The Z39.50 ASN.1 module provides you with a set of C struct definitions for the various PDUs of the Z39.50 protocol, as well as for the complex types appearing within the PDUs. For the primitive data types, the C representation often takes the form of an ordinary C language type, such as int. For ASN.1 constructs that have no direct representation in C, such as general octet strings and bit strings, the ODR module (see section The ODR Module) provides auxiliary definitions.
The Z39.50 ASN.1 module is located in sub directory z39.50. There you'll find C files that implements encoders and decoders for the Z39.50 types. You'll also find the protocol definitions: z3950v3.asn, esupdate.asn, and others.