Usenet 1994 January/usenetsourcesnewsgroupsinfomagicjanuary1994.iso/sources/unix/volume6/rpc2/part03/rpc/doc/xdr.spec.p1

Text File | 1986-11-30 | 48.6 KB | 1,995 lines

.\" @(#)c3.xdr.spec 1.1 86/02/25 SMI; for 3.0 FCS
.PL RIGHT
.EQ
delim $$
.EN
.TL
External Data Representation
.br
Protocol Specification
.bp
.NH
Introduction
.LP
This manual describes library routines that allow a C programmer to
describe arbitrary data structures in a machine-independent fashion.
The eXternal Data Representation (XDR) standard
is the backbone of Sun's Remote Procedure Call package,
in the sense that data for remote procedure calls
is transmitted using the standard.
XDR library routines should be used to transmit data
that is accessed (read or written) by more than one type of machine.
.LP
This manual contains a description of XDR library routines,
a guide to accessing currently available XDR streams,
information on defining new streams and data types,
and a formal definition of the XDR standard.
XDR was designed to work across different languages,
operating systems, and machine architectures.
Most users (particularly RPC users)
only need the information in sections 2 and 3 of this document.
Programmers wishing to implement RPC and XDR on new machines
will need the information in sections 4 through 6.
Advanced topics, not necessary for all implementations,
are covered in section 7.
.LP
On Sun systems,
C programs that want to use XDR routines
must include the file
.LW <rpc/rpc.h> ,
which contains all the necessary interfaces to the XDR system.
Since the C library
.LW libc.a
contains all the XDR routines,
compile as normal.
.LS
% \f(LBcc\0\fIprogram\fP.c\fL
.Lf
.NH 2
Justification
.LP
Consider the following two programs,
.LW writer :
.LS
#include <stdio.h>
.sp.5
main()            /* writer.c */
{
    long i;
.sp.5
    for (i = 0; i < 8; i++) {
        if (fwrite((char *)&i, sizeof(i), 1, stdout) != 1) {
            fprintf(stderr, "failed!\en");
            exit(1);
        }
    }
}
.Lf
and
.LW reader :
.LS
#include <stdio.h>
.sp.5
main()            /* reader.c */
{
    long i, j;
.sp.5
    for (j = 0; j < 8; j++) {
        if (fread((char *)&i, sizeof (i), 1, stdin) != 1) {
            fprintf(stderr, "failed!\en");
            exit(1);
        }
        printf("%ld ", i);
    }
    printf("\en");
}
.Lf
The two programs appear to be portable, because
(a) they pass
.LW lint
checking, and
(b) they exhibit the same behavior when executed
on two different hardware architectures, a Sun and a VAX.
.LP
Piping the output of the
.LW writer
program to the
.LW reader
program gives identical results on a Sun or a VAX.\(dd
.FS \(dd
VAX is a trademark of Digital Equipment Corporation.
.FE
.LS
sun% writer | reader
0 1 2 3 4 5 6 7
sun%
.Lf
.LS
vax% writer | reader
0 1 2 3 4 5 6 7
vax%
.Lf
With the advent of local area networks and Berkeley's 4.2 BSD
.UX
came the concept of ``network pipes'' \(em
a process produces data on one machine,
and a second process consumes data on another machine.
A network pipe can be constructed with
.LW writer
and
.LW reader .
Here are the results if the first produces data on a Sun,
and the second consumes data on a VAX.
.LS
sun% writer | rsh vax reader
0 16777216 33554432 50331648 67108864 83886080 100663296
117440512
sun%
.Lf
Identical results can be obtained by executing
.LW writer
on the VAX and
.LW reader
on the Sun.
These results occur because the byte ordering
of long integers differs between the VAX and the Sun,
even though word size is the same.
Note that $16777216$ is $2 sup 24$ \(em
when four bytes are reversed, the 1 winds up in the 24th bit.
.LP
Whenever data is shared by two or more machine types,
there is a need for portable data.
Programs can be made data-portable by replacing the
.LW read()
and
.LW write()
calls with calls to an XDR library routine
.LW xdr_long() ,
a filter that knows the standard representation
of a long integer in its external form.
Here are the revised versions of
.LW writer :
.LS
#include <stdio.h>
#include <rpc/rpc.h>    /* xdr is a sub-library of rpc */
.sp.5
main()        /* writer.c */
{
    XDR xdrs;
    long i;
.sp.5
    xdrstdio_create(&xdrs, stdout, XDR_ENCODE);
    for (i = 0; i < 8; i++) {
        if (!xdr_long(&xdrs, &i)) {
            fprintf(stderr, "failed!\en");
            exit(1);
        }
    }
}
.Lf
and
.LW reader :
.LS
#include <stdio.h>
#include <rpc/rpc.h>    /* xdr is a sub-library of rpc */
.sp.5
main()        /* reader.c */
{
    XDR xdrs;
    long i, j;
.sp.5
    xdrstdio_create(&xdrs, stdin, XDR_DECODE);
    for (j = 0; j < 8; j++) {
        if (!xdr_long(&xdrs, &i)) {
            fprintf(stderr, "failed!\en");
            exit(1);
        }
        printf("%ld ", i);
    }
    printf("\en");
}
.Lf
The new programs were executed on a Sun,
on a VAX, and from a Sun to a VAX;
the results are shown below.
.LS
sun% writer | reader
0 1 2 3 4 5 6 7
sun%
.Lf
.LS
vax% writer | reader
0 1 2 3 4 5 6 7
vax%
.Lf
.LS
sun% writer | rsh vax reader
0 1 2 3 4 5 6 7
sun%
.Lf
Dealing with integers is just the tip of the portable-data iceberg.
Arbitrary data structures present portability problems,
particularly with respect to alignment and pointers.
Alignment on word boundaries may cause the
size of a structure to vary from machine to machine.
Pointers are convenient to use,
but have no meaning outside the machine where they are defined.
.NH 2
The XDR Library
.LP
The XDR library solves data portability problems.
It allows you to write and read arbitrary C constructs
in a consistent, specified, well-documented manner.
Thus, it makes sense to use the library even when the data
is not shared among machines on a network.
.LP
The XDR library has filter routines for
strings (null-terminated arrays of bytes),
structures, unions, and arrays, to name a few.
Using more primitive routines,
you can write your own specific XDR routines
to describe arbitrary data structures,
including elements of arrays, arms of unions,
or objects pointed at from other structures.
The structures themselves may contain arrays of arbitrary elements,
or pointers to other structures.
.LP
Let's examine the two programs more closely.
