Re: [PATCH 01/10] ivec: introduce the C side of ivec

From: Phillip Wood <phillip.wood123@gmail.com>
Date: Jan 16, 2026, 10:39 UTC
Message-ID: <c2d9a432-0753-4786-8de9-c3dcfe69ac36@gmail.com>
In-Reply-To: <CAH=ZcbA_HgEO2T2smn4Yg6gf4sm4jrR8A0ek1v9nqsa1MXbRJw@mail.gmail.com>

I've Cc'd Peff and René for a second opinion if you have time please.

On 15/01/2026 15:55, Ezekiel Newren wrote:

> On Thu, Jan 8, 2026 at 7:34 AM Phillip Wood <phillip.wood123@gmail.com> wrote:

Show 12 quoted lines

>>> +static void _set_capacity(void *self_, size_t new_capacity)
>>> +{
>>> +     struct IVec_c_void *self = self_;
>>
>> Passing any of the ivec variants defined below to this function invokes
>> undefined behavior because we're not casting the pointer back to the
>> orginal type. However I think on the platforms we care about
>> sizeof(void*) == sizeof(T*) for all T so maybe we can look the other way.
> 
> If someone finds that this code does not work because of this
> assumption I'd like to know. But I can't fathom a case where it
> wouldn't work.

So we have two different structs

struct IVec_c_void {
	void *ptr;
	size_t length;
	size_t capacity;
	size_t element_size;
}

and

struct Ivec_u8 {
	uint8_t *ptr;
	size_t length;
	size_t capacity;
	size_t element_size;
}

One the platforms we care about they will have the same memory layout as all pointers have the same representation. However I don't think they are "compatible types" in the language of the C standard because the type of the "ptr" member differs. That means casting IVec_u8* to IVec_c_void* either directly or via void* is undefined and so

	struct IVec_u8 vec;
	ivec_init(&vec, sizeof(*vec.ptr));

is undefined. For the compiler to see the undefined cast it needs to look across translation units because the implementation of ivec_init() will be in a separate file to where it is called. Maybe that and the fact they have the same memory layout saves us from having to worry too much though I'm always nervous of undefined behavior.

An alternative would be to pass the individual struct members as function parameters

	void ivec_init(void **vec, size_t &length, size_t &capacity,
		       size_t &element_size_, size_t element_size)
	{
		*vec = NULL;
		*length = 0;
		*capacity = 0;
		*element_size_ = element_size;
	}

and have DEFINE_IVEC_TYPE create typesafe wrappers

	static inline void ivec_u8_init(struct IVec_u8 *vec)
	{
		void *ptr = vec->ptr;
		ivec_init(&ptr, &v->length, &v->capacity,
			  &v->element_size, sizeof(*(v->ptr));
		vec->ptr = ptr;
	}

That's safe because we cast the "ptr" member to "void*" and then back to the original type. On the rust side the implementation of IVec<T> would also need to split out the individual struct members when it calls ivec_init() etc. It's all a bit more effort but the benefit is that we don't have any undefined behavior and we have a nice typesafe C interface to 'struct IVec_*'.

Thanks

Phillip