How to Handle Pre ANSI-C 0 Pointers in ANSI C

Описание: Discover how to properly handle `0 pointers` in ANSI C with our clear guide on converting older code structures.
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This video is based on the question https://stackoverflow.com/q/62801691/ asked by the user 'M.E.' ( https://stackoverflow.com/u/5328289/ ) and on the answer https://stackoverflow.com/a/62801914/ provided by the user 'Nate Eldredge' ( https://stackoverflow.com/u/634919/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding 0 Pointers in ANSI C

When working with legacy C code, especially pre ANSI-C code, you might encounter various programming practices that have since changed. One such practice involves the use of 0 pointers. If you're diving into old code and see constructs like (Symbol *)0, you might find yourself wondering how they should be expressed in ANSI C.

In this guide, we will explore how to write these pointers correctly in ANSI C while preserving the original code's functionality. Let's break down this process step by step.

The Legacy Code Example

Consider the following function from an older C codebase:

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

In this code:

The function lookup searches for a Symbol by its name.

The loop continues as long as sp is not equal to (Symbol *)0.

Transitioning to ANSI C

Declaring Function Parameters

In ANSI C, a more straightforward declaration for the function's parameters is possible. The lookup function can be rewritten as:

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

Handling 0 Pointers in ANSI C

The next question is about the 0 pointers. The expression sp != (Symbol *)0 remains valid in ANSI/ISO C. According to the ANSI C standard (C17 6.3.2.3), any integer constant expression with the value 0 can be utilized as a null pointer constant.

Recommended Practices

While (Symbol *)0 is valid, it is advisable for clarity and convention to use NULL. Here’s how you could rewrite the loop:

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

This improves readability, indicating that sp should not be a null pointer.

An Even More Concise Approach

If you want to write more concise code, you can take advantage of the fact that the condition in the loop inherently checks for null pointers. You can rewrite the for loop like this:

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

This will effectively continue looping until sp becomes a null pointer, making your code not only shorter but arguably cleaner.

Conclusion

When converting older C code to ANSI C, handling 0 pointers may seem daunting, but it is relatively simple. Just remember:

It's perfectly valid to use (Symbol *)0 in ANSI C.

Utilizing NULL is clearer and more conventional.

You can write concise loops that check for NULL without needing an explicit comparison.

This way, you can modernize legacy code while maintaining its intended functionality.



Thank you for reading! If you have any questions or suggestions regarding working with pointers in ANSI C, feel free to leave a comment below!