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Browse Prior Art Database

Method for Sharing File Handles in a Dos Environment

IP.com Disclosure Number: IPCOM000113971D
Original Publication Date: 1994-Oct-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 4 page(s) / 112K

Publishing Venue

IBM

Related People

Feigenbaum, B: AUTHOR [+3]

Abstract

Disclosed is a method for sharing file handles in a DOS environment, so that two file systems can operate concurrently while appearing as a single file system to applications. With this feature, a version of DOS can be booted into an OS/2* virtual machine session. While user applications may run in this session using the booted DOS operating system with its file system, the DOS file system is not given access to the actual hardfiles of the system, which are only accessed through the OS/2 file system.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Method for Sharing File Handles in a Dos Environment

      Disclosed is a method for sharing file handles in a DOS
environment, so that two file systems can operate concurrently while
appearing as a single file system to applications.  With this
feature, a version of DOS can be booted into an OS/2* virtual machine
session.  While user applications may run in this session using the
booted DOS operating system with its file system, the DOS file system
is not given access to the actual hardfiles of the system, which are
only accessed through the OS/2 file system.

      System applications needing to provide additional file system
support in networking environments often do so by hooking the DOS INT
21 interface.  With this approach, the application intercepts all INT

21 calls to determine which file system will handle the call, using
routines that check the drive number, file handle, or
file-specification parameters passed along in the call.  With a drive
number or file specification, it is straightforward to determine
whether a hardfile is being referenced.  On the other hand, since it
is difficult to determine with which drive a file handle is
associated, the entire file handle range is often divided into two
separate ranges by using the "sign" bit of the handle.  However, with
this technique, fewer files may be opened than otherwise allowed.
For example, if the file handle range is split at the half-way point,
only half as many files can be opened.  Also, many user applications
expect to receive certain file handles from DOS, or file handles
supplied in a contiguous order.  Furthermore, multiple applications
using this technique may collide with each other when they attempt to
use overlapping ranges.

      These difficulties are overcome using a DOS device driver,
named FSFILTER.SYS, allowing applications running in an OS/2-specific
DOS session to access the hardfiles of the system by intercepting all
INT 21h function calls and by rerouting the calls referencing mapped
drives to the OS/2 file system.

      Fig. 1 summarizes the assignment of various file numbers as a
result of a number of different commands within two DOS applications,
process 1 and process 2, where C: is mapped to OS/2, but A: is not.
The handle number, or Job File Number (JFN).

      Fig. 2 shows the Process Data Block (PDB) for each of the
applications of Fig. 1.  There may be several PDBs, each of which is
a structure contain...