Three Methods to Import Functions from a DLL

The information in this article applies to:

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

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

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.