libc/stdlib/mallocr.c (unlink)

[J. Johnston]

I mean circular in that you do not traverse the list
until you reach a NULL pointer as one should not exist.  The last element
will end up pointing forward to the first and vice-versa.  If there is one element,
it should point to itself and cannot be removed via unlink.

Take a look at the first() and last() operators for bins.

/* field-extraction macros */

#define first(b) ((b)->fd)
#define last(b)  ((b)->bk)

There is also a comment about the chunk list being circular:

     Chunks always begin on even word boundries, so the mem portion
     (which is returned to the user) is also on an even word boundary, and
     thus double-word aligned.

     Free chunks are stored in circular doubly-linked lists, and look like this:

As Chris has pointed out, we need to understand why you are seeing a NULL
pointer in the list.  There may be a serious logic flaw occuring or something
in your program may be stepping on storage.

-- Jeff J.



Earnie Boyd wrote:
> What do you mean by ``circular''?  If the pointer value is capable of 
> being zero it will SIGSEGV, on some systems, if referenced.
> 
> Earnie.
> 
> J. Johnston wrote:
> 
>> Actually, you are entirely correct.  I had forgotten that "both" the 
>> chunk
>> and bin lists were circular.  The change will be reverted.
>>
>> Earnie, you are going to have to do some more investigating.
>>
>> -- Jeff J.
>>
>> Christopher Faylor wrote:
>>
>>> On Tue, Feb 18, 2003 at 05:35:14PM -0500, J. Johnston wrote:
>>>
>>>> Patch checked in, thanks.
>>>
>>>
>>>
>>>
>>> Isn't this a "why is the pointer NULL?" type of situation?  It seems 
>>> like
>>> checking for null is papering over a possible problem.
>>>
>>> cgf
>>>
>>>
>>>>> ------------------------------------------------------------------------ 
>>>>>
>>>>>
>>>>> 2003.02.18  Earnie Boyd  <earnie@users.sf.net>
>>>>>
>>>>>     * libc/stdlib/mallocr.c (unlink): Don't assign a value to a 
>>>>> pointer
>>>>>     with a NULL value.
>>>>>
>>>>> Index: mallocr.c
>>>>> ===================================================================
>>>>> RCS file: /cvs/src/src/newlib/libc/stdlib/mallocr.c,v
>>>>> retrieving revision 1.9
>>>>> diff -u -3 -p -r1.9 mallocr.c
>>>>> --- mallocr.c    11 Oct 2002 10:28:30 -0000    1.9
>>>>> +++ mallocr.c    18 Feb 2003 21:40:05 -0000
>>>>> @@ -1936,8 +1936,8 @@ static void do_check_malloced_chunk(p, s
>>>>> {                                                                         
>>>>> \
>>>>>  BK = 
>>>>> P->bk;                                                              \
>>>>>  FD = 
>>>>> P->fd;                                                              \
>>>>> -  FD->bk = 
>>>>> BK;                                                            \
>>>>> -  BK->fd = 
>>>>> FD;                                                            \
>>>>> +  if (FD) FD->bk = 
>>>>> BK;                                                    \
>>>>> +  if (BK) BK->fd = 
>>>>> FD;                                                    \
>>>>> }                                                                         
>>>>> \
>>>>>
>>>>> /* Place p as the last remainder */
>>>>
>>>>
>>>>
>>
>>
>