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.OH 'External Data Representation Standard''Page %'

.EH 'Page %''External Data Representation Standard'

.IX "External Data Representation"

.if \\n%=1 .bp

.SH

\&External Data Representation Standard: Protocol Specification

.IX XDR RFC

.IX XDR "protocol specification"

.LP

.NH 0

\&Status of this Standard

.nr OF 1

.IX XDR "RFC status"

.LP

Note: This chapter specifies a protocol that Sun Microsystems, Inc., and

others are using.  It has been designated RFC1014 by the ARPA Network

Information Center.

.NH 1

Introduction

\&

.LP

XDR is a standard for the description and encoding of data.  It is

useful for transferring data between different computer

architectures, and has been used to communicate data between such

diverse machines as the Sun Workstation, VAX, IBM-PC, and Cray.

XDR fits into the ISO presentation layer, and is roughly analogous in

purpose to X.409, ISO Abstract Syntax Notation.  The major difference

between these two is that XDR uses implicit typing, while X.409 uses

explicit typing.

.LP

XDR uses a language to describe data formats.  The language can only

be used only to describe data; it is not a programming language.

This language allows one to describe intricate data formats in a

concise manner. The alternative of using graphical representations

(itself an informal language) quickly becomes incomprehensible when

faced with complexity.  The XDR language itself is similar to the C

language [1], just as Courier [4] is similar to Mesa. Protocols such

as Sun RPC (Remote Procedure Call) and the NFS (Network File System)

use XDR to describe the format of their data.

.LP

The XDR standard makes the following assumption: that bytes (or

octets) are portable, where a byte is defined to be 8 bits of data.

A given hardware device should encode the bytes onto the various

media in such a way that other hardware devices may decode the bytes

without loss of meaning.  For example, the Ethernet standard

suggests that bytes be encoded in "little-endian" style [2], or least

significant bit first.

.NH 2

\&Basic Block Size

.IX XDR "basic block size"

.IX XDR "block size"

.LP

The representation of all items requires a multiple of four 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.  If the n bytes needed to contain the data are not

a multiple of four, then the n bytes are followed by enough (0 to 3)

residual zero bytes, r, to make the total byte count a multiple of 4.

.LP

We include the familiar graphic box notation for illustration and

comparison.  In most illustrations, each box (delimited by a plus

sign at the 4 corners and vertical bars and dashes) depicts a byte.

Ellipses (...) between boxes show zero or more additional bytes where

required.

.ie t .DS

.el .DS L

\fIA Block\fP

\f(CW+--------+--------+...+--------+--------+...+--------+

| byte 0 | byte 1 |...|byte n-1|    0   |...|    0   |

+--------+--------+...+--------+--------+...+--------+

|<-----------n bytes---------->|<------r bytes------>|

|<-----------n+r (where (n+r) mod 4 = 0)>----------->|\fP

.DE

.NH 1

\&XDR Data Types

.IX XDR "data types"

.IX "XDR data types"

.LP

Each of the sections that follow describes a data type defined in the

XDR standard, shows how it is declared in the language, and includes

a graphic illustration of its encoding.

.LP

For each data type in the language we show a general paradigm

declaration.  Note that angle brackets (< and >) denote

variable length sequences of data and square brackets ([ and ]) denote

fixed-length sequences of data.  "n", "m" and "r" denote integers.

For the full language specification and more formal definitions of

terms such as "identifier" and "declaration", refer to

.I "The XDR Language Specification" ,

below.

.LP

For some data types, more specific examples are included.

A more extensive example of a data description is in

.I "An Example of an XDR Data Description"

below.

.NH 2

\&Integer

.IX XDR integer

.LP

An XDR signed integer is a 32-bit datum that encodes an integer in

the range [-2147483648,2147483647].  The integer is represented in

two's complement notation.  The most and least significant bytes are

.ie t .DS

.el .DS L

\fIInteger\fP

\f(CW(MSB)                   (LSB)

+-------+-------+-------+-------+

|byte 0 |byte 1 |byte 2 |byte 3 |

+-------+-------+-------+-------+

<------------32 bits------------>\fP

.DE

.NH 2

\&Unsigned Integer

.IX XDR "unsigned integer"

.IX XDR "integer, unsigned"

