rfc1832.txt

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Srinivasan                  Standards Track                     [Page 6]

RFC 1832       XDR: External Data Representation Standard    August 1995


   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 interpreted within XDR
   as anything other than "NaN".

3.8 Quadruple-precision Floating-point

   The standard defines the encoding for the quadruple-precision
   floating-point data type "quadruple" (128 bits or 16 bytes).  The
   encoding used is designed to be a simple analog of of the encoding
   used for single and double-precision floating-point numbers using one
   form of IEEE double extended precision. The standard encodes the
   following three fields, which describe the quadruple-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.  15 bits are devoted to
         this field.  The exponent is biased by 16383.

      F: The fractional part of the number's mantissa, base 2.  112 bits
         are devoted to this field.

   Therefore, the floating-point number is described by:

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

   It is declared as follows:

         quadruple identifier;

         +------+------+------+------+------+------+-...--+------+
         |byte 0|byte 1|byte 2|byte 3|byte 4|byte 5| ...  |byte15|
         S|    E       |                  F                      |
         +------+------+------+------+------+------+-...--+------+
         1|<----15---->|<-------------112 bits------------------>|
         <-----------------------128 bits------------------------>
                                      QUADRUPLE-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 quadruple-precision
   floating-point number are 0 and 127.  The beginning bit (and most



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RFC 1832       XDR: External Data Representation Standard    August 1995


   significant bit) offsets of S, E , and F are 0, 1, and 16,
   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 encoding for signed zero, signed infinity (overflow), and
   denormalized numbers are analogs of the corresponding encodings for
   single and double-precision floating-point numbers [5], [6].  The
   "NaN" encoding as it applies to quadruple-precision floating-point
   numbers is system dependent and should not be interpreted within XDR
   as anything other than "NaN".

3.9 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

3.10 Variable-length Opaque Data

   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.













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RFC 1832       XDR: External Data Representation Standard    August 1995


   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.

3.11 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:

         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



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RFC 1832       XDR: External Data Representation Standard    August 1995


   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.

3.12 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

3.13 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:




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RFC 1832       XDR: External Data Representation Standard    August 1995


         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.

3.14 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
         +-------------+-------------+...

3.15 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
   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:



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RFC 1832       XDR: External Data Representation Standard    August 1995


            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--->|

3.16 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

3.17 Constant

   The data declaration for a constant follows this form:

         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.




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