How Can I Effectively Link Multiple Static Libraries Together?

Linking Static Libraries: A Comprehensive Guide

Many software projects depend on multiple static libraries for essential functionality. These libraries provide precompiled and linked code that can be utilized by developers to build their own applications. However, there are scenarios where it becomes necessary to create a new static library that combines functionality from multiple existing libraries. This article explores the challenges and solutions related to linking static libraries together.

Problem: Linking Static Libraries with Dependencies

Consider a situation where a static library named X relies on several other static libraries (a_1, a_2, ..., a_n). When you create a sample program that uses a function from library X and try to link it to X, you may encounter errors indicating missing symbols from the dependent libraries (a_1 - a_n). This issue arises because static libraries do not link with each other.

Solution: Creating a Combined Static Library

To resolve this problem, you can create a new static library, Y, that contains the functionality of both library X and the necessary dependencies from libraries a_1 - a_n. This allows you to distribute Y and make it available for other developers to link their programs to, ensuring that they have all the required functionality without needing to manage multiple libraries.

Method: Concatenating Libraries Using an Archiver

One approach to create a combined static library is to use an archiver tool like ar on Linux. This involves concatenating the individual libraries into a single new library:

ar rcs Y.a X.a a_1.a a_2.a ... a_n.a

By doing this, you effectively include all the object files and symbols from the dependent libraries into the new library Y. This ensures that your sample program can successfully link against Y because it contains all the necessary functionality.

Challenge: Including Only Required Symbols

While the concatenation approach provides a functional solution, it may include excessive symbols that are not strictly required by your application. To address this challenge, it is desirable to create a combined library Y that contains only the essential symbols needed by X and its dependencies.

Manual Selection of Object Files

Regrettably, there is no straightforward automated approach to identify and include only the required symbols. A manual approach involves carefully examining the object files (.o) from the dependent libraries and selecting only those that are essential for X's functionality. This process can be time-consuming and requires a deep understanding of the code.

Conclusion

Linking static libraries together can be a complex task, but it is essential for combining functionality from multiple sources. By concatenating libraries using an archiver, you can create a single static library that includes all the necessary dependencies. However, it is important to note that this approach may include unnecessary symbols. Manual selection of object files remains the only way to create a combined library with minimal footprint, but it is a challenging and error-prone process.

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