Microsoft KB Archive/43052

From BetaArchive Wiki

Windows Does Not Support OS/2 Family API Calls


The information in this article applies to:

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


Windows and Windows 95 do not support 16-bit Windows-based applications coded with OS/2 Family API (FAPI) calls.


FAPI works in the following manner:

  1. The code contains references to OS/2 FAPI calls (such as DosOpen, DosRead, and so on).
  2. When the program is loaded into OS/2 protected mode, the system loader dynamically links the FAPI calls to the OS/2 system-services DLLs (Dynamic-Link Libraries).
  3. When the program is loaded into real mode (MS-DOS or OS/2 compatibility box), what actually gets loaded is a small program called the "FAPI Loader and Linker." It is this program that loads the real code; it dynamically links the FAPI calls to a special library of support routines that translate FAPI calls into 80x86 code and MS-DOS interrupts (INT 21H Function xx).

This process is also described on Page 251 of Gordon Letwin's book titled "Inside OS/2" (Microsoft Press, 1988).

FAPI works well for programs that need to run in MS-DOS and OS/2 protected mode. The problem is that Windows uses the "New EXE Format" for programs, bypassing the standard MS-DOS entry point. For example, if a Windows program is run outside of Windows (in MS-DOS), the following message appears, and the program terminates:

This program requires Microsoft Windows

MS-DOS is not responsible for this message; the Windows program itself is responsible. The way the Windows program works is very similar to OS/2: it uses dual entry points into the .EXE file. In MS-DOS, a short program that prints the above message runs; however, in Windows, a true Windows-based application runs using the other entry point in the .EXE file.

Therefore, the problem is narrowed down to the following: if the FAPI Loader and Linker program is run using the standard MS-DOS .EXE file entry point, and Windows starts an application using a different entry point, the dynamic linking of the FAPI routines will not occur.

Therefore, FAPI calls cannot be used in Windows-based applications.

To avoid this problem, do the following:

Instead of using low-level MS-DOS calls (INT 21H Function xx) in a Windows-based application and OS/2 API calls (DosRead, DosOpen, and so on) in a Presentation Manager (PM) application, use the C run-time I/O routines for all of these applications.

This will work because the Microsoft C Compiler and the run-time libraries supply versions of the libraries that work in both OS/2 and MS-DOS. By moving C code to PM and to Windows, it will not be necessary to rewrite it, and the appropriate conversion routines will be supplied at link time.

Be sure not to use high-level (stream) I/O routines in C with Windows; just use the low-level (handles) versions. This topic is discussed in Charles Petzold's book titled "Programming Windows" (Microsoft Press, 1988), and in other articles in the Microsoft Knowledge Base.

Additional query words: 3.00 no32bit 3.10

Keywords : kb16bitonly
Issue type :
Technology : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300 kbWinSDK310

Last Reviewed: November 5, 1999
© 2001 Microsoft Corporation. All rights reserved. Terms of Use.