My Free C

My Free C
The C library function void free(void *ptr) deallocates the memory previously allocated by a call to calloc, malloc, or realloc.
Following is the declaration for free() function.
ptr − This is the pointer to a memory block previously allocated with malloc, calloc or realloc to be deallocated. If a null pointer is passed as argument, no action occurs.
This function does not return any value.
The following example shows the usage of free() function.
Let us compile and run the above program that will produce the following result −
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I have been trying to free memory allocated via malloc() using free() .
Some of the structs it does free but leaves some the way they were and they also remain linked to their children. It also never frees the root (gRootPtr) for a binary tree.
I am using Xcode to find out if the memory used by the binary tree has been freed and also use the if statement.
Code I am using to free the memory:
Code I am using to see if the memory has been freed.
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First, free does not change the pointer itself.
If you want the pointer to go back to NULL, you must do this yourself.
Second, there are no guarentees about what will happen to the memory pointed to by the pointer after the free:
Note that this means that you cannot actually test if free() works. Strictly speaking, it's legal for free() to be implemented by doing absolutely nothing (although you'll run out of memory eventually if this is the case, of course).
That check won't check if the variable is freed. Note that free(pointer) does not set that pointer to NULL. If you want that to be the case, you have to set it yourself, and it is a common idiom in C:
to signal that you already freed that pointer.
The only reason free() would fail is if the pointer you give it does not dereference to allocated heap. This behavior is clearly defined, free() will either work, or your program will halt due to an access violation.
It is good practice to re-initialize your pointers after freeing them, for just this purpose. This lets you:
Make sure you don't allocate on top of an already allocated pointer (thus losing the reference to the original blocks and causing a leak) (realloc() notwithstanding).
Make sure you don't free recently freed memory, or memory that was never allocated
Both become easy by testing to see if the pointer is initialized (or, NULL).
It's best to just do this manually and get in the habit of doing so. I have seen some very convoluted ways of re-implementing free() so that re-initializes the pointer automatically, like this little gem that also attempts avoid freeing memory that wasn't allocated:
Please, don't use that code, it will break horribly on strict platforms due to dereferencing a type punned pointer. Additionally, what if the pointer is a string literal?
Instead, just make sure you keep track of your pointers and initialize them after freeing.
The function free takes a pointer to allocated memory, it does not however set that pointer to NULL , in fact there is no way it could do so (it would need to take the address of a pointer for that).
Typical use case in this scenario is:
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