Developing Network Applications for Windows 3.1

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As local area networks (LANs) become increasingly common, application developers should ensure that their applications run properly in a network environment.

There are two areas that developers should keep in mind:

• Applications should be well behaved when shared by multiple users.
• Applications that access network software directly might require special measures to be compatible with protected (standard or 386 enhanced) mode.

There are other forms of networking available to developer which provide a layer of transparency from the protected vs real mode issues of Windows network programming. These network interfaces are not discussed in this article, but may be found in other articles throughout the Microsoft Knowledge Base. They are: Remote Procedure Calls (RPC), Windows Sockets, named pipes, mailslots, and NetDDE.

                   BEING SHARED BY MULTIPLE USERS

Many corporations are choosing to have their computer users share a single copy of an application that resides on a network server. Microsoft Windows version 3.1 can be run this way. The Windows Setup /n (network) option configures the user's system, which ensures that most Windows files are used directly off the network, while the user's personal files and configuration information are stored in a private Windows directory. (See the "Microsoft Windows User's Guide" for more information on using a shared copy of Windows.)

If the developer intends to allow shared copies of an application, then the developer must make sure that two users running the same application will not interfere with each other. The following sections present rules that should help the developer design the application for network support.

CREATING PERMANENT FILES

Many applications store configuration files in the same directory as the application's own executable file. This method will not work for multiple users because the application will store each user's information in the same directory, overwriting other users' information.

Instead, the application should use the GetWindowsDirectory() function to find the user's private Windows directory. This location is guaranteed to be unique for each user.

Refer to the "Microsoft Windows Software Development Kit Reference" for more information on the GetWindowsDirectory() function.

CREATING TEMPORARY FILES

When creating temporary files, use the GetTempFilename() function to determine the name and location of the file. This function will ensure that temporary filenames will not conflict, even if multiple users are sharing the same temporary storage directory.

USING PROFILE STRINGS

Profile and configuration information is usually stored in initialization (.INI) files using Windows functions such as WriteProfileString(). These functions fall into two categories: those that access WIN.INI, and those that access another .INI file specified by the program.

The following functions access WIN.INI:

• GetProfileString()
• GetProfileInt()
• WriteProfileString()

The following functions access other initialization files:

• GetPrivateProfileString()
• GetPrivateProfileInt()
• WritePrivateProfileString()

Initialization files that need to be shared among all users are the exception to this rule. Make sure that such files cannot be left in an inconsistent state if multiple users update them simultaneously.

For a full description of the initialization file functions, see the Windows SDK (Software Development Kit) reference manual.

             CALLING NETWORK SOFTWARE IN PROTECTED MODE

Windows-based applications running in protected mode require special support whenever they make a function call to real-mode software. This includes calls to MS-DOS, the BIOS, or a network. However, applications that are not based on Windows but are running with Windows do not require this special support because they always run in real or virtual-8086 mode.

The main support required is called "API mapping." If the arguments to the call include pointers to data, that data should be copied down into the first 1 MB of address space to ensure that the real-mode software can access it. The processor is then switched into real or virtual-8086 mode to ensure that the real-mode software can process the call. Finally, when that call returns, any data it modified is copied back up to the caller's protected-mode address.

Fortunately, most applications interact with the network only indirectly, by using MS-DOS functions to manipulate files on redirected drive letters, or by using MS-DOS or BIOS calls to print to a remote printer using redirected printer ports. Windows-based applications can continue to perform these functions normally because Windows automatically maps standard MS-DOS and BIOS calls.

However, some applications need to use functions that are specific to a particular network or networking protocol. Some part of the software must map these APIs, and in some cases this may require special procedures on the part of the programmer.

The remainder of this document describes programming considerations for designing Windows-based applications that use the following networking protocols and networks:

• Microsoft Networks and MS-DOS network functions
• NetBIOS functions
• Microsoft LAN Manager-based networks
• Novell NetWare
• Ungermann-Bass Net/One
• Banyan VINES

MICROSOFT NETWORKS AND MS-DOS NETWORK FUNCTIONS

Many networks on the market today are based on the Microsoft Network standard, also known as MS-NET. These networks support a set of standard MS- DOS functions that perform network activities, such as redirecting drive letters.

Windows automatically handles these MS-DOS functions. However, to maintain compatibility with future Windows products, the application should not make MS-DOS calls by executing an INT 21H instruction. Instead, it should set up all the registers for executing the INT 21H and then make a far call to the Windows DOS3Call() function.

See the Windows SDK reference manual for a full description of the DOS3Call() function. See the "MS-DOS Encyclopedia," available from Microsoft Press, for more information on Microsoft Network functions.

NETBIOS FUNCTIONS

NetBIOS is the most widely used networking API. These functions are normally called using INT 5CH. Windows handles most NetBIOS functions. However, to maintain compatibility with future Windows products, the application should not make the NetBIOS call by executing an INT 5CH instruction. Instead, it should set up all the registers for executing the INT 5CH, and then make a far call to the Windows NetBIOSCall() function.

The following rarely-used NetBIOS functions are not supported:

   Function  Description
   --------  -----------

   71H       Send.No.Ack
   72H       Chain.Send.No.Ack
   73H       Lan.Status.Alert
   78H       Find.Name
   79H       Trace

LAN MANAGER-BASED NETWORKS

Networks based on Microsoft LAN Manager can be installed in either Basic or Enhanced versions.

All versions of LAN Manager support MS-NET and NetBIOS functions. However, if LAN Manager Enhanced is being used with the API option, applications can also use a powerful set of networking functions.

Applications that are not based on Windows can call these functions by linking with DOSNET.LIB, a static library provided with the network software. However, Windows-based applications must use DLLs (Dynamic-Link Libraries) called NETAPI.DLL and PMSPL.DLL, which are distributed on every LAN Manager version 2.x Enhanced workstation. Note that these DLLs will not run on LAN Manager version 1.x or 2.x Basic; therefore, these functions may only be used on LAN Manager version 2.x Enhanced.

See the "Microsoft LAN Manager Programmer's Reference" for more details on writing Windows-based applications for LAN Manager.

NOVELL NETWARE

Novell NetWare supports MS-NET and, optionally, NetBIOS functions, which are described earlier in this document.

In addition, Novell NetWare also supports the NetWare and IPX/SPX API sets, all based on INT 21H.

Windows-based applications can make NetWare calls; however, this cannot be done by executing the INT 21H directly because this method is not supported in all Windows operating modes. Instead, the INT 21H instruction should be replaced with a far call to the NetWareRequest() function. This function is exported by name from the NetWare DLL, and should be imported to the module- definition (.DEF) file as NetWare.NetWareRequest.

UNGERMANN-BASS NET/ONE

Ungermann-Bass Net/One is based upon the Microsoft Network standard. It supports standard MS-NET and most NetBIOS functions, which are described earlier in this document.

Net/One also supports private extensions to the NetBIOS function set (functions 72H-7DH). These functions are supported by the standard Windows product. These functions can be called as standard NetBIOS functions would be called, by making a far call to the NetBIOSCall function.

BANYAN VINES

Banyan VINES supports the standard MS-NET and, optionally, NetBIOS functions. There is also a toolkit for applications that write directly to the VINES API.

Windows-based applications can call the MS-NET and NetBIOS functions as previously described.