XDR Data Description (ONC+ Developer's Guide)

ONC+ Developer's Guide

XDR Data Description

Here is a short XDR data description of a file data structure, which might be used to transfer files from one machine to another.

Example C-2 XDR File Data Structur

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 interpreter<MAXNAMELEN>; /*proginterptr*/
};

/* 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:

Table C-2 XDR Data Description Example


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 interpreter = 4
24	6c 69 73 70	lisp	Interpreter 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	(qu	File data bytes ...
44	74 29 00 00	t)..	... and 2 zero-bytes of fill