Creating a Single Import Library for Multiple DLLs (68081)

SUMMARY

A single import library that contains information about multiple dynamic-link libraries (DLLs) can sometimes be useful. One example in the Microsoft Windows graphical environment is LIBW.LIB, which serves as the single import library through which an application can link to the Windows system DLLs GDI.DLL, KERNEL.DLL, and USER.DLL.

An import library for many DLLs can be created by supplying multiple module definition (DEF) files as input parameters to the IMPLIB utility. However, one must make sure that none of the exported functions is defined in more than one DEF file. IMPLIB issues an error message when a function name is defined more than once, which occurs most often with the Windows exit procedure (WEP) because each DLL must export its own WEP in its DEF file.

To avoid an IMPLIB error indicating that the WEP routine is defined many times, create modified DEF files that do not contain EXPORT statements for the WEP routine and specify these files as input to IMPLIB.

MORE INFORMATION

Defining a function in an import library more than once creates an ambiguity when the loader resolves references to functions exported by other modules. For example, suppose two DLLs that share a common import library export an Open function. When an application refers to a function in a DLL, the loader resolves a reference to the function (known as a fix-up) to its true address (which is not known until run time). The DLL containing the function must be loaded to provide a true address for its functions. However, in this example, this poses a dilemma: because both DLLs contain an Open function, which one should be used? Because the application's source code makes no distinction between the two Open functions, the loader cannot resolve the reference correctly.

To avoid the potential for ambiguity, IMPLIB issues an error message for each function that is defined in multiple DEF files and does not create a combined import library. Two DLLs that share a common function name cannot share a single import library. The one exception to this rule is the WEP function, which every DLL must export. Because an application does not call the WEP, the function need not be listed in the combined import library.

To create a single import library, MYIMPLIB.LIB, that can be used to link to the three DLLs (DLL1, DLL2, and DLL3), perform the following four steps in this order:

Compile and link all three DLLs as outlined in Chapter 20 of the "Microsoft Windows Software Development Kit: Guide to Programming" for versions 3.0 and 3.1. Use the original DEF file to link each DLL. Each file must list the WEP in its EXPORTS section with the RESIDENTNAME qualifier to properly export the WEP function.
Copy the module definition files for each DLL (DLL1.DEF, DLL2.DEF, and DLL3.DEF) into three dummy DEF files (DUMMY1.DEF, DUMMY2.DEF, and DUMMY3.DEF). These dummy DEF files are used to create the combined import library as outlined in Step 3 below. Do not link any DLL using its dummy DEF file.
Remove the WEP from the EXPORTS statement of each dummy DEF file. Only Windows calls the WEP procedure; the application does not. Therefore, the WEP does not need to be defined in the import library. Removing the WEP entries prevents an IMPLIB error about multiple definitions of the WEP routine.
Use DUMMY1.DEF, DUMMY2.DEF, and DUMMY3.DEF as input parameters to the IMPLIB utility, as follows:
```
      IMPLIB MYIMPLIB.LIB DUMMY1.DEF DUMMY2.DEF DUMMY3.DEF
```

The four steps above build a combined import library that contains the linkage information required for all of the exported functions in each of the three DLLs. It does not contain any information about any of the WEP functions. Because applications do not call the WEP function, this omission does not cause any linkage problems.

A dummy DEF file must not be used to link a DLL because a dummy DEF file does not export the WEP function. Every DLL must export a WEP that Windows can call before removing the DLL from memory.