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rfc:rfc1014

Network Working Group Sun Microsystems, Inc. Request for Comments: 1014 June 1987

             XDR: External Data Representation Standard

STATUS OF THIS MEMO

 This RFC describes a standard that Sun Microsystems, Inc., and others
 are using, one we wish to propose for the Internet's consideration.
 Distribution of this memo is unlimited.

1. INTRODUCTION

 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.
 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.
 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.

2. BASIC BLOCK SIZE

 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)

SUN Microsystems [Page 1] RFC 1014 External Data Representation June 1987

 residual zero bytes, r, to make the total byte count a multiple of 4.
 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.
      +--------+--------+...+--------+--------+...+--------+
      | byte 0 | byte 1 |...|byte n-1|    0   |...|    0   |   BLOCK
      +--------+--------+...+--------+--------+...+--------+
      |<-----------n bytes---------->|<------r bytes------>|
      |<-----------n+r (where (n+r) mod 4 = 0)>----------->|

3. XDR DATA TYPES

 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.
 For each data type in the language we show a general paradigm
 declaration.  Note that angle brackets (< and >) denote
 variablelength 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 section 5:
 "The XDR Language Specification".
 For some data types, more specific examples are included.  A more
 extensive example of a data description is in section 6:  "An Example
 of an XDR Data Description".

3.1 Integer

 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:
       int identifier;
         (MSB)                   (LSB)
       +-------+-------+-------+-------+
       |byte 0 |byte 1 |byte 2 |byte 3 |                      INTEGER
       +-------+-------+-------+-------+
       <------------32 bits------------>

SUN Microsystems [Page 2] RFC 1014 External Data Representation June 1987

3.2.Unsigned Integer

 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:
       unsigned int identifier;
         (MSB)                   (LSB)
       +-------+-------+-------+-------+
       |byte 0 |byte 1 |byte 2 |byte 3 |             UNSIGNED INTEGER
       +-------+-------+-------+-------+
       <------------32 bits------------>

3.3 Enumeration

 Enumerations have the same representation as signed integers.
 Enumerations are handy for describing subsets of the integers.
 Enumerated data is declared as follows:
       enum { name-identifier = constant, ... } identifier;
 For example, the three colors red, yellow, and blue could be
 described by an enumerated type:
       enum { RED = 2, YELLOW = 3, BLUE = 5 } colors;
 It is an error to encode as an enum any other integer than those that
 have been given assignments in the enum declaration.

3.4 Boolean

 Booleans are important enough and occur frequently enough to warrant
 their own explicit type in the standard.  Booleans are declared as
 follows:
    bool identifier;
    This is equivalent to:
       enum { FALSE = 0, TRUE = 1 } identifier;

SUN Microsystems [Page 3] RFC 1014 External Data Representation June 1987

3.5 Hyper Integer and Unsigned Hyper Integer

 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:
 hyper identifier; unsigned hyper identifier;
      (MSB)                                                   (LSB)
    +-------+-------+-------+-------+-------+-------+-------+-------+
    |byte 0 |byte 1 |byte 2 |byte 3 |byte 4 |byte 5 |byte 6 |byte 7 |
    +-------+-------+-------+-------+-------+-------+-------+-------+
    <----------------------------64 bits---------------------------->
                                               HYPER INTEGER
                                               UNSIGNED HYPER INTEGER

3.6 Floating-point

 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:
    S: The sign of the number.  Values 0 and 1 represent positive and
       negative, respectively.  One bit.
    E: The exponent of the number, base 2.  8 bits are devoted to this
       field.  The exponent is biased by 127.
    F: The fractional part of the number's mantissa, base 2.  23 bits
       are devoted to this field.
 Therefore, the floating-point number is described by:
       (-1)**S * 2**(E-Bias) * 1.F

SUN Microsystems [Page 4] RFC 1014 External Data Representation June 1987

 It is declared as follows:
       float identifier;
       +-------+-------+-------+-------+
       |byte 0 |byte 1 |byte 2 |byte 3 |              SINGLE-PRECISION
       S|   E   |           F          |         FLOATING-POINT NUMBER
       +-------+-------+-------+-------+
       1|<- 8 ->|<-------23 bits------>|
       <------------32 bits------------>
 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).
 The EEE 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.

