[Solved] Corrupted size Vs. prev_size Error in Python

One might have encountered an error message while running a code involving malloc() or related to memory allocation like – Corrupted size vs. prev_size.

Corrupted size vs. prev_size Error in Python
You can notice a statement in the image below which says ‘Corrupted size vs. prev_size’.

What do you think this error message is trying to convey and what causes this error? Many a time programmers generally face a common issue. They find it difficult to get the real meaning of the error message that pops up randomly while running their lines of code.

The answer to the above question is simple. The practical cause of this error is that when you overwrite the memory chunk control fields in the next upcoming memory chunk due to out-of-bounds access by the code.

Understanding Corrupted size vs. prev_size error in Python

Let’s say you allocate x bytes to a pointer p, but end up exceeding the value to be written for the value of x. You will see the above-discussed error message. This indicates that the current memory chunk size isn’t the same as the one specified in the next memory control structure. When you overwrite the memory allocation size, this happens.

We can refer to this issue as heap corruption. The problem doesn’t actually lie in the memory allocation as such. The trouble arises when you attempt to access the memory that is not allocated.

Glibc’s implementation has introduced a security check with its function to handle this error.

Unlink (function)- Corrupted size vs. prev_size (error)

The main task of the above-mentioned function is to check whether the memory chunk size is equivalent to the previously mentioned size set in the succeeding chunk.

Corrupted size vs. prev_size – Merge sort example

Corrupted size vs. prev_size - Merge sort example
Corrupted size vs. prev_size - Merge sort example 2
The program shows the concept of merge sort.

While running the above program, you will notice an error appear.

Corrupted size vs. prev_size - Merge sort example output

The major mistake sighted in the above code is that the Merge() defined in the program has a variable k which is incremented to a higher value than it should actually be.

When you add an else statement by mutually making the if statements exclusive. This could be a fix to the problem seen here.

The rectified statements can be seen in the below image with its output.

Fix Corrupted size vs. prev_size - Merge sort example
The corrections are denoted by the comments attached to the programs.
Fix Corrupted size vs. prev_size - Merge sort example output
It can be witnessed that the desired output is obtained successfully after making the corrections.

Corrupted size vs. prev_size aborted(core dumped) c

There are various ways in which this kind of error occurs. Another scenario where this happens to appear is when you allocate dynamic memory in a thread.

While allocating the memory location in the main() function, the code works fine. But on allocating the memory in a thread, that’s where the problem occurs.

Corrupted size vs. prev_size aborted(core dumped) c

By choosing an alternative line of code with the above program, one can avoid this error.

x->array = malloc(x->size*sizeof(int));

Through this line of code, you allocate memory space for x->size integers. malloc accepts the value of the memory to be allocated.

And as in the case of n int, n times the size of a single int value is required. By rewriting the code with the specified corrections, one can successfully run the program with no errors.

Crucial facts to keep in mind

To handle these kinds of errors one should always keep in mind the objectives of your code. Make sure your lines of code implement all that you really require your code to function.

When you insert a function or a method to perform certain operations or tasks, understand the salient features of the function used. Try to figure out if the used function really satisfies your requirements.

Do always remember to know the requirements of the functions you intend to use too. On ignoring this point, many face issues in running the program successfully. Moreover, clear unwanted memory from your python code may help to avoid this error.

If you fail to specify the requirements of the function or the keyword used, there are high chances of encountering an error similar to what we are discussing in this article.

Heap corruption

What could be the cause for heap corruption?

  • The main or the most common mistake one commits (with reference to the error discusssed here) is writing outside the allocated memory space.
  • When the pointers stay uninitialised. Initialising a pointer is a key factor while writing your code. It is generally indicated in the error message while running the code.
  • Typo errors committed while using identifiers or keywords. This mainly leads to a great dilemma when the programmer sits hopelessly looking for flaws in the logic prescribed rather than noticing the mistakes occured due to typo.
  • When the pointer specified points to garbage
  • When you call delete multiple times on the data.

It’s important to know that you can avoid encountering errors by analyzing and having a better knowledge of using keywords at the right place.

For example, do not confuse with the delete statements. While using new, to relieve the variables allocated in it, you should use delete.

While using new[], to relieve the array allocated in it, and use delete[].

Corrupted size vs. prev_size realloc

The main task of realloc is to return the pointer to the new memory chunk.

It can either expand the present memory block or will add a new one to copy the old contents to the new memory block.

Let’s see how this error pops up when using realloc through a shortcode.

Corrupted size vs. prev_size realloc

On executing the above program, the following error occurs.

Corrupted size vs. prev_size realloc output

A possible fix to the above problem could be through going for an alternative line of code which goes like:

string=realloc(string,(i+2) * sizeof(char));

The previous code could work well with maybe fewer characters. For a lengthier set of characters, the alternative code along with the other statements would execute perfectly.

fix Corrupted size vs. prev_size realloc
The rectified program
fix Corrupted size vs. prev_size realloc output
The code gave the desired output as shown in the image

In the previous code which gave an error, the memory space allocated was very limited. The space extended up to fewer characters to hold. In order to expand the memory space even further, the memory block is pushed to get reallocated elsewhere. When you never update the pointer you can only try to have access to the memory you no longer can have access to.

What does corrupted size vs. prev_size mean?

It’s a common error sighted while running codes working with malloc() or memory allocations in general. These tend to occur to overwriting the memory spaces of the blocks and expecting to return values with more memory space than allocated.

How to fix corrupted size vs. prev_size?

This error can be easily fixed by reallocating the memory spaces. You can try to update the pointer to have access to the memory locations that your memory blocks can have access to.

What does malloc corrupted top size mean?

This error possibly occurs when you try to access dynamic memory by using functions like malloc(), free(). While using them, you tend to overrun a buffer, call free() multiple times on a pointer, or call free() with an improper value.


It’s highly essential to keenly understand the core concepts before structuring the code you are intending to execute. Corrupted size vs. prev_size error practically occurs due to overwriting the next memory space of the allocated memory location without considering the current size of the memory block.

Through constant practice and abundant analysis of the functions used one can easily get over these errors and run the code successfully.


  • Memory Management in Python: Memory management in Python involves a private heap containing all Python objects and data structures.
  • PYTHONMALLOC: Set the Python memory allocators and/or install debug hooks.

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