[U-Boot] [PATCH] Prevent malloc with size 0

Graeme Russ
2012-04-02T20:28:46Z

On 04/02/2012 05:40 PM, Joakim Tjernlund wrote:
> Hi Grame
>
> Graeme Russ <graeme.russ@gmail.com> wrote on 2012/04/02 09:17:44:
>>
>> Hi Joakim,
>> On Apr 2, 2012 4:55 PM, "Joakim Tjernlund" <joakim.tjernlund@transmode.se> wrote:
>>>
>>>>
>>>> Hi Marek,
>>>>
>>>> On Mon, Apr 2, 2012 at 1:36 PM, Marek Vasut <marek.vasut@gmail.com> wrote:
>>>>> Dear Mike Frysinger,
>>>>>
>>>>>> On Sunday 01 April 2012 20:25:44 Graeme Russ wrote:
>>>>>>> b) The code calling malloc(0) is making a perfectly legitimate assumption
>>>>>>>
>>>>>>> based on how glibc handles malloc(0)
>>>>>>
>>>>>> not really. POSIX says malloc(0) is implementation defined (so it may
>>>>>> return a unique address, or it may return NULL). no userspace code
>>>>>> assuming malloc(0) will return non-NULL is correct.
>>>>>
>>>>> Which is your implementation-defined ;-) But I have to agree with this one. So
>>>>> my vote is for returning NULL.
>>>>
>>>> Also, no userspace code assuming malloc(0) will return NULL is correct
>>>>
>>>> Point being, no matter which implementation is chosen, it is up to the
>>>> caller to not assume that the choice that was made was, in fact, the
>>>> choice that was made.
>>>>
>>>> I.e. the behaviour of malloc(0) should be able to be changed on a whim
>>>> with no side-effects
>>>>
>>>> So I think I should change my vote to returning NULL for one reason and
>>>> one reason only - It is faster during run-time
>>>
>>> Then u-boot will be incompatible with both glibc and the linux kernel, it seems
>> Forget aboug other implementations...
>> What matters is that the fact that the behaviour is undefined and it is up to the caller to take that into account
>
> Well, u-boot borrows code from both kernel and user space so it would make sense if
> malloc(0) behaved the same. Especially for kernel code which tend to depend on the
> kernels impl.(just look at Scotts example)
>
>>> to me that any modern impl. of malloc(0) will return a non NULL ptr.
>>>
>>> It does need to be slower, just return ~0 instead, the kernel does something similar:
>>> if (!size)
>>> return ZERO_SIZE_PTR;
>> That could work, but technically I don't think it complies as it is not a pointer to allocated memory...
>
> It doesn't not have to be allocated memory, just a ptr != NULL which you can do free() on.

As per the spec:

The malloc function returns either a null pointer or a pointer to the
allocated space.

The amount of storage allocated by a successful call to the calloc, malloc,
or realloc function when 0 bytes was requested (7.22.3).

The way I read that, if NULL is not returned, then what is returned is a
pointer to allocated space. If malloc(0) is called, the amount of space
allocated is not determined by the spec

Regards,

Graeme