.LP

An XDR unsigned integer is a 32-bit datum that encodes a nonnegative

integer in the range [0,4294967295].  It is represented by an

unsigned binary number whose most and least significant bytes are 0

and 3, respectively.  An unsigned integer is declared as follows:

.ie t .DS

.el .DS L

\fIUnsigned Integer\fP

\f(CW(MSB)                   (LSB)

+-------+-------+-------+-------+

|byte 0 |byte 1 |byte 2 |byte 3 |

+-------+-------+-------+-------+

<------------32 bits------------>\fP

.DE

.NH 2

\&Enumeration

.IX XDR enumeration

.LP

Enumerations have the same representation as signed integers.

Enumerations are handy for describing subsets of the integers.

Enumerated data is declared as follows:

.ft CW

.DS

enum { name-identifier = constant, ... } identifier;

.DE

For example, the three colors red, yellow, and blue could be

described by an enumerated type:

.DS

.ft CW

enum { RED = 2, YELLOW = 3, BLUE = 5 } colors;

.DE

It is an error to encode as an enum any other integer than those that

have been given assignments in the enum declaration.

.NH 2

\&Boolean

.IX XDR boolean

.LP

Booleans are important enough and occur frequently enough to warrant

their own explicit type in the standard.  Booleans are declared as

follows:

.DS

.ft CW

bool identifier;

.DE

This is equivalent to:

.DS

.ft CW

enum { FALSE = 0, TRUE = 1 } identifier;

.DE

.NH 2

\&Hyper Integer and Unsigned Hyper Integer

.IX XDR "hyper integer"

.IX XDR "integer, hyper"

.LP

The standard also defines 64-bit (8-byte) numbers called hyper

integer and unsigned hyper integer.  Their representations are the

obvious extensions of integer and unsigned integer defined above.

They are represented in two's complement notation.  The most and

least significant bytes are 0 and 7, respectively.  Their

declarations:

.ie t .DS

.el .DS L

\fIHyper Integer\fP

\fIUnsigned Hyper Integer\fP

\f(CW(MSB)                                                   (LSB)

+-------+-------+-------+-------+-------+-------+-------+-------+

|byte 0 |byte 1 |byte 2 |byte 3 |byte 4 |byte 5 |byte 6 |byte 7 |

+-------+-------+-------+-------+-------+-------+-------+-------+

<----------------------------64 bits---------------------------->\fP

.DE

.NH 2

\&Floating-point

.IX XDR "integer, floating point"

.IX XDR "floating-point integer"

.LP

The standard defines the floating-point data type "float" (32 bits or

4 bytes).  The encoding used is the IEEE standard for normalized

single-precision floating-point numbers [3].  The following three

fields describe the single-precision floating-point number:

.RS

.IP \fBS\fP:

The sign of the number.  Values 0 and  1 represent  positive and

negative, respectively.  One bit.

.IP \fBE\fP:

The exponent of the number, base 2.  8  bits are devoted to this

field.  The exponent is biased by 127.

.IP \fBF\fP:

The fractional part of the number's mantissa,  base 2.   23 bits

are devoted to this field.

.RE

.LP

Therefore, the floating-point number is described by:

.DS

(-1)**S * 2**(E-Bias) * 1.F

.DE

It is declared as follows:

.ie t .DS

.el .DS L

\fISingle-Precision Floating-Point\fP

\f(CW+-------+-------+-------+-------+

|byte 0 |byte 1 |byte 2 |byte 3 |

S|   E   |           F          |

+-------+-------+-------+-------+

1|<- 8 ->|<-------23 bits------>|

<------------32 bits------------>\fP

.DE

Just as the most and least significant bytes of a number are 0 and 3,

the most and least significant bits of a single-precision floating-

point number are 0 and 31.  The beginning bit (and most significant

bit) offsets of S, E, and F are 0, 1, and 9, respectively.  Note that

these numbers refer to the mathematical positions of the bits, and

NOT to their actual physical locations (which vary from medium to

medium).

.LP

The IEEE specifications should be consulted concerning the encoding

for signed zero, signed infinity (overflow), and denormalized numbers

(underflow) [3].  According to IEEE specifications, the "NaN" (not a

number) is system dependent and should not be used externally.

