About

ASN.1 to C compiler takes the ASN.1 module files (example) and generates the C++ compatible C source code. That code can be used to serialize the native C structures into compact and unambiguous BER/OER/PER/XER-based data files, and deserialize the files back.

Various ASN.1 based formats are widely used in the industry, such as to encode the X.509 certificates employed in the HTTPS handshake, to exchange control data between mobile phones and cellular networks, to perform car-to-car communication in intelligent transportation networks.

The ASN.1 family of standards is large and complex, and no open source compiler supports it in its entirety. The asn1c is arguably the most evolved open source ASN.1 compiler.

ASN.1 Transfer Syntaxes

ASN.1 encodings interoperability table

The ASN.1 family of standards define a number of ways to encode data, including byte-oriented (e.g., BER), bit-oriented (e.g., PER), and textual (e.g., XER). Some encoding variants (e.g., DER) are just stricter variants of the more general encodings (e.g., BER).

The interoperability table below specifies which API functions can be used to exchange data in a compatible manner. If you need to produce data conforming to the standard specified in the column 1, use the API function in the column 2. If you need to process data conforming to the standard(s) specified in the column 3, use the API function specified in column 4. See the doc/asn1c-usage.pdf for details.

Encoding	API function	Understood by	API function
BER	der_encode()	BER	ber_decode()
DER	der_encode()	DER, BER	ber_decode()
CER	not supported	CER, BER	ber_decode()
BASIC-OER	oer_encode()	*-OER	oer_decode()
CANONICAL-OER	oer_encode()	*-OER	oer_decode()
BASIC-UPER	uper_encode()	*-UPER	uper_decode()
CANONICAL-UPER	uper_encode()	*-UPER	uper_decode()
*-APER	not supported	*-APER	not supported
BASIC-XER	xer_encode(XER_F_BASIC)	*-XER	xer_decode()
CANONICAL-XER	xer_encode(XER_F_CANONICAL)	*-XER	xer_decode()

*) Asterisk means both BASIC and CANONICAL variants.

Build and Install

If you haven't installed the asn1c yet, read the INSTALL.md file for a short installation guide.

Documentation

For the list of asn1c command line options, see asn1c -h or man asn1c.

The comprehensive documentation on this compiler is in doc/asn1c-usage.pdf.

Please also read the FAQ file.

An excellent book on ASN.1 is written by Olivier Dubuisson: "ASN.1 Communication between heterogeneous systems", ISBN:0-12-6333361-0.

Quick start

(also check out doc/asn1c-quick.pdf)

After installing the compiler (see INSTALL.md), you may use the asn1c command to compile the ASN.1 specification:

asn1c <module.asn1>                         # Compile module

If several specifications contain interdependencies, all of them must be specified at the same time:

asn1c <module1.asn1> <module2.asn1> ...     # Compile interdependent modules

The asn1c source tarball contains the examples/ directory with several ASN.1 modules and a script to extract the ASN.1 modules from RFC documents. Refer to the examples/README file in that directory.

To compile the X.509 PKI module:

./asn1c/asn1c -P ./examples/rfc3280-*.asn1  # Compile-n-print

In this example, the -P option is to print the compiled text on the standard output. The default behavior is that asn1c compiler creates multiple .c and .h files for every ASN.1 type found inside the specified ASN.1 modules.

The compiler's -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 be useful for checking whether a particular syntactic construction is properly supported by the compiler.

asn1c -EF <module-to-test.asn1>             # Check semantic validity

Model of operation

The asn1c compiler works by processing the ASN.1 module specifications in several stages:

During the first stage, the ASN.1 file is parsed. (Parsing produces an ASN.1 syntax tree for the subsequent levels)
During the second stage, the syntax tree is "fixed". (Fixing is a process of checking the tree for semantic errors, accompanied by the tree transformation into the canonical form)
During the third stage, the syntax tree is compiled into the target language.

There are several command-line options reserved for printing the results after each stage of operation:

<parser> => print                                       (-E)
<parser> => <fixer> => print                            (-E -F)
<parser> => <fixer> => <compiler> => print              (-P)
<parser> => <fixer> => <compiler> => save-compiled      [default]

-- Lev Walkin vlm@lionet.info

osmocom/asn1c