On Apr 3, 2012 6:57 AM, "Joakim Tjernlund" <joakim.tjernl...@transmode.se> wrote: > > Graeme Russ <graeme.r...@gmail.com> wrote on 2012/04/02 22:28:46: > > From: Graeme Russ <graeme.r...@gmail.com> > > > > On 04/02/2012 05:40 PM, Joakim Tjernlund wrote: > > > Hi Grame > > > > > > Graeme Russ <graeme.r...@gmail.com> wrote on 2012/04/02 09:17:44: > > >> > > >> Hi Joakim, > > >> On Apr 2, 2012 4:55 PM, "Joakim Tjernlund" < joakim.tjernl...@transmode.se> wrote: > > >>> > > >>>> > > >>>> Hi Marek, > > >>>> > > >>>> On Mon, Apr 2, 2012 at 1:36 PM, Marek Vasut <marek.va...@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 > > Please read http://lwn.net/Articles/236920/ > They have a different view.
Yes, I read that. They also have a compelling argument. Bottom line is, all three solutions are valid because, at the end of the day, it's up to the caller to handle the unspecified behaviour. Regards, Graeme
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