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-rw-r--r-- | share/doc/psd/25.xdrrfc/xdr.rfc.ms | 1060 |
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diff --git a/share/doc/psd/25.xdrrfc/Makefile b/share/doc/psd/25.xdrrfc/Makefile index 105135e..7045d45 100644 --- a/share/doc/psd/25.xdrrfc/Makefile +++ b/share/doc/psd/25.xdrrfc/Makefile @@ -4,6 +4,5 @@ VOLUME= psd/25.xdrrfc SRCS= stubs xdr.rfc.ms MACROS= -ms USE_TBL= -SRCDIR= ${.CURDIR}/../../../../lib/libc/rpc/PSD.doc .include <bsd.doc.mk> diff --git a/share/doc/psd/25.xdrrfc/stubs b/share/doc/psd/25.xdrrfc/stubs new file mode 100644 index 0000000..78b0a2c --- /dev/null +++ b/share/doc/psd/25.xdrrfc/stubs @@ -0,0 +1,3 @@ +.\" $FreeBSD$ +.\" +.if t .ftr L CR diff --git a/share/doc/psd/25.xdrrfc/xdr.rfc.ms b/share/doc/psd/25.xdrrfc/xdr.rfc.ms new file mode 100644 index 0000000..480a339 --- /dev/null +++ b/share/doc/psd/25.xdrrfc/xdr.rfc.ms @@ -0,0 +1,1060 @@ +.\" +.\" Must use -- tbl -- with this one +.\" +.\" @(#)xdr.rfc.ms 2.2 88/08/05 4.0 RPCSRC +.\" $FreeBSD$ +.\" +.de BT +.if \\n%=1 .tl ''- % -'' +.. +.ND +.\" prevent excess underlining in nroff +.if n .fp 2 R +.OH 'External Data Representation Standard''Page %' +.EH 'Page %''External Data Representation Standard' +.if \n%=1 .bp +.SH +\&External Data Representation Standard: Protocol Specification +.IX "External Data Representation" +.IX XDR RFC +.IX XDR "protocol specification" +.LP +.NH 0 +\&Status of this Standard +.nr OF 1 +.IX XDR "RFC status" +.LP +Note: This chapter specifies a protocol that Sun Microsystems, Inc., and +others are using. It has been designated RFC1014 by the ARPA Network +Information Center. +.NH 1 +Introduction +\& +.LP +XDR is a standard for the description and encoding of data. It is +useful for transferring data between different computer +architectures, and has been used to communicate data between such +diverse machines as the Sun Workstation, VAX, IBM-PC, and Cray. +XDR fits into the ISO presentation layer, and is roughly analogous in +purpose to X.409, ISO Abstract Syntax Notation. The major difference +between these two is that XDR uses implicit typing, while X.409 uses +explicit typing. +.LP +XDR uses a language to describe data formats. The language can only +be used only to describe data; it is not a programming language. +This language allows one to describe intricate data formats in a +concise manner. The alternative of using graphical representations +(itself an informal language) quickly becomes incomprehensible when +faced with complexity. The XDR language itself is similar to the C +language [1], just as Courier [4] is similar to Mesa. Protocols such +as Sun RPC (Remote Procedure Call) and the NFS (Network File System) +use XDR to describe the format of their data. +.LP +The XDR standard makes the following assumption: that bytes (or +octets) are portable, where a byte is defined to be 8 bits of data. +A given hardware device should encode the bytes onto the various +media in such a way that other hardware devices may decode the bytes +without loss of meaning. For example, the Ethernet standard +suggests that bytes be encoded in "little-endian" style [2], or least +significant bit first. +.NH 2 +\&Basic Block Size +.IX XDR "basic block size" +.IX XDR "block size" +.LP +The representation of all items requires a multiple of four bytes (or +32 bits) of data. The bytes are numbered 0 through n-1. The bytes +are read or written to some byte stream such that byte m always +precedes byte m+1. If the n bytes needed to contain the data are not +a multiple of four, then the n bytes are followed by enough (0 to 3) +residual zero bytes, r, to make the total byte count a multiple of 4. +.LP +We include the familiar graphic box notation for illustration and +comparison. In most illustrations, each box (delimited by a plus +sign at the 4 corners and vertical bars and dashes) depicts a byte. +Ellipses (...) between boxes show zero or more additional bytes where +required. +.ie t .DS +.el .DS L +\fIA Block\fP + +\f(CW+--------+--------+...+--------+--------+...+--------+ +| byte 0 | byte 1 |...|byte n-1| 0 |...| 0 | ++--------+--------+...+--------+--------+...