Windows for Workgroups: How VSHARE.386 Manages File Sharing

• Microsoft Windows for Workgroups, version 3.1

SUMMARY

The Windows for Workgroups file sharer, VSHARE.386, contains several improvements over the MS-DOS file sharing utility, SHARE.EXE. These improvements allow Windows for Workgroups to share files in a manner similar to other network operating systems.

This article defines the file sharer (NOT the file server) and its function in the operating system.

MORE INFORMATION

To be able to allow multiple users or programs concurrent access to files and resources, an operating system must provide two basic services:

• Arbitration of access requests
• Updating of in-memory information that describes resources

VSHARE.386 is intended as a drop-in replacement for SHARE.EXE. Because VSHARE is an Enhanced Mode virtual device driver (VxD), it has features which improve file sharing under Enhanced Mode. Because it was designed to work with Windows for Workgroups, VSHARE has features to make it "friendlier" than SHARE.EXE, especially in the area of access control. If SHARE.EXE is loaded and you start Windows for Workgroups in Enhanced Mode, VSHARE takes over as file sharer until you exit Windows. If you usually run Windows for Workgroups in Enhanced Mode, you may not need to run SHARE.EXE at all (this can save 5K or more of conventional memory).

For each file open on the local computer, MS-DOS maintains a copy of information about the file in memory. This includes both global information that pertains to the file on disk, such as the file's location on disk and its size, and local information that pertains to each open instance of the file, such as the application's file pointer. If a file is opened by multiple programs, and one application changes a global attribute of the file, the sharer updates the information for all file handles open on that file.

NOTE: The file handles that applications use to keep track of files point to the copies of information about files maintained by MS-DOS in memory.

If the real mode sharer SHARE.EXE is present without VSHARE, then all information needed to open a file must be in memory at all times, even when Enhanced Mode virtual memory is being used. (Virtual memory cannot be swapped into or out of memory while MS-DOS is busy with a file system request.) When you use VSHARE instead of (or in addition to) SHARE, file open information for multiple Virtual Machines (VMs) does not need to be in memory at the same time, only when each VM is active. (MS-DOS applications run in separate VMs, and the Windows for Workgroups file server runs its own separate VM.)

NOTE: This means you can use the Enhanced Mode PerVMFiles setting with VSHARE, which means you can open more files using VSHARE than using SHARE when running Windows for Workgroups in Enhanced Mode. See the "Microsoft Windows Resource Kit," version 3.1, for a discussion of the PerVMFiles setting.

Unlike SHARE (which uses global conventional memory), VSHARE uses extended memory to store its copy of open file information. This greatly increases the number of open files that Windows for Workgroups, especially the server, can work with at once. Applications which use file locking, such as Microsoft Mail, are less likely to run out of file locks with VSHARE than with SHARE.

Because Windows for Workgroups is designed to function as a peer server, the access control rules used by SHARE have been relaxed somewhat in VSHARE. SHARE forbids the opening of "compatibility mode" files while a "sharing" file is open, and vice versa. SHARE forbids the opening of "compatibility mode" files from different machines or VMs. Since a large percentage of MS-DOS applications and Windows applications still use compatibility mode, SHARE prevents the Windows for Workgroups server from operating effectively with several clients connected to one server (the only way SHARE could operate in this environment is if all the files on the server had read-only attributes). This makes it inconvenient for network administrators to set up directories of shared files on a network file server.

Using VSHARE, a compatibility mode read request can be active from different machines and at the same time as most sharing requests. This makes it easy to set up a directory of shared files under Windows for Workgroups. The access rules are only relaxed for "safe" access requests. If a process tries to open a file to write to it, for example, the more restrictive set of rules is used.

This new file access behavior is known as "softcompat mode." There may be a reason to disable this, if there is a conflict with an older application that has problems with the new access rules. This can be done by placing "SOFTCOMPATMODE=FALSE" in the [386ENH] section of the Windows SYSTEM.INI file. There are no applications known to require this setting.

It is necessary to use a file sharer on any machine being used as a file server or peer server. If an access violation happens on a server and "Sharing Violation" error message appears, it interferes with server performance. This is because MS-DOS is in a critical section while the error message pop-up is visible. For this reason, VSHARE disables sharing violation pop-up messages on MS-DOS versions 4.01 and greater. If this causes problems, you can add the "ENABLESHARINGPOPUPS=TRUE" setting to the [386ENH] section of the Windows SYSTEM.INI file. There are no applications known to require this setting.

The file sharer can also be used to control access to MS-DOS device drivers, if these are opened using MS-DOS file system calls. With SHARE present, it is possible to get sharing violations on the NUL and CON devices. VSHARE does not generate sharing violation error messages when you access these devices. You can restore SHARE device sharing functionality by adding "TRADITIONALDEVICESHARING=TRUE" to the [386ENH] section of the Windows SYSTEM.INI file. There are no applications known to require this setting.

Some applications are incompatible with file access checking. In general, it is not recommended that you run such applications under Windows for Workgroups, especially while the file server is running. You can use the SYSTEM.INI switch "IGNORESHARINGVIOLATIONS=TRUE" to diagnose such problems that occur when VSHARE is installed. This switch turns off file access checking for all files and devices in the system. Use this switch to diagnose problems only; you should not use it to correct problems. It usually causes more problems than it solves. If any application refuses to run with SHARE or VSHARE installed, contact the application vendor for assistance.

Sometimes problems occur where a "network aware" version of an application does not keep application documents open. These applications attempt to arbitrate document access by using a semaphore or marker in the document file or on disk. This technique usually fails in a multi-user network environment due to synchronization problems. In order to write an application properly to handle multi- user or multi-application access to data files, the first instance of the application accessing any data file should KEEP THE FILE OPEN to allow the file sharer and MS-DOS to communicate to subsequent applications that the file is in use. This suggestion applies equally to SHARE and VSHARE. There is no way that the file sharer can know that a file is "busy" unless it is actually kept open by the application.

Troubleshooting Tip

If problems with file sharing are suspected, remember that the file sharer responsible for a specific file is the sharer running on the machine where the file is physically located. If file sharing problems occur while you access a file on a network drive, modifying the VSHARE settings on the local machine will not help.