Q74360: Three Methods to Import Functions from a DLL

Article: Q74360
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.0,3.1
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 05-NOV-1999

-------------------------------------------------------------------------------
The information in this article applies to:

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

SUMMARY
=======

In the Microsoft Windows graphical environment, an application can import a
function from a dynamic-link library (DLL) in three different ways:

1. Implicitly at link time

2. Explicitly at link time

3. Dynamically at run time

Importing at link time is the simplest and the most commonly used.

While importing at run time is cumbersome, it is the most powerful and it should
be used if the link is not certain to succeed.

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

An application implicitly imports a DLL function at link time when the import
library for the DLL is listed on the linker command line for the application.
For example, an application implicitly links to Windows functions because its
linker command line includes LIBW.LIB.

An application explicitly imports a DLL function at link time when the IMPORTS
section of the application's module definition (DEF) file includes the name of
the function.

Both methods are easy to use because the application can call an imported
function as if it were part of the application.

However, in certain situations, importing at link time has drawbacks. For
example, if the DLL is not found when Windows loads the application, Windows
displays a system-modal message box: "please insert (a disk with the DLL) in
drive A." Therefore, an application should not perform an implicit import at
link time if the DLL may not be present.

Another situation in which importing at link time is not appropriate occurs when
the application needs to import a routine that is not exported by all versions
of the DLL. An attempt to link to an invalid export ordinal results in an
unrecoverable application error (UAE).

Both of these situations are addressed by importing the function at run time. An
application does this by calling the LoadLibrary function to load the DLL and
then calling the GetProcAddress function to retrieve the address of the required
function. The application should check the values returned from LoadLibrary and
from GetProcAddress to ensure that these functions succeed. If the DLL is not
found or does not export the desired function, one of these calls fails, and the
application can handle the error gracefully.

Additional query words: no32bit 3.00 3.10

======================================================================
Keywords          : kb16bitonly 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300 kbWinSDK310
Version           : WINDOWS:3.0,3.1

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.