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| pdfauthor={Lev Walkin}, |
| pdftitle={Using the Open Source ASN.1 Compiler}, |
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| \begin{document} |
| |
| \title{Using the Open Source ASN.1 Compiler} |
| |
| |
| \author{Lev Walkin <\href{mailto:vlm@lionet.info?Subject=asn1c}{vlm@lionet.info}>} |
| |
| \maketitle |
| \lhead{This document describes \href{http://lionet.info/asn1c}{asn1c-0.9.21}} |
| |
| \rhead{$Revision$} |
| |
| \tableofcontents{} |
| |
| \pagestyle{headings} |
| |
| |
| \part{Using the ASN.1 Compiler} |
| |
| |
| \chapter{Introduction to the ASN.1 Compiler} |
| |
| The purpose of the ASN.1 compiler is to convert the specifications |
| in ASN.1 notation into some other language. At this moment, only C |
| and C++ target languages are supported, the latter is in upward compatibility |
| mode. |
| |
| The compiler reads the specification and emits a series of target |
| language structures (C structs, unions, enums) describing the corresponding |
| ASN.1 types. The compiler also creates the code which allows automatic |
| serialization and deserialization of these structures using several |
| standardized encoding rules (BER, DER, XER, PER). |
| |
| For example, suppose the following ASN.1 module is given% |
| \footnote{Part \vref{par:ASN.1-Basics} provides a quick reference |
| on the ASN.1 notation.}: |
| \begin{lyxcode} |
| RectangleTest~DEFINITIONS~::= |
| |
| BEGIN |
| |
| ~ |
| |
| Rectangle~::=~SEQUENCE~\{ |
| |
| ~~~~height~~INTEGER,~~~~~~~~-{}-~Height~of~the~rectangle |
| |
| ~~~~width~~~INTEGER~~~~~~~~~-{}-~Width~of~the~rectangle |
| |
| \} |
| |
| ~ |
| |
| END |
| \end{lyxcode} |
| The compiler would read this ASN.1 definition and produce the following |
| C type% |
| \footnote{\emph{-fnative-types} compiler option is used to produce basic C \emph{int} |
| types instead of infinite width INTEGER\_t structures. See Section |
| \vref{sec:Command-line-options}.% |
| }: |
| \begin{lyxcode} |
| typedef~struct~Rectangle\_s~\{ |
| |
| ~~~~int~height; |
| |
| ~~~~int~width; |
| |
| \}~Rectangle\_t; |
| \end{lyxcode} |
| It would also create the code for converting this structure into platform-independent |
| wire representation (a serializer API) and the decoder of such wire |
| representation back into local, machine-specific type (a deserializer |
| API). |
| |
| |
| \section{Quick start with asn1c} |
| |
| After building and installing the compiler, the \emph{asn1c}% |
| \footnote{The 1 symbol in asn\textbf{1}c is a digit, not an ``ell'' letter.% |
| } command may be used to compile the ASN.1 module% |
| \footnote{This is probably \textbf{not} what you want to try out right now. Read through the rest of this chapter and check the Section \vref{sec:Command-line-options} |
| to find out about \textbf{-P} and \textbf{-R} options.% |
| }: |
| \begin{lyxcode} |
| asn1c~\emph{<module.asn1>} |
| \end{lyxcode} |
| If several ASN.1 modules contain interdependencies, all of the files |
| must be specified altogether: |
| \begin{lyxcode} |
| asn1c~\emph{<module1.asn1>~<module2.asn1>~...} |
| \end{lyxcode} |
| The compiler \textbf{-E} and \textbf{-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, if \textbf{-F} |
| is involved) ASN.1 specification as it was ``understood'' |
| by the compiler. It might be useful to check whether a particular |
| syntactic construction is properly supported by the compiler. |
| \begin{lyxcode} |
| asn1c~\textbf{-EF}~\emph{<module-to-test.asn1>} |
| \end{lyxcode} |
| The \textbf{-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 \textbf{-P} |
| option instead of creating a mess in your current directory. Another |
| option, \textbf{-R}, asks compiler to only generate the files which |
| need to be generated, and supress linking in the numerous support |
| files. |
| |
| Print the compiled output instead of creating multiple source files: |
| \begin{lyxcode} |
| asn1c~\textbf{-P}~\emph{<module-to-compile-and-print.asn1>} |
| \end{lyxcode} |
| |
| \section{Recognizing compiler output} |
| |
| After compiling, the following entities will be created in your current |
| directory: |
| \begin{itemize} |
| \item 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{Rectangle.c} and \emph{Rectangle.h} for the |
| RectangleTest ASN.1 module defined in the beginning of this document). |
| \item 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 compilation will be rather small anyway. |
| \item A \emph{Makefile.am.sample} file mentioning all the files created |
| at the earlier steps. This file is suitable for either automake suite |
| or the plain `make` utility. |
| \end{itemize} |
| It is your responsibility to create .c file with the \emph{int main()} |
| routine. |
| |
| In other words, after compiling the Rectangle module, you have the |
| following set of files: \{~Makefile.am.sample, Rectangle.c, Rectangle.h, |
| \textbf{\ldots{}} \}, where ``\textbf{\ldots{}}'' stands for the |
| set of additional ``helper'' files created by the compiler. If you |
| add a simple file with the \emph{int main()} routine, it would even |
| be possible to compile everything with the single instruction: |
| \begin{lyxcode} |
| cc~-I.~-o~rectangle.exe~{*}.c~~~\#~It~could~be~\emph{that}~simple |
| \end{lyxcode} |
| Refer to the Chapter \vref{cha:Step-by-step-examples} for a sample |
| \emph{int main()} routine. |
| |
| |
| \section{\label{sec:Command-line-options}Command line options} |
| |
| The following table summarizes the asn1c command line options. |
| |
| \renewcommand{\arraystretch}{1.33} |
| \begin{longtable}{lp{4in}} |
| \toprule |
| \textbf{\small Overall Options} & \textbf{\small Description}\tabularnewline |
| \midrule |
| {\small -E} & {\small Stop after the parsing stage and print the reconstructed ASN.1 |
| specification code to the standard output.}\tabularnewline |
| {\small -F} & {\small 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.}\tabularnewline |
