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Some bits on malloc(0) in C being allowed to return NULL

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Re: Some bits on malloc(0) in C being allowed to return NULL

#31

Post by MaxBarraclough »

masfuerte wrote: post #20
bobmcnamara wrote: post #13

Earlier quoted context omitted.

> Noncompliant, since `malloc(0)` is specified to return a unique pointer if it's not `NULL`. I know I've seen that somewhere, but may I ask what standard you're referring to?

It's POSIX. > Each [...] allocation shall yield a pointer to an object disjoint from any other object. The pointer returned points to the start (lowest byte address) of the allocated space. If the space cannot be allocated, a null pointer shall be returned. If the size of the space requested is 0, the behavior is implementation-defined: either a null pointer shall be returned, or the behavior shall be as if the size…

Not just POSIX, also the ISO C standard itself. https://en.cppreference.com/w/c/memory/malloc

Re: Some bits on malloc(0) in C being allowed to return NULL

#32

Post by randomNumber7 »

I never had the use case to allocate 0 bytes of memory.

If I would allocate 0 bytes of memory and get a pointer to it, I wouldn't care what the value of the pointer is since I am not allowed to dereference it anyways.

But then again, why would I allocate 0 bytes of memory?

Re: Some bits on malloc(0) in C being allowed to return NULL

#33

Post by o11c »

bobmcnamara wrote: post #12
o11c wrote: post #3

Earlier quoted context omitted.

Noncompliant, since `malloc(0)` is specified to return a unique pointer if it's not `NULL`. On most platforms an implementation could just return adjacent addresses from the top half of the address space. On 32-bit platforms it doesn't take long to run out of such address space however, and you don't want to waste the space for a bitmap allocator. I suppose you could just use a counter for each 64K region or somethin…

> Noncompliant, since `malloc(0)` is specified to return a unique pointer if it's not `NULL`. I know I've seen that somewhere, but may I ask what standard you're referring to? If I recall correctly, this was an archaic stackless microcontroller. The heap support was mostly a marketing claim.

(you duped your comment under the other subthread)

From C89, §7.10.3 "Memory management functions":

> If the size of the space requested is > zero, the behavior is implementation-defined; the value returned shall be either a null pointer or a > unique pointer.

The wording is different for C99 and POSIX, but I went back as far as possible (despite the poor source material; unlike later standards C89 is only accessible in scans and bad OCR, and also has catastrophic numbering differences). K&R C specifies nothing (it's often quite useless; people didn't actually write against K&R C but against the common subset of extensions of platforms they cared about), but its example implementation adds a block header without checking for 0 so it ends up doing the "unique non-NULL pointer" thing.

Re: Some bits on malloc(0) in C being allowed to return NULL

#34

Post by masfuerte »

MaxBarraclough wrote: post #31
masfuerte wrote: post #20

Earlier quoted context omitted.

It's POSIX. > Each [...] allocation shall yield a pointer to an object disjoint from any other object. The pointer returned points to the start (lowest byte address) of the allocated space. If the space cannot be allocated, a null pointer shall be returned. If the size of the space requested is 0, the behavior is implementation-defined: either a null pointer shall be returned, or the behavior shall be as if the size…

Not just POSIX, also the ISO C standard itself. https://en.cppreference.com/w/c/memory/malloc

That doesn't say the pointer has to be unique.

Re: Some bits on malloc(0) in C being allowed to return NULL

#35

Post by Joker_vD »

DSMan195276 wrote: post #22
commandlinefan wrote: post #14

Earlier quoted context omitted.

> return a valid unique pointer A pointer to what, though? If the requester asked for 0 bytes of memory, you'd either be pointing to memory allocated for another purpose (!) or allocating a few bytes that weren't asked for. > This makes people unhappy for various reasons I read through all the links trying to figure out what those reasons might be and came up empty, I'm still curious why anybody would expect or rely…

> allocating a few bytes that weren't asked for. FWIW the alignment guarantees of `malloc()` mean it often will have to allocate more than you ask for (before C23 anyway). You can't 'legally' use this space, but `malloc()` also can't repurpose it for other allocations because it's not suitably aligned. That said I still agree it's a hack compared to just using `malloc(1)` for this purpose, it's well-defined and funct…

> before C23 anyway

Did they change "suitably aligned for any object type" to "suitably aligned for any object type with size less than or equal to what was requested" or something like in C23?

Re: Some bits on malloc(0) in C being allowed to return NULL

#36

Post by Lvl999Noob »

Can someone tell me a usecase where you want multiple allocations of size 0, each one with a unique address, and each one unique from any other allocation (hence necessarily removing that pointer from being allocated to anything else) but can't use malloc(1) instead?

I think it would be much better if malloc(0) just returned 1 or -1 or something constant. If the programmer needs the allocation to have a unique address, they can call malloc(1) instead.

Re: Some bits on malloc(0) in C being allowed to return NULL

#38

Post by [deleted] »

MaxBarraclough wrote: post #31
masfuerte wrote: post #20

Earlier quoted context omitted.

It's POSIX. > Each [...] allocation shall yield a pointer to an object disjoint from any other object. The pointer returned points to the start (lowest byte address) of the allocated space. If the space cannot be allocated, a null pointer shall be returned. If the size of the space requested is 0, the behavior is implementation-defined: either a null pointer shall be returned, or the behavior shall be as if the size…

Not just POSIX, also the ISO C standard itself. https://en.cppreference.com/w/c/memory/malloc

[deleted]

Re: Some bits on malloc(0) in C being allowed to return NULL

#40

Post by jcranmer »

masfuerte wrote: post #34
MaxBarraclough wrote: post #31

Earlier quoted context omitted.

Not just POSIX, also the ISO C standard itself. https://en.cppreference.com/w/c/memory/malloc

That doesn't say the pointer has to be unique.

cppreference isn't the standard, and while the text they write looks like it's the same verbiage that would be authoritative, it's not. (And there's some criticism of it from standards committee members in that regard).

The current C standard text says:

> The order and contiguity of storage allocated by successive calls to the aligned_alloc, calloc, malloc, and realloc functions is unspecified. The pointer returned if the allocation succeeds is suitably aligned so that it can be assigned to a pointer to any type of object with a fundamental alignment requirement and size less than or equal to the size requested. It can then be used to access such an object or an array of such objects in the space allocated (until the space is explicitly deallocated). The lifetime of an allocated object extends from the allocation until the deallocation. Each such allocation shall yield a pointer to an object disjoint from any other object. The pointer returned points to the start (lowest byte address) of the allocated space. If the space cannot be allocated, a null pointer is returned. If the size of the space requested is zero, the behavior is implementation-defined: either a null pointer is returned to indicate an error, or the behavior is as if the size were some nonzero value, except that the returned pointer shall not be used to access an object.

So yeah, the allocations are required to be unique (at least until it's free'd).

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