KnowledgeBase Archive

An Archive of Early Microsoft KnowledgeBase Articles

View on GitHub

Q81577: Limits on the Number of Open Files

Article: Q81577
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.0,3.1
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 06-NOV-1999

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) versions 3.0, 3.1 
-------------------------------------------------------------------------------

SUMMARY
=======

Because an application developed for the Windows graphical environment runs only
within the MS-DOS environment, the number of files that the application can open
is subject to two distinct limits imposed by MS-DOS. This article discusses
these limits.

MORE INFORMATION
================

The first limit on the number of open files is imposed by a table of SFT data
structures (System File Tables) within MS-DOS. The initial number of SFTs is
specified in the FILES= line of the CONFIG.SYS file. MS-DOS version 5.0 does not
provide a function to change the number of SFTs in the table. However, some
applications, including Windows, contain the information necessary to change the
size of the table.

The SFT table is global, that is, it is shared by all applications or tasks that
are active in the system. (Each task is represented by a PSP or program segment
prefix.) The exception to the global nature of the SFT table, and the associated
limit on the number of open files, in introduced by Windows. Different groupings
of applications, called virtual machines (or VMs), have a "per VM" address
space. This allows a virtual machine to have a local portion of the SFT table,
which exists only in that VM. Because all graphical applications in the Windows
environment run in a single VM, only the MS-DOS (not graphical) applications run
in separate VMs.

MS-DOS does not support more than 255 SFTs. Therefore, it does not support more
than 255 open files at any time in any specific VM. The SHARE utility, which
extends MS-DOS core functionality, does not support the ability to increase the
size of the SFT table on a "per VM" basis under Windows, because SHARE must, at
any time, in any VM, be able to enumerate all open SFTs in the entire system.

The second limit on the number of open files involves a "per application" table,
called the JFN table, which is stored in each task's PSP. By default, the JFN in
the PSP has room for 20 entries, which limits each application to 20 open files.
An application running on MS-DOS version 3.3 and subsequent versions can change
the size of the JFN by calling MS-DOS INT 21h function 67h. This call allocates
a new JFN table and modifies values in the PSP to indicate the larger size of
the JFN table. This allows the application corresponding to a particular PSP to
open more than 20 files, provided that the global SFT table, shared by all
applications, has available SFTs.

The two limits work together as follows. First, to open a file, the application
must have a place available in its JFN table. Second, MS-DOS must have an
available SFT in its internal SFT table. Note that if either of these
requirements is not met, the OPEN (or CREATE) INT 21h call will fail with error
4.

Additional query words: 3.00 3.10

======================================================================
Keywords          : kb16bitonly 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300 kbWinSDK310
Version           : WINDOWS:3.0,3.1

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.