3.7 Double-precision Floating-point

 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:
    S: The sign of the number.  Values 0 and 1 represent positive and
       negative, respectively.  One bit.
    E: The exponent of the number, base 2.  11 bits are devoted to
       this field.  The exponent is biased by 1023.
    F: The fractional part of the number's mantissa, base 2.  52 bits
       are devoted to this field.
 Therefore, the floating-point number is described by:
       (-1)**S * 2**(E-Bias) * 1.F

SUN Microsystems [Page 5] RFC 1014 External Data Representation June 1987

 It is declared as follows:
       double identifier;
       +------+------+------+------+------+------+------+------+
       |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------------------------->
                                      DOUBLE-PRECISION FLOATING-POINT
 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).
 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.

3.8 Fixed-length Opaque Data

 At times, fixed-length uninterpreted data needs to be passed among
 machines.  This data is called "opaque" and is declared as follows:
       opaque identifier[n];
 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.
        0        1     ...
    +--------+--------+...+--------+--------+...+--------+
    | byte 0 | byte 1 |...|byte n-1|    0   |...|    0   |
    +--------+--------+...+--------+--------+...+--------+
    |<-----------n bytes---------->|<------r bytes------>|
    |<-----------n+r (where (n+r) mod 4 = 0)------------>|
                                                 FIXED-LENGTH OPAQUE

3.9 Variable-length Opaque Data

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

SUN Microsystems [Page 6] RFC 1014 External Data Representation June 1987

 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.
 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).
 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.  Variable-length opaque data is declared in the
 following way:
       opaque identifier<m>;
    or
       opaque identifier<>;
 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:
       opaque filedata<8192>;
          0     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)---->|
                                                VARIABLE-LENGTH OPAQUE
 It is an error to encode a length greater than the maximum described
 in the specification.

3.10 String

 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:

SUN Microsystems [Page 7] RFC 1014 External Data Representation June 1987

       string object<m>;
    or
       string object<>;
 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:
       string filename<255>;
          0     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)---->|
                                                                STRING
 It is an error to encode a length greater than the maximum described
 in the specification.

3.11 Fixed-length Array

 Declarations for fixed-length arrays of homogeneous elements are in
 the following form:
       type-name identifier[n];
 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.
       +---+---+---+---+---+---+---+---+...+---+---+---+---+
       |   element 0   |   element 1   |...|  element n-1  |
       +---+---+---+---+---+---+---+---+...+---+---+---+---+
       |<--------------------n elements------------------->|
                                             FIXED-LENGTH ARRAY

SUN Microsystems [Page 8] RFC 1014 External Data Representation June 1987

3.12 Variable-length Array

 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:
       type-name identifier<m>;
    or
       type-name identifier<>;
 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.
         0  1  2  3
       +--+--+--+--+--+--+--+--+--+--+--+--+...+--+--+--+--+
       |     n     | element 0 | element 1 |...|element n-1|
       +--+--+--+--+--+--+--+--+--+--+--+--+...+--+--+--+--+
       |<-4 bytes->|<--------------n elements------------->|
                                                       COUNTED ARRAY
 It is an error to encode a value of n that is greater than the
 maximum described in the specification.

3.13 Structure

 Structures are declared as follows:
       struct {
          component-declaration-A;
          component-declaration-B;
          ...
       } identifier;
 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.
       +-------------+-------------+...
       | component A | component B |...                      STRUCTURE
       +-------------+-------------+...