| {\small -P} & {\small Dump the compiled output to the standard output instead of |
| creating the target language files on disk.}\tabularnewline |
| {\small -R} & {\small Restrict the compiler to generate only the ASN.1 tables, omitting the usual support code.}\tabularnewline |
| {\small -S }\emph{\small <directory>} & {\small Use the specified directory with ASN.1 skeleton files.}\tabularnewline |
| {\small -X} & {\small Generate the XML DTD for the specified ASN.1 modules.}\tabularnewline |
| \midrule\tabularnewline |
| \textbf{\small Warning Options} & \textbf{\small Description}\tabularnewline |
| \midrule |
| {\small -Werror} & {\small Treat warnings as errors; abort if any warning is produced.}\tabularnewline |
| {\small -Wdebug-lexer} & {\small Enable lexer debugging during the ASN.1 parsing stage.}\tabularnewline |
| {\small -Wdebug-fixer} & {\small Enable ASN.1 syntax tree fixer debugging during the |
| fixing stage.}\tabularnewline |
| {\small -Wdebug-compiler} & {\small Enable debugging during the actual compile time.}\tabularnewline |
| \midrule\tabularnewline |
| \textbf{\small Language Options} & \textbf{\small Description}\tabularnewline |
| \midrule |
| {\small -fbless-SIZE} & {\small 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.}\tabularnewline |
| {\small -fcompound-names} & {\small Use complex names for C structures. Using complex names prevents |
| name clashes in case the module reuses the same identifiers in multiple |
| contexts.}\tabularnewline |
| {\small -findirect-choice} & {\small When generating code for a CHOICE type, compile the CHOICE |
| members as indirect pointers instead of declaring them inline. Consider |
| using this option together with }\textbf{\small -fno-include-deps}{\small{} |
| to prevent circular references. }\tabularnewline |
| {\small -fknown-extern-type=}\emph{\small <name>} & {\small Pretend the specified type is known. The compiler will assume |
| the target language source files for the given type have been provided |
| manually. }\tabularnewline |
| {\small -fnative-types} & {\small Use the native machine's data types (int, double) whenever |
| possible, instead of the compound INTEGER\_t, ENUMERATED\_t and REAL\_t |
| types. }\tabularnewline |
| {\small -fno-constraints} & {\small Do not generate ASN.1 subtype constraint checking code. This |
| may produce a shorter executable.}\tabularnewline |
| {\small -fno-include-deps} & {\small Do not generate courtesy \#include lines for non-critical |
| dependencies.}\tabularnewline |
| {\small -funnamed-unions} & {\small Enable unnamed unions in the definitions of target language's |
| structures.}\tabularnewline |
| {\small -fskeletons-copy} & {\small Copy support files rather than symlink them.}\tabularnewline |
| \midrule\tabularnewline |
| \textbf{\small Codecs Generation Options} & \textbf{\small Description}\tabularnewline |
| \midrule |
| {\small -gen-PER} & {\small Generate Packed Encoding Rules (PER) support code.}\tabularnewline |
| {\small -pdu=}\emph{\small auto} & {\small Generate PDU tables by discovering Protocol Data Units automatically.}\tabularnewline |
| \midrule\tabularnewline |
| \textbf{\small Output Options} & \textbf{\small Description}\tabularnewline |
| \midrule |
| {\small -print-constraints} & {\small When -EF are also specified, this option forces the compiler |
| to explain its internal understanding of subtype constraints.}\tabularnewline |
| {\small -print-lines} & {\small Generate ``-{}- \#line'' comments |
| in -E output.}\tabularnewline |
| \bottomrule |
| \end{longtable} |
| \renewcommand{\arraystretch}{1} |
| |
| |
| \chapter{Using the ASN.1 Compiler} |
| |
| |
| \section[Invoking the helper code]{Invoking the ASN.1 helper code} |
| |
| First of all, you should include one or more header files into your |
| application. Typically, it is enough to include the header file of |
| the main PDU type. For our Rectangle module, including the Rectangle.h |
| file is sufficient: |
| \begin{lyxcode} |
| \#include~<Rectangle.h> |
| \end{lyxcode} |
| 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: |
| \begin{lyxcode} |
| Rectangle\_t~{*}rect~=~...; |
| |
| ~ |
| |
| asn\_DEF\_Rectangle.free\_struct(\&asn\_DEF\_Rectangle, rect,~0); |
| \end{lyxcode} |
| This code defines a \emph{rect} pointer which points to the Rectangle\_t |
| structure which needs to be freed. The second line invokes the generic |
| \emph{free\_struct()} routine created specifically for this Rectangle\_t |
| structure. The \emph{asn\_DEF\_Rectangle} is the type descriptor, |
| which holds a collection of routines to deal with the Rectangle\_t |
| structure. |
| |
| The following member functions of the asn\_DEF\_Rectangle type descriptor |
| are of interest: |
| \begin{description} |
| \item [{ber\_decoder}] This is the generic \emph{restartable}% |
| \footnote{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.% |
| } BER decoder (Basic Encoding Rules). This decoder would create and/or |
| fill the target structure for you. See Section \vref{sub:Decoding-BER}. |
| \item [{der\_encoder}] This is the generic DER encoder (Distinguished Encoding |
| Rules). This encoder will take the target structure and encode it |
| into a series of bytes. See Section \vref{sub:Encoding-DER}. NOTE: |
| DER encoding is a subset of BER. Any BER decoder should be able to |
| handle DER input. |
| \item [{xer\_decoder}] This is the generic XER decoder. It takes both BASIC-XER |
| or CANONICAL-XER encodings and deserializes the data into a local, |
| machine-dependent representation. See Section \vref{sub:Decoding-XER}. |
| \item [{xer\_encoder}] This is the XER encoder (XML Encoding Rules). This |
| encoder will take the target structure and represent it as an XML |
| (text) document using either BASIC-XER or CANONICAL-XER encoding rules. |
| See Section \vref{sub:Encoding-XER}. |
| \item [{uper\_decoder}] This is the Unaligned PER decoder. |
