Debug Assertion error when freeing memory

This is a discussion on Debug Assertion error when freeing memory within the C Programming forums, part of the General Programming Boards category; This is invariably to do with the way I am freeing memory here, can anyone tell me exactly how/why? This ...

  1. #1
    Registered User foniks munkee's Avatar
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    Nov 2001
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    Debug Assertion error when freeing memory

    This is invariably to do with the way I am freeing memory here, can anyone tell me exactly how/why?

    This could be anything, but I am hopped up on goof ballz trying to keep the flu away at the moment.. Not thinking straight!

    Using MS VC++ V6.0
    Windows XP

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    
    typedef struct 
    {
    	void *foo;
    } MYSTRUCT1;
    
    typedef struct
    {
    	int data;
    
    } MYSTRUCT2;
    
    int main(void)
    {
    	MYSTRUCT1 *struct1;
    	MYSTRUCT2 *struct2;
    
    	struct1 = malloc (sizeof(MYSTRUCT2));
    	struct2 = malloc (sizeof(MYSTRUCT2));
    
    	struct1->foo = malloc (sizeof(MYSTRUCT2));
    
    	printf("Enter a number: ");
    	scanf("%d", &struct2->data);
    
    	struct1->foo = struct2;
    
    	
    	free(struct1->foo);
    	free(struct2);
    	free(struct1);
    
    	return 0;
    }
    This is the error (in a popup window)

    Debug Assertion Failed!

    Program c:\programming\foo\Debug\foo.exe
    File: dbgheap.c
    Line: 1017
    Expression: _BLOCK_TYPE_IS_INVALID(pHead->nBlockUse)

  2. #2
    B26354 Deckard's Avatar
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    Jan 2002
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    633
    Foniks,

    I would expect free() to cause a nasty error if you attempted to deallocate a memory address that was never allocated. Let's take a code walk, shall we?

    struct1 = malloc (sizeof(MYSTRUCT2));

    Assuming malloc() doesn't fail, struct1 now holds an address on the heap.

    struct2 = malloc (sizeof(MYSTRUCT2));

    Assuming malloc() doesn't fail, struct2 now holds a different address on the heap.

    struct1->foo = struct2;

    struct->foo now holds the address of struct2. They both point to the same location on the heap.

    free(struct1->foo);
    free(struct2);


    The first call to free() succeeds; however the second call will fail because you are trying to free a location of memory that is no longer allocated. This has the same effect as:

    free(struct2);
    free(struct2);


    You may also want to get into the habit of checking the return value of malloc(). If malloc() fails, it will return NULL.

    Cheers,
    Jason Deckard

  3. #3
    Registered User foniks munkee's Avatar
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    Nov 2001
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    Cheers!

    Oh, and don't worry, I usually test to ensure that the call to malloc has succeeded.

    Thanks again - I appreciate the help!
    Last edited by foniks munkee; 03-01-2002 at 04:48 AM.

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