.NH 2

\&Double-precision Floating-point

.IX XDR "integer, double-precision floating point"

.IX XDR "double-precision floating-point integer"

.LP

The standard defines the encoding for the double-precision floating-

point data type "double" (64 bits or 8 bytes).  The encoding used is

the IEEE standard for normalized double-precision floating-point

numbers [3].  The standard encodes the following three fields, which

describe the double-precision floating-point number:

.RS

.IP \fBS\fP:

The sign of the number.  Values  0 and 1  represent positive and

negative, respectively.  One bit.

.IP \fBE\fP:

The exponent of the number, base 2.  11 bits are devoted to this

field.  The exponent is biased by 1023.

.IP \fBF\fP:

The fractional part of the number's  mantissa, base 2.   52 bits

are devoted to this field.

.RE

.LP

Therefore, the floating-point number is described by:

.DS

(-1)**S * 2**(E-Bias) * 1.F

.DE

It is declared as follows:

.ie t .DS

.el .DS L

\fIDouble-Precision Floating-Point\fP

\f(CW+------+------+------+------+------+------+------+------+

|byte 0|byte 1|byte 2|byte 3|byte 4|byte 5|byte 6|byte 7|

S|    E   |                    F                        |

+------+------+------+------+------+------+------+------+

1|<--11-->|<-----------------52 bits------------------->|

<-----------------------64 bits------------------------->\fP

.DE

Just as the most and least significant bytes of a number are 0 and 3,

the most and least significant bits of a double-precision floating-

point number are 0 and 63.  The beginning bit (and most significant

bit) offsets of S, E , and F are 0, 1, and 12, respectively.  Note

that these numbers refer to the mathematical positions of the bits,

and NOT to their actual physical locations (which vary from medium to

medium).

.LP

The IEEE specifications should be consulted concerning the encoding

for signed zero, signed infinity (overflow), and denormalized numbers

(underflow) [3].  According to IEEE specifications, the "NaN" (not a

number) is system dependent and should not be used externally.

.NH 2

\&Fixed-length Opaque Data

.IX XDR "fixed-length opaque data"

.IX XDR "opaque data, fixed length"

.LP

At times, fixed-length uninterpreted data needs to be passed among

machines.  This data is called "opaque" and is declared as follows:

.DS

.ft CW

opaque identifier[n];

.DE

where the constant n is the (static) number of bytes necessary to

contain the opaque data.  If n is not a multiple of four, then the n

bytes are followed by enough (0 to 3) residual zero bytes, r, to make

the total byte count of the opaque object a multiple of four.

.ie t .DS

.el .DS L

\fIFixed-Length Opaque\fP

\f(CW0        1     ...

+--------+--------+...+--------+--------+...+--------+

| byte 0 | byte 1 |...|byte n-1|    0   |...|    0   |

+--------+--------+...+--------+--------+...+--------+

|<-----------n bytes---------->|<------r bytes------>|

|<-----------n+r (where (n+r) mod 4 = 0)------------>|\fP

.DE

.NH 2

\&Variable-length Opaque Data

.IX XDR "variable-length opaque data"

.IX XDR "opaque data, variable length"

.LP

The standard also provides for variable-length (counted) opaque data,

defined as a sequence of n (numbered 0 through n-1) arbitrary bytes

to be the number n encoded as an unsigned integer (as described

below), and followed by the n bytes of the sequence.

.LP

Byte m of the sequence always precedes byte m+1 of the sequence, and

byte 0 of the sequence always follows the sequence's length (count).

enough (0 to 3) residual zero bytes, r, to make the total byte count

a multiple of four.  Variable-length opaque data is declared in the

following way:

.DS

.ft CW

opaque identifier;

.DE

or

.DS

.ft CW

opaque identifier<>;

.DE

The constant m denotes an upper bound of the number of bytes that the

sequence may contain.  If m is not specified, as in the second

declaration, it is assumed to be (2**32) - 1, the maximum length.

The constant m would normally be found in a protocol specification.

For example, a filing protocol may state that the maximum data

transfer size is 8192 bytes, as follows:

.DS

.ft CW

opaque filedata<8192>;

.DE

This can be illustrated as follows:

.ie t .DS

.el .DS L

\fIVariable-Length Opaque\fP

\f(CW0     1     2     3     4     5   ...

