#LyX 1.3 created this file. For more info see http://www.lyx.org/ \lyxformat 221 \textclass book \language english \inputencoding latin1 \fontscheme ae \graphics default \paperfontsize default \spacing single \papersize Default \paperpackage a4 \use_geometry 0 \use_amsmath 0 \use_natbib 0 \use_numerical_citations 0 \paperorientation portrait \secnumdepth 3 \tocdepth 3 \paragraph_separation indent \defskip medskip \quotes_language swedish \quotes_times 2 \papercolumns 1 \papersides 1 \paperpagestyle default \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 \layout Standard If several specifications contain interdependencies, all of them must be specified: \layout LyX-Code asn1c \emph on ... \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 \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 \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; /* */ \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