3.14 Discriminated Union

 A discriminated union is a type composed of a discriminant followed
 by a type selected from a set of prearranged types according to the

SUN Microsystems [Page 9] RFC 1014 External Data Representation June 1987

 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:
       union switch (discriminant-declaration) {
       case discriminant-value-A:
          arm-declaration-A;
       case discriminant-value-B:
          arm-declaration-B;
       ...
       default: default-declaration;
       } identifier;
 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.
 The discriminated union is encoded as its discriminant followed by
 the encoding of the implied arm.
         0   1   2   3
       +---+---+---+---+---+---+---+---+
       |  discriminant |  implied arm  |          DISCRIMINATED UNION
       +---+---+---+---+---+---+---+---+
       |<---4 bytes--->|

3.15 Void

 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:
       void;
 Voids are illustrated as follows:
         ++
         ||                                                     VOID
         ++
       --><-- 0 bytes

3.16 Constant

 The data declaration for a constant follows this form:

SUN Microsystems [Page 10] RFC 1014 External Data Representation June 1987

       const name-identifier = n;
 "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.
       const DOZEN = 12;

3.17 Typedef

 "typedef" does not declare any data either, but serves to define new
 identifiers for declaring data. The syntax is:
       typedef declaration;
 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":
       typedef egg eggbox[DOZEN];
 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":
       eggbox  fresheggs;
       egg     fresheggs[DOZEN];
 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:
       typedef <<struct, union, or enum definition>> identifier;
 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":

SUN Microsystems [Page 11] RFC 1014 External Data Representation June 1987

       typedef enum {    /* using typedef */
          FALSE = 0,
          TRUE = 1
       } bool;
       enum bool {       /* preferred alternative */
          FALSE = 0,
          TRUE = 1
       };
 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.

3.18 Optional-data

 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:
       type-name *identifier;
 This is equivalent to the following union:
       union switch (bool opted) {
       case TRUE:
          type-name element;
       case FALSE:
          void;
       } identifier;
 It is also equivalent to the following variable-length array
 declaration, since the boolean "opted" can be interpreted as the
 length of the array:
       type-name identifier<1>;
 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:
       struct *stringlist {
          string item<>;
          stringlist next;
       };

SUN Microsystems [Page 12] RFC 1014 External Data Representation June 1987

 It could have been equivalently declared as the following union:
       union stringlist switch (bool opted) {
       case TRUE:
          struct {
             string item<>;
             stringlist next;
          } element;
       case FALSE:
          void;
       };
    or as a variable-length array:
       struct stringlist<1> {
          string item<>;
          stringlist next;
       };
 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.

3.19 Areas for Future Enhancement

 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.
 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.
 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.

SUN Microsystems [Page 13] RFC 1014 External Data Representation June 1987

4. DISCUSSION

 (1) Why use a language for describing data?  What's wrong with
 diagrams?
 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.
 (2) Why is there only one byte-order for an XDR unit?
 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.
 (3) Why is the XDR byte-order big-endian instead of little-endian?
 Isn't this unfair to little-endian machines such as the VAX(r), which
 has to convert from one form to the other?
 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.
 (4) Why is the XDR unit four bytes wide?
 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.

SUN Microsystems [Page 14] RFC 1014 External Data Representation June 1987

 (5) Why must variable-length data be padded with zeros?
 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.
 (6) Why is there no explicit data-typing?
 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.

5. THE XDR LANGUAGE SPECIFICATION

 5.1 Notational Conventions
 This specification uses an extended Back-Naur Form notation for
 describing the XDR language.  Here is a brief description of the
 notation:
 (1) The characters '|', '(', ')', '[', ']', '"', and '*' are special.
 (2) Terminal symbols are strings of any characters surrounded by
 double quotes.
 (3) Non-terminal symbols are strings of non-special characters.
 (4) Alternative items are separated by a vertical bar ("|").
 (5) Optional items are enclosed in brackets.
 (6) Items are grouped together by enclosing them in parentheses.
 (7) A '*' following an item means 0 or more occurrences of that item.
 For example,  consider  the  following pattern:
       "a " "very" (", " "very")* [" cold " "and "]  " rainy "
       ("day" | "night")
 An infinite number of strings match this pattern. A few of them are:

SUN Microsystems [Page 15] RFC 1014 External Data Representation June 1987

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

5.2 Lexical Notes

 (1) Comments begin with '/*' and terminate with '*/'.
 (2) White space serves to separate items and is otherwise ignored.
 (3) An identifier is a letter followed by an optional sequence of
 letters, digits or underbar ('_'). The case of identifiers is not
 ignored.
 (4) A constant is a sequence of one or more decimal digits,
 optionally preceded by a minus-sign ('-').

