asn1ct

asn1ct

asn1ct
ASN.1 compiler and compile-time support functions

The ASN.1 compiler takes an ASN.1 module as input and generates a corresponding Erlang module, which can encode and decode the specified data types. Alternatively, the compiler takes a specification module specifying all input modules, and generates a module with encode/decode functions. In addition, some generic functions can be used during development of applications that handles ASN.1 data (encoded as BER or PER).

Note

By default in OTP 17, the representation of the BIT STRING and OCTET STRING types as Erlang terms were changed. BIT STRING values are now Erlang bit strings and OCTET STRING values are binaries. Also, an undecoded open type is now wrapped in an asn1_OPENTYPE tuple. For details, see BIT STRING, OCTET STRING, and ASN.1 Information Objects in the User's Guide.

To revert to the old representation of the types, use option legacy_erlang_types.

Note

In OTP R16, the options were simplified. The back end is chosen using one of the options ber, per, uper or jer. Options optimize, nif, and driver options are no longer necessary (and the ASN.1 compiler generates a warning if they are used). Options ber_bin, per_bin, and uper_bin options still work, but generates a warning.

Another change in OTP R16 is that the generated function encode/2 always returns a binary. Function encode/2 for the BER back end used to return an iolist.

Types

Asn1module = atom() | string()
Options = [Option| OldOption]
Option = ber | per | uper | jer | der | compact_bit_string | legacy_bit_string | legacy_erlang_types | noobj | {n2n, EnumTypeName} |{outdir, Dir} | {i, IncludeDir} | asn1config | undec_rest | no_ok_wrapper | {macro_name_prefix, Prefix} | {record_name_prefix, Prefix} | verbose | warnings_as_errors | deterministic
OldOption = ber | per
Reason = term()
Prefix = string()

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 that 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). Asn1module can be a full pathname (relative or absolute) including filename with (or without) extension.

If it is needed to compile a set of ASN.1 modules into an Erlang file with encode/decode functions, ensure 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"). List the input file names within quotation marks (""), one at each row in the file. If the input files are File1.asn, File2.asn, and File3.asn, the configuration file must look as follows:

File1.asn
File2.asn
File3.asn

The output files in this case get their names from the configuration file. If the configuration file is named SetOfFiles.set.asn, the names of the output files are SetOfFiles.hrl, SetOfFiles.erl, and SetOfFiles.asn1db.

Sometimes in a system of ASN.1 modules, different default tag modes, for example, AUTOMATIC, IMPLICIT, or EXPLICIT. The multi-file compilation resolves the default tagging as if the modules were compiled separately.

Name collisions is another unwanted effect that can occur in multi file-compilation. The compiler solves this problem in one of two ways:

  • If the definitions are identical, the output module keeps only one definition with the original name.
  • If the definitions have the same name and differs in the definition, they are renamed. The new names are the definition name and the original module name concatenated.

If a name collision occurs, 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 ASN.1 compiler and options that are applied to the Erlang compiler. The latter are not recognized as ASN.1 specific. The available options are as follows:

ber | per | uper | jer

The encoding rule to be used. The supported encoding rules are Basic Encoding Rules (ber), Packed Encoding Rules (per) aligned, PER unaligned (uper) and JSON Encoding Rules (jer). The jer option can be used by itself to generate a module that only supports encoding/decoding to JER or it can be used as a supplementary option to ber, per and uper. In the latter case a module with for both the main encoding rules and JER will be generated. The exported functions for JER will then be jer_encode(Type, Value) and jer_decode(Type, Bytes).

The jer encoding rules (ITU-T X.697) are experimental in OTP 22. There is support for a subset of the X.697 standard, for example there is no support for:

  • JER encoding instructions
  • the REAL type

Also note that when using the jer encoding rules the generated module will get a dependency to an external json component. The generated code is currently tested together with:

  • jsx which currently is the default.
  • jsone can be chosen instead of jsx by providing the option {d,jsone}.

If the encoding rule option is omitted, ber is the default.

The generated Erlang module always gets the same name as the ASN.1 module. Therefore, only one encoding rule per ASN.1 module can be used at runtime.

der

With this option the Distinguished Encoding Rules (der) is chosen. DER is regarded as a specialized variant of the BER encoding rule. Therefore, this option only makes sense together with option ber. This option sometimes adds sorting and value checks when encoding, which implies a slower encoding. The decoding routines are the same as for ber.

maps

This option changes the representation of the types SEQUENCE and SET to use maps (instead of records). This option also suppresses the generation of .hrl files.

