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# asn1ct (asn1 v5.4.2)

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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 Erlang/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`.

# Summary

## Functions

compile(Asn1Module)

Equivalent to `compile(Asn1Module, [])`.

compile(Asn1Module, Options)

Compiles the ASN.1 module `Asn1Module` and generates an Erlang module `Asn1Module.erl` with encode and decode functions for all types defined in the ASN.1 module.

test(Module)

Tests encoding and decoding of all types in `Module`.

test/2

Tests encoding and decoding of `Module`.

test/3

Performs a test of encode and decode of types in `Module`.

value(Module, Type)

Returns an Erlang term that is an example of a valid Erlang representation of a value of the ASN.1 type `Type`.

# Functions

# compile(Asn1Module)

    -spec compile(Asn1Module) -> ok | {error, Reason} when Asn1Module :: atom() | string(), Reason :: term().

Equivalent to `compile(Asn1Module, [])`.

# compile(Asn1Module, Options)

    -spec compile(Asn1Module, Options) -> ok | {error, Reason}
                     when
                         Asn1Module :: atom() | string(),
                         Options :: [Option | OldOption],
                         Option ::
                             ber | per | uper | jer | der | compact_bit_string | legacy_bit_string |
                             legacy_erlang_types | noobj |
                             {n2n, EnumTypeName :: term()} |
                             {outdir, Dir :: term()} |
                             {i, IncludeDir :: term()} |
                             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 all types defined in the ASN.1 module.

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, list all involved files in a configuration file, one line per file. This configuration file must have a double extension `".set.asn1"` (`".asn1"` can alternatively be `".asn"` or `".py"`). If the input files are `File1.asn1`, `File2.asn1`, and `File3.asn1`, the configuration file should look as follows:

    File1.asn1
    File2.asn1
    File3.asn1

The output files in this case get their names from the configuration file. If the configuration file is named `SetOfFiles.set.asn1`, the names of the output files are `SetOfFiles.hrl, SetOfFiles.erl, and SetOfFiles.asn1db`.

Sometimes in a system of `ASN.1` modules, different modules can have different default tag modes, for example, one uses `AUTOMATIC` and another `IMPLICIT`. The multi-file compilation resolves the default tagging as if the modules were compiled separately.

Name collisions is an 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 ASN.1 compiler passes on any unrecognized options to the Erlang compiler. 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 of JER, or it can be used as a supplementary option to `ber`, `per`, and `uper`, in which case a module that handles both the main encoding rules and JER will be generated. In that case, the exported functions for JER will be `jer_encode(Type, Value)` and `jer_decode(Type, Bytes)`.

  JER (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

  #### Change

  In Erlang/OTP 27 and later, module `json` in STDLIB is used for encoding and decoding JSON. Before Erlang/OTP 27, it was necessary to provide an external JSON library.

  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 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.

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

  This option implies option `legacy_erlang_types`, and it cannot be combined with option `maps`.

- `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.

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

  This option implies option `legacy_erlang_types`, and it cannot be combined with option `maps`.

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

  This option is not recommended for new code.

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

  This option cannot be combined with 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` - By default when decoding, any bytes following the end of an ASN.1 data structure are discarded. If an ASN.1 module is compiled with option `undec_rest`, the decode function returns a tuple `{ok, Value, Rest}`, where `Rest` is the bytes following the ASN.1 data structure. `Rest` can be a list or a binary.

- `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.

Unrecognized options are passed on to the Erlang compiler 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.

# test(Module)

    -spec test(Module) -> ok | {error, Reason} when Module :: module(), Reason :: term().

Tests encoding and decoding of all types in `Module`.

For more details, see `test/3`.

# test/2

    -spec test(Module, Type | Options) -> ok | {error, Reason}
                  when
                      Module :: module(),
                      Type :: atom(),
                      Options :: [{i, IncludeDir :: term()}],
                      Reason :: term().

Tests encoding and decoding of `Module`.

If the second argument is given as atom `Type`, that type is tested.

If the second argument is given as list `Options`, that are the options that are used for testing all types in the module.

For more details, see `test/3`.

# test/3

    -spec test(Module, Type, Value | Options) -> ok | {error, Reason}
                  when
                      Module :: module(),
                      Type :: atom(),
                      Value :: term(),
                      Options :: [{i, IncludeDir :: term()}],
                      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 for testing to ensure 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.

# value(Module, Type)

    -spec value(Module, Type) -> {ok, Value} | {error, Reason}
                   when Module :: 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.