| \item [{uper\_encoder}] This is the Unaligned Basic PER encoder. This encoder |
| will take the target structure and encode it into a series of bytes. |
| \item [{check\_constraints}] Check that the contents of the target structure |
| are semantically valid and constrained to appropriate implicit or |
| explicit subtype constraints. See Section \vref{sub:Validating-the-target}. |
| \item [{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. See Section |
| \vref{sub:Printing-the-target}. |
| \item [{free\_struct}] This is a generic disposal which frees the target |
| structure. See Section \vref{sub:Freeing-the-target}. |
| \end{description} |
| Each of the above function takes the type descriptor (\emph{asn\_DEF\_\ldots{}}) |
| and the target structure (\emph{rect}, in the above example). |
| |
| |
| \subsection{\label{sub:Decoding-BER}Decoding BER} |
| |
| The Basic Encoding Rules describe the most widely used (by the ASN.1 |
| community) way to encode and decode a given structure in a machine-independent |
| way. 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. |
| |
| \emph{The ASN.1 compiler provides the generic BER decoder which is |
| implicitly capable of decoding BER, CER and DER encoded data.} |
| |
| 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 there is available and ask for more data to be provided. |
| Please note that the decoder may actually process less data than it |
| was given in the buffer, which means that you must be able to make |
| the next buffer contain the unprocessed part of the previous buffer. |
| |
| Suppose, you have two buffers of encoded data: 100 bytes and 200 bytes. |
| \begin{itemize} |
| \item You may concatenate these buffers and feed the BER decoder with 300 |
| bytes of data, or |
| \item 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 additional 200 bytes from the second |
| buffer. |
| \end{itemize} |
| This is not as convenient as it could be (like, the BER encoder could |
| consume the whole 100 bytes and keep these 5 bytes in some temporary |
| storage), but in case of existing stream based processing it might |
| actually fit well into existing algorithm. Suggestions are welcome. |
| |
| Here is the simplest example of BER decoding. |
| \begin{lyxcode} |
| Rectangle\_t~{*} |
| |
| simple\_deserializer(const~void~{*}buffer,~size\_t~buf\_size)~\{ |
| |
| ~~~~Rectangle\_t~{*}rect~=~0;~~~~/{*}~Note~this~0!~{*}/ |
| |
| ~~~~asn\_dec\_rval\_t~rval; |
| |
| ~ |
| |
| ~~~~rval~=~\textbf{asn\_DEF\_Rectangle.ber\_decoder}(0, |
| |
| ~~~~~~~~~~\&asn\_DEF\_Rectangle, |
| |
| ~~~~~~~~~~(void~{*}{*})\&rect, |
| |
| ~~~~~~~~~~buffer,~buf\_size, |
| |
| ~~~~~~~~~~0); |
| |
| ~ |
| |
| ~~~~if(rval\textbf{.code}~==~RC\_OK)~\{ |
| |
| ~~~~~~~~return~rect;~~~~~~~~~~/{*}~Decoding~succeeded~{*}/ |
| |
| ~~~~\}~else~\{ |
| |
| ~~~~~~~~/{*}~Free~partially~decoded~rect~{*}/ |
| |
| ~~~~~~~~asn\_DEF\_Rectangle.free\_struct(\&asn\_DEF\_Rectangle,~rect,~0); |
| |
| ~~~~~~~~return~0; |
| |
| ~~~~\} |
| |
| \} |
| \end{lyxcode} |
| The code above defines a function, \emph{simple\_deserializer}, which |
| takes a buffer and its length and is 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 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{ber\_decoder} routine and returns 0 (no data). (This \textbf{freeing |
| is necessary} 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{ber\_decoder()} 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. |
| |
| A little less wordy would be to invoke a globally available \emph{ber\_decode()} |
| function instead of dereferencing the asn\_DEF\_Rectangle type descriptor: |
| \begin{lyxcode} |
| rval~=~ber\_decode(0,~\&asn\_DEF\_Rectangle,~(void~{*}{*})\&rect, |
| |
| ~~~~buffer,~buf\_size); |
| \end{lyxcode} |
| Note that the initial (asn\_DEF\_Rectangle.ber\_decoder) reference |
| is gone, and also the last argument (0) is no longer necessary. |
| |
| These two ways of BER decoder invocations are fully equivalent. |
| |
| The BER de\emph{coder} may fail because of (\emph{the following RC\_\ldots{} |
| codes are defined in ber\_decoder.h}): |
| \begin{itemize} |
| \item RC\_WMORE: There is more data expected than it is provided (stream |
| mode continuation feature); |
| \item RC\_FAIL: General failure to decode the buffer; |
| \item \ldots{} other codes may be defined as well. |
| \end{itemize} |
| Together with the return code (.code) the asn\_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 \textbf{always} |
| valid, even if the decoder succeeds or fails with any other return |
| code. |
| |
| Look into ber\_decoder.h for the precise definition of ber\_decode() |
| and related types. |
| |
| |
| \subsection{\label{sub:Encoding-DER}Encoding DER} |
| |
| The Distinguished Encoding Rules is the \emph{canonical} variant of |
| BER encoding rules. The DER is best suited to encode the structures |
| where all the lengths are 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. Among other uses, the DER encoding is used |
| to encode X.509 certificates. |
| |
| As with BER decoder, the DER encoder may be invoked either directly |
| from the ASN.1 type descriptor (asn\_DEF\_Rectangle) or from the stand-alone |
| function, which is somewhat simpler: |
| \begin{lyxcode} |
| ~ |
| |
| /{*} |
| |
| ~{*}~This~is~the~serializer~itself, |
| |
| ~{*}~it~supplies~the~DER~encoder~with~the |
| |
| ~{*}~pointer~to~the~custom~output~function. |
| |
| ~{*}/ |
| |
| ssize\_t |
| |
| simple\_serializer(FILE~{*}ostream,~Rectangle\_t~{*}rect)~\{ |
| |
| ~~~~asn\_enc\_rval\_t~er;~~/{*}~Encoder~return~value~{*}/ |
| |
| ~ |
| |
| ~~~~er~=~der\_encode(\&asn\_DEF\_Rect,~rect, |
| |
| ~~~~~~~~write\_stream,~ostream); |
| |
| ~~~~if(er.\textbf{encoded}~==~-1)~\{ |
| |
| ~~~~~~~~/{*} |
| |
| ~~~~~~~~~{*}~Failed~to~encode~the~rectangle~data. |
| |
| ~~~~~~~~~{*}/ |
| |
| ~~~~~~~~fprintf(stderr,~''Cannot~encode~\%s:~\%s\textbackslash{}n'', |
| |