5.3 Syntax Information

    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"
    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 )

SUN Microsystems [Page 16] RFC 1014 External Data Representation June 1987

          ( "," 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 *

5.4 Syntax Notes

 (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".
 (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.

SUN Microsystems [Page 17] RFC 1014 External Data Representation June 1987

 (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.
 (4) Similarly, variable names must  be unique within  the scope  of
 struct and union declarations. Nested struct and union declarations
 create new scopes.
 (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.

6. AN EXAMPLE OF AN XDR DATA DESCRIPTION

 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.
       const MAXUSERNAME = 32;     /* max length of a user name */
       const MAXFILELEN = 65535;   /* max length of a file      */
       const MAXNAMELEN = 255;     /* max length of a file name */
       /*
        * Types of files:
        */
       enum filekind {
          TEXT = 0,       /* ascii data */
          DATA = 1,       /* raw data   */
          EXEC = 2        /* executable */
       };
       /*
        * File information, per kind of file:
        */
       union filetype switch (filekind kind) {
       case TEXT:
          void;                           /* no extra information */
       case DATA:
          string creator<MAXNAMELEN>;     /* data creator         */
       case EXEC:
          string interpretor<MAXNAMELEN>; /* program interpretor  */
       };

SUN Microsystems [Page 18] RFC 1014 External Data Representation June 1987

       /*
        * A complete file:
        */
       struct file {
          string filename<MAXNAMELEN>; /* name of file    */
          filetype type;               /* info about file */
          string owner<MAXUSERNAME>;   /* owner of file   */
          opaque data<MAXFILELEN>;     /* file data       */
       };
 Suppose now that there is a user named "john" who wants to store his
 lisp program "sillyprog" that contains just the data "(quit)".  His
 file would be encoded as follows:
     OFFSET  HEX BYTES       ASCII    COMMENTS
     ------  ---------       -----    --------
      0      00 00 00 09     ....     -- length of filename = 9
      4      73 69 6c 6c     sill     -- filename characters
      8      79 70 72 6f     ypro     -- ... and more characters ...
     12      67 00 00 00     g...     -- ... and 3 zero-bytes of fill
     16      00 00 00 02     ....     -- filekind is EXEC = 2
     20      00 00 00 04     ....     -- length of interpretor = 4
     24      6c 69 73 70     lisp     -- interpretor characters
     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 ...
     44      74 29 00 00     t)..     -- ... and 2 zero-bytes of fill

7. REFERENCES

 [1]  Brian W. Kernighan & Dennis M. Ritchie, "The C Programming
      Language", Bell Laboratories, Murray Hill, New Jersey, 1978.
 [2]  Danny Cohen, "On Holy Wars and a Plea for Peace", IEEE Computer,
      October 1981.
 [3]  "IEEE Standard for Binary Floating-Point Arithmetic", ANSI/IEEE
      Standard 754-1985, Institute of Electrical and Electronics
      Engineers, August 1985.
 [4]  "Courier: The Remote Procedure Call Protocol", XEROX
      Corporation, XSIS 038112, December 1981.

SUN Microsystems [Page 19] RFC 1014 External Data Representation June 1987

8. TRADEMARKS AND OWNERS

      SUN WORKSTATION  Sun Microsystems, Inc.
      VAX              Digital Equipment Corporation
      IBM-PC           International Business Machines Corporation
      Cray             Cray Research
      NFS              Sun Microsystems, Inc.
      Ethernet         Xerox Corporation.
      Motorola 68000   Motorola, Inc.
      IBM 370          International Business Machines Corporation

SUN Microsystems [Page 20]

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