DOCUMENT:Q100659  02-MAY-2002  [visualc]
TITLE   :INFO: Exporting Callback Functions Not Required in Win32 Apps
PRODUCT :Microsoft C Compiler
PROD/VER::1.0,2.0,2.1,4.0,5.0,6.0
OPER/SYS:
KEYWORDS:kbGenInfo kbVC kbVC100 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 kbDSupport kbArtTypeINF

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The information in this article applies to:

 - Microsoft Visual C++, versions 1.0, 2.0, 2.1, 4.0 
 - Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
 - Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
 - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
 - Microsoft Visual C++.NET (2002) 
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SUMMARY
=======

In the Win32 operating system, an application is not required to export callback
functions. In 16-bit Windows 3.1 and prior versions, an application must export
callback functions primarily to allow them to fix up references to global data
and to ensure that any required EMS memory is available. Neither of these
situations applies to applications running under the Windows NT 32-bit operating
system.

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

It is necessary to export a function to make it available to the
GetProcAddress() function (for dynamic linking) or to list it in an import
library (for static linking). Either of these linking methods require
information about the function name or ordinal number. Exporting the function
places the name or ordinal number into the application's exported entry points
table and allows Windows to determine the associated address at run time.

The loader performs load-time linking by looking up the imported entry points an
application requires to load. Typically, it looks up this information by ordinal
number. An application performs dynamic linking with the GetProcAddress()
function which instructs the system to look up the desired function in the
desired dynamic-link library (DLL) exports table by ordinal number or by name.
It is not necessary to export functions in the Win32 application programming
interface (API).

In 16-bit Windows, the linker automatically fixes exported entries to load the
appropriate value into the DS register. Exporting an entry in Win32 simply adds
the entry to the application's exported names and ordinal numbers table; the
linker need not "fix" them. To maintain source code compatibility with 16-bit
Windows, you may want to continue using the MakeProcInstance() function and
exporting all callback functions.

The following table summarizes this information:

Function Type      Windows 3.x (16-bit Windows)     Windows NT (Win32)  
Callback           Exported or MakeProcInstance     Address of fn() *
GetProcAddress     Must be exported                 Must be exported
Static linking     Must be exported                 Must be exported

* Because the MakeProcInstance() macro has no effect, it works as well.

Additional query words: exporting vcnt

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Keywords          : kbGenInfo kbVC kbVC100 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 kbDSupport kbArtTypeINF 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbvc100 kbVC500 kbVC600 kbVC200 kbVC210 kbVC32bitSearch kbVCNET kbVC500Search
Version           : :1.0,2.0,2.1,4.0,5.0,6.0
Issue type        : kbinfo

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