+-----+-----+-----+-----+-----+-----+...+-----+-----+...+-----+

|        length n       |byte0|byte1|...| n-1 |  0  |...|  0  |

+-----+-----+-----+-----+-----+-----+...+-----+-----+...+-----+

|<-------4 bytes------->|<------n bytes------>|<---r bytes--->|

|<----n+r (where (n+r) mod 4 = 0)---->|\fP

.DE

.LP

It   is  an error  to  encode  a  length  greater  than the maximum

described in the specification.

.NH 2

\&String

.IX XDR string

.LP

The standard defines a string of n (numbered 0 through n-1) ASCII

bytes to be the number n encoded as an unsigned integer (as described

above), and followed by the n bytes of the string.  Byte m of the

string always precedes byte m+1 of the string, and byte 0 of the

string always follows the string's length.  If n is not a multiple of

four, then the n bytes are followed by enough (0 to 3) residual zero

bytes, r, to make the total byte count a multiple of four.  Counted

byte strings are declared as follows:

.DS

.ft CW

string object;

.DE

or

.DS

.ft CW

string object<>;

.DE

The constant m denotes an upper bound of the number of bytes that a

string may contain.  If m is not specified, as in the second

declaration, it is assumed to be (2**32) - 1, the maximum length.

The constant m would normally be found in a protocol specification.

For example, a filing protocol may state that a file name can be no

longer than 255 bytes, as follows:

.DS

.ft CW

string filename<255>;

.DE

Which can be illustrated as:

.ie t .DS

.el .DS L

\fIA String\fP

\f(CW0     1     2     3     4     5   ...

+-----+-----+-----+-----+-----+-----+...+-----+-----+...+-----+

|        length n       |byte0|byte1|...| n-1 |  0  |...|  0  |

+-----+-----+-----+-----+-----+-----+...+-----+-----+...+-----+

|<-------4 bytes------->|<------n bytes------>|<---r bytes--->|

|<----n+r (where (n+r) mod 4 = 0)---->|\fP

.DE

.LP

It   is an  error  to  encode  a length greater  than   the maximum

described in the specification.

.NH 2

\&Fixed-length Array

.IX XDR "fixed-length array"

.IX XDR "array, fixed length"

.LP

Declarations for fixed-length arrays of homogeneous elements are in

the following form:

.DS

.ft CW

type-name identifier[n];

.DE

Fixed-length arrays of elements numbered 0 through n-1 are encoded by

individually encoding the elements of the array in their natural

order, 0 through n-1.  Each element's size is a multiple of four

bytes. Though all elements are of the same type, the elements may

have different sizes.  For example, in a fixed-length array of

strings, all elements are of type "string", yet each element will

vary in its length.

.ie t .DS

.el .DS L

\fIFixed-Length Array\fP

\f(CW+---+---+---+---+---+---+---+---+...+---+---+---+---+

|   element 0   |   element 1   |...|  element n-1  |

+---+---+---+---+---+---+---+---+...+---+---+---+---+

|<--------------------n elements------------------->|\fP

.DE

.NH 2

\&Variable-length Array

.IX XDR "variable-length array"

.IX XDR "array, variable length"

.LP

Counted arrays provide the ability to encode variable-length arrays

of homogeneous elements.  The array is encoded as the element count n

(an unsigned integer) followed by the encoding of each of the array's

elements, starting with element 0 and progressing through element n-

1.  The declaration for variable-length arrays follows this form:

.DS

.ft CW

type-name identifier;

.DE

or

.DS

.ft CW

type-name identifier<>;

.DE

The constant m specifies the maximum acceptable element count of an

array; if  m is not specified, as  in the second declaration, it is

assumed to be (2**32) - 1.

.ie t .DS

.el .DS L

\fICounted Array\fP

\f(CW0  1  2  3

+--+--+--+--+--+--+--+--+--+--+--+--+...+--+--+--+--+

|     n     | element 0 | element 1 |...|element n-1|

+--+--+--+--+--+--+--+--+--+--+--+--+...+--+--+--+--+

|<-4 bytes->|<--------------n elements------------->|\fP

.DE

It is  an error to  encode  a  value of n that  is greater than the

maximum described in the specification.