| ~~~~~~~~~~~~er.\textbf{failed\_type}->name, strerror(errno)); |
| |
| ~~~~~~~~return~-1; |
| |
| ~~~~\}~else~\{ |
| |
| ~~~~~~~~/{*}~Return~the~number~of~bytes~{*}/ |
| |
| ~~~~~~~~return~er.encoded; |
| |
| ~~~~\} |
| |
| \} |
| \end{lyxcode} |
| 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{app\_key} is opaque for the DER encoder code and just |
| used by \emph{\_write\_stream()} as the pointer to the appropriate |
| output stream to be used. |
| |
| 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% |
| \footnote{It is actually faster too: the encoder might skip over some computations |
| which aren't important for the size determination.% |
| }. |
| |
| Look into der\_encoder.h for the precise definition of der\_encode() |
| and related types. |
| |
| |
| \subsection{\label{sub:Encoding-XER}Encoding XER} |
| |
| The XER stands for XML Encoding Rules, where XML, in turn, is eXtensible |
| Markup Language, a text-based format for information exchange. The |
| encoder routine API comes in two flavors: stdio-based and callback-based. |
| With the callback-based encoder, the encoding process is very similar |
| to the DER one, described in Section \vref{sub:Encoding-DER}. The |
| following example uses the definition of write\_stream() from up there. |
| \begin{lyxcode} |
| /{*} |
| |
| ~{*}~This~procedure~generates~the~XML~document |
| |
| ~{*}~by~invoking~the~XER~encoder. |
| |
| ~{*}~NOTE:~Do~not~copy~this~code~verbatim! |
| |
| ~{*}~~~~~~~If~the~stdio~output~is~necessary, |
| |
| ~{*}~~~~~~~use~the~xer\_fprint()~procedure~instead. |
| |
| ~{*}~~~~~~~See~Section~\vref{sub:Printing-the-target}. |
| |
| ~{*}/ |
| |
| int |
| |
| print\_as\_XML(FILE~{*}ostream,~Rectangle\_t~{*}rect)~\{ |
| |
| ~~~~asn\_enc\_rval\_t~er;~~/{*}~Encoder~return~value~{*}/ |
| |
| ~ |
| |
| ~~~~er~=~xer\_encode(\&asn\_DEF\_Rectangle,~rect, |
| |
| ~~~~~~~~XER\_F\_BASIC,~/{*}~BASIC-XER~or~CANONICAL-XER~{*}/ |
| |
| ~~~~~~~~write\_stream,~ostream); |
| |
| ~ |
| |
| ~~~~return~(er.encoded~==~-1)~?~-1~:~0; |
| |
| \} |
| \end{lyxcode} |
| Look into xer\_encoder.h for the precise definition of xer\_encode() |
| and related types. |
| |
| See Section \ref{sub:Printing-the-target} for the example of stdio-based |
| XML encoder and other pretty-printing suggestions. |
| |
| |
| \subsection{\label{sub:Decoding-XER}Decoding XER} |
| |
| The data encoded using the XER rules can be subsequently decoded using |
| the xer\_decode() API call: |
| \begin{lyxcode} |
| Rectangle\_t~{*} |
| |
| XML\_to\_Rectangle(const~void~{*}buffer,~size\_t~buf\_size)~\{ |
| |
| ~~~~Rectangle\_t~{*}rect~=~0;~/{*}~Note~this~0!~{*}/ |
| |
| ~~~~asn\_dec\_rval\_t~rval; |
| |
| ~~ |
| |
| ~~~~rval~=~xer\_decode(0,~\&asn\_DEF\_Rectangle,~(void~{*}{*})\&rect, |
| |
| ~~~~~~~~buffer,~buf\_size); |
| |
| ~~~~if(rval\textbf{.code}~==~RC\_OK)~\{ |
| |
| ~~~~~~~~return~rect;~~~~~~~~~~/{*}~Decoding~succeeded~{*}/ |
| |
| ~~~~\}~else~\{ |
| |
| ~~~~~~~~/{*}~Free~partially~decoded~rect~{*}/ |
| |
| ~~~~~~~~asn\_DEF\_Rectangle.free\_struct(\&asn\_DEF\_Rectangle,~rect,~0); |
| |
| ~~~~~~~~return~0; |
| |
| ~~~~\} |
| |
| \} |
| \end{lyxcode} |
| The decoder takes both BASIC-XER and CANONICAL-XER encodings. |
| |
| The decoder shares its data consumption properties with BER decoder; |
| please read the Section \vref{sub:Decoding-BER} to know more. |
| |
| Look into xer\_decoder.h for the precise definition of xer\_decode() |
| and related types. |
| |
| |
| \subsection{\label{sub:Validating-the-target}Validating the target structure} |
| |
| 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. |
| |
| 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. |
| |
| Look into constraints.h for the precise definition of asn\_check\_constraints() |
| and related types. |
| |
| |
| \subsection{\label{sub:Printing-the-target}Printing the target structure} |
| |
| 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: |
| \begin{lyxcode} |
| asn\_fprint(stdout,~\&asn\_DEF\_Rectangle,~rect); |
| \end{lyxcode} |
| Look into constr\_TYPE.h for the precise definition of asn\_fprint() |
| and related types. |
| |
| Another practical alternative to this custom format printing would |
| be to invoke XER encoder. The default BASIC-XER encoder performs reasonable |
| formatting for the output to be useful and human readable. To invoke |
| the XER decoder in a manner similar to asn\_fprint(), use the xer\_fprint() |
| call: |
| \begin{lyxcode} |
| xer\_fprint(stdout,~\&asn\_DEF\_Rectangle,~rect); |
| \end{lyxcode} |
| See Section \vref{sub:Encoding-XER} for XML-related details. |
| |
| |
| \subsection{\label{sub:Freeing-the-target}Freeing the target structure} |
| |
| 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 are recursively freed as well. But it might not |
| be feasible to free the structure itself. Consider the following case: |
| \begin{lyxcode} |
| struct~my\_figure~\{~~~~~~~/{*}~The~custom~structure~{*}/ |
| |
| ~~~~int~flags;~~~~~~~~~~~/{*}~<some~custom~member>~{*}/ |
| |
| ~~~~/{*}~The~type~is~generated~by~the~ASN.1~compiler~{*}/ |
| |
| ~~~~\emph{Rectangle\_t~rect;} |
| |
| ~~~~/{*}~other~members~of~the~structure~{*}/ |
| |
| \}; |
| \end{lyxcode} |
| 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 the memory |
| allocation routine, but instead lies within a block allocated for |
| the my\_figure structure. |
| |
| To solve this problem, the free\_struct routine has the additional |
| argument (besides the obvious 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). |
| \begin{lyxcode} |
| \textbf{/{*}~1.~Rectangle\_t~is~defined~within~my\_figure~{*}/} |
| |
| struct~my\_figure~\{ |
| |
| ~~~~Rectangle\_t~rect; |
| |
| \}~{*}mf~=~\textbf{...}; |
| |
| /{*} |
| |
| ~{*}~Freeing~the~Rectangle\_t |
| |
| ~{*}~without~freeing~the~mf->rect~area |
| |
| ~{*}/ |
| |
| asn\_DEF\_Rectangle.free\_struct( |
| |
| ~~~~\&asn\_DEF\_Rectangle,~\&mf->rect,~\textbf{1}~\textbf{/{*}~!free~{*}/}); |
| |
