X.693 specifies XER ("XML Encoding Rules"). There are three variants of XER given: BASIC-XER (often just XER for short), canonical XER, and EXTENDED-XER. Into this mix, Objective Systems has added its own encoding rules which we'll call OSys-XER. OSys-XER is very similar to XER, but has a few variations that are meant to produce XML documents more closely aligned with what you might get if you were using XML Schema to specify your abstract syntax. Generally, OSys-XER produces fewer tags. The differences between these two sets of encoding rules are discussed in more detail below.

ASN1C supports BASIC-XER, canonical XER, and OSys-XER. It has for some time supported EXTENDED-XER via direct compilation of XSD. In version 6.5.0, we have begun to add direct support for EXTENDED-XER by adding support for some of the XER encoding instructions. Nonetheless, EXTENDED-XER is most fully support today via direct compilation of XSD. By compiling XSD, you can obtain behavior much the same as with OSys-XER, and more.

Prior to version 6.5.1, ASN1C generated two different styles of code and had two different runtime layers for the various XML encoding rules. In support of XER (basic and canonical), there was the XER style of generated code and the XER runtime. In support of OSys-XER and extended-XER (via direct compilation of XSD), there was what we called simply the XML style of generated code and the XML runtime. These two styles of code generation had different encode and decode method signatures, and used different XML parsing techniques (SAX vs. pull-parser). As of version 6.5.1, these have been merged together. The old style of XER code generation has now been deprecated. XER is now supported using the same style of code and the same runtime that was previously used for OSys-XER and extended-XER (via direct XSD compilation).

Depending on the circumstances, the generated code may support more than one set of encoding rules. In these cases, the OSASN1XER and OSXMLC14N flags (set using rtxCtxtSetFlag) are used to choose, at runtime, which encoding rules to follow. OSASN1XER should be set when using BASIC-XER, canonical XER, or EXTENDED-XER. For canonical XER, OSXMLC14N must also be set.. The table below describes the possibilities.

Note that you may use the -xsd switch when generating XML encoders and decoders. The XML schema produced from the ASN.1 specification using the -xsd switch can be used to validate the XML messages generated using the XML encode functions. Similarly, an XML instance can be validated using the generated XML schema prior to decoding.