.NH 2

\&Structure

.IX XDR structure

.LP

Structures are declared as follows:

.DS

.ft CW

struct {

        component-declaration-A;

        component-declaration-B;

        \&...

} identifier;

.DE

The components of the structure are encoded in the order of their

declaration in the structure.  Each component's size is a multiple of

four bytes, though the components may be different sizes.

.ie t .DS

.el .DS L

\fIStructure\fP

\f(CW+-------------+-------------+...

| component A | component B |...

+-------------+-------------+...\fP

.DE

.NH 2

\&Discriminated Union

.IX XDR "discriminated union"

.IX XDR union discriminated

.LP

A discriminated union is a type composed of a discriminant followed

by a type selected from a set of prearranged types according to the

value of the discriminant.  The type of discriminant is either "int",

"unsigned int", or an enumerated type, such as "bool".  The component

types are called "arms" of the union, and are preceded by the value

of the discriminant which implies their encoding.  Discriminated

unions are declared as follows:

.DS

.ft CW

union switch (discriminant-declaration) {

        case discriminant-value-A:

        arm-declaration-A;

        case discriminant-value-B:

        arm-declaration-B;

        \&...

        default: default-declaration;

} identifier;

.DE

Each "case" keyword is followed by a legal value of the discriminant.

The default arm is optional.  If it is not specified, then a valid

encoding of the union cannot take on unspecified discriminant values.

The size of the implied arm is always a multiple of four bytes.

.LP

The discriminated union is encoded as its discriminant followed by

the encoding of the implied arm.

.ie t .DS

.el .DS L

\fIDiscriminated Union\fP

\f(CW0   1   2   3

+---+---+---+---+---+---+---+---+

|  discriminant |  implied arm  |

+---+---+---+---+---+---+---+---+

|<---4 bytes--->|\fP

.DE

.NH 2

\&Void

.IX XDR void

.LP

An XDR void is a 0-byte quantity.  Voids are useful for describing

operations that take no data as input or no data as output. They are

also useful in unions, where some arms may contain data and others do

not.  The declaration is simply as follows:

.DS

.ft CW

void;

.DE

Voids are illustrated as follows:

.ie t .DS

.el .DS L

\fIVoid\fP

\f(CW  ++

  ||

  ++

--><-- 0 bytes\fP

.DE

.NH 2

\&Constant

.IX XDR constant

.LP

The data declaration for a constant follows this form:

.DS

.ft CW

const name-identifier = n;

.DE

"const" is used to define a symbolic name for a constant; it does not

declare any data.  The symbolic constant may be used anywhere a

regular constant may be used.  For example, the following defines a

symbolic constant DOZEN, equal to 12.

.DS

.ft CW

const DOZEN = 12;

.DE

.NH 2

\&Typedef

.IX XDR typedef

.LP

"typedef" does not declare any data either, but serves to define new

identifiers for declaring data. The syntax is:

.DS

.ft CW

typedef declaration;

.DE

The new type name is actually the variable name in the declaration

part of the typedef.  For example, the following defines a new type

called "eggbox" using an existing type called "egg":

.DS

.ft CW

typedef egg eggbox[DOZEN];

.DE

Variables declared using the new type name have the same type as the

new type name would have in the typedef, if it was considered a

variable.  For example, the following two declarations are equivalent

in declaring the variable "fresheggs":

.DS

.ft CW

eggbox  fresheggs;

egg     fresheggs[DOZEN];

.DE

When a typedef involves a struct, enum, or union definition, there is

another (preferred) syntax that may be used to define the same type.

In general, a typedef of the following form:

.DS

.ft CW

typedef <> identifier;

.DE

may be converted to the alternative form by removing the "typedef"

part and placing the identifier after the "struct", "union", or

"enum" keyword, instead of at the end.  For example, here are the two

ways to define the type "bool":

.DS

.ft CW

typedef enum {    /* \fIusing typedef\fP */

        FALSE = 0,

        TRUE = 1

        } bool;

enum bool {       /* \fIpreferred alternative\fP */

        FALSE = 0,

        TRUE = 1

        };

.DE

The reason this syntax is preferred is one does not have to wait

until the end of a declaration to figure out the name of the new

type.

