Marek Vasut marek.vasut@gmail.com wrote on 2012/04/02 18:39:33:
From: Marek Vasut marek.vasut@gmail.com
Dear Joakim Tjernlund,
Marek Vasut marek.vasut@gmail.com wrote on 2012/04/02 17:23:03:
Dear Joakim Tjernlund,
Marek Vasut marek.vasut@gmail.com wrote on 2012/04/02 16:42:30:
Dear Joakim Tjernlund,
Marek Vasut marek.vasut@gmail.com wrote on 2012/04/02 16:05:13: > Dear Joakim Tjernlund, > > > 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. > > But kernel has something mapped there to trap these pointers I > believe.
So? That only means that the kernel has extra protection if someone tries to deference such a ptr. You are not required to do that(nice to have though) You don have any protection for deferencing NULL either I think?
Can't GCC track it?
Track what? NULL ptrs? I don't think so. Possibly when you have a static NULL ptr but not in the general case.
Well of course.
What did you mean then with "Can't GCC track it?" then? Just a bad joke?
Never mind, didn't finish my train of thought.
I almost figured that ...
I am getting tired of this discussion now. I am just trying to tell you that no sane impl. of malloc() these days return NULL for malloc(0).
And I got your point. Though for u-boot, this would be the best solution actually. Anyone who uses memory allocated by malloc(0) is insane.
No, you don't get the point. If you did you would not have have made the "insane" remark.
No, relying on malloc(0) returning something sane is messed up.
Depends, if writing generic code for lots of OS:es you cannot rely malloc(0). Writing kernel code you can. Not to mention those devs that don't know better and just assumes that what works in glibc/kernel works every where.
From Scotts example we already know there is kernel code that relies on malloc(0)
not returning NULL.
Your argument seems to boil down to "relying on malloc(0) returning something sane is messed up" so therefore u-boot malloc should take the easy route and just return NULL for malloc(0).
Jocke