Re: realloc problem, corrupt last item

K. Jennings wrote:
> On Thu, 08 May 2008 14:27:23 +0000, Richard Heathfield wrote:
> 
>> Igal said:
>>
>>> hay, i have this werid problem with my book adding function, this how
>>> it looks
>>>
>>> book* AddBook(book *bp, unsigned *size) { ...
>>> //then i use realloc to allocate space for the new item in the bp
>>> pointer
>>> bp = (book*)realloc(bp, sizeof(book)); ...
>>> }
>> You have not provided sufficient information for me to help you
>> effectively. On general principles, lose the cast, replace sizeof(book)
>> with sizeof *bp, 
> 
> 	Why? Under what cir***stances they would not return the same 
> value?

     When bp turns out not to be a book* but a book** or a
booklet* or a Book* or ...

     Suppose that the compiler raises no objections to

	ptr = malloc(sizeof(struct messagedetail));

Can you tell whether the malloc() call requests the correct
amount of memory for ptr to point at?  Not until you scroll
around for a while to find the declaration of ptr to make
sure it's not a `struct messagedigest*' instead.  But if
it's written as

	ptr = malloc(sizeof *ptr);

.... you can tell at a glance that it's correct.

-- 
Eric.Sosman@[EMAIL PROTECTED]