+--------+ +|<-----------n bytes---------->|<------r bytes------>| +|<-----------n+r (where (n+r) mod 4 = 0)>----------->|\fP + +.DE +.NH 1 +\&XDR Data Types +.IX XDR "data types" +.IX "XDR data types" +.LP +Each of the sections that follow describes a data type defined in the +XDR standard, shows how it is declared in the language, and includes +a graphic illustration of its encoding. +.LP +For each data type in the language we show a general paradigm +declaration. Note that angle brackets (< and >) denote +variable length sequences of data and square brackets ([ and ]) denote +fixed-length sequences of data. "n", "m" and "r" denote integers. +For the full language specification and more formal definitions of +terms such as "identifier" and "declaration", refer to +.I "The XDR Language Specification" , +below. +.LP +For some data types, more specific examples are included. +A more extensive example of a data description is in +.I "An Example of an XDR Data Description" +below. +.NH 2 +\&Integer +.IX XDR integer +.LP +An XDR signed integer is a 32-bit datum that encodes an integer in +the range [-2147483648,2147483647]. The integer is represented in +two's complement notation. The most and least significant bytes are +0 and 3, respectively. Integers are declared as follows: +.ie t .DS +.el .DS L +\fIInteger\fP + +\f(CW(MSB) (LSB) ++-------+-------+-------+-------+ +|byte 0 |byte 1 |byte 2 |byte 3 | ++-------+-------+-------+-------+ +<------------32 bits------------>\fP +.DE +.NH 2 +\&Unsigned Integer +.IX XDR "unsigned integer" +.IX XDR "integer, unsigned" +.LP +An XDR unsigned integer is a 32-bit datum that encodes a nonnegative +integer in the range [0,4294967295]. It is represented by an +unsigned binary number whose most and least significant bytes are 0 +and 3, respectively. An unsigned integer is declared as follows: +.ie t .DS +.el .DS L +\fIUnsigned Integer\fP + +\f(CW(MSB) (LSB) ++-------+-------+-------+-------+ +|byte 0 |byte 1 |byte 2 |byte 3 | ++-------+-------+-------+-------+ +<------------32 bits------------>\fP +.DE +.NH 2 +\&Enumeration +.IX XDR enumeration +.LP +Enumerations have the same representation as signed integers. +Enumerations are handy for describing subsets of the integers. +Enumerated data is declared as follows: +.ft CW +.DS +enum { name-identifier = constant, ... } identifier; +.DE +For example, the three colors red, yellow, and blue could be +described by an enumerated type: +.DS +.ft CW +enum { RED = 2, YELLOW = 3, BLUE = 5 } colors; +.DE +It is an error to encode as an enum any other integer than those that +have been given assignments in the enum declaration. +.NH 2 +\&Boolean +.IX XDR boolean +.LP +Booleans are important enough and occur frequently enough to warrant +their own explicit type in the standard. Booleans are declared as +follows: +.DS +.ft CW +bool identifier; +.DE +This is equivalent to: +.DS +.ft CW +enum { FALSE = 0, TRUE = 1 } identifier; +.DE +.NH 2 +\&Hyper Integer and Unsigned Hyper Integer +.IX XDR "hyper integer" +.IX XDR "integer, hyper" +.LP +The standard also defines 64-bit (8-byte) numbers called hyper +integer and unsigned hyper integer. Their representations are the +obvious extensions of integer and unsigned integer defined above. +They are represented in two's complement notation. The most and +least significant bytes are 0 and 7, respectively. Their +declarations: +.ie t .DS +.el .DS L +\fIHyper Integer\fP +\fIUnsigned Hyper Integer\fP + +\f(CW(MSB) (LSB) ++-------+-------+-------+-------+-------+-------+-------+-------+ +|byte 0 |byte 1 |byte 2 |byte 3 |byte 4 |byte 5 |byte 6 |byte 7 | ++-------+-------+-------+-------+-------+-------+-------+-------+ +<----------------------------64 bits---------------------------->\fP +.DE +.NH 2 +\&Floating-point +.IX XDR "integer, floating point" +.IX XDR "floating-point integer" +.LP +The standard defines the floating-point data type "float" (32 bits or +4 bytes). The encoding used is the IEEE standard for normalized +single-precision floating-point numbers [3]. The following three +fields describe the single-precision floating-point number: +.RS +.IP \fBS\fP: +The sign of the number. Values 0 and 1 represent positive and +negative, respectively. One bit. +.IP \fBE\fP: +The exponent of the number, base 2. 8 bits are devoted to this +field. The exponent is biased by 127. +.IP \fBF\fP: +The fractional part of the number's mantissa, base 2. 