There is a family of XDR stream creation routines
in which each member treats the stream of bits differently.
In our example, data is manipulated using standard I/O routines,
so we use
.LW xdrstdio_create() .
The parameters to XDR stream creation routines
vary according to their function.
In our example,
.LW xdrstdio_create()
takes a pointer to an XDR structure that it initializes,
a pointer to a
.LW FILE
that the input or output is performed on, and the operation.
The operation may be
.LW XDR_ENCODE
for serializing in the
.LW writer
program, or
.LW XDR_DECODE
for deserializing in the
.LW reader
program.
.LP
Note: RPC clients never need to create XDR streams;
the RPC system itself creates these streams,
which are then passed to the clients.
.LP
The
.LW xdr_long()
primitive is characteristic of most XDR library
primitives and all client XDR routines.
First, the routine returns
.LW FALSE
(0) if it fails, and
.LW TRUE
(1) if it succeeds.
Second, for each data type,
.LW xxx ,
there is an associated XDR routine of the form:
.LS
xdr_xxx(xdrs, fp)
    XDR *xdrs;
    xxx *fp;
{
}
.Lf
In our case,
.LW xxx
is long, and the corresponding XDR routine is
a primitive,
.LW xdr_long .
The client could also define an arbitrary structure
.LW xxx
in which case the client would also supply the routine
.LW xdr_xxx ,
describing each field by calling XDR routines
of the appropriate type.
In all cases the first parameter,
.LW xdrs
can be treated as an opaque handle,
and passed to the primitive routines.
.LP
XDR routines are direction independent;
that is, the same routines are called to serialize or deserialize data.
This feature is critical to software engineering of portable data.
The idea is to call the same routine for either operation \(em
this almost guarantees that serialized data can also be deserialized.
One routine is used by both producer and consumer of networked data.
This is implemented by always passing the address
of an object rather than the object itself \(em
only in the case of deserialization is the object modified.
This feature is not shown in our trivial example,
but its value becomes obvious when nontrivial data structures
are passed among machines.
If needed, you can obtain the direction of the XDR operation.
See section 3.7 for details.
.LP
Let's look at a slightly more complicated example.
Assume that a person's gross assets and liabilities
are to be exchanged among processes.
Also assume that these values are important enough
to warrant their own data type:
.LS
struct gnumbers {
    long g_assets;
    long g_liabilities;
};
.Lf
The corresponding XDR routine describing this structure would be:
.LS
bool_t          /* TRUE is success, FALSE is failure */
xdr_gnumbers(xdrs, gp)
    XDR *xdrs;
    struct gnumbers *gp;
{
    if (xdr_long(xdrs, &gp->g_assets) &&
        xdr_long(xdrs, &gp->g_liabilities))
        return(TRUE);
    return(FALSE);
}
.Lf
Note that the parameter
.LW xdrs
is never inspected or modified;
it is only passed on to the subcomponent routines.
It is imperative to inspect the return value of each XDR routine call,
and to give up immediately and return
.LW FALSE
if the subroutine fails.
.LP
This example also shows that the type
.LW bool_t
is declared as an integer whose only values are
.LW TRUE
(1) and
.LW FALSE
(0).  This document uses the following definitions:
.LS
#define bool_t    int
#define TRUE    1
#define FALSE    0
.sp.5
#define enum_t int    /* enum_t used for generic enums */
.Lf
.LP
Keeping these conventions in mind,
.LW xdr_gnumbers()
can be rewritten as follows:
.LS
xdr_gnumbers(xdrs, gp)
    XDR *xdrs;
    struct gnumbers *gp;
{
    return(xdr_long(xdrs, &gp->g_assets) &&
        xdr_long(xdrs, &gp->g_liabilities));
}
.Lf
This document uses both coding styles.
.bp
.NH
XDR Library Primitives
.LP
This section gives a synopsis of each XDR primitive.
It starts with basic data types and moves on to constructed data types.
Finally, XDR utilities are discussed.
The interface to these primitives
and utilities is defined in the include file
.LW <rpc/xdr.h> ,
automatically included by
.LW <rpc/rpc.h> .
.NH 2
Number Filters
.LP
The XDR library provides primitives to translate between numbers
and their corresponding external representations.
Primitives cover the set of numbers in:
.EQ
[signed, unsigned] * [short, int, long]
.EN
Specifically, the six primitives are:
.LS
bool_t xdr_int(xdrs, ip)
    XDR *xdrs;
    int *ip;
.sp.5
bool_t xdr_u_int(xdrs, up)
    XDR *xdrs;
    unsigned *up;
.sp.5
bool_t xdr_long(xdrs, lip)
    XDR *xdrs;
    long *lip;
.sp.5
bool_t xdr_u_long(xdrs, lup)
    XDR *xdrs;
    u_long *lup;
.sp.5
bool_t xdr_short(xdrs, sip)
    XDR *xdrs;
    short *sip;
.sp.5
bool_t xdr_u_short(xdrs, sup)
    XDR *xdrs;
    u_short *sup;
.Lf
The first parameter,
.LW xdrs ,
is an XDR stream handle.
The second parameter is the address of the number
that provides data to the stream or receives data from it.
All routines return
.LW TRUE
if they complete successfully, and
.LW FALSE
otherwise.
.NH 2
Floating Point Filters
.LP
The XDR library also provides primitive routines
for C's floating point types:
.LS
bool_t xdr_float(xdrs, fp)
    XDR *xdrs;
    float *fp;
.sp.5
bool_t xdr_double(xdrs, dp)
    XDR *xdrs;
    double *dp;
.Lf
The first parameter,
.LW xdrs
is an XDR stream handle.
The second parameter is the address
of the floating point number that provides data to the stream
or receives data from it.
All routines return
.LW TRUE
if they complete successfully, and
.LW FALSE
otherwise.
.LP
Note: Since the numbers are represented in IEEE floating point,
routines may fail when decoding a valid IEEE representation
into a machine-specific representation, or vice-versa.
.NH 2
Enumeration Filters
.LP
The XDR library provides a primitive for generic enumerations.
The primitive assumes that a C
.LW enum
has the same representation inside the machine as a C integer.
The boolean type is an important instance of the
.LW enum .
The external representation of a boolean is always one
.LW TRUE ) (
or zero
.LW FALSE ). (
.LS
#define bool_t    int
#define FALSE    0
#define TRUE    1
.sp.5
#define enum_t int
.sp.5
bool_t xdr_enum(xdrs, ep)
    XDR *xdrs;
    enum_t *ep;
.sp.5
bool_t xdr_bool(xdrs, bp)
    XDR *xdrs;
    bool_t *bp;
.Lf
The second parameters
.LW ep
and
.LW bp
are addresses of the associated type
that provides data to, or receives data from, the stream
.LW xdrs .
