Issue with free() for linked list implementation

[Steven Schveighoffer via Digitalmars-d-learn]

On 4/3/15 6:08 PM, Kitt wrote:
> On Friday, 3 April 2015 at 22:06:06 UTC, Namespace wrote:
>> On Friday, 3 April 2015 at 22:02:13 UTC, Kitt wrote:
>>> Hello. I’m trying to write my own version of a list that doesn’t rely
>>> on the garbage collector. I’m working on a very bare bones
>>> implementation using malloc and free, but I’m running into an
>>> exception when I attempt to call free. Here is a very minimal code
>>> sample to illustrate the issue:
>>>
>>> // Some constant values we can use
>>> static const int two = 2, ten = 10;
>>>
>>> // Get memory for two new nodes
>>> Node* head = cast(Node*)malloc(two.sizeof);
>>> Node* node1 = cast(Node*)malloc(ten.sizeof);
>>>
>>> // Initialize the nodes
>>> node1.value = ten;
>>> node1.next = null;
>>> head.value = two;
>>> head.next = node1;
>>>
>>> // Attempt to free the head node
>>> Node* temp = head.next;
>>> head.next = null;
>>> free(head); // Exception right here
>>> head = temp;
>>>
>>> Note, if I comment out the line ‘head.next = node1’, this code works.
>>> Does anyone know what I’m doing wrong with my manual memory management?
>>
>> Why did you allocate only 2 / 10 bytes and not Node.sizeof bytes?
>> Since your Node struct has at least one pointer (nexT) and a value (I
>> assume of type int) you must allocate at least 8 bytes for one Node.
>> I'm sure that is at least one of your problems.
>
> Wow, I can't even begin to explain how red my cheeks are right now.
> You're completely right; I have no idea what my head was thinking. Sure
> enough, call malloc with the correct type, and the error goes away =P
>
> Thanks for the help =) I guess I've been in C# land at work for way too
> long now, my low level C skills are evaporating!

I'm not here to redden your cheeks any further, but I did want to make 
sure you understood what actually was happening above:

1. you have established 2 integers named 'two' and 'ten'. These are 
simply integers.
2. When you malloc, you use 'two.sizeof' and 'ten.sizeof'. Integers are 
4 bytes, so you were allocating 4 bytes for each of these (not 2 or 10 
bytes as is alluded to above).
3. Then you are casting the resulting pointer as pointing at a "Node *". 
I'm assuming, having implemented linked lists many times and seeing your 
usage of Node, that it has at least a pointer and a value. Best case, 
this needs at least 8 bytes of space (32-bit CPU), and worst case 16 
bytes (64-bit CPU).
4. When you access the "Node *" flavored pointer to your 4-byte block, 
you were corrupting memory in any case.

Why does the free fail? Probably due to corrupted memory, be careful 
when using casts and C malloc.

-Steve