23 bits +are devoted to this field. +.RE +.LP +Therefore, the floating-point number is described by: +.DS +(-1)**S * 2**(E-Bias) * 1.F +.DE +It is declared as follows: +.ie t .DS +.el .DS L +\fISingle-Precision Floating-Point\fP + +\f(CW+-------+-------+-------+-------+ +|byte 0 |byte 1 |byte 2 |byte 3 | +S| E | F | ++-------+-------+-------+-------+ +1|<- 8 ->|<-------23 bits------>| +<------------32 bits------------>\fP +.DE +Just as the most and least significant bytes of a number are 0 and 3, +the most and least significant bits of a single-precision floating- +point number are 0 and 31. The beginning bit (and most significant +bit) offsets of S, E, and F are 0, 1, and 9, respectively. Note that +these numbers refer to the mathematical positions of the bits, and +NOT to their actual physical locations (which vary from medium to +medium). +.LP +The IEEE specifications should be consulted concerning the encoding +for signed zero, signed infinity (overflow), and denormalized numbers +(underflow) [3]. According to IEEE specifications, the "NaN" (not a +number) is system dependent and should not be used externally. +.NH 2 +\&Double-precision Floating-point +.IX XDR "integer, double-precision floating point" +.IX XDR "double-precision floating-point integer" +.LP +The standard defines the encoding for the double-precision floating- +point data type "double" (64 bits or 8 bytes). The encoding used is +the IEEE standard for normalized double-precision floating-point +numbers [3]. The standard encodes the following three fields, which +describe the double-precision floating-point number: +.RS +.IP \fBS\fP: +The sign of the number. Values 0 and 1 represent positive and +negative, respectively. One bit. +.IP \fBE\fP: +The exponent of the number, base 2. 11 bits are devoted to this +field. The exponent is biased by 1023. +.IP \fBF\fP: +The fractional part of the number's mantissa, base 2. 52 bits +are devoted to this field. +.RE +.LP +Therefore, the floating-point number is described by: +.DS +(-1)**S * 2**(E-Bias) * 1.F +.DE +It is declared as follows: +.ie t .DS +.el .DS L +\fIDouble-Precision Floating-Point\fP + +\f(CW+------+------+------+------+------+------+------+------+ +|byte 0|byte 1|byte 2|byte 3|byte 4|byte 5|byte 6|byte 7| +S| E | F | ++------+------+------+------+------+------+------+------+ +1|<--11-->|<-----------------52 bits------------------->| +<-----------------------64 bits------------------------->\fP +.DE +Just as the most and least significant bytes of a number are 0 and 3, +the most and least significant bits of a double-precision floating- +point number are 0 and 63. The beginning bit (and most significant +bit) offsets of S, E , and F are 0, 1, and 12, respectively. Note +that these numbers refer to the mathematical positions of the bits, +and NOT to their actual physical locations (which vary from medium to +medium). +.LP +The IEEE specifications should be consulted concerning the encoding +for signed zero, signed infinity (overflow), and denormalized numbers +(underflow) [3]. According to IEEE specifications, the "NaN" (not a +number) is system dependent and should not be used externally. +.NH 2 +\&Fixed-length Opaque Data +.IX XDR "fixed-length opaque data" +.IX XDR "opaque data, fixed length" +.LP +At times, fixed-length uninterpreted data needs to be passed among +machines. This data is called "opaque" and is declared as follows: +.DS +.ft CW +opaque identifier[n]; +.DE +where the constant n is the (static) number of bytes necessary to +contain the opaque data. If n is not a multiple of four, then the n +bytes are followed by enough (0 to 3) residual zero bytes, r, to make +the total byte count of the opaque object a multiple of four. +.ie t .DS +.el .DS L +\fIFixed-Length Opaque\fP + +\f(CW0 1 ... ++--------+--------+...+--------+--------+...+--------+ +| byte 0 | byte 1 |...|byte n-1| 0 |...| 0 | ++--------+--------+...+--------+--------+...+--------+ +|<-----------n bytes---------->|<------r bytes------>| +|<-----------n+r (where (n+r) mod 4 = 0)------------>|\fP +.DE +.NH 2 +\&Variable-length Opaque Data +.IX XDR "variable-length opaque data" +.IX XDR "opaque data, variable length" +.LP +The standard also provides for variable-length (counted) opaque data, +defined as a sequence of n (numbered 0 through n-1) arbitrary bytes +to be the number n encoded as an unsigned integer (as described +below), and followed by the n bytes of the sequence. +.LP +Byte m of the sequence always precedes byte m+1 of the sequence, and +byte 0 of the sequence always follows the sequence's length (count). +enough (0 to 3) residual zero bytes, r, to make the total byte count +a multiple of four. Variable-length opaque data is declared in the +following way: +.DS +.ft CW +opaque identifier<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 +his lisp program "sillyprog" that contains just the data "(quit)". +His file would be encoded as follows: +.TS +box tab (&) ; +lfI lfI lfI lfI +rfL rfL rfL l . +Offset&Hex Bytes&ASCII&Description +_ +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 +.TE +.NH 1 +\&References +.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. |