Why should it be allowed to return a valid pointers anyways? Surely it should always return NULL?
replies(5):
Another use was to use it to mint unique cookies/addresses, but malloc(1) works for this just as well.
Additionally the actual amount of memory malloc allocates is implementation-defined so long as it is not less than the amount requested, but accessing this extra memory is undefined behavior since processes don't know if it exists or not. a non-NULL return could be interpreted as malloc(0) allocating more than zero bytes.
Some implementations don't actually perform the allocation until theres a pagefault from the process writing to or reading from that memory so in that sense a non-NULL return is valid too.
I'd argue that malloc(0)==NULL makes less sense because there's no distinction between failure and success.
The only real problem is specifying two alternate behaviors and declaring them both to be equally valid.
The point of the original C Standard was to make rules about these things AND not break existing implementations. They recognized that (a) and (b) were in existing implementations and were reasonable, and they chose not to break the existing implementations when writing the standard.
This is similar to the extremely unfortunate definition of the NULL macro. There were two existing styles of implementation (bare literal 0 and (void *) 0) and the Standard allows either style. Which means the NULL macro is not entirely safe to use in portable code.
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 on anything except a null pointer in this instance.
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 functionally equivalent if you're looking for a unique address. The fact that you don't know what `malloc(0)` is going to do makes it pretty useless anyway.
If you’re pointing to a zero sized data it shouldn’t matter what it’s pointing to. Even outside valid address space. Because you shouldn’t be reading or writing more than 0 bytes anyway.
You are always allocating bytes you weren't asked for: the allocation metadata and some extra bytes to satisfy the alignment requirement. If you absolutely don't want to allocate memory, you probably shouldn't have called malloc() in the first place :)
By extension, if x == 0, doesn't it make sense for the system to either return NULL OR to return a pointer to 0 bytes of memory which the program can use? So the standard promises exactly that: to return either NULL or else a unique pointer where that the program has permission to use zero bytes starting at that pointer.
malloc(0) is allowed to return non-NULL because the standard decrees it.
One way of thinking is that all mallocated pointers must always be freed exactly once. Then you're portable.