Communications Programming Concepts

XDR Language Specification

The eXternal Data Representation (XDR) language specification uses an extended Backus Naur form notation for describing the XDR language. The following is a brief description of the notation:

• The following characters are special characters:
  

  |	A vertical bar separates alternative items.
  ( )	Parentheses enclose items that are grouped together.
  [ ]	Brackets enclose optional items.
  ,	A comma separates more than one variable.
  *	An asterisk following an item means 0 or more occurrences of the item.

• Terminal symbols are strings of special and nonspecial characters surrounded by " " (double quotation marks).
• Nonterminal symbols are strings of nonspecial characters.

The following specification illustrates the XDR notation:

"a" "very" ("," "very")* ["cold" "and"] "rainy" ("day" | "night")

An infinite number of strings match this pattern, including the following examples:

• "a very rainy day"
• "a very, very rainy day"
• "a very, cold and rainy day"
• "a very, very, very cold and rainy night"

Lexical Notes

The following lexical notes apply to XDR language specification:

• Comments begin with a /* (backslash, asterisk) and terminate with an */ (asterisk, backslash).
• White space is used to separate items and is otherwise ignored.
• An identifier is a letter followed by an optional sequence of letters, digits, or an _ (underscore). Identifiers are case-sensitive.
• A constant is a sequence of one or more decimal digits, optionally preceded by a - (minus sign).

Declarations, Enumerations, Structures, and Unions

The following XDR syntax describes declarations, enumerations, structures, and unions:

declaration:type-specifier identifier

| type-specifier identifier "[" value "]"

| type-specifier identifier "<" [ value ] "<"

| "opaque" identifier "[" value "]"

| "string" identifier "[" value "]"

| type-specifier "*" identifier

|"void"

value:

constant

 | identifier

type-specifier:

[ "unsigned" ] "int"

| [ "unsigned" ] "hyper"

| "float"

| "double"

| "bool"

| enum-type-spec

| struct-type-spec

| union-type-spec

| identifier

enum-type-spec:

"enum" enum-body

enum-body:

"{"

( identifier "=" value )

("," identifier "=" value )*

"}"

struct-type-spec:

"struct" struct-body

struct-body:

"{"

( declaration ";" )

( declaration ";" )*

"}"

union-type-spec:

"union" union-body

union-body:

"switch" "(" declaration ")" "{"

( "case" value ":" declaration ";" )

( "case" value ":" declaration ";" )*

[ "default" ":" declaration ";" ]

"}"

constant-def:

"const" identifier "=" constant ";"

type-def

"typedef" declaration ";"

| "enum" identifier enum-body ";"

| "struct" identifier struct-body ";"

| "union" identifier union-body ";"

definition:

type-def

| constant-def

specification:

definition *

Syntax Notes

The following considerations pertain to XDR language syntax:

• The following keywords cannot be used as identifiers:
  • bool
  • case
  • const
  • default
  • double
  • enum
  • float
  • hyper
  • opaque
  • string
  • struct
  • switch
  • typedef
  • union
  • unsigned
  • void
• Only unsigned constants can be used as size specifications for arrays. If an identifier is used, it must be declared previously as an unsigned constant in a const definition.
• In the scope of a specification, constant and type identifiers are in the same name space and must be declared uniquely.
• Variable names must be unique in the scope of struct and union declarations. Nested struct and union declarations create new scopes.