Re: How do I represent a convenient two-step allocation using ffi/unsafe?

Hi Matthew,

That makes sense, thanks. I have a follow-up question, and forgive me for the 180-degree turn.

I realized I was wrong to convey that I want the wrapper functions to help with allocations. My situation is that I have 346 generated procedure bindings for Vulkan, and 1342 bindings total. The more "helpful" the bindings I generate, the more differences I have to document, the more corner cases I have to support in the code generator, and the higher the learning curve for my users. I think it would be wiser to keep the bindings entirely consistent with their C equivalents and hand-write helpers on top to capture common patterns.

So if I take the `vkEnumerateInstanceLayerProperties/private` you showed me and change the (_ptr io _uint32_t) to (_cpointer _uint32_t), will it have any other noticeable usability differences for someone thinking like a C programmer? Do I have to use _ptr at all to do what I intend to do?

~slg

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Sunday, October 20, 2019 9:10 AM, Matthew Flatt mflatt-sDh8Nw2yj/[email protected] wrote:

> I don't think `ffi/unsafe` can generate that kind of wrapper for you.
> I'd write something like this:
> (define-vulkan vkEnumerateInstanceLayerProperties/private
> (_fun (o0 : (_ptr io _uint32_t))
> _pointer
> -> (r : _VkResult)
>
>          -> (begin
>
>               (check-vkResult r 'vkEnumerateInstanceLayerProperties)
>               o0))
>
>
> #:c-id vkEnumerateInstanceLayerProperties)
> (define (vkEnumerