Скачать или смотреть Understanding and Fixing Heap Corruption in Merge Sort with C

Discover how to troubleshoot and fix `heap corruption` issues when implementing the merge sort algorithm in C. Learn the correct memory allocation techniques and key concepts related to `malloc` and array slicing.
---
This video is based on the question https://stackoverflow.com/q/63383784/ asked by the user 'Anton Tretyakov' ( https://stackoverflow.com/u/12320997/ ) and on the answer https://stackoverflow.com/a/63402337/ 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.

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: Writing outside heap in mergesort

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 and Fixing Heap Corruption in Merge Sort with C

When programming in C, particularly during complex operations like sorting algorithms, memory management can become tricky. One frequent issue developers face is "heap corruption"—a situation that leads to invalid reads and writes in memory, which can crash your program. This guide explores a specific case in implementing a merge sort algorithm in C, where improper memory allocation using malloc resulted in heap corruption.

The Problem

In this example, a developer attempted to create a temporary array for merging elements within a sort function using malloc. However, they encountered invalid memory access errors that persisted even after free operations were accounted for. The core issues involved:

Incorrect calculation of the temporary array size.

Misalignment in pointer arithmetic that led to writing beyond allocated memory bounds.

The use of a hardcoded array that limited the program's length and caused it to fail with larger datasets.

Original Code Snippet

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

In the above line, the developer allocates space using end - strt, which does not account for the correct indexing of the last slice element, causing potential overwriting and memory corruption.

The Solution

Correcting Memory Allocation

The first step in resolving heap corruption lies in altering how we allocate memory for our temporary array. The correct size for memory allocation should be:

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

This adjustment accounts for the last element by ensuring that the allocated size properly reflects the number of elements intended for sorting.

Understanding Array Indexing

It's crucial to understand that in C, arrays are zero-indexed. This means that when you refer to an array segment from strt to end, you are actually working with a range that includes strt and excludes end. Therefore, the allocation should allow for the last index, hence the + 1.

Updated Code Implementation

Here’s an updated merge_sort implementation that takes these fixes into account:

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

In the merge function, utilizing a safer memory allocation and correct indices will eliminate the heap corruption:

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

Conclusion

Proper memory management and understanding array boundaries are vital when programming in C, especially when dealing with dynamic memory allocation. By correcting the memory allocation size and adhering to array indexing rules, you can avoid heap corruption and ensure that your merge sort implementation runs smoothly.

If you encounter issues related to memory allocation and heap corruption, remember to revisit your calculations and carefully inspect your pointer arithmetic. Happy coding!