1062 lines
32 KiB
Plaintext
1062 lines
32 KiB
Plaintext
.\"
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.\" Must use -- tbl -- with this one
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.\"
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.\" @(#)xdr.rfc.ms 2.2 88/08/05 4.0 RPCSRC
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.\" $FreeBSD$
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.\"
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.so stubs
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.de BT
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.if \\n%=1 .tl ''- % -''
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..
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.ND
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.\" prevent excess underlining in nroff
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.if n .fp 2 R
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.OH 'External Data Representation Standard''Page %'
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.EH 'Page %''External Data Representation Standard'
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.if \n%=1 .bp
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.SH
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\&External Data Representation Standard: Protocol Specification
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.IX "External Data Representation"
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.IX XDR RFC
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.IX XDR "protocol specification"
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.LP
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.NH 0
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\&Status of this Standard
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.nr OF 1
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.IX XDR "RFC status"
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.LP
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Note: This chapter specifies a protocol that Sun Microsystems, Inc., and
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others are using. It has been designated RFC1014 by the ARPA Network
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Information Center.
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.NH 1
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Introduction
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\&
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.LP
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XDR is a standard for the description and encoding of data. It is
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useful for transferring data between different computer
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|
architectures, and has been used to communicate data between such
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|
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
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explicit typing.
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.LP
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|
XDR uses a language to describe data formats. The language can only
|
|
be used only to describe data; it is not a programming language.
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|
This language allows one to describe intricate data formats in a
|
|
concise manner. The alternative of using graphical representations
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(itself an informal language) quickly becomes incomprehensible when
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|
faced with complexity. The XDR language itself is similar to the C
|
|
language [1], just as Courier [4] is similar to Mesa. Protocols such
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|
as Sun RPC (Remote Procedure Call) and the NFS (Network File System)
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|
use XDR to describe the format of their data.
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|
.LP
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|
The XDR standard makes the following assumption: that bytes (or
|
|
octets) are portable, where a byte is defined to be 8 bits of data.
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|
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.
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|
.NH 2
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|
\&Basic Block Size
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|
.IX XDR "basic block size"
|
|
.IX XDR "block size"
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|
.LP
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|
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.
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|
.LP
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|
We include the familiar graphic box notation for illustration and
|
|
comparison. In most illustrations, each box (delimited by a plus
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|
sign at the 4 corners and vertical bars and dashes) depicts a byte.
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|
Ellipses (...) between boxes show zero or more additional bytes where
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required.
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.ie t .DS
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.el .DS L
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\fIA Block\fP
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|
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|
\f(CW+--------+--------+...+--------+--------+...+--------+
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|
| byte 0 | byte 1 |...|byte n-1| 0 |...| 0 |
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|
+--------+--------+...+--------+--------+...+--------+
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|
|<-----------n bytes---------->|<------r bytes------>|
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|<-----------n+r (where (n+r) mod 4 = 0)>----------->|\fP
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.DE
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|
.NH 1
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|
\&XDR Data Types
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|
.IX XDR "data types"
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|
.IX "XDR data types"
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|
.LP
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|
Each of the sections that follow describes a data type defined in the
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|
XDR standard, shows how it is declared in the language, and includes
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|
a graphic illustration of its encoding.
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|
.LP
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|
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.
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|
For the full language specification and more formal definitions of
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|
terms such as "identifier" and "declaration", refer to
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|
.I "The XDR Language Specification" ,
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|
below.
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|
.LP
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|
For some data types, more specific examples are included.
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|
A more extensive example of a data description is in
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|
.I "An Example of an XDR Data Description"
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|
below.
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|
.NH 2
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|
\&Integer
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|
.IX XDR integer
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|
.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
|
|
0 and 3, respectively. Integers are declared as follows:
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|
.ie t .DS
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|
.el .DS L
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|
\fIInteger\fP
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|
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|
\f(CW(MSB) (LSB)
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|
+-------+-------+-------+-------+
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|
|byte 0 |byte 1 |byte 2 |byte 3 |
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|
+-------+-------+-------+-------+
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|
<------------32 bits------------>\fP
|
|
.DE
|
|
.NH 2
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|
\&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:
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|
.ie t .DS
|
|
.el .DS L
|
|
\fIUnsigned Integer\fP
|
|
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|
\f(CW(MSB) (LSB)
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|
+-------+-------+-------+-------+
|
|
|byte 0 |byte 1 |byte 2 |byte 3 |
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|
+-------+-------+-------+-------+
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|
<------------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:
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|
.ft CW
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|
.DS
|
|
enum { name-identifier = constant, ... } identifier;
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|
.DE
|
|
For example, the three colors red, yellow, and blue could be
|
|
described by an enumerated type:
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|
.DS
|
|
.ft CW
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|
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
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|
\&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
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|
.ft CW
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|
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<m>;
|
|
.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<m>;
|
|
.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<m>;
|
|
.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 <<struct, union, or enum definition>> 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 *
|
|
.DE
|
|
.NH 3
|
|
\&Syntax Notes
|
|
.IX "XDR language" syntax
|
|
.LP
|
|
.IP 1.
