How to Forcibly Link Dynamic Library to Static Library Global Variables

Learn how to ensure global variables from a static library are linked correctly to a dynamic library in C++.
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How to Forcibly Link Dynamic Library to Static Library Global Variables

When working with C++ and building libraries, developers often encounter issues related to the proper linking of static and shared libraries. One common problem is ensuring that global variables defined in static libraries are also recognized in dynamic libraries. This guide will address how to tackle such an issue effectively.

The Problem

Imagine you've created a .c file that has a global string variable and compiled it into a static library. After linking this library to a shared library, you find that the global string symbol is missing. This issue occurs because the variable was not referenced within the dynamic library, leading the linker to discard it.

For example, you may have the following code definitions:

Static Library File

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Shared Library File

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You can see that even though you're declaring the external variable in the shared library, it doesn't show up in the .data section of the shared library as intended. The common approach using --whole-archive is not an option here.

The Solution: Making Symbols Visible

To ensure that the global variable from the static library appears in the shared library, you need to use specific techniques to reference it so that the linker keeps it in mind. Here’s a better method to do this:

Step 1: Define the Global Variable

Instead of directly defining a global variable, you can create a function that uses it. This usage guarantees that the linker does not discard the variable as unused.

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Step 2: Use Attributes for Weak Linking

Another effective approach is to declare the global variable using the weak attribute provided by GCC. This allows the linker to treat it differently, granting it the potential to be overridden while still being included.

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Step 3: For Windows Users

If you are working on a Windows platform, you can use the selectany specifier, which allows the symbol to be included but can also be optimized based on the compiler flags.

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Conclusion

Linking global variables from static libraries to dynamic libraries can be tricky but is certainly manageable. By forcing the usage of these variables through function references or using compiler attributes, you can ensure that crucial symbols are retained and recognized during linking. Always test your implementation thoroughly to confirm that the variable appears correctly in the shared library's .data section.

By employing the right techniques, you can effectively eliminate any linking issues that arise when integrating static and dynamic libraries in C++.

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