How Do Import Libraries Facilitate Implicit DLL Linking?

How Do Import Libraries Work?

An import library, often provided with DLLs, serves as an intermediary between the main application and the DLLs. It contains stubs that facilitate the implicit loading of the DLL at link time. These stubs contain crucial information, such as:

• The full path of the corresponding DLL, enabling the runtime to load it
• The relative address of each exported DLL method's entry point, ensuring correct calls and jumps

Through these stubs, the calling sequence becomes:

My main application --> stub in the LIB --> real target DLL

Tools for Inspecting Import Libraries

• Dependency Walker (depends.exe): Identifies DLLs used at link time and run time.

Explicit vs. Implicit Linking: Why Import Libraries Matter

Implicit linking occurs at build time, while explicit linking happens at run time. Import libraries play a critical role in implicit linking by providing stubs that act as placeholders for DLL exports. This allows the program to be linked to the correct DLLs during the build process, ensuring that the stubs are present in the final EXE file.

Benefits of Dynamic Linking

• Reduces the size of the executable file
• Enables sharing of code pages among multiple processes running multiple applications
• Allows for third-party library updates without recompiling or relinking applications
• Facilitates extension mechanisms through explicit loading of DLLs

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