Munmap_chunk() invalid pointer Fixed

Munmap_chunk() invalid pointer Fixed

posted 8 min read

The "Munmap_chunk(): Invalid Pointer" error typically appears during the runtime of a program while executing it. It arises when a program tries to free a particular memory portion that has either never been allocated or has already been freed. To resolve this error we need to be careful with memory management , and in this article, we will discuss the error cases deeply and how to resolve them.

Understanding the Error

The "munmap_chunk(): invalid pointer" error typically occurs when there's an attempt to free a pointer that was not allocated through malloc, calloc, or realloc functions, or if the pointer has already been freed. In simpler terms, this error arises when you're trying to return memory that wasn't yours to begin with or no longer belongs to you.

This error can also occur due to buffer overflow, where your program writes more data into a buffer than it can hold, thus overwriting other information. Additionally, it can be caused by memory corruption, where your program erroneously alters the metadata that the dynamic memory allocator uses to keep track of allocated blocks.

Error Triggering Cases

Case 1: Invalid Pointer

In this case we are trying to free a pointer that is not assigned to anything. We only delcared the pointer but didn't define it by intialzing it or assigning a memory address to it. Take a look at the following code:

#include <stdlib.h>

int main(void)
{
        char *ptr; // this is the line that causes the error to happen
        int toallocate = 0;

        if (toallocate)
        {
                ptr = malloc(10);
                free(ptr);
        }
        free(ptr); //This is the line that raises the error

        return 0;
}

Here, we have declared a pointer "ptr", then we are checking with an if condition if it's the right situation to allocate memory and assign the address to "ptr", and after that we called the free function to free the allocated memory

Case 2: Double Free

In this case, we are freeing a memory that we already have freed before and that happens more in complicated programs with a lot of files and modules, so let's check a simple snippet code:

#include <stdlib.h>

int main(void)
{
    char *ptr = malloc(10);
    int isallocate = 0;

    // some code here

    free(ptr); // Correctly freeing the pointer.

    // some other code

    if (isallocate)
        ptr = malloc(15);

    free(ptr); // freeing the ptr again 

    return 0;
}

Of course, it won't be as simple as the above code, but I made it simple to understand it easier and I can assure you it will be the same flow every time. So in the above code, we allocated a memory for some reason, We finished using it and we decided to free it, with no problems till now, but what if we tried it to free the same pointer again ?. That will trigger the error because we can't free a pointer that is already freed.

Case 3: Freeing Part of Allocated Memory

We can't free just a portion of a block of memory. It's all or nothing. So we shouldn't pass a pointer pointing at  a memory address other than the one that the memory block starts with. The following code demonstrate the case and where is the line that causes the error:

#include <stdlib.h>

int main(void)
{
    char *ptr = malloc(50); // Allocate memory

    free(ptr + 10); // This line triggers the error
    return 0;
}

So what's happening here? We're allocating a block of memory for ptr using malloc(). Then, we're trying to free() a part of that memory block. Specifically, we are trying to free the memory block after its strating byte by 10 more bytes, but that won't work, because it's to free all the block code or nothing from it, so that will trigger the error.

Walking Through The Solutions

Review Your Code

Start by thoroughly reviewing your code, especially areas where you're freeing memory. Ensure that every pointer you're attempting to free was indeed allocated using malloc, calloc, or realloc. Also, check to make sure you're not freeing the same pointer twice.

char* ptr = malloc(10);
// ... some code ..
free(ptr);
// ... some more code ...
free(ptr); // this will cause an error

In the example above, the pointer ptr is freed twice, which will lead to the "munmap_chunk(): invalid pointer" error.

Debug Your Code

Debugging your code can help identify problematic sections. Tools like Valgrind or GDB (GNU Debugger) are extremely helpful for this purpose.

Valgrind is a programming tool that helps detect memory leaks and memory management issues. Running your program through Valgrind can help pinpoint the exact location of the error:

valgrind ./your_program
Tip: Use the -g option when you compile the code to get useful info like which line that caused the error

GDB is another powerful debugging tool that allows you to see what is going on 'inside' your program while it executes.

gdb your_program
run

Case 1: Solution: Invalid Pointer

In this case, we need to initialize the "ptr" with NULL  and that will make it safe to pass it to the free() function without raising any errors unless the program entered the if block, allocated a memory block, and then calling the free two times with same pointer, which will take us to next case and the solution for Double free.

The invalid pointer fix will be like the following code:

#include <stdlib.h>

int main(void)
{
        char *ptr = NULL; // fixed line
        int allocate = 0;

        if (allocate)
        {
                ptr = malloc(10);
                free(ptr);
        }
        free(ptr);

        return 0;
}

Case 2: Solution: Double Free

In the case, we need to assign NULL to "ptr" after calling the free() so if there is any additional attempt to free "ptr", it will run safely without any problems. So the code after the fix will be like the following:

#include <stdlib.h>

int main(void)
{
    char *ptr = malloc(10);
    int isallocate = 0;

    // some code here

    free(ptr); // Correctly freeing the pointer.
    ptr = NULL; // fixed line

    // some other code

    if (isallocate)
        ptr = malloc(15);

    free(ptr); // freeing the ptr again but safely this time

    return 0;
}
Tip: Always after freeing a pointer, assign NULL to the pointer to avoid problems if there is any additional attempts to free the same pointer

Case 3: Solution: Freeing Part of Allocated Memory

In this case, it's obvious that the solution is not to pass a random memory address of memory block or a pointer pointing at a random memory address of the memory block, but the correct method is to pass the memory address that begins the memory block or the pointer that pointing at it, and we get it as the return value of functions that allocate memory dynamiclly like malloc, alloc, etc...the

So the code after the fix should be like the following:

#include <stdlib.h>
int main(void) {
    char *ptr = malloc(50); // Allocate memory

    free(ptr); // Fixed to pass the first of the block memory
    return 0;
}

Fix Buffer Overflows and Memory Corruption

If the error persists even after ensuring correct memory allocation and deallocation, you might be dealing with a buffer overflow or memory corruption.

In case of a buffer overflow, ensure that you're not writing beyond the memory you've allocated. For instance, if you've allocated an array of size 10, make sure you're not trying to access or modify the 11th element.

For memory corruption, the solution is trickier as it involves understanding how dynamic memory management works in C. However, tools like Valgrind can help identify memory corruption issues as well.

It's a good practice to always check the return value of the dynamic allocations functions, like malloc before using it to handle the failure if you are out of memory, you shouldexit the program with a user-friendly error message. better than continue running the program until it crashes, so instead of the following code:

ptr = malloc(10);

// doing something with ptr

We write it correctly like:

ptr = malloc(10);
if (ptr == NULL)
{
   fprintf(stderr, "allocating failed: out of memory");
   exit(1);
}

// doing something with ptr

In the above code, we checked if ptr is NULL. If that condition is true which means that malloc failed to allocate memory, then we print a message to the standard error and exit with 1 which means somethinng wrong happened caused to exit.

Conclusion

In conclusion, while the "munmap_chunk(): invalid pointer" error can initially seem daunting, understanding its root causes and knowing how to debug your code effectively can take you a long way in resolving it.

We have discussed the importance of reviewing the code and using tools to help us like Valgrind and GDB, how to nullish the pointer after freeing it to avoid any potential errors, to pass to the free function the start of the memory block only, and the correct way to handle the failing of allocating memory dynamically. Watch out for buffer overflows, and use tools like Valgrind and GDB to help you along the way. Happy coding!

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