The routines return
.LW TRUE
if they complete successfully, and
.LW FALSE
otherwise.
.NH 2
No Data
.LP
Occasionally, an XDR routine must be supplied to the RPC system,
even when no data is passed or required.
The library provides such a routine:
.LS
bool_t xdr_void();  /* always returns TRUE */
.Lf
.NH 2
Constructed Data Type Filters
.LP
Constructed or compound data type primitives
require more parameters and perform more complicated functions
then the primitives discussed above.
This section includes primitives for
strings, arrays, unions, and pointers to structures.
.LP
Constructed data type primitives may use memory management.
In many cases, memory is allocated when deserializing data with
.LW XDR_DECODE .
Therefore, the XDR package must provide means to deallocate memory.
This is done by an XDR operation,
.LW XDR_FREE .
To review, the three XDR directional operations are
.LW XDR_ENCODE ,
.LW XDR_DECODE ,
and
.LW XDR_FREE .
.NH 3
Strings
.LP
In C, a string is defined as a sequence of bytes
terminated by a null byte,
which is not considered when calculating string length.
However, when a string is passed or manipulated,
a pointer to it is employed.
Therefore, the XDR library defines a string to be a
.LW "char *" ,
and not a sequence of characters.
The external representation of a string is drastically different
from its internal representation.
Externally, strings are represented as
sequences of ASCII characters,
while internally, they are represented with character pointers.
Conversion between the two representations
is accomplished with the routine
.LW xdr_string() :
.LS
bool_t xdr_string(xdrs, sp, maxlength)
    XDR *xdrs;
    char **sp;
    u_int maxlength;
.Lf
The first parameter
.LW xdrs
is the XDR stream handle.
The second parameter
.LW sp
is a pointer to a string (type
.LW "char **" ).
The third parameter
.LW maxlength
specifies the maximum number of bytes allowed during encoding or decoding;
its value is usually specified by a protocol.
For example, a protocol specification may say
that a file name may be no longer than 255 characters.
The routine returns
.LW FALSE
if the number of characters exceeds
.LW maxlength ,
and
.LW TRUE
if it doesn't.
.LP
The behavior of
.LW xdr_string()
is similar to the behavior of other routines
discussed in this section.  The direction
.LW XDR_ENCODE
is easiest to understand.  The parameter
.LW sp
points to a string of a certain length;
if it does not exceed
.LW maxlength ,
the bytes are serialized.
.LP
The effect of deserializing a string is subtle.
First the length of the incoming string is determined;
it must not exceed
.LW maxlength .
Next
.LW sp
is dereferenced; if the the value is
.LW NULL ,
then a string of the appropriate length is allocated and
.LW *sp
is set to this string.
If the original value of
.LW *sp
is non-null, then the XDR package assumes
that a target area has been allocated,
which can hold strings no longer than
.LW maxlength .
In either case, the string is decoded into the target area.
The routine then appends a null character to the string.
.LP
In the
.LW XDR_FREE
operation, the string is obtained by dereferencing
.LW sp .
If the string is not
.LW NULL ,
it is freed and
.LW *sp
is set to
.LW NULL .
In this operation,
.LW xdr_string
ignores the
.LW maxlength
parameter.
.NH 3
Byte Arrays
.LP
Often variable-length arrays of bytes are preferable to strings.
Byte arrays differ from strings in the following three ways:
1) the length of the array (the byte count) is explicitly
located in an unsigned integer,
2) the byte sequence is not terminated by a null character, and
3) the external representation of the bytes is the same as their
internal representation.
The primitive
.LW xdr_bytes()
converts between the internal and external
representations of byte arrays:
.LS
bool_t xdr_bytes(xdrs, bpp, lp, maxlength)
    XDR *xdrs;
    char **bpp;
    u_int *lp;
    u_int maxlength;
.Lf
The usage of the first, second and fourth parameters
are identical to the first, second and third parameters of
.LW xdr_string() ,
respectively.
The length of the byte area is obtained by dereferencing
.LW lp
when serializing;
.LW *lp
is set to the byte length when deserializing.
.NH 3
Arrays
.LP
The XDR library package provides a primitive
for handling arrays of arbitrary elements.
The
.LW xdr_bytes()
routine treats a subset of generic arrays,
in which the size of array elements is known to be 1,
and the external description of each element is built-in.
The generic array primitive,
.LW xdr_array()
requires parameters identical to those of
.LW xdr_bytes()
plus two more:
the size of array elements,
and an XDR routine to handle each of the elements.
This routine is called to encode or decode
each element of the array.
.LS
bool_t
xdr_array(xdrs, ap, lp, maxlength, elementsiz, xdr_element)
    XDR *xdrs;
    char **ap;
    u_int *lp;
    u_int maxlength;
    u_int elementsiz;
    bool_t (*xdr_element)();
.Lf
The parameter
.LW ap
is the address of the pointer to the array.
If
.LW *ap
is
.LW NULL
when the array is being deserialized,
XDR allocates an array of the appropriate size and sets
.LW *ap
to that array.
The element count of the array is obtained from
.LW *lp
when the array is serialized;
.LW *lp
is set to the array length when the array is deserialized.
The parameter
.LW maxlength
is the maximum number of elements that the array is allowed to have;
.LW elementsiz
is the byte size of each element of the array
(the C function
.LW sizeof()
can be used to obtain this value).
The routine
.LW xdr_element
is called to serialize, deserialize, or free
each element of the array.
.NH 4
Examples
.LP
Before defining more constructed data types,
it is appropriate to present three examples.
.LP
.I "Example A"
.LP
A user on a networked machine can be identified by
(a) the machine name, such as
.LW krypton :
see
.I gethostname (3);
(b) the user's UID: see
.I geteuid (2);
and (c) the group numbers to which the user belongs: see
.I getgroups (2).
A structure with this information and its associated XDR routine
could be coded like this:
.LS
struct netuser {
    char    *nu_machinename;
    int     nu_uid;
    u_int    nu_glen;
    int     *nu_gids;
};
#define NLEN 255    /* machine names < 256 chars */
#define NGRPS 20    /* user can't be in > 20 groups */
.sp.5
bool_t
xdr_netuser(xdrs, nup)
    XDR *xdrs;
    struct netuser *nup;
{
    return(xdr_string(xdrs, &nup->nu_machinename, NLEN) &&
        xdr_int(xdrs, &nup->nu_uid) &&
        xdr_array(xdrs, &nup->nu_gids, &nup->nu_glen, NGRPS,
        sizeof (int), xdr_int));
}
.Lf
.LP
.I "Example B"
.LP
A party of network users could be implemented
as an array of
.LW netuser
structure.
