Скачать или смотреть How to Access C++ DLL Variables in a C Application

Struggling to access a variable in your C++ DLL from a C app? This guide outlines the necessary steps and code to overcome common issues such as linker errors.
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This video is based on the question https://stackoverflow.com/q/56500/ asked by the user 'Ian Horwill' ( https://stackoverflow.com/u/5816/ ) and on the answer https://stackoverflow.com/a/56514/ provided by the user 'botismarius' ( https://stackoverflow.com/u/4528/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Access C++ DLL Variables in a C Application: A Comprehensive Guide

If you're working on a legacy application, it's not uncommon to encounter issues while trying to access variables defined in a C++ Dynamic Link Library (DLL) from a C application. This can often result in confusing linker errors that can stall your progress. In this guide, we will address a specific scenario: how to access a variable in a C++ DLL from a C app and provide a clear step-by-step solution.

The Problem

You may have defined a variable in your C++ DLL like this:

[[See Video to Reveal this Text or Code Snippet]]

While this code should make MyNewVariable available without name decoration (which is generally helpful), you're left unable to access it from your C source file. Attempts to declare the variable in your C code, such as the following, result in unresolved external symbol errors:

[[See Video to Reveal this Text or Code Snippet]]

The error messages indicating unresolved symbols can be particularly frustrating, as they suggest a breakdown in the linkage between your C application and the DLL.

The Solution

To successfully access variables from a C++ DLL in a C application, follow these steps:

Step 1: Linking the .lib File

The first thing you need to do is ensure that your C application links against the appropriate .lib file generated during the compilation of your DLL. This .lib file contains the necessary references to functions and variables defined in the DLL.

Find the .lib file: After compiling your DLL, locate its corresponding .lib file.

Add it to your linker options: In your C project, go to your project settings and include the .lib file in the linker options.

Step 2: Declare the Variable Correctly

Next, you'll need to declare the variable in your C source file correctly to ensure that it matches the way it's defined in the C++ DLL. Use the following declaration:

[[See Video to Reveal this Text or Code Snippet]]

extern "C" – This tells the compiler to use C linkage for MyNewVariable, preventing name decoration.

__declspec(dllimport) – This keyword indicates that you're importing a variable from a DLL rather than exporting it.

Step 3: Ensure Consistency

Consistency is key to successful linkage:

Ensure that the type of MyNewVariable matches in both your C++ and C declarations.

Double-check that you are not mistakenly using different versions of the DLL.

Conclusion

Accessing variables defined in a C++ DLL from a C application can initially seem daunting, especially when confronted with linker errors and name decoration issues. However, by correctly linking against the generated .lib file and carefully declaring the variables with proper linkage specifications, you can navigate these challenges with ease.

Follow these steps, and you should find yourself successfully interfacing between your C and C++ codebases without further hiccups. Happy coding!