| ~~~~ |
| |
| ~~ |
| |
| \textbf{/{*}~2.~Rectangle\_t~is~a~stand-alone~pointer~{*}/} |
| |
| Rectangle\_t~{*}rect~=~\textbf{...}; |
| |
| /{*} |
| |
| ~{*}~Freeing~the~Rectangle\_t |
| |
| ~{*}~and~freeing~the~rect~pointer |
| |
| ~{*}/ |
| |
| asn\_DEF\_Rectangle.free\_struct( |
| |
| ~~~~\&asn\_DEF\_Rectangle,~rect,~\textbf{0}~\textbf{/{*}~free~the~pointer~too~{*}/}); |
| \end{lyxcode} |
| It is safe to invoke the \emph{free\_struct} function with the target |
| structure pointer set to 0 (NULL), the function will do nothing. |
| |
| For the programmer's convenience, the following macros are available: |
| \begin{lyxcode} |
| ASN\_STRUCT\_FREE(asn\_DEF,~ptr); |
| |
| ASN\_STRUCT\_FREE\_CONTENTS\_ONLY(asn\_DEF,~ptr); |
| \end{lyxcode} |
| These macros bear the same semantics as the \emph{free\_struct} function |
| invocation, discussed above. |
| |
| |
| \chapter{\label{cha:Step-by-step-examples}Step by step examples} |
| |
| |
| \section{A ``Rectangle'' Encoder} |
| |
| This example will help you create a simple BER and XER encoder of |
| a ``Rectangle'' type used throughout this document. |
| \begin{enumerate} |
| \item Create a file named \textbf{rectangle.asn1} with the following contents: |
| |
| \begin{lyxcode} |
| RectangleModule1~DEFINITIONS~::= |
| |
| BEGIN |
| |
| ~ |
| |
| Rectangle~::=~SEQUENCE~\{ |
| |
| ~~~~height~~INTEGER, |
| |
| ~~~~width~~~INTEGER |
| |
| \} |
| |
| ~ |
| |
| END |
| \end{lyxcode} |
| \item Compile it into the set of .c and .h files using asn1c compiler \cite{ASN1C}: |
| |
| \begin{lyxcode} |
| \emph{asn1c~-fnative-types}~\textbf{rectangle.asn1} |
| \end{lyxcode} |
| \item Alternatively, use the Online ASN.1 compiler \cite{AONL} by uploading |
| the \textbf{rectangle.asn1} file into the Web form and unpacking the |
| produced archive on your computer. |
| \item By this time, you should have gotten multiple files in the current |
| directory, including the \textbf{Rectangle.c} and \textbf{Rectangle.h}. |
| \item Create a main() routine which creates the Rectangle\_t structure in |
| memory and encodes it using BER and XER encoding rules. Let's name |
| the file \textbf{main.c}:\clearpage{} |
| |
| \begin{lyxcode} |
| {\small \#include~<stdio.h>}{\small \par} |
| |
| {\small \#include~<sys/types.h>}{\small \par} |
| |
| {\small \#include~<Rectangle.h>~~~/{*}~Rectangle~ASN.1~type~~{*}/}{\small \par} |
| |
| ~ |
| |
| {\small /{*}}{\small \par} |
| |
| {\small{}~{*}~This~is~a~custom~function~which~writes~the}{\small \par} |
| |
| {\small{}~{*}~encoded~output~into~some~FILE~stream.}{\small \par} |
| |
| {\small{}~{*}/}{\small \par} |
| |
| {\small static~int}{\small \par} |
| |
| {\small write\_out(const~void~{*}buffer,~size\_t~size,~void~{*}app\_key)~\{}{\small \par} |
| |
| {\small{}~~~~FILE~{*}out\_fp~=~app\_key;}{\small \par} |
| |
| {\small{}~~~~size\_t~wrote;}{\small \par} |
| |
| {\small{}~}{\small \par} |
| |
| {\small{}~~~~wrote~=~fwrite(buffer,~1,~size,~out\_fp);}{\small \par} |
| |
| {\small{}~}{\small \par} |
| |
| {\small{}~~~~return~(wrote~==~size)~?~0~:~-1;}{\small \par} |
| |
| {\small \}}{\small \par} |
| |
| ~ |
| |
| {\small int~main(int~ac,~char~{*}{*}av)~\{}{\small \par} |
| |
| {\small{}~~~~Rectangle\_t~{*}rectangle;~/{*}~Type~to~encode~~~~~~~~{*}/}{\small \par} |
| |
| {\small{}~~~~asn\_enc\_rval\_t~ec;~~~~~~/{*}~Encoder~return~value~~{*}/}{\small \par} |
| |
| {\small{}~}{\small \par} |
| |
| {\small{}~~~~/{*}~Allocate~the~Rectangle\_t~{*}/}{\small \par} |
| |
| {\small{}~~~~rectangle~=~calloc(1,~sizeof(Rectangle\_t));~/{*}~not~malloc!~{*}/}{\small \par} |
| |
| {\small{}~~~~if(!rectangle)~\{}{\small \par} |
| |
| {\small{}~~~~~~perror(''calloc()~failed'');}{\small \par} |
| |
| {\small{}~~~~~~exit(71);~/{*}~better,~EX\_OSERR~{*}/}{\small \par} |
| |
| {\small{}~~~~\}}{\small \par} |
| |
| {\small{}~}{\small \par} |
| |
| {\small{}~~~~/{*}~Initialize~the~Rectangle~members~{*}/}{\small \par} |
| |
| {\small{}~~~~rectangle->height~=~42;~~/{*}~any~random~value~{*}/}{\small \par} |
| |
| {\small{}~~~~rectangle->width~~=~23;~~/{*}~any~random~value~{*}/}{\small \par} |
| |
| {\small{}~~~~~}{\small \par} |
| |
| {\small{}~~~~/{*}~BER~encode~the~data~if~filename~is~given~{*}/}{\small \par} |
| |
| {\small{}~~~~if(ac~<~2)~\{}{\small \par} |
| |
| {\small{}~~~~~~fprintf(stderr,~''Specify~filename~for~BER~output\textbackslash{}n'');}{\small \par} |
| |
| {\small{}~~~~\}~else~\{}{\small \par} |
| |
| {\small{}~~~~~~const~char~{*}filename~=~av{[}1{]};}{\small \par} |
| |
| {\small{}~~~~~~FILE~{*}fp~=~fopen(filename,~''wb'');~~~/{*}~for~BER~output~{*}/}{\small \par} |
| |
| ~ |
| |
| {\small{}~~~~~~if(!fp)~\{}{\small \par} |
| |
| {\small{}~~~~~~~~perror(filename);}{\small \par} |
| |
| {\small{}~~~~~~~~exit(71);~/{*}~better,~EX\_OSERR~{*}/}{\small \par} |
| |
| {\small{}~~~~~~\}}{\small \par} |
| |
| {\small{}~~}{\small \par} |
| |
| {\small{}~~~~~~/{*}~Encode~the~Rectangle~type~as~BER~(DER)~{*}/}{\small \par} |
| |
| {\small{}~~~~~~ec~=~der\_encode(\&asn\_DEF\_Rectangle,}{\small \par} |
| |
| {\small{}~~~~~~~~~~~~rectangle,~write\_out,~fp);}{\small \par} |
| |
| {\small{}~~~~~~fclose(fp);}{\small \par} |
| |
| {\small{}~~~~~~if(ec.encoded~==~-1)~\{}{\small \par} |
| |
| {\small{}~~~~~~~~fprintf(stderr,}{\small \par} |
| |
| {\small{}~~~~~~~~~~''Could~not~encode~Rectangle~(at~\%s)\textbackslash{}n'',}{\small \par} |
| |
| {\small{}~~~~~~~~~~ec.failed\_type~?~ec.failed\_type->name~:~''unknown'');}{\small \par} |
| |
| {\small{}~~~~~~~~exit(65);~/{*}~better,~EX\_DATAERR~{*}/}{\small \par} |
| |
| {\small{}~~~~~~\}~else~\{}{\small \par} |
| |
| {\small{}~~~~~~~~fprintf(stderr,~''Created~\%s~with~BER~encoded~Rectangle\textbackslash{}n'',}{\small \par} |
| |
| {\small{}~~~~~~~~~~filename);}{\small \par} |
| |
| {\small{}~~~~~~\}}{\small \par} |
| |
| {\small{}~~~~\}}{\small \par} |
| |
| {\small{}~}{\small \par} |
| |
| {\small{}~~~~/{*}~Also~print~the~constructed~Rectangle~XER~encoded~(XML)~{*}/}{\small \par} |
