Overview of ASN1C

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The ASN1C code generation tool translates an Abstract Syntax Notation 1 (ASN.1) source file into computer language source files that allow ASN.1 data to be encoded/decoded. This release of the compiler includes options to generate code in three different languages: C, C++, or Java. This manual discusses the C and C++ code generation capabilities. The ASN1C Java User's Manual discusses the Java code generation capability.

Each ASN.1 module that is encountered in an ASN.1 source file results in the generation of the following two types of C/C++ language files:

1. An include (.h) file containing C/C++ typedefs and classes that represent each of the ASN.1 productions listed in the ASN.1 source file, and

2. A C/C++ source (.c or .cpp) file containing C/C++ encode and decode functions. One encode and decode function is generated for each ASN.1 production.

These files, when compiled and linked with the ASN.1 low-level encode/decode function library, provide a complete package for working with ASN.1 encoded data.

ASN1C works with the version of ASN.1 specified in ITU-T international standards X.680 through X.683. It generates code for encoding/decoding data in accordance with the following encoding rules:

• Basic Encoding Rules (BER) or Distinguished Encoding Rules (DER) as published in the ITU-T X.690 standard.
  
• Packed Encoding Rules (PER) as published in the ITU-T X.691 standard.
  
• XML Encoding Rules (XER) as published in the ITU-T X.693 standard.
  

The compiler is capable of parsing all ASN.1 syntax as defined in the standards. It is capable of parsing advanced syntax including Information Object Specifications as defined in the ITU-T X.681 standard as well as Parameterized Types as defined in ITU-T X.683. This version of the compiler is also capable of using table constraints as define it ITU-T X.682 to generate single-step encoders and decoders that can encode or decode multi-part messages in a single function call.

This release of the compiler contains a special executable (asn1c90.exe) that is backward compatible with deprecated features from the older X.208 and X.209 standards. These include the ANY data type and unnamed fields in SEQUENCE, SET, and CHOICE types. This version can also parse type syntax from common macro definitions such as ROSE.