Скачать или смотреть Understanding Undefined Behavior in C: Why Code Works in Debugger but Not When Running Normally

Explore the common pitfalls of undefined behavior in C programming, focusing on why specific code may work properly in debugging mode, yet fail during normal execution. Learn how to resolve these issues effectively.
---
This video is based on the question https://stackoverflow.com/q/66369190/ asked by the user 'Mark' ( https://stackoverflow.com/u/13137150/ ) and on the answer https://stackoverflow.com/a/66369213/ provided by the user 'MikeCAT' ( https://stackoverflow.com/u/4062354/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Why does this C code work fine when debugging but not when run normally?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding Undefined Behavior in C: Why Code Works in Debugger but Not When Running Normally

In the world of C programming, encountering issues that lead to unexpected results is not uncommon. One particularly perplexing scenario occurs when code behaves as intended while debugging, yet produces erratic outcomes during standard execution. A classic example of this phenomenon is the problem surrounding the use of uninitialized local variables, which can invoke what is known as undefined behavior. Let's dive into this topic to understand both the issue and how to mitigate it effectively.

The Problem

Consider the following function that checks for duplicate characters in a given string:

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

Expected vs Actual Output

When executed with a test string like fhfkkdkdjrbrhrjrotorrjekwl, you may observe unexpected output such as:

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

On the contrary, running the same function step-by-step in a debugger reveals the expected output of zeros followed by increasing counts for each character observed.

Why the Discrepancy?

The key to understanding this difference lies in the behavior of the seen array. When an array in C is declared but not initialized, it contains indeterminate values. This is what's happening in your code:

The int seen[256]; array is declared but not initialized.

When you increment its elements, you are adding to garbage values, leading to unpredictable results.

The Solution: Proper Initialization

The fix is straightforward: initializing the array properly before use. This ensures that all elements start with a known value (typically zero).

Initialization Example

Change the array declaration to initialize all elements to zero like so:

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

This simple change will prevent undefined behavior and ensure that when you perform the count of characters, the array behaves as expected.

Conclusion

Undefined behavior in C can be tricky, especially when debugging tools provide a false sense of security. By understanding the importance of variable initialization, you can avoid many of these pitfalls and enhance the reliability of your programs. Remember, always initialize your variables before use!

By taking these steps, you'll ensure the integrity of your code execution in both debugging and normal run modes. Happy coding!