The declaration and its associated XDR routines
are as follows:
.LS
struct party {
    u_int p_len;
    struct netuser *p_nusers;
};
#define PLEN 500 /* max number of users in a party */
.sp.5
bool_t
xdr_party(xdrs, pp)
    XDR *xdrs;
    struct party *pp;
{
    return(xdr_array(xdrs, &pp->p_nusers, &pp->p_len, PLEN,
        sizeof (struct netuser), xdr_netuser));
}
.Lf
.LP
.I "Example C"
.LP
The well-known parameters to
.LW main() ,
.LW argc
and
.LW argv
can be combined into a structure.
An array of these structures can make up a history of commands.
The declarations and XDR routines might look like:
.LS no
struct cmd {
    u_int c_argc;
    char **c_argv;
};
#define ALEN 1000    /* args cannot be > 1000 chars */
#define NARGC 100    /* commands cannot have > 100 args */
.sp.5
struct history {
    u_int h_len;
    struct cmd *h_cmds;
};
#define NCMDS 75  /* history is no more than 75 commands */
.sp.5
bool_t
xdr_wrap_string(xdrs, sp)
    XDR *xdrs;
    char **sp;
{
    return(xdr_string(xdrs, sp, ALEN));
}
.sp.5
bool_t
xdr_cmd(xdrs, cp)
    XDR *xdrs;
    struct cmd *cp;
{
    return(xdr_array(xdrs, &cp->c_argv, &cp->c_argc, NARGC,
        sizeof (char *), xdr_wrap_string));
}
.sp.5
bool_t
xdr_history(xdrs, hp)
    XDR *xdrs;
    struct history *hp;
{
    return(xdr_array(xdrs, &hp->h_cmds, &hp->h_len, NCMDS,
        sizeof (struct cmd), xdr_cmd));
}
.Lf
The most confusing part of this example is that the routine
.LW xdr_wrap_string()
is needed to package the
.LW xdr_string()
routine, because the implementation of
.LW xdr_array()
only passes two parameters to the array element description routine;
.LW xdr_wrap_string()
supplies the third parameter to
.LW xdr_string() .
.LP
By now the recursive nature of the XDR library should be obvious.
Let's continue with more constructed data types.
.NH 3
Opaque Data
.LP
In some protocols, handles are passed from a server to client.
The client passes the handle back to the server at some later time.
Handles are never inspected by clients;
they are obtained and submitted.
That is to say, handles are opaque.
The primitive
.LW xdr_opaque()
is used for describing fixed sized, opaque bytes.
.LS
bool_t xdr_opaque(xdrs, p, len)
    XDR *xdrs;
    char *p;
    u_int len;
.Lf
The parameter
.LW p
is the location of the bytes;
.LW len
is the number of bytes in the opaque object.
By definition, the actual data
contained in the opaque object are not machine portable.
.NH 3
Fixed Sized Arrays
.LP
The XDR library does not provide a primitive for fixed-length arrays
(the primitive
.LW xdr_array()
is for varying-length arrays).
Example A could be rewritten to use fixed-sized arrays
in the following fashion:
.LS no
#define NLEN 255    /* machine names must be < 256 chars */
#define NGRPS 20    /* user can't belong to > 20 groups */
.sp.5
struct netuser {
    char *nu_machinename;
    int nu_uid;
    int nu_gids[NGRPS];
};
.sp.5
bool_t
xdr_netuser(xdrs, nup)
    XDR *xdrs;
    struct netuser *nup;
{
    int i;
.sp.5
    if (!xdr_string(xdrs, &nup->nu_machinename, NLEN))
        return(FALSE);
    if (!xdr_int(xdrs, &nup->nu_uid))
        return(FALSE);
    for (i = 0; i < NGRPS; i++) {
        if (!xdr_int(xdrs, &nup->nu_gids[i]))
            return(FALSE);
    }
    return(TRUE);
}
.Lf
.LP
Exercise:
Rewrite example A so that it uses varying-length arrays and so that the
.LW netuser
structure contains the actual
.LW nu_gids
array body as in the example above.
.NH 3
Discriminated Unions
.LP
The XDR library supports discriminated unions.
A discriminated union is a C union and an
.LW enum_t
value that selects an ``arm'' of the union.
.LS
struct xdr_discrim {
    enum_t value;
    bool_t (*proc)();
};
.sp.5
bool_t xdr_union(xdrs, dscmp, unp, arms, defaultarm)
    XDR *xdrs;
    enum_t *dscmp;
    char *unp;
    struct xdr_discrim *arms;
    bool_t (*defaultarm)();  /* may equal NULL */
.Lf
First the routine translates the discriminant of the union located at
.LW *dscmp .
The discriminant is always an
.LW enum_t .
Next the union located at
.LW *unp
is translated.
The parameter
.LW arms
is a pointer to an array of
.LW xdr_discrim
structures.
Each structure contains an order pair of
.LW [value,proc] .
If the union's discriminant is equal to the associated
.LW value ,
then the
.LW proc
is called to translate the union.
The end of the
.LW xdr_discrim
structure array is denoted by a routine of value
.LW NULL
(0).  If the discriminant is not found in the
.LW arms
array, then the
.LW defaultarm
procedure is called if it is non-null;
otherwise the routine returns
.LW FALSE .
.LP
.I "Example D"
.LP
Suppose the type of a union may be integer,
character pointer (a string), or a
.LW gnumbers
structure.
Also, assume the union and its current type
are declared in a structure.
The declaration is:
.LS
enum utype { INTEGER=1, STRING=2, GNUMBERS=3 };
.sp.5
struct u_tag {
    enum utype utype;    /* the union's discriminant */
    union {
        int ival;
        char *pval;
        struct gnumbers gn;
    } uval;
};
.Lf
The following constructs and XDR procedure (de)serialize
the discriminated union:
.LS
struct xdr_discrim u_tag_arms[4] = {
    { INTEGER, xdr_int },
    { GNUMBERS, xdr_gnumbers }
    { STRING, xdr_wrap_string },
    { __dontcare__, NULL }
    /* always terminate arms with a NULL xdr_proc */
}
.sp.5
bool_t
xdr_u_tag(xdrs, utp)
    XDR *xdrs;
    struct u_tag *utp;
{
    return(xdr_union(xdrs, &utp->utype, &utp->uval,
        u_tag_arms, NULL));
}
.Lf
The routine
.LW xdr_gnumbers()
was presented in Section 2;
.LW xdr_wrap_string()
was presented in example C.