| |
| {\small{}~~~~xer\_fprint(stdout,~\&asn\_DEF\_Rectangle,~rectangle);}{\small \par} |
| |
| {\small{}~}{\small \par} |
| |
| {\small{}~~~~return~0;~/{*}~Encoding~finished~successfully~{*}/}{\small \par} |
| |
| {\small \}}{\small \par} |
| \end{lyxcode} |
| \item Compile all files together using C compiler (varies by platform): |
| |
| \begin{lyxcode} |
| \emph{cc~-I.~-o}~\textbf{\emph{rencode}}~\emph{{*}.c} |
| \end{lyxcode} |
| \item Voila! You have just created the BER and XER encoder of a Rectangle |
| type, named \textbf{rencode}! |
| \end{enumerate} |
| \clearpage{} |
| |
| |
| \section{\label{sec:A-Rectangle-Decoder}A ``Rectangle'' Decoder} |
| |
| This example will help you to create a simple BER decoder of a simple |
| ``Rectangle'' type used throughout this document. |
| \begin{enumerate} |
| \item Create a file named \textbf{rectangle.asn1} with the following contents: |
| |
| \begin{lyxcode} |
| RectangleModule1~DEFINITIONS~::= |
| |
| BEGIN |
| |
| ~ |
| |
| Rectangle~::=~SEQUENCE~\{ |
| |
| ~~~~height~~INTEGER, |
| |
| ~~~~width~~~INTEGER |
| |
| \} |
| |
| ~ |
| |
| END |
| \end{lyxcode} |
| \item Compile it into the set of .c and .h files using asn1c compiler \cite{ASN1C}: |
| |
| \begin{lyxcode} |
| \emph{asn1c~-fnative-types}~\textbf{rectangle.asn1} |
| \end{lyxcode} |
| \item Alternatively, use the Online ASN.1 compiler \cite{AONL} by uploading |
| the \textbf{rectangle.asn1} file into the Web form and unpacking the |
| produced archive on your computer. |
| \item By this time, you should have gotten multiple files in the current |
| directory, including the \textbf{Rectangle.c} and \textbf{Rectangle.h}. |
| \item Create a main() routine which takes the binary input file, decodes |
| it as it were a BER-encoded Rectangle type, and prints out the text |
| (XML) representation of the Rectangle type. Let's name the file \textbf{main.c}:\clearpage{} |
| |
| \begin{lyxcode} |
| {\small \#include~<stdio.h>}{\small \par} |
| |
| {\small \#include~<sys/types.h>}{\small \par} |
| |
| {\small \#include~<Rectangle.h>~~~/{*}~Rectangle~ASN.1~type~~{*}/}{\small \par} |
| |
| {\small{}~}{\small \par} |
| |
| {\small int~main(int~ac,~char~{*}{*}av)~\{}{\small \par} |
| |
| {\small{}~~~~char~buf{[}1024{]};~~~~~~/{*}~Temporary~buffer~~~~~~{*}/}{\small \par} |
| |
| {\small{}~~~~Rectangle\_t~{*}rectangle~=~0;~/{*}~Type~to~decode~{*}/}{\small \par} |
| |
| {\small{}~~~~asn\_dec\_rval\_t~rval;~/{*}~Decoder~return~value~~{*}/}{\small \par} |
| |
| {\small{}~~~~FILE~{*}fp;~~~~~~~~~~~~/{*}~Input~file~handler~~~~{*}/}{\small \par} |
| |
| {\small{}~~~~size\_t~size;~~~~~~~~~/{*}~Number~of~bytes~read~~{*}/}{\small \par} |
| |
| {\small{}~~~~char~{*}filename;~~~~~~/{*}~Input~file~name~{*}/}{\small \par} |
| |
| {\small{}~}{\small \par} |
| |
| {\small{}~~~~/{*}~Require~a~single~filename~argument~{*}/}{\small \par} |
| |
| {\small{}~~~~if(ac~!=~2)~\{}{\small \par} |
| |
| {\small{}~~~~~~fprintf(stderr,~''Usage:~\%s~<file.ber>\textbackslash{}n'',~av{[}0{]});}{\small \par} |
| |
| {\small{}~~~~~~exit(64);~/{*}~better,~EX\_USAGE~{*}/}{\small \par} |
| |
| {\small{}~~~~\}~else~\{}{\small \par} |
| |
| {\small{}~~~~~~filename~=~av{[}1{]};}{\small \par} |
| |
| {\small{}~~~~\}}{\small \par} |
| |
| {\small{}~}{\small \par} |
| |
| {\small{}~~~~/{*}~Open~input~file~as~read-only~binary~{*}/}{\small \par} |
| |
| {\small{}~~~~fp~=~fopen(filename,~''rb'');}{\small \par} |
| |
| {\small{}~~~~if(!fp)~\{}{\small \par} |
| |
| {\small{}~~~~~~perror(filename);}{\small \par} |
| |
| {\small{}~~~~~~exit(66);~/{*}~better,~EX\_NOINPUT~{*}/}{\small \par} |
| |
| {\small{}~~~~\}}{\small \par} |
| |
| {\small{}~~}{\small \par} |
| |
| {\small{}~~~~/{*}~Read~up~to~the~buffer~size~{*}/}{\small \par} |
| |
| {\small{}~~~~size~=~fread(buf,~1,~sizeof(buf),~fp);}{\small \par} |
| |
| {\small{}~~~~fclose(fp);}{\small \par} |
| |
| {\small{}~~~~if(!size)~\{}{\small \par} |
| |
| {\small{}~~~~~~fprintf(stderr,~''\%s:~Empty~or~broken\textbackslash{}n'',~filename);}{\small \par} |
| |
| {\small{}~~~~~~exit(65);~/{*}~better,~EX\_DATAERR~{*}/}{\small \par} |
| |
| {\small{}~~~~\}}{\small \par} |
| |
| {\small{}~}{\small \par} |
| |
| {\small{}~~~~/{*}~Decode~the~input~buffer~as~Rectangle~type~{*}/}{\small \par} |
| |
| {\small{}~~~~rval~=~ber\_decode(0,~\&asn\_DEF\_Rectangle,}{\small \par} |
| |
| {\small{}~~~~~~(void~{*}{*})\&rectangle,~buf,~size);}{\small \par} |
| |
| {\small{}~~~~if(rval.code~!=~RC\_OK)~\{}{\small \par} |
| |
| {\small{}~~~~~~fprintf(stderr,}{\small \par} |
| |
| {\small{}~~~~~~~~''\%s:~Broken~Rectangle~encoding~at~byte~\%ld\textbackslash{}n'',}{\small \par} |
| |
| {\small{}~~~~~~~~filename,~(long)rval.consumed);}{\small \par} |
| |
| {\small{}~~~~~~exit(65);~/{*}~better,~EX\_DATAERR~{*}/}{\small \par} |
| |
| {\small{}~~~~\}}{\small \par} |
| |
| {\small{}~}{\small \par} |
| |
| {\small{}~~~~/{*}~Print~the~decoded~Rectangle~type~as~XML~{*}/}{\small \par} |
| |
| {\small{}~~~~xer\_fprint(stdout,~\&asn\_DEF\_Rectangle,~rectangle);}{\small \par} |
| |
| {\small{}~}{\small \par} |
| |
| {\small{}~~~~return~0;~/{*}~Decoding~finished~successfully~{*}/}{\small \par} |
| |
| {\small \}}{\small \par} |
| \end{lyxcode} |
| \item Compile all files together using C compiler (varies by platform): |
| |
| \begin{lyxcode} |
| \emph{cc~-I.~-o}~\textbf{\emph{rdecode}}~\emph{{*}.c} |
| \end{lyxcode} |
| \item Voila! You have just created the BER decoder of a Rectangle type, |
| named \textbf{rdecode}! |
| \end{enumerate} |
| |
| \chapter{Constraint validation examples} |
| |
| This chapter shows how to define ASN.1 constraints and use the generated |
| validation code. |
| |
| |
| \section{Adding constraints into ``Rectangle'' type} |
| |
| This example shows how to add basic constraints to the ASN.1 specification |
| and how to invoke the constraints validation code in your application. |
| \begin{enumerate} |
| \item Create a file named \textbf{rectangle.asn1} with the following contents: |