For details, see Section Map representation for SEQUENCE and SET in the User's Guide.

compact_bit_string

The BIT STRING type is decoded to "compact notation". This option is not recommended for new code. This option cannot be combined with the option maps.

For details, see Section BIT STRING in the User's Guide.

This option implies option legacy_erlang_types.

legacy_bit_string

The BIT STRING type is decoded to the legacy format, that is, a list of zeroes and ones. This option is not recommended for new code. This option cannot be combined with the option maps.

For details, see Section BIT STRING in the User's Guide

This option implies option legacy_erlang_types.

legacy_erlang_types

Use the same Erlang types to represent BIT STRING and OCTET STRING as in OTP R16.

For details, see Section BIT STRING and Section OCTET STRING in the User's Guide.

This option is not recommended for new code. This option cannot be combined with the option maps.

{n2n, EnumTypeName}

Tells the compiler to generate functions for conversion between names (as atoms) and numbers and conversely for the specified EnumTypeName. There can be multiple occurrences of this option to specify several type names. The type names must be declared as ENUMERATIONS in the ASN.1 specification.

If EnumTypeName does not exist in the ASN.1 specification, the compilation stops with an error code.

The generated conversion functions are named name2num_EnumTypeName/1 and num2name_EnumTypeName/1.

noobj

Do not compile (that is, do not produce object code) the generated .erl file. If this option is omitted, the generated Erlang module is compiled.

{i, IncludeDir}

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

{outdir, Dir}

Specifies directory Dir where all generated files are to be placed. If this option is omitted, the files are placed in the current directory.

asn1config

When using one of the specialized decodes, exclusive or selective decode, instructions must be given in a configuration file. Option asn1config enables specialized decodes and takes the configuration file in concern. The configuration file has the same name as the ASN.1 specification, but with extension .asn1config.

For instructions for exclusive decode, see Section Exclusive Decode in the User's Guide.

For instructions for selective decode, see Section Selective Decode in the User's Guide.

undec_rest

A buffer that holds a message, being decoded it can also have some following bytes. Those following bytes can now be returned together with the decoded value. If an ASN.1 specification is compiled with this option, a tuple {ok, Value, Rest} is returned. Rest can be a list or a binary. Earlier versions of the compiler ignored those following bytes.

no_ok_wrapper

With this option, the generated encode/2 and decode/2 functions do not wrap a successful return value in an {ok,...} tuple. If any error occurs, an exception will be raised.

{macro_name_prefix, Prefix}

All macro names generated by the compiler are prefixed with Prefix. This is useful when multiple protocols that contain macros with identical names are included in a single module.

{record_name_prefix, Prefix}

All record names generated by the compiler are prefixed with Prefix. This is useful when multiple protocols that contain records with identical names are included in a single module.

verbose

Causes more verbose information from the compiler describing what it is doing.

warnings_as_errors

Causes warnings to be treated as errors.

deterministic

Causes all non-deterministic options to be stripped from the -asn1_info() attribute.

Any more option that is applied is passed to the final step when the generated .erl file is compiled.

The compiler generates the following files:

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

Types

Module = Type = atom()
Value = term()
Reason = term()

Returns an Erlang term that 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.

Note

Currently, the value function has many limitations. Essentially, it will mostly work for old specifications based on the 1997 standard for ASN.1, but not for most modern-style applications. Another limitation is that the value function may not work if options that change code generations strategies such as the options macro_name_prefix and record_name_prefix have been used.

Types

Module = Type = atom()
Value = term()
Options = [{i, IncludeDir}]
Reason = term()

Performs a test of encode and decode of 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 as well as the general runtime support work as expected.

Note

Currently, the test functions have many limitations. Essentially, they will mostly work for old specifications based on the 1997 standard for ASN.1, but not for most modern-style applications. Another limitation is that the test functions may not work if options that change code generations strategies such as the options macro_name_prefix and record_name_prefix have been used.

  • test/1 iterates over all types in Module.

  • test/2 tests type Type with a random value.

  • test/3 tests type Type with Value.

Schematically, the following occurs for each type in the module:

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

The test functions use the *.asn1db files for all included modules. If they are located in a different directory than the current working directory, use the include option to add paths. This is only needed when automatically generating values. For static values using Value no options are needed.

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