For each ASN.1 production defined in an ASN.1 source file, a C decode function is generated. This function will decode an ASN.1 message into a C variable of the given type.

If C++ code generation is specified, a control class is generated for certain types (so-called "PDU" types). The control class is a wrapper around the C functions. The Decode function is called to decode an ASN.1 message into the variable referenced in the msgData component of the class.