| |
| \begin{lyxcode} |
| RectangleModuleWithConstraints~DEFINITIONS~::= |
| |
| BEGIN |
| |
| ~ |
| |
| Rectangle~::=~SEQUENCE~\{ |
| |
| ~~~~height~~INTEGER~(0..100),~-{}-~Value~range~constraint |
| |
| ~~~~width~~~INTEGER~(0..MAX)~~-{}-~Makes~width~non-negative~ |
| |
| \} |
| |
| ~ |
| |
| END |
| \end{lyxcode} |
| \item Compile the file according to procedures shown in the previous chapter. |
| \item Modify the Rectangle type processing routine (you can start with the |
| main() routine shown in the Section \vref{sec:A-Rectangle-Decoder}) |
| by placing the following snippet of code \emph{before} encoding and/or |
| \emph{after} decoding the Rectangle type% |
| \footnote{Placing the constraint checking code \emph{before} encoding helps |
| to make sure you know the data is correct and within constraints before |
| sharing the data with anyone else. |
| |
| Placing the constraint checking code \emph{after} decoding, but before |
| any further action depending on the decoded data, helps to make sure |
| the application got the valid contents before making use of it.% |
| }:\clearpage{} |
| |
| \begin{lyxcode} |
| {\small int~ret;~~~~~~~~~~~/{*}~Return~value~{*}/}{\small \par} |
| |
| {\small char~errbuf{[}128{]};~~/{*}~Buffer~for~error~message~{*}/}{\small \par} |
| |
| {\small size\_t~errlen~=~sizeof(errbuf);~~/{*}~Size~of~the~buffer~{*}/}{\small \par} |
| |
| {\small{}~~}{\small \par} |
| |
| {\small /{*}~...~here~may~go~Rectangle~decoding~code~...~{*}/}{\small \par} |
| |
| {\small{}~}{\small \par} |
| |
| {\small ret~=~asn\_check\_constraints(\&asn\_DEF\_Rectangle,}{\small \par} |
| |
| {\small{}~~~~~~~~rectangle,~errbuf,~\&errlen);}{\small \par} |
| |
| {\small /{*}~assert(errlen~<~sizeof(errbuf));~//~you~may~rely~on~that~{*}/}{\small \par} |
| |
| {\small if(ret)~\{}{\small \par} |
| |
| {\small{}~~~~~~~~fprintf(stderr,~''Constraint~validation~failed:~\%s\textbackslash{}n'',}{\small \par} |
| |
| {\small{}~~~~~~~~~~errbuf~~~/{*}~errbuf~is~properly~nul-terminated~{*}/}{\small \par} |
| |
| {\small{}~~~~~~~~);}{\small \par} |
| |
| {\small{}~~~~~~~~/{*}~exit(...);~//~Replace~with~appropriate~action~{*}/}{\small \par} |
| |
| {\small \}}{\small \par} |
| |
| {\small{}~}{\small \par} |
| |
| {\small /{*}~...~here~may~go~Rectangle~encoding~code~...~{*}/}{\small \par} |
| \end{lyxcode} |
| \item Compile the resulting C code as shown in the previous chapters. |
| \item Try to test the constraints checking code by assigning integer value |
| 101 to the \textbf{.height} member of the Rectangle structure, or |
| a negative value to the \textbf{.width} member. In either case, the |
| program should print ``Constraint validation failed'' message, followed |
| by the short explanation why validation did not succeed. |
| \item Done. |
| \end{enumerate} |
| |
| \part{\label{par:ASN.1-Basics}ASN.1 Basics} |
| |
| |
| \chapter{\label{cha:Abstract-Syntax-Notation:}Abstract Syntax Notation: ASN.1} |
| |
| \emph{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 \cite{Dub00} or the ASN.1 body |
| of standards itself \cite{ITU-T/ASN.1}.} |
| |
| 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 are |
| used to achieve the following: |
| \begin{itemize} |
| \item The specification expressed in the ASN.1 notation is a formal and |
| precise way to communicate the data semantics to human readers; |
| \item 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). |
| \end{itemize} |
| Consider the following example: |
| \begin{lyxcode} |
| Rectangle~::=~SEQUENCE~\{ |
| |
| ~~~~height~~INTEGER, |
| |
| ~~~~width~~~INTEGER |
| |
| \} |
| \end{lyxcode} |
| This ASN.1 specification describes a constructed type, \emph{Rectangle}, |
| containing two integer fields. This specification may tell the reader |
| that there exists this kind of data structure and that some entity |
| may be prepared to send or receive it. The question on \emph{how} |
| that entity is going to send or receive the \emph{encoded data} 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, PER, XER |
| and others, including CER and DER derivatives from BER. |
| |
| The complete specification must be wrapped in a module, which looks |
| like this: |
| \begin{lyxcode} |
| RectangleModule1 |
| |
| ~~~~\{~iso~org(3)~dod(6)~internet(1)~private(4) |
| |
| ~~~~~~enterprise(1)~spelio(9363)~software(1) |
| |
| ~~~~~~asn1c(5)~docs(2)~rectangle(1)~1~\}~ |
| |
| ~~~~DEFINITIONS~AUTOMATIC~TAGS~::= |
| |
| BEGIN |
| |
| ~ |
| |
| -{}-~This~is~a~comment~which~describes~nothing. |
| |
| Rectangle~::=~SEQUENCE~\{ |
| |
| ~~~~height~~INTEGER,~~~~~~~~-{}-~Height~of~the~rectangle |
| |
| ~~~~width~~~INTEGER~~~~~~~~~-{}-~Width~of~the~rectangle |
| |
| \} |
| |
| ~ |
| |
| END |
| \end{lyxcode} |
| The module header consists of module name (RectangleModule1), the |
| module object identifier (\{...\}), a keyword ``DEFINITIONS'', a |
| set of module flags (AUTOMATIC TAGS) and ``::= BEGIN''. The module |
| ends with an ``END'' statement. |
| |
| |
| \section{Some of the ASN.1 Basic Types} |
| |
| |
| \subsection{The BOOLEAN type} |
| |
| The BOOLEAN type models the simple binary TRUE/FALSE, YES/NO, ON/OFF |
| or a similar kind of two-way choice. |
| |
| |
| \subsection{The INTEGER type} |
| |
| 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. |
| \begin{lyxcode} |
| SimpleInteger~::=~INTEGER |
| |
| ~ |
| |
| -{}-~An~integer~with~a~very~limited~range |
| |
| SmallPositiveInt~::=~INTEGER~(0..127) |
| |
| ~ |
| |
| -{}-~Integer,~negative |
| |
| NegativeInt~::=~INTEGER~(MIN..0) |
| \end{lyxcode} |
| |
| \subsection{The ENUMERATED type} |
| |
| The ENUMERATED type is semantically equivalent to the INTEGER type |
| with some integer values explicitly named. |
| \begin{lyxcode} |
| FruitId~::=~ENUMERATED~\{~apple(1),~orange(2)~\} |
| |
| ~ |
| |
| -{}-~The~numbers~in~braces~are~optional, |
| |
| -{}-~the~enumeration~can~be~performed |
| |
| -{}-~automatically~by~the~compiler |
