Скачать или смотреть Understanding the memcpy Mistakes: Custom Function Issues and Solutions

Learn how to troubleshoot your custom `memcpy` function that prints extra characters due to null termination errors. Discover the correct approach and tips to fix your code.
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This video is based on the question https://stackoverflow.com/q/72585241/ asked by the user 'Karna Rai' ( https://stackoverflow.com/u/19319892/ ) and on the answer https://stackoverflow.com/a/72585380/ provided by the user 'chqrlie' ( https://stackoverflow.com/u/4593267/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the memcpy Mistakes: Custom Function Issues and Solutions

When programming in C, handling memory effectively is crucial, especially when dealing with string manipulation. In this guide, we will explore a common issue faced when creating a custom memcpy function and how to resolve it. In particular, we will discuss unexpected characters appearing in the output after attempting to copy a string.

The Problem

You have recently implemented your own version of the memcpy function and observed something strange in the output. The destination string printed unexpected characters after what you thought was a proper null termination.

When running your program, you encountered an output that looks something like this:

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

Clearly, there's an issue since you would typically expect the second line to simply display "HELLO WORD". So what went wrong?

Analyzing the Code

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

Identifying the Problems

Incorrect Null Termination:
The line dst[j] = "\0"; attempts to assign a string pointer to a character, which leads to undefined behavior. The correct approach is to use a character constant: dst[j] = '\0';.

Exceeding Bounds:
The mymemcpy function does not prevent writes beyond the destination buffer, which may also cause issues if not carefully managed.

The Solution

To fix these problems, let's walk through the improved version step-by-step.

Adjustments to the mymemcpy Function

Here is how you should structure your function:

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

Key Changes

Use of Character Constant:
Replace dst[j] = "\0"; with the correct termination dst[j] = '\0'; to ensure that you set the null terminator properly.

Using Proper Data Types:
Utilize unsigned char for both the source and destination pointers, which is more aligned with the way memory is handled.

Safety Measures:
In your main function or as parameters to printf, manage how many characters to retrieve to avoid accessing memory beyond what was allocated.

Final Code Example

Here’s how your final program should look:

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

Conclusion

By addressing the null pointer issue and ensuring you're not writing beyond the allocated size of your destination buffer, you can avoid strange output errors. The use of custom memory handling functions like mymemcpy allows for greater control but also requires careful programming to avoid pitfalls. Be sure to test your code thoroughly and keep these tips in mind whenever manipulating memory in C!