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Network Working Group                             Sun Microsystems, Inc.Request for Comments: 1014                                     June 1987XDR: External Data Representation StandardSTATUS 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 tosection 5:   "The XDR Language Specification".   For some data types, more specific examples are included.  A more   extensive example of a data description is insection 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 19873.2.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 19873.5 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 INTEGER3.6 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.FSUN 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.FSUN 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 OPAQUE3.9 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 ARRAYSUN Microsystems                                                [Page 8]

RFC 1014              External Data Representation             June 19873.12 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 theSUN 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 bytes3.16 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 19874. 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 fill16      00 00 00 02     ....     -- filekind is EXEC = 220      00 00 00 04     ....     -- length of interpretor = 4       24      6c 69 73 70     lisp     -- interpretor characters28      00 00 00 04     ....     -- length of owner = 4       32      6a 6f 68 6e     john     -- owner characters36      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 fill7. 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 19878. 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 CorporationSUN Microsystems                                               [Page 20]