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ONC+ Developer's Guide     Oracle Solaris 11.1 Information Library
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Document Information


1.  Introduction to ONC+ Technologies

2.  Introduction to TI-RPC

3.  rpcgen Programming Guide

4.  Programmer's Interface to RPC

5.  Advanced RPC Programming Techniques

6.  Porting From TS-RPC to TI-RPC

7.  Multithreaded RPC Programming

8.  Extensions to the Oracle Solaris RPC Library

A.  XDR Technical Note

B.  RPC Protocol and Language Specification

C.  XDR Protocol Specification

XDR Protocol Introduction

Graphic Box Notation

Basic Block Size

XDR Data Type Declarations

Signed Integer


Signed Integer Encoding

Unsigned Integer


Unsigned Integer Encoding



Hyper Integer and Unsigned Hyper Integer


Hyper Integer Encoding

Floating Point


Double-Precision Floating Point Encoding

Quadruple-Precision Floating Point


Quadruple-Precision Floating Point Encoding

Fixed-Length Opaque Data


Fixed-Length Opaque Encoding

Variable-Length Opaque Data


Variable-Length Opaque Encoding

Counted Byte Strings


String Encoding

Fixed-Length Array


Fixed-Length Array Encoding

Variable-Length Array


Counted Array Encoding



Structure Encoding

Discriminated Union


Discriminated Union Encoding







XDR Language Specification

Notational Conventions

Lexical Notes

Syntax Notes

XDR Data Description

RPC Language Reference

D.  RPC Code Examples

E.  portmap Utility



XDR Protocol Introduction

External data representation (XDR) is a standard for the description and encoding of data. The XDR protocol is useful for transferring data between different computer architectures and has been used to communicate data between very diverse machines. XDR fits into the ISO reference model's presentation layer (layer 6) and is roughly analogous in purpose to X.409, ISO Abstract Syntax Notation. The major difference between the two is that XDR uses implicit typing, while X.409 uses explicit typing.

XDR uses a language to describe data formats and can only be used to describe data. It is not a programming language. This language enables you to describe intricate data formats in a concise manner. The XDR language is similar to the C language. Protocols such as RPC and NFS use XDR to describe the format of their data.

The XDR standard assumes that bytes, or octets, are portable and that a byte is defined to be 8 bits of data.

Graphic Box Notation

This appendix uses graphic box notation for illustration and comparison. In most illustrations, each box depicts a byte. The representation of all items requires a multiple of 4 bytes (or 32 bits) of data. The bytes are numbered 0 through n -1. The bytes are read or written to some byte stream such that byte m always precedes byte m+1. The n bytes are followed by enough (0 to 3) residual zero bytes, r, to make the total byte count a multiple of four. Ellipses (...) between boxes show zero or more additional bytes where required, as shown in the following illustration.

image:This graphic depicts a bytestream consisting of n+r bytes, where (n+r)mod4 = 0.

Basic Block Size

Choosing the XDR block size requires a tradeoff. Choosing a small size such as 2 makes the encoded data small, but causes alignment problems for machines that are not aligned on these boundaries. A large size such as 8 means the data is aligned on virtually every machine, but causes the encoded data to grow too large. Four was chosen as a compromise. Four is big enough to support most architectures efficiently.

This basic block size of 4 does not mean that the computers cannot utilize standard XDR, just that they do so at a greater overhead per data item than 4-byte (32-bit) architectures. Four is also small enough to keep the encoded data restricted to a reasonable size.

The same data should encode into an equivalent result on all machines so that encoded data can be compared or checksummed. So, variable-length data must be padded with trailing zeros.