The default arm parameter to
.LW xdr_union()
(the last parameter) is
.LW NULL
in this example.  Therefore the value of the union's discriminant
may legally take on only values listed in the
.LW u_tag_arms
array.  This example also demonstrates that
the elements of the arm's array do not need to be sorted.
.LP
It is worth pointing out that the values of the discriminant
may be sparse, though in this example they are not.
It is always good
practice to assign explicitly integer values to each element of the
discriminant's type.
This practice both documents the external
representation of the discriminant and guarantees that different
C compilers emit identical discriminant values.
.LP
Exercise: Implement
.LW xdr_union()
using the other primitives in this section.
.NH 3
Pointers
.LP
In C it is often convenient to put pointers
to another structure within a structure.
The primitive
.LW xdr_reference()
makes it easy to serialize, deserialize, and free
these referenced structures.
.LS
bool_t xdr_reference(xdrs, pp, size, proc)
    XDR *xdrs;
    char **pp;
    u_int ssize;
    bool_t (*proc)();
.Lf
.LP
Parameter
.LW pp
is the address of
the pointer to the structure;
parameter
.LW ssize
is the size in bytes of the structure
(use the C function
.LW sizeof()
to obtain this value); and
.LW proc
is the XDR routine that describes the structure.
When decoding data, storage is allocated if
.LW *pp
is
.LW NULL .
.LP
There is no need for a primitive
.LW xdr_struct()
to describe structures within structures,
because pointers are always sufficient.
.LP
Exercise: Implement
.LW xdr_reference()
using
.LW xdr_array() .
Warning:
.LW xdr_reference()
and
.LW xdr_array()
are NOT interchangeable external representations of data.
.LP
.I "Example E"
.LP
Suppose there is a structure containing a person's name
and a pointer to a
.LW gnumbers
structure containing the person's gross assets and liabilities.
The construct is:
.LS
struct pgn {
    char *name;
    struct gnumbers *gnp;
};
.Lf
The corresponding XDR routine for this structure is:
.LS
bool_t
xdr_pgn(xdrs, pp)
    XDR *xdrs;
    struct pgn *pp;
{
    if (xdr_string(xdrs, &pp->name, NLEN) &&
      xdr_reference(xdrs, &pp->gnp,
      sizeof(struct gnumbers), xdr_gnumbers))
        return(TRUE);
    return(FALSE);
}
.Lf
.NH 4
Pointer Semantics and XDR
.LP
In many applications,
C programmers attach double meaning to the values of a pointer.
Typically the value
.LW NULL
(or zero) means data is not needed,
yet some application-specific interpretation applies.
In essence, the C programmer is encoding
a discriminated union efficiently
by overloading the interpretation of the value of a pointer.
For instance, in example E a
.LW NULL
pointer value for
.LW gnp
could indicate that
the person's assets and liabilities are unknown.
That is, the pointer value encodes two things:
whether or not the data is known;
and if it is known, where it is located in memory.
Linked lists are an extreme example of the use
of application-specific pointer interpretation.
.LP
The primitive
.LW xdr_reference()
cannot and does not attach any special
meaning to a null-value pointer during serialization.
That is, passing an address of a pointer whose value is
.LW NULL
to
.LW xdr_reference()
when serialing data will most likely cause a memory fault and, on
.UX ,
a core dump for debugging.
.LP
It is the explicit responsibility of the programmer
to expand non-dereferenceable pointers into their specific semantics.
This usually involves describing data with a two-armed discriminated union.
One arm is used when the pointer is valid;
the other is used when the pointer is invalid
.LW NULL ). (
Section 7 has an example (linked lists encoding) that deals
with invalid pointer interpretation.
.LP
Exercise:
After reading Section 7, return here and extend example E so that
it can correctly deal with null pointer values.
.LP
Exercise:
Using the
.LW xdr_union() ,
.LW xdr_reference()
and
.LW xdr_void()
primitives, implement a generic pointer handling primitive
that implicitly deals with
.LW NULL
pointers.  The XDR library does not provide such a primitive
because it does not want to give the illusion
that pointers have meaning in the external world.
.NH 2
Non-filter Primitives
.LP
XDR streams can be manipulated with
the primitives discussed in this section.
.LS
u_int xdr_getpos(xdrs)
    XDR *xdrs;
.sp.5
bool_t xdr_setpos(xdrs, pos)
    XDR *xdrs;
    u_int pos;
.sp.5
xdr_destroy(xdrs)
    XDR *xdrs;
.Lf
The routine
.LW xdr_getpos()
returns an unsigned integer
that describes the current position in the data stream.
Warning: In some XDR streams, the returned value of
.LW xdr_getpos()
is meaningless;
the routine returns a \-1 in this case
(though \-1 should be a legitimate value).
.LP
The routine
.LW xdr_setpos()
sets a stream position to
.LW pos .
Warning: In some XDR streams, setting a position is impossible;
in such cases,
.LW xdr_setpos()
will return
.LW FALSE .
This routine will also fail if the requested position is out-of-bounds.
The definition of bounds varies from stream to stream.
.LP
The
.LW xdr_destroy()
primitive destroys the XDR stream.
Usage of the stream
after calling this routine is undefined.
.NH 2
XDR Operation Directions
.LP
At times you may wish to optimize XDR routines by taking
advantage of the direction of the operation \(em
.LW XDR_ENCODE ,
.LW XDR_DECODE ,
or
.LW XDR_FREE .
The value
.LW xdrs->x_op
always contains the
direction of the XDR operation.
Programmers are not encouraged to take advantage of this information.
Therefore, no example is presented here.
However, an example in Section 7
demonstrates the usefulness of the
.LW xdrs->x_op
field.
.bp
.NH
XDR Stream Access
.LP
An XDR stream is obtained by calling the appropriate creation routine.
These creation routines take arguments that are tailored to the
specific properties of the stream.
.LP
Streams currently exist for (de)serialization of data to or from
standard I/O
.LW FILE
streams, TCP/IP connections and
.UX
files, and memory.
Section 5 documents the XDR object and how to make
new XDR streams when they are required.
.NH 2
Standard I/O Streams
.LP
XDR streams can be interfaced to standard I/O using the
.LW xdrstdio_create()
routine as follows:
.LS
#include <stdio.h>
#include <rpc/rpc.h>    /* xdr streams part of rpc */
.sp.5
void
xdrstdio_create(xdrs, fp, x_op)
    XDR *xdrs;
    FILE *fp;
    enum xdr_op x_op;
.Lf
The routine
.LW xdrstdio_create()
initializes an XDR stream pointed to by
.LW xdrs .
The XDR stream interfaces to the standard I/O library.
Parameter
.LW fp
is an open file, and
.LW x_op
is an XDR direction.
.NH 2
Memory Streams
.LP
Memory streams allow the streaming of data into or out of
a specified area of memory:
.LS
#include <rpc/rpc.h>
.sp.5
void
xdrmem_create(xdrs, addr, len, x_op)
    XDR *xdrs;
    char *addr;
    u_int len;
    enum xdr_op x_op;
.Lf
The routine
.LW xdrmem_create()
initializes an XDR stream in local memory.
The memory is pointed to by parameter
.LW addr ;
parameter
.LW len
is the length in bytes of the memory.
The parameters
.LW xdrs
and
.LW x_op
are identical to the corresponding parameters of
.LW xdrstdio_create() .
Currently, the UDP/IP implementation of RPC uses
.LW xdrmem_create() .
Complete call or result messages are built in memory before calling the
.LW sendto()
system routine.
.NH 2
Record (TCP/IP) Streams
.LP
A record stream is an XDR stream built on top of
a record marking standard that is built on top of the
.UX
file or 4.2 BSD connection interface.
.LS
#include <rpc/rpc.h>    /* xdr streams part of rpc */
.sp.5
xdrrec_create(xdrs,
  sendsize, recvsize, iohandle, readproc, writeproc)
    XDR *xdrs;
    u_int sendsize, recvsize;
    char *iohandle;
    int (*readproc)(), (*writeproc)();
.Lf
The routine
.LW xdrrec_create()
provides an XDR stream interface that allows for a bidirectional,
arbitrarily long sequence of records.
The contents of the records are meant to be data in XDR form.
The stream's primary use is for interfacing RPC to TCP connections.
However, it can be used to stream data into or out of normal
.UX
files.
.LP
The parameter
.LW xdrs
is similar to the corresponding parameter described above.
The stream does its own data buffering similar to that of standard I/O.
The parameters
.LW sendsize
and
.LW recvsize
determine the size in bytes of the output and input buffers, respectively;
if their values are zero (0), then predetermined defaults are used.
When a buffer needs to be filled or flushed, the routine
.LW readproc
or
.LW writeproc
is called, respectively.
The usage and behavior of these
routines are similar to the
.UX
system calls
.LW read()
and
.LW write() .
However,
the first parameter to each of these routines is the opaque parameter
.LW iohandle .
The other two parameters
.LW buf "" (
and
.LW nbytes )
and the results
(byte count) are identical to the system routines.
If
.LW xxx
is
.LW readproc
or
.LW writeproc ,
then it has the following form:
.LS
/*
 * returns the actual number of bytes transferred.
 * -1 is an error
 */
int
xxx(iohandle, buf, len)
    char *iohandle;
    char *buf;
    int nbytes;
.Lf
The XDR stream provides means for delimiting records in the byte stream.
The implementation details of delimiting records in a stream
are discussed in appendix 1.
The primitives that are specific to record streams are as follows:
.LS
bool_t
xdrrec_endofrecord(xdrs, flushnow)
    XDR *xdrs;
    bool_t flushnow;
.sp.5
bool_t
xdrrec_skiprecord(xdrs)
    XDR *xdrs;
.sp.5
bool_t
xdrrec_eof(xdrs)
    XDR *xdrs;
.Lf
The routine
.LW xdrrec_endofrecord()
causes the current outgoing data to be marked as a record.
If the parameter
.LW flushnow
is
.LW TRUE ,
then the stream's
.LW writeproc()
will be called; otherwise,
.LW writeproc()
will be called when the output buffer has been filled.
.LP
The routine
.LW xdrrec_skiprecord()
causes an input stream's position to be moved past
the current record boundary and onto the
beginning of the next record in the stream.
.LP
If there is no more data in the stream's input buffer,
then the routine
.LW xdrrec_eof()
returns
.LW TRUE .
That is not to say that there is no more data
in the underlying file descriptor.
.bp
.NH
XDR Stream Implementation
.LP
This section provides the abstract data types needed
to implement new instances of XDR streams.
.NH 2
The XDR Object
.LP
The following structure defines the interface to an XDR stream:
.LS
enum xdr_op { XDR_ENCODE=0, XDR_DECODE=1, XDR_FREE=2 };
.sp.5
typedef struct {
    enum xdr_op x_op;    /* operation; fast added param */
    struct xdr_ops {
        bool_t  (*x_getlong)();  /* get long from stream */
        bool_t  (*x_putlong)();  /* put long to stream */
        bool_t  (*x_getbytes)(); /* get bytes from stream */
        bool_t  (*x_putbytes)(); /* put bytes to stream */
        u_int   (*x_getpostn)(); /* return stream offset */
        bool_t  (*x_setpostn)(); /* reposition offset */
        caddr_t (*x_inline)();   /* ptr to buffered data */
        VOID    (*x_destroy)();  /* free private area */
    } *x_ops;
    caddr_t    x_public;    /* users' data */
    caddr_t    x_private;    /* pointer to private data */
    caddr_t    x_base;        /* private for position info */
    int        x_handy;    /* extra private word */
} XDR;
.Lf
The
.LW x_op
field is the current operation being performed on the stream.
This field is important to the XDR primitives,
but should not affect a stream's implementation.
That is, a stream's implementation should not depend
on this value.
The fields
.LW x_private ,
.LW x_base ,
and
.LW x_handy
are private to the particular
stream's implementation.
The field
.LW x_public
is for the XDR client and should never be used by
the XDR stream implementations or the XDR primitives.
.LP
Macros for accessing  operations
.LW x_getpostn() ,
.LW x_setpostn() ,
and
.LW x_destroy()
were defined in Section 3.6.
The operation
.LW x_inline()
takes two parameters:
an XDR *, and an unsigned integer, which is a byte count.
The routine returns a pointer to a piece of
the stream's internal buffer.
The caller can then use the buffer segment for any purpose.
From the stream's point of view, the bytes in the
buffer segment have been consumed or put.
The routine may return
.LW NULL
if it cannot return a buffer segment of the requested size.
(The
.LW x_inline
routine is for cycle squeezers.