| |
| ComputerOSType~::=~ENUMERATED~\{ |
| |
| ~~~~FreeBSD,~~~~~~~~~~-{}-~acquires~value~0 |
| |
| ~~~~Windows,~~~~~~~~~~-{}-~acquires~value~1 |
| |
| ~~~~Solaris(5),~~~~~~~-{}-~remains~5 |
| |
| ~~~~Linux,~~~~~~~~~~~~-{}-~becomes~6 |
| |
| ~~~~MacOS~~~~~~~~~~~~~-{}-~becomes~7 |
| |
| \} |
| \end{lyxcode} |
| |
| \subsection{The OCTET STRING type} |
| |
| 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). |
| |
| |
| \subsection{The OBJECT IDENTIFIER type} |
| |
| 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 \url{http://www.iana.org/protocols/forms.htm}. |
| |
| For example, the very first ASN.1 module in this Chapter (RectangleModule1) |
| has the following OBJECT IDENTIFIER: 1 3 6 1 4 1 9363 1 5 2 1 1. |
| \begin{lyxcode} |
| ExampleOID~::=~OBJECT~IDENTIFIER |
| |
| ~ |
| |
| rectangleModule1-oid~ExampleOID |
| |
| ~~::=~\{~1~3~6~1~4~1~9363~1~5~2~1~1~\} |
| |
| ~ |
| |
| -{}-~An~identifier~of~the~Internet. |
| |
| internet-id~OBJECT~IDENTIFIER |
| |
| ~~::=~\{~iso(1)~identified-organization(3) |
| |
| ~~~~~~~~dod(6)~internet(1)~\} |
| \end{lyxcode} |
| As you see, names are optional. |
| |
| |
| \subsection{The RELATIVE-OID type} |
| |
| 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. |
| \begin{lyxcode} |
| this-document~RELATIVE-OID~::=~\{~docs(2)~usage(1)~\} |
| |
| ~ |
| |
| this-example~RELATIVE-OID~::=~\{ |
| |
| ~~~~this-document~assorted-examples(0)~this-example(1)~\} |
| \end{lyxcode} |
| |
| \section{Some of the ASN.1 String Types} |
| |
| |
| \subsection{The IA5String type} |
| |
| This is essentially the ASCII, with 128 character codes available |
| (7 lower bits of an 8-bit byte). |
| |
| |
| \subsection{The UTF8String type} |
| |
| This is the character string which encodes the full Unicode range |
| (4 bytes) using multibyte character sequences. |
| |
| |
| \subsection{The NumericString type} |
| |
| This type represents the character string with the alphabet consisting |
| of numbers (``0'' to ``9'') and a space. |
| |
| |
| \subsection{The PrintableString type} |
| |
| The character string with the following alphabet: space, ``\textbf{'}'' |
| (single quote), ``\textbf{(}'', ``\textbf{)}'', ``\textbf{+}'', |
| ``\textbf{,}'' (comma), ``\textbf{-}'', ``\textbf{.}'', ``\textbf{/}'', |
| digits (``0'' to ``9''), ``\textbf{:}'', ``\textbf{=}'', ``\textbf{?}'', |
| upper-case and lower-case letters (``A'' to ``Z'' and ``a'' |
| to ``z''). |
| |
| |
| \subsection{The VisibleString type} |
| |
| The character string with the alphabet which is more or less a subset |
| of ASCII between the space and the ``\textbf{\textasciitilde{}}'' |
| symbol (tilde). |
| |
| Alternatively, the alphabet may be described as the PrintableString |
| alphabet presented earlier, plus the following characters: ``\textbf{!}'', |
| ``\textbf{``}'', ``\textbf{\#}'', ``\textbf{\$}'', ``\textbf{\%}'', |
| ``\textbf{\&}'', ``\textbf{{*}}'', ``\textbf{;}'', ``\textbf{<}'', |
| ``\textbf{>}'', ``\textbf{{[}}'', ``\textbf{\textbackslash{}}'', |
| ``\textbf{{]}}'', ``\textbf{\textasciicircum{}}'', ``\textbf{\_}'', |
| ``\textbf{`}`` (single left quote), ``\textbf{\{}'', ``\textbf{|}'', |
| ``\textbf{\}}'', ``\textbf{\textasciitilde{}}''. |
| |
| |
| \section{ASN.1 Constructed Types} |
| |
| |
| \subsection{The SEQUENCE type} |
| |
| This is an ordered collection of other simple or constructed types. |
| The SEQUENCE constructed type resembles the C ``struct'' statement. |
| \begin{lyxcode} |
| Address~::=~SEQUENCE~\{ |
| |
| ~~~~-{}-~The~apartment~number~may~be~omitted |
| |
| ~~~~apartmentNumber~~~~~~NumericString~OPTIONAL, |
| |
| ~~~~streetName~~~~~~~~~~~PrintableString, |
| |
| ~~~~cityName~~~~~~~~~~~~~PrintableString, |
| |
| ~~~~stateName~~~~~~~~~~~~PrintableString, |
| |
| ~~~~-{}-~This~one~may~be~omitted~too |
| |
| ~~~~zipNo~~~~~~~~~~~~~~~~NumericString~OPTIONAL |
| |
| \} |
| \end{lyxcode} |
| |
| \subsection{The SET type} |
| |
| 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. |
| |
| |
| \subsection{The CHOICE type} |
| |
| 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. |
| |
| The following type defines a response code, which may be either an |
| integer code or a boolean ``true''/``false'' code. |
| \begin{lyxcode} |
| ResponseCode~::=~CHOICE~\{ |
| |
| ~~~~intCode~~~~INTEGER, |
| |
| ~~~~boolCode~~~BOOLEAN |
| |
| \} |
| |
| |
| \end{lyxcode} |
| |
| \subsection{The SEQUENCE OF type} |
| |
| This one is the list (array) of simple or constructed types: |
| \begin{lyxcode} |
| -{}-~Example~1 |
| |
| ManyIntegers~::=~SEQUENCE~OF~INTEGER |
| |
| ~ |
| |
| -{}-~Example~2 |
| |
| ManyRectangles~::=~SEQUENCE~OF~Rectangle |
| |
| ~ |
| |
| -{}-~More~complex~example: |
| |
| -{}-~an~array~of~structures~defined~in~place. |
| |
| ManyCircles~::=~SEQUENCE~OF~SEQUENCE~\{ |
| |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~radius~INTEGER |
| |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~\} |
| \end{lyxcode} |
| |
| \subsection{The SET OF type} |
| |
| 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. |
| \begin{lyxcode} |
| -{}-~A~set~of~structures~defined~elsewhere |
| |
| SetOfApples~::~SET~OF~Apple |
| |
| ~ |
| |
| -{}-~Set~of~integers~encoding~the~kind~of~a~fruit |
| |
| FruitBag~::=~SET~OF~ENUMERATED~\{~apple,~orange~\}\end{lyxcode} |
| \begin{thebibliography}{ITU-T/ASN.1} |
| \bibitem[ASN1C]{ASN1C}The Open Source ASN.1 Compiler. \url{http://lionet.info/asn1c} |
| |
| \bibitem[AONL]{AONL}Online ASN.1 Compiler. \url{http://lionet.info/asn1c/asn1c.cgi} |
| |
| \bibitem[Dub00]{Dub00}Olivier Dubuisson --- \emph{ASN.1 Communication |
| between heterogeneous systems}~---~Morgan Kaufmann Publishers, 2000. |
| \url{http://asn1.elibel.tm.fr/en/book/}. ISBN:0-12-6333361-0. |
| |
| \bibitem[ITU-T/ASN.1]{ITU-T/ASN.1}ITU-T Study Group 17 --- Languages |
| for Telecommunication Systems \url{http://www.itu.int/ITU-T/studygroups/com17/languages/} |
| \end{thebibliography} |
| |
| \end{document} |