.NH 2

\&Optional-data

.IX XDR "optional data"

.IX XDR "data, optional"

.LP

Optional-data is one kind of union that occurs so frequently that we

give it a special syntax of its own for declaring it.  It is declared

as follows:

.DS

.ft CW

type-name *identifier;

.DE

This is equivalent to the following union:

.DS

.ft CW

union switch (bool opted) {

        case TRUE:

        type-name element;

        case FALSE:

        void;

} identifier;

.DE

It is also equivalent to the following variable-length array

declaration, since the boolean "opted" can be interpreted as the

length of the array:

.DS

.ft CW

type-name identifier<1>;

.DE

Optional-data is not so interesting in itself, but it is very useful

for describing recursive data-structures such as linked-lists and

trees.  For example, the following defines a type "stringlist" that

encodes lists of arbitrary length strings:

.DS

.ft CW

struct *stringlist {

        string item<>;

        stringlist next;

};

.DE

It could have been equivalently declared as the following union:

.DS

.ft CW

union stringlist switch (bool opted) {

        case TRUE:

                struct {

                        string item<>;

                        stringlist next;

                } element;

        case FALSE:

                void;

};

.DE

or as a variable-length array:

.DS

.ft CW

struct stringlist<1> {

        string item<>;

        stringlist next;

};

.DE

Both of these declarations obscure the intention of the stringlist

type, so the optional-data declaration is preferred over both of

them.  The optional-data type also has a close correlation to how

recursive data structures are represented in high-level languages

such as Pascal or C by use of pointers. In fact, the syntax is the

same as that of the C language for pointers.

.NH 2

\&Areas for Future Enhancement

.IX XDR futures

.LP

The XDR standard lacks representations for bit fields and bitmaps,

since the standard is based on bytes.  Also missing are packed (or

binary-coded) decimals.

.LP

The intent of the XDR standard was not to describe every kind of data

that people have ever sent or will ever want to send from machine to

machine. Rather, it only describes the most commonly used data-types

of high-level languages such as Pascal or C so that applications

written in these languages will be able to communicate easily over

some medium.

.LP

One could imagine extensions to XDR that would let it describe almost

any existing protocol, such as TCP.  The minimum necessary for this

are support for different block sizes and byte-orders.  The XDR

discussed here could then be considered the 4-byte big-endian member

of a larger XDR family.

.NH 1

\&Discussion

.sp 2

.NH 2

\&Why a Language for Describing Data?

.IX XDR language

.LP

There are many advantages in using a data-description language such

as  XDR  versus using  diagrams.   Languages are  more  formal than

diagrams   and   lead  to less  ambiguous   descriptions  of  data.

Languages are also easier  to understand and allow  one to think of

other   issues instead of  the   low-level details of bit-encoding.

Also,  there is  a close analogy  between the  types  of XDR and  a

high-level language   such  as C   or    Pascal.   This makes   the

implementation of XDR encoding and decoding modules an easier task.

Finally, the language specification itself  is an ASCII string that

can be passed from  machine to machine  to perform  on-the-fly data

interpretation.

.NH 2

\&Why Only one Byte-Order for an XDR Unit?

.IX XDR "byte order"

.LP

Supporting two byte-orderings requires a higher level protocol for

determining in which byte-order the data is encoded.  Since XDR is

not a protocol, this can't be done.  The advantage of this, though,

is that data in XDR format can be written to a magnetic tape, for

example, and any machine will be able to interpret it, since no

higher level protocol is necessary for determining the byte-order.

.NH 2

\&Why does XDR use Big-Endian Byte-Order?

.LP

Yes, it is unfair, but having only one byte-order means you have to

be unfair to somebody.  Many architectures, such as the Motorola

68000 and IBM 370, support the big-endian byte-order.

.NH 2

\&Why is the XDR Unit Four Bytes Wide?

.LP

There is a tradeoff in choosing the XDR unit size.  Choosing a small

size such as two makes the encoded data small, but causes alignment

problems for machines that aren't aligned on these boundaries.  A

large size such as eight means the data will be aligned on virtually

every machine, but causes the encoded data to grow too big.  We chose

four as a compromise.  Four is big enough to support most

architectures efficiently, except for rare machines such as the

eight-byte aligned Cray.  Four is also small enough to keep the

encoded data restricted to a reasonable size.