|
|
The following are keywords and cannot be used as identifiers:
|
|
"bool", "case", "const", "default", "double", "enum", "float",
|
|
"hyper", "opaque", "string", "struct", "switch", "typedef", "union",
|
|
"unsigned" and "void".
|
|
.IP 2.
|
|
Only unsigned constants may be used as size specifications for
|
|
arrays. If an identifier is used, it must have been declared
|
|
previously as an unsigned constant in a "const" definition.
|
|
.IP 3.
|
|
Constant and type identifiers within the scope of a specification
|
|
are in the same name space and must be declared uniquely within this
|
|
scope.
|
|
.IP 4.
|
|
Similarly, variable names must be unique within the scope of
|
|
struct and union declarations. Nested struct and union declarations
|
|
create new scopes.
|
|
.IP 5.
|
|
The discriminant of a union must be of a type that evaluates to
|
|
an integer. That is, "int", "unsigned int", "bool", an enumerated
|
|
type or any typedefed type that evaluates to one of these is legal.
|
|
Also, the case values must be one of the legal values of the
|
|
discriminant. Finally, a case value may not be specified more than
|
|
once within the scope of a union declaration.
|
|
.NH 1
|
|
\&An Example of an XDR Data Description
|
|
.LP
|
|
Here is a short XDR data description of a thing called a "file",
|
|
which might be used to transfer files from one machine to another.
|
|
.ie t .DS
|
|
.el .DS L
|
|
.ft CW
|
|
|
|
const MAXUSERNAME = 32; /*\fI max length of a user name \fP*/
|
|
const MAXFILELEN = 65535; /*\fI max length of a file \fP*/
|
|
const MAXNAMELEN = 255; /*\fI max length of a file name \fP*/
|
|
|
|
.ft I
|
|
/*
|
|
* Types of files:
|
|
*/
|
|
.ft CW
|
|
|
|
enum filekind {
|
|
TEXT = 0, /*\fI ascii data \fP*/
|
|
DATA = 1, /*\fI raw data \fP*/
|
|
EXEC = 2 /*\fI executable \fP*/
|
|
};
|
|
|
|
.ft I
|
|
/*
|
|
* File information, per kind of file:
|
|
*/
|
|
.ft CW
|
|
|
|
union filetype switch (filekind kind) {
|
|
case TEXT:
|
|
void; /*\fI no extra information \fP*/
|
|
case DATA:
|
|
string creator<MAXNAMELEN>; /*\fI data creator \fP*/
|
|
case EXEC:
|
|
string interpretor<MAXNAMELEN>; /*\fI program interpretor \fP*/
|
|
};
|
|
|
|
.ft I
|
|
/*
|
|
* A complete file:
|
|
*/
|
|
.ft CW
|
|
|
|
struct file {
|
|
string filename<MAXNAMELEN>; /*\fI name of file \fP*/
|
|
filetype type; /*\fI info about file \fP*/
|
|
string owner<MAXUSERNAME>; /*\fI owner of file \fP*/
|
|
opaque data<MAXFILELEN>; /*\fI file data \fP*/
|
|
};
|
|
.DE
|
|
.LP
|
|
Suppose now that there is a user named "john" who wants to store
|
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his lisp program "sillyprog" that contains just the data "(quit)".
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His file would be encoded as follows:
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|
.TS
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|
box tab (&) ;
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|
lfI lfI lfI lfI
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|
rfL rfL rfL l .
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|
Offset&Hex Bytes&ASCII&Description
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|
_
|
|
0&00 00 00 09&....&Length of filename = 9
|
|
4&73 69 6c 6c&sill&Filename characters
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|
8&79 70 72 6f&ypro& ... and more characters ...
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|
12&67 00 00 00&g...& ... and 3 zero-bytes of fill
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|
16&00 00 00 02&....&Filekind is EXEC = 2
|
|
20&00 00 00 04&....&Length of interpretor = 4
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|
24&6c 69 73 70&lisp&Interpretor characters
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|
28&00 00 00 04&....&Length of owner = 4
|
|
32&6a 6f 68 6e&john&Owner characters
|
|
36&00 00 00 06&....&Length of file data = 6
|
|
40&28 71 75 69&(qui&File data bytes ...
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|
44&74 29 00 00&t)..& ... and 2 zero-bytes of fill
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|
.TE
|
|
.NH 1
|
|
\&References
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|
.LP
|
|
[1] Brian W. Kernighan & Dennis M. Ritchie, "The C Programming
|
|
Language", Bell Laboratories, Murray Hill, New Jersey, 1978.
|
|
.LP
|
|
[2] Danny Cohen, "On Holy Wars and a Plea for Peace", IEEE Computer,
|
|
October 1981.
|
|
.LP
|
|
[3] "IEEE Standard for Binary Floating-Point Arithmetic", ANSI/IEEE
|
|
Standard 754-1985, Institute of Electrical and Electronics
|
|
Engineers, August 1985.
|
|
.LP
|
|
[4] "Courier: The Remote Procedure Call Protocol", XEROX
|
|
Corporation, XSIS 038112, December 1981.
|