Скачать или смотреть Understanding the pointer being freed was not allocated Error in C

This guide breaks down the common error of `pointer being freed was not allocated` in C programming, explaining its causes and providing solutions for new programmers.
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Understanding the pointer being freed was not allocated Error in C

As a new programmer diving into C, encountering errors can be a frustrating part of the learning process. One such common error is pointer being freed was not allocated. This message can be perplexing, especially when you think you've correctly used memory allocation functions like malloc and free. In this post, we'll explore why this error occurs and how you can avoid it in your programs.

The Problem

In the question posed by our fellow programmer, they are building a map/hashtable using SDL (Simple DirectMedia Layer). After successfully creating the map and adding key-value pairs to it, they attempt to free memory using free, only to be met with the dreaded error:

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This indicates that the program is trying to free a pointer that was either not allocated dynamically or has already been freed.

Understanding Memory Allocation

When working with dynamic memory in C, it’s crucial to understand how memory allocation works. Functions like malloc and free are used to reserve and release memory, respectively.

Memory Allocation Steps:

Allocating Memory:

A programmer allocates memory using malloc (or SDL_malloc in this case), which reserves space on the heap.

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Using Pointers:

These pointers (itemA and itemB) should point to the allocated memory blocks.

However, the programmer then replaces these pointers with string literals:

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The Mistake

By assigning string literals to itemA and itemB, the programmer loses the reference to the originally allocated memory. The allocated memory is thus “leaked” (you can no longer access it to free it), leading to an attempt to free memory that was never dynamically allocated.

The Solution

To resolve this issue and properly handle memory allocation, the programmer should copy the strings into the allocated memory instead of overwriting the pointer. This can be accomplished using strcpy as shown below:

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Key Takeaways:

Do not overwrite pointers that point to dynamically allocated memory.

Always copy data into the allocated memory instead of pointing to new objects (like string literals).

Ensure that every malloc has a corresponding free to avoid memory leaks.

Conclusion

The pointer being freed was not allocated error is a common stumbling block for programmers new to C. By understanding how memory allocation works and being cautious with pointer assignments, you can avoid this error and write more robust code. Remember to always manage your memory wisely, and as you continue learning, you'll find that C programming becomes clearer and more manageable. Happy coding!