.NH 2

\&Why must Variable-Length Data be Padded with Zeros?

.IX XDR "variable-length data"

.LP

It is desirable that the same data encode into the same thing on all

machines, so that encoded data can be meaningfully compared or

checksummed.  Forcing the padded bytes to be zero ensures this.

.NH 2

\&Why is there No Explicit Data-Typing?

.LP

Data-typing has a relatively high cost for what small advantages it

may have.  One cost is the expansion of data due to the inserted type

fields.  Another is the added cost of interpreting these type fields

and acting accordingly.  And most protocols already know what type

they expect, so data-typing supplies only redundant information.

However, one can still get the benefits of data-typing using XDR. One

way is to encode two things: first a string which is the XDR data

description of the encoded data, and then the encoded data itself.

Another way is to assign a value to all the types in XDR, and then

define a universal type which takes this value as its discriminant

and for each value, describes the corresponding data type.

.NH 1

\&The XDR Language Specification

.IX XDR language

.sp 1

.NH 2

\&Notational Conventions

.IX "XDR language" notation

.LP

This specification  uses an extended Backus-Naur Form  notation for

describing the XDR language.   Here is  a brief description  of the

notation:

.IP  1.

The characters

.I | ,

.I ( ,

.I ) ,

.I [ ,

.I ] ,

.I " ,

and

.I *

are special.

.IP  2.

Terminal symbols are  strings of any  characters surrounded by

double quotes.

.IP  3.

Non-terminal symbols are strings of non-special characters.

.IP  4.

Alternative items are separated by a vertical bar ("\fI|\fP").

.IP  5.

Optional items are enclosed in brackets.

.IP  6.

Items are grouped together by enclosing them in parentheses.

.IP  7.

A

.I *

following an item means  0 or more  occurrences of that item.

.LP

For example,  consider  the  following pattern:

.DS L

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

.DE

.LP

An infinite  number of  strings match  this pattern. A few  of them

are:

.DS

"a very rainy day"

"a very, very rainy day"

"a very cold and  rainy day"

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

.DE

.NH 2

\&Lexical Notes

.IP  1.

Comments begin with '/*' and terminate with '*/'.

.IP  2.

White space serves to separate items and is otherwise ignored.

.IP  3.

An identifier is a letter followed by  an optional sequence of

letters, digits or underbar ('_').  The case of identifiers is

not ignored.

.IP  4.

A  constant is  a  sequence  of  one  or  more decimal digits,

optionally preceded by a minus-sign ('-').

.NH 2

\&Syntax Information

.IX "XDR language" syntax

.DS

.ft CW

declaration:

        type-specifier identifier

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

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

        | "opaque" identifier "[" value "]"

        | "opaque" identifier "<" [ value ] ">"

        | "string" identifier "<" [ value ] ">"

        | type-specifier "*" identifier

        | "void"

.DE

.DS

.ft CW

value:

        constant

        | identifier

type-specifier:

          [ "unsigned" ] "int"

        | [ "unsigned" ] "hyper"

        | "float"

        | "double"

        | "bool"

        | enum-type-spec

        | struct-type-spec

        | union-type-spec

        | identifier

.DE

.DS

.ft CW

enum-type-spec:

        "enum" enum-body

enum-body:

        "{"

        ( identifier "=" value )

        ( "," identifier "=" value )*

        "}"

.DE

.DS

.ft CW

struct-type-spec:

        "struct" struct-body

struct-body:

        "{"

        ( declaration ";" )

        ( declaration ";" )*

        "}"

.DE

.DS

.ft CW

union-type-spec:

        "union" union-body

union-body:

        "switch" "(" declaration ")" "{"

        ( "case" value ":" declaration ";" )

        ( "case" value ":" declaration ";" )*

        [ "default" ":" declaration ";" ]

        "}"

constant-def:

        "const" identifier "=" constant ";"

.DE

.DS

.ft CW

type-def:

        "typedef" declaration ";"

        | "enum" identifier enum-body ";"

        | "struct" identifier struct-body ";"

        | "union" identifier union-body ";"

definition:

        type-def

        | constant-def

specification:

        definition *