asn1ct

asn1ct

ASN.1 compiler and compile-time support functions

The ASN.1 compiler takes an ASN.1 module as input and genarates a corresponding Erlang module which can encode and decode the datatypes specified. Alternatively the compiler takes a specification module (se below) specifying all input modules and generates one module with encode/decode functions. There are also some generic functions which can be used in during development of applications which handles ASN.1 data (encoded as BER or PER).

Types:

Compiles the ASN.1 module Asn1module and generates an Erlang module Asn1module.erl with encode and decode functions for the types defined in Asn1module. For each ASN.1 value defined in the module an Erlang function which returns the value in Erlang representation is generated.

If Asn1module is a filename without extension first ".asn1" is assumed, then ".asn" and finally ".py" (to be compatible with the old ASN.1 compiler). Of course Asn1module can be a full pathname (relative or absolute) including filename with (or without) extension.

If one wishes to compile a set of Asn1 modules into one Erlang file with encode/decode functions one has to list all involved files in a configuration file. This configuration file must have a double extension ".set.asn", (".asn" can alternatively be ".asn1" or ".py"). The input files' names must be listed, within qoutation marks (""), one at each row in the file. If the input files are File1.asn, File2.asn and File3.asn the configuration file shall look like:

File1.asn
File2.asn
File3.asn

The output files will in this case get their names from the configuration file. If the configuration file has the name SetOfFiles.set.asn the name of the output files will be SetOfFiles.hrl, SetOfFiles.hrl and SetOfFiles.asn1db.

Sometimes in a system of ASN.1 modules there are different default tag modes, e.g. AUTOMATIC, IMPLICIT or EXPLICIT. The multi file compilation resolves the default tagging as if the modules were compiled separetely.

Another unwanted effect that may occure in multi file compilation is name collisions. The compiler solves this problem in two ways: If the definitions are identical then the output module keeps only one definition with the original name. But if definitions only have same name and differs in the definition, then they will be renamed. The new names will be the definition name and the original module name concatenated.

If any name collision have occured the compiler reports a "NOTICE: ..." message that tells if a definition was renamed, and the new name that must be used to encode/decode data.

Options is a list with options specific for the asn1 compiler and options that are applied to the Erlang compiler. The latter are those that not is recognized as asn1 specific. Available options are:

ber | ber_bin | per | per_bin

The encoding rule to be used. EncodingRule is ber or per. If this option is omitted ber is the default. The per option means the aligned variant, the unaligned variant of PER is not supported in this version of the compiler. The generated Erlang module always gets the same name as the ASN.1 module and as a consequence of this only one encoding rule per ASN.1 module can be used at runtime.
The ber_bin and per_bin options are equivalent with the ber and per options with the difference that the generated encoding/decoding functions take advantage of the bit syntax, which in most cases increases the performance considerably. The result from encoding is a list (mayby nested) with Erlang terms, including binaries. Note that the erlang virtual machine that will execute the generated code must be of version R7 or higher.

der

By this option the Distinguished Encoding Rule (DER) is chosed. DER is regarded as a specialized variant of the BER encoding rule, therefore the der option only makes sense when the ber or ber_bin option is used. This option sometimes adds sorting and value checks when encoding, which implies a slower encoding. The decoding rutines are the same as for ber.

compact_bit_string

Makes it possible to use a compact notation for values of the BIT STRING type in Erlang. The notation:

BitString = {Unused,Binary},
Unused = integer(),
Binary = binary()

Unused must be a number in the range 0 to 7. It tells how many bits in the least significant byte in Binary that is unused. For details see BIT STRING type section in users guide.

{i,IncludeDir}

Adds IncludeDir to the search-path for .asn1db and asn1 source files. The compiler tries to open a .asn1db file when a module imports definitions from another ASN.1 module. If no .asn1db file is found the asn1 source file is parsed. Several {i,IncludeDir} can be given.

noobj

Do not compile (i.e do not produce object code) the generated .erl file. If this option is omitted the generated Erlang module will be compiled.

{out_dir,Dir}

Specifies the directory Dir where all generated files shall be placed. If omitted the files are placed in the current directory.

optimize

This option is only valid together with one of the per_bin or ber_bin option. It gives time optimized code generated and it uses another runtime module and in the per_bin case a linked-in driver. The result in the per_bin case from an encode when compiled with this option will be a binary.

driver

Option valid together with ber_bin and optimize options. It enables the use of a linked-in driver that gives considerable faster decode. In ber_bin the driver is enabled only by explicit use of the option driver.

asn1config

When one of the specialized decodes, exclusive or selective decode, is wanted one has to give instructions in a configuration file. The option asn1config enables specialized decodes and takes the configuration file, which has the same name as the ASN.1 spec but with extension .asn1config, in concern.
The instructions for exclusive decode must follow the instruction and grammar in the User's Guide.
You can also find the instructions for selctive decode in the User's Guide.

undec_rest

A buffer that holds a message, beeing decoded may also have some following bytes. Now it is possible to get those following bytes returned together with the decoded value. If an asn1 spec is compiled with this option a tuple {ok,Value,Rest} is returned. Rest may be a list or a binary. Earlier versions of the compiler ignored those following bytes.

{inline,OutputName}

Compiling with this option gives one output module containing all asn1 run-time functionality. The asn1 specs are provided in a target module Module.set.asn as described above. The name of the resulting module containing generated encode/decode functions and inlined run-time functions will be OutputName.erl. The merging/inlining of code is done by the igor module of syntax_tools. By default the functions generated from the first asn1 spec in the .set.asn are exported, unless a {export,[atom()]} or {export_all,true} option are provided. The list of atoms are names of choosen asn1 specs from the .set.asn file.

Any additional option that is applied will be passed to the final step when the Erlang compiler is invoked.

The compiler generates the following files:

• Asn1module.hrl (if any SET or SEQUENCE is defined)
  
• Asn1module.erl the Erlang module with encode, decode and value functions.
  
• Asn1module.asn1db intermediate format used by the compiler when modules IMPORTS definitions from each other.
  

Types:

Encodes Value of Type defined in the ASN.1 module Module. Returns a list of bytes if successful. To get as fast execution as possible the encode function only performs rudimentary tests that the input Value is a correct instance of Type. The length of strings is for example not always checked. Returns {ok,Bytes} if successful or {error,Reason} if an error occured.

Types:

Decodes Type from Module from the list of bytes Bytes. Returns {ok,Value} if successful.

Types:

Validates that Value conforms to Type from Module. Not implemented in this version of the ASN.1 application.

Types:

Returns an Erlang term which is an example of a valid Erlang representation of a value of the ASN.1 type Type. The value is a random value and subsequent calls to this function will for most types return different values.

Performs a test of encode and decode of all types in Module. The generated functions are called by this function. This function is useful during test to secure that the generated encode and decode functions and the general runtime support work as expected.
test/1 iterates over all types in Module.
test/2 tests type Type with a random value.
test/3 tests type <c>Type with Value.
Schematically the following happens for each type in the module.

{ok,Value} = asn1ct:value(Module,Type),
{ok,Bytes} = asn1ct:encode(Module,Type,Value),
{ok,Value} = asn1:decode(Module,Type,Bytes). 

Kenneth Lundin - support@erlang.ericsson.se
Bertil Karlsson - support@erlang.ericsson.se