Limits on the Number of Open Files (81577)

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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.