Use of the resulting buffer is not data-portable.
Users are encouraged not to use this feature.)
.LP
The operations
.LW x_getbytes()
and
.LW x_putbytes()
blindly get and put sequences of bytes
from or to the underlying stream;
they return
.LW TRUE
if they are successful, and
.LW FALSE
otherwise.  The routines have identical parameters (replace
.LW xxx ):
.LS
bool_t
xxxbytes(xdrs, buf, bytecount)
    XDR *xdrs;
    char *buf;
    u_int bytecount;
.Lf
The operations
.LW x_getlong()
and
.LW x_putlong()
receive and put
long numbers from and to the data stream.
It is the responsibility of these routines
to translate the numbers between the machine representation
and the (standard) external representation.
The
.UX
primitives
.LW htonl()
and
.LW ntohl()
can be helpful in accomplishing this.
Section 6 defines the standard representation of numbers.
The higher-level XDR implementation assumes that
signed and unsigned long integers contain the same number of bits,
and that nonnegative integers
have the same bit representations as unsigned integers.
The routines return
.I TRUE
if they succeed, and
.LW FALSE
otherwise.  They have identical parameters:
.LS
bool_t
xxxlong(xdrs, lp)
    XDR *xdrs;
    long *lp;
.Lf
Implementors of new XDR streams must make an XDR structure
(with new operation routines) available to clients,
using some kind of create routine.
.bp
.NH
XDR Standard
.LP
This section defines the external data representation standard.
The standard is independent of languages,
operating systems and hardware architectures.
Once data is shared among machines, it should not matter that the data
was produced on a Sun, but is consumed by a VAX (or vice versa).
Similarly the choice of operating systems should have no influence
on how the data is represented externally.
For programming languages,
data produced by a C program should be readable
by a Fortran or Pascal program.
.LP
The external data representation standard depends on the assumption that
bytes (or octets) are portable.
A byte is defined to be eight bits of data.
It is assumed that hardware
that encodes bytes onto various media
will preserve the bytes' meanings
across hardware boundaries.
For example, the Ethernet standard suggests that bytes be
encoded ``little endian'' style.
Both Sun and VAX hardware implementations
adhere to the standard.
.LP
The XDR standard also suggests a language used to describe data.
The language is a bastardized C;
it is a data description language, not a programming language.
(The Xerox Courier Standard uses bastardized Mesa
as its data description language.)
.NH 2
Basic 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 $n-1$, where $(n ~ \fRmod\fP ~ 4) = 0$.
The bytes are read or written to some byte stream
such that byte $m$ always precedes byte $m+1$.
.NH 2
Integer
.LP
An XDR signed integer is a 32-bit datum
that encodes an integer in the range
.LW [-2147483648,2147483647] .
The integer is represented in two's complement notation.
The most and least significant bytes are 0 and 3, respectively.
The data description of integers is
.LW integer .
.NH 2
Unsigned Integer
.LP
An XDR unsigned integer is a 32-bit datum
that encodes a nonnegative integer in the range
.LW [0,4294967295] .
It is represented by an unsigned binary number whose most
and least significant bytes are 0 and 3, respectively.
The data description of unsigned integers is
.LW unsigned .
.NH 2
Enumerations
.LP
Enumerations have the same representation as integers.
Enumerations are handy for describing subsets of the integers.
The data description of enumerated data is as follows:
.LS
typedef enum { name = value, .... } type-name;
.Lf
For example the three colors red, yellow and blue
could be described by an enumerated type:
.LS
typedef enum { RED = 2, YELLOW = 3, BLUE = 5 } colors;
.Lf
.NH 2
Booleans
.LP
Booleans are important enough and occur frequently enough
to warrant their own explicit type in the standard.
Boolean is an enumeration with the
following form:
.LS
typedef enum { FALSE = 0, TRUE = 1 } boolean;
.Lf
.NH 2
Hyper Integer and Hyper Unsigned
.LP
The standard also defines 64-bit (8-byte) numbers called
.LW "hyper integer"
and
.LW "hyper unsigned" .
Their representations are the obvious extensions of
the integer and unsigned defined above.
The most and least significant bytes are 0 and 7, respectively.
.NH 2
Floating Point and Double Precision
.LP
The standard defines the encoding for the floating point data types
.LW float
(32 bits or 4 bytes) and
.LW double
(64 bits or 8 bytes).
The encoding used is the IEEE standard for normalized
single- and double-precision floating point numbers.
See the IEEE floating point standard for more information.
The standard encodes the following three fields,
which describe the floating point number:
.IP \fIS\fP
The sign of the number.
Values 0 and 1 represent
positive and negative, respectively.
.IP \fIE\fP
The exponent of the number, base 2.
Floats devote 8 bits to this field,
while doubles devote 11 bits.
The exponents for float and double are
biased by 127 and 1023, respectively.
.IP \fIF\fP
The fractional part of the number's mantissa, base 2.
Floats devote 23 bits to this field,
while doubles devote 52 bits.
.LP
Therefore, the floating point number is described by:
.EQ
(-1) sup S * 2 sup { E - Bias } * 1.F
.EN
.LP
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.
.LP
Doubles have the analogous extensions.
The beginning bit (and
most significant bit) offsets of $S$, $E$, and $F$
are 0, 1, and 12, respectively.
.LP
The IEEE specification should be consulted concerning the encoding for
signed zero, signed infinity (overflow), and denormalized numbers (underflow).
Under IEEE specifications, the ``NaN'' (not a number)
is system dependent and should not be used.
.NH 2
Opaque Data
.LP
At times fixed-sized uninterpreted data
needs to be passed among machines.
This data is called
.LW opaque
and is described as:
.LS
typedef opaque type-name[n];
opaque name[n];
.Lf
where
.LW n
is the (static) number of bytes necessary to contain the opaque data.
If
.LW n
is not a multiple of four, then the
.LW n
bytes are followed by enough (up to 3) zero-valued bytes
to make the total byte count of the opaque object a multiple of four.
.NH 2
Counted Byte Strings
.LP
The standard defines a string of $n$ (numbered $0$ through $n-1$)
bytes to be the number $n$ encoded as
.LW unsigned ,
and followed by the $n$ bytes of the string.
If $n$ is not a multiple of four,
then the $n$ bytes are followed by
enough (up to 3) zero-valued bytes
to make the total byte count a multiple of four.
The data description of strings is as follows:
.LS
typedef string type-name<N>;
typedef string type-name<>;
string name<N>;
string name<>;
.Lf
Note that the data description language uses angle brackets (< and >)
to denote anything that is varying-length
(as opposed to square brackets to denote fixed-length sequences of data).
.LP
The constant
.LW N
denotes an upper bound of the number of bytes that a
string may contain.
If
.LW N
is not specified, it is assumed to be $2 sup 32 - 1$,
the maximum length.
The constant
.LW N
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, such as:
.LS
string filename<255>;
.Lf
.LP
The XDR specification does not say what the
individual bytes of a string represent;
this important information is left to higher-level specifications.
A reasonable default is to assume
that the bytes encode ASCII characters.
.NH 2
Fixed Arrays
.LP
The data description for fixed-size arrays of
homogeneous elements is as follows:
.LS
typedef elementtype type-name[n];
elementtype name[n];
.Lf
Fixed-size arrays of elements numbered $0$ through $n-1$
are encoded by individually encoding the elements of the array
in their natural order, $0$ through $n-1$.
.NH 2
Counted Arrays
.LP
Counted arrays provide the ability to encode varyiable-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 data description for counted arrays
is similar to that of counted strings:
.LS
typedef elementtype type-name<N>;
typedef elementtype type-name<>;
elementtype name<N>;
elementtype name<>;
.Lf
Again, the constant
.LW N
specifies the maximum acceptable
element count of an array; if
.LW N
is  not specified, it is assumed to be $2 sup 32 - 1$.
.NH 2
Structures
.LP
The data description for structures is very similar to
that of standard C:
.LS
typedef struct {
    component-type component-name;
    ...
} type-name;
.Lf
The components of the structure are encoded
in the order of their declaration in the structure.
.NH 2
Discriminated Unions
.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 the discriminant is always an enumeration.
The component types are called ``arms'' of the union.
The discriminated union is encoded as its discriminant followed by
the encoding of the implied arm.
The data description for discriminated unions is as follows:
.LS
typedef union switch (discriminant-type) {
    discriminant-value: arm-type;
    ...
    default: default-arm-type;
} type-name;
.Lf
The default arm is optional.
If it is not specified, then a valid
encoding of the union cannot take on unspecified discriminant values.
Most specifications neither need nor use default arms.
.NH 2
Missing Specifications
.LP
The standard lacks representations for bit fields and bitmaps,
since the standard is based on bytes.
This is not to say that no specification should be attempted.
.NH 2
Library Primitive / XDR Standard Cross Reference
.LP
The following table describes the association between
the C library primitives discussed in Section 3,
and the standard data types defined in this section:
.LP
.TS
box;
cfBI s s
cfI cfI cfI
rfL | cfL | l.
.sp.1
Primitives and Data Types
.sp.1
_
C Primitive    XDR Type    Sections
_
xdr_int
xdr_long    integer    3.1, 6.2
xdr_short
_
xdr_u_int
xdr_u_long    unsigned    3.1, 6.3
xdr_u_short
_
-    hyper integer    6.6
    hyper unsigned
_
xdr_float    float    3.2, 6.7
_
xdr_double    double    3.2, 6.7
_
xdr_enum    enum_t    3.3, 6.4
_
xdr_bool    bool_t    3.3, 6.5
_
xdr_string    string    3.5.1, 6.9
xdr_bytes        3.5.2
_
xdr_array    \fR(varying arrays)\fP    3.5.3, 6.11
_
-    \fR(fixed arrays)\fP    3.5.5, 6.10
_
xdr_opaque    opaque    3.5.4, 6.8
_
xdr_union    union    3.5.6, 6.13
_
xdr_reference    -    3.5.7
_
-    struct    6.6
.TE
.TN "Primitives and Data Types"
.bp
.NH
Advanced Topics
.LP
This section describes techniques for passing data structures
that are not covered in the preceding sections.
Such structures include linked lists (of arbitrary lengths).
Unlike the simpler examples covered in the earlier sections,
the following examples are written using both
the XDR C library routines and the XDR data description language.
Section 6 describes the XDR data definition language used below.
.NH 2
Linked Lists
.LP
The last example in Section 2 presented a C data structure and its
associated XDR routines for a person's gross assets and liabilities.
The example is duplicated below:
.LS
struct gnumbers {
    long g_assets;
    long g_liabilities;
};
.sp.5
bool_t
xdr_gnumbers(xdrs, gp)
    XDR *xdrs;
    struct gnumbers *gp;
{
    if (xdr_long(xdrs, &(gp->g_assets)))
        return(xdr_long(xdrs, &(gp->g_liabilities)));
    return(FALSE);
}
.Lf
Now assume that we wish to implement a linked list of such information.
A data structure could be constructed as follows:
.LS
typedef struct gnnode {
    struct gnumbers gn_numbers;
    struct gnnode *nxt;
};
.sp.5
typedef struct gnnode *gnumbers_list;
.Lf
The head of the linked list can be thought of as the data object;
that is, the head is not merely a convenient shorthand for a structure.
Similarly the
.LW nxt
field is used to indicate whether or not the object has terminated.
Unfortunately, if the object continues, the
.LW nxt
field is also the address
of where it continues.
The link addresses carry no useful information when
the object is serialized.
.LP
The XDR data description of this linked list is described by the
recursive type declaration of gnumbers_list:
.LS
struct gnumbers {
    unsigned g_assets;
    unsigned g_liabilities;
};
.Lf
.LS
typedef union switch (boolean) {
    case TRUE: struct {
        struct gnumbers current_element;
        gnumbers_list rest_of_list;
    };
    case FALSE: struct {};
} gnumbers_list;
.Lf
In this description,
the boolean indicates whether there is more data following it.
If the boolean is
.LW FALSE ,
then it is the last data field of the structure.
If it is
.LW TRUE ,
then it is followed by a
.LW gnumbers
structure and (recursively) by a
.LW gnumbers_list
(the rest of the object).
Note that the C declaration has no boolean explicitly declared in it
(though the
.LW nxt
field implicitly carries the information), while
the XDR data description has no pointer explicitly declared in it.
.LP
Hints for writing a set of XDR routines to successfully (de)serialize
a linked list of entries can be taken
from the XDR description of the pointer-less data.
The set consists of the mutually recursive routines
.LW xdr_gnumbers_list ,
.LW xdr_wrap_list ,
and
.LW xdr_gnnode .
.LS
bool_t
xdr_gnnode(xdrs, gp)
    XDR *xdrs;
    struct gnnode *gp;
{
    return(xdr_gnumbers(xdrs, &(gp->gn_numbers)) &&
        xdr_gnumbers_list(xdrs, &(gp->nxt)) );
}
.Lf