How to Change an extern Function Pointer to an extern Function Using Preprocessor Hacks

Описание: Discover how to convert an `extern` function pointer to an actual `extern` function using clever preprocessor techniques in C while working with unchangeable library source files.
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How to Change an extern Function Pointer to an extern Function Using Preprocessor Hacks

In C programming, there's a common dilemma programmers face when dealing with third-party libraries. You may come across a situation where you need to modify an existing extern function pointer to an actual extern function. This situation raises two fundamental questions:

Is it possible to change the way a library exposes its functions without modifying its source code?

How can you ensure that modifications do not result in unexpected run-time behaviors?

In this post, we will explore a method to achieve this using the C preprocessor without altering the library's files directly.

Understanding the Problem

To illustrate the issue, let’s set the stage with a simplified version of what your library might look like:

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This library uses an extern pointer to a function called some_func. Your goal is to ensure that some_func behaves like a regular extern function, effectively preventing any changes to this global pointer. The advantages?

Save runtime and code space.

Minimize accidental modifications to the global function pointer by other code.

The Solution: Using C Preprocessor Tricks

Here's an approach that allows you to define some_func as a constant function rather than a pointer to a function. This way, it will be initialized with the desired function's address, and any attempt to modify it later will raise a run-time error.

Step 1: Define Your Own Header

Create an empty file named my_file.h. The intent here is to prevent conflictions with any changes in the library's headers.

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Step 2: Implement Your Main Logic

You will write your own implementation that replaces the pointer to a function with a defined constant function.

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Step 3: Create an Offending Module

This module will attempt to modify some_func, allowing us to test whether our solution truly works.

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Step 4: Build and Test

Compile your code:

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Step 5: Run the Executable

Once compiled, run your executable. The expected output should show that it correctly initializes the function call:

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Followed by an error when trying to modify some_func:

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Conclusion

By employing a clever workaround using the C preprocessor, we successfully modified the behavior of an extern function pointer, establishing it as a constant function. This method efficiently prevents unwanted changes while keeping the library files intact, demonstrating both the flexibility and power of C programming.

This technique emphasizes a critical development principle: Working within constraints can lead to innovative solutions. If you find yourself needing to work around limitations in a third-party library, consider using preprocessor directives to your advantage.

By carefully employing these methods, you can maintain stability and performance in your applications while working with external code!