>_write_stream()</I> as the pointer to the appropriate
output stream to be used.

<P>
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<A NAME="tex2html9"
  HREF="#foot313"><SUP>4.3</SUP></A>.

<P>
Please look into der_encoder.h for the precise definition of der_encode()
and related types.

<P>

<H2><A NAME="SECTION03333000000000000000"></A><A NAME="sub:Validating-the-target"></A><BR>
Validating the target structure
</H2>

<P>
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.

<P>
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.

<P>
Please look into constraints.h for the precise definition of asn_check_constraints()
and related types.

<P>

<H2><A NAME="SECTION03334000000000000000"></A><A NAME="sub:Printing-the-target"></A><BR>
Printing the target structure
</H2>

<P>
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:

<P>

<BLOCKQUOTE><PRE>
asn_fprint(stdout,&nbsp;&amp;asn1_DEF_Rectangle,&nbsp;rect);
</PRE>
</BLOCKQUOTE>
Please look into constr_TYPE.h for the precise definition of asn_fprint()
and related types.

<P>

<H2><A NAME="SECTION03335000000000000000"></A><A NAME="sub:Freeing-the-target"></A><BR>
Freeing the target structure
</H2>

<P>
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:

<P>

<BLOCKQUOTE><PRE>
struct&nbsp;my_figure&nbsp;{&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;/*&nbsp;The&nbsp;custom&nbsp;structure&nbsp;*/
&nbsp;&nbsp;&nbsp;&nbsp;int&nbsp;flags;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;/*&nbsp;&lt;some&nbsp;custom&nbsp;member&gt;&nbsp;*/
&nbsp;&nbsp;&nbsp;&nbsp;/*&nbsp;The&nbsp;type&nbsp;is&nbsp;generated&nbsp;by&nbsp;the&nbsp;ASN.1&nbsp;compiler&nbsp;*/
&nbsp;&nbsp;&nbsp;&nbsp;<I>Rectangle_t&nbsp;rect;</I>
&nbsp;&nbsp;&nbsp;&nbsp;/*&nbsp;other&nbsp;members&nbsp;of&nbsp;the&nbsp;structure&nbsp;*/
};
</PRE>
</BLOCKQUOTE>
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 &amp;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.

<P>
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).

<P>

<BLOCKQUOTE><PRE>
/*&nbsp;Rectangle_t&nbsp;is&nbsp;defined&nbsp;within&nbsp;my_figure&nbsp;*/
struct&nbsp;my_figure&nbsp;*mf&nbsp;=&nbsp;<B>...</B>;
/*
&nbsp;*&nbsp;Freeing&nbsp;the&nbsp;Rectangle_td
&nbsp;*&nbsp;without&nbsp;freeing&nbsp;the&nbsp;mf-&gt;rect&nbsp;pointer
&nbsp;*/
asn1_DEF_Rectangle-&gt;free_struct(
&nbsp;&nbsp;&nbsp;&nbsp;&amp;asn1_DEF_Rectangle,&nbsp;&amp;mf-&gt;rect,&nbsp;<I>1</I>&nbsp;/*&nbsp;!free&nbsp;*/);
&nbsp;
/*&nbsp;Rectangle_t&nbsp;is&nbsp;a&nbsp;stand-alone&nbsp;pointer&nbsp;*/
Rectangle_t&nbsp;*rect&nbsp;=&nbsp;<B>...</B>;
/*
&nbsp;*&nbsp;Freeing&nbsp;the&nbsp;Rectangle_t
&nbsp;*&nbsp;and&nbsp;freeing&nbsp;the&nbsp;rect&nbsp;pointer
&nbsp;*/
asn1_DEF_Rectangle-&gt;free_struct(
&nbsp;&nbsp;&nbsp;&nbsp;&amp;asn1_DEF_Rectangle,&nbsp;rect,&nbsp;<I>0</I>&nbsp;/*&nbsp;free&nbsp;the&nbsp;pointer&nbsp;too&nbsp;*/);
</PRE>
</BLOCKQUOTE>
It is safe to invoke the <I>free_struct</I> function with the target
structure pointer set to 0 (NULL), the function will do nothing.

<P>
 
<H2><A NAME="SECTION04000000000000000000">
Bibliography</A>
</H2><DL COMPACT><DD><P></P><DT><A NAME="ASN1C">ASN1C</A>
<DD>Free ASN.1 Compiler. http://lionet.info/asn1/
<P></P><DT><A NAME="Dub00">Dub00</A>
<DD>Olivier Dubuisson - <I>ASN.1 Communication between heterogeneous
systems</I> - Morgan Kaufmann Publishers, 2000. http://asn1.elibel.tm.fr/en/book/.
ISBN:0-12-6333361-0.
<P></P><DT><A NAME="ITU-T_ASN.1">ITU-T/ASN.1</A>
<DD>ITU-T Study Group 17 - Languages for Telecommunication Systems http://www.itu.int/ITU-T/studygroups/com17/languages/</DL>

<P>
<BR><HR><H4>Footnotes</H4>
<DL>
<DT><A NAME="foot144">... supported</A><A
 HREF="asn1c-usage.html#tex2html1"><SUP>2.1</SUP></A></DT>
<DD>C++ is ''supported'' too, as long as an class-based approach is
not a definitive factor.

</DD>
<DT><A NAME="foot379">... this</A><A
 HREF="asn1c-usage.html#tex2html2"><SUP>2.2</SUP></A></DT>
<DD><I>-fnative-integers</I> compiler option is used to produce basic
C <I>int</I> types instead of infinite width INTEGER_t structures.
See <A HREF=#Table1>Table 1</A>.

</DD>
<DT><A NAME="foot151">... binary</A><A
 HREF="asn1c-usage.html#tex2html3"><SUP>2.3</SUP></A></DT>
<DD>BER, CER and DER encodings are binary. However, the XER encoding is
text (XML) based.

</DD>
<DT><A NAME="foot380">...asn1c</A><A
 HREF="asn1c-usage.html#tex2html4"><SUP>3.1</SUP></A></DT>
<DD>The 1 symbol in asn<B>1</B>c is a digit, not an ''ell'' letter.

</DD>
<DT><A NAME="foot381">... specification</A><A
 HREF="asn1c-usage.html#tex2html5"><SUP>3.2</SUP></A></DT>
<DD>This is probably <B>not</B> what you want to try out right now -
read through the rest of this chapter to find out about <B>-P</B>
and <B>-R</B> options.

</DD>
<DT><A NAME="foot387">...that&nbsp;simple</A><A
 HREF="asn1c-usage.html#tex2html7"><SUP>4.1</SUP></A></DT>
<DD>Provided that you've also created a .c file with the <I>int main()</I>
routine.

</DD>
<DT><A NAME="foot241">...restartable</A><A
 HREF="asn1c-usage.html#tex2html8"><SUP>4.2</SUP></A></DT>
<DD>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.

</DD>
<DT><A NAME="foot313">... encoding</A><A
 HREF="asn1c-usage.html#tex2html9"><SUP>4.3</SUP></A></DT>
<DD>It is actually faster too: the encoder might skip over some computations
which aren't important for the size determination.

</DD>
</DL><BR><HR>
<ADDRESS>
Lev Walkin
2004-08-23
</ADDRESS>
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