Three Methods to Import Functions from a DLL
ID: Q74360
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The information in this article applies to:
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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:
- Implicitly at link time
- Explicitly at link time
- 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
Version : WINDOWS:3.0,3.1
Platform : WINDOWS
Issue type :