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)

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,

Cheers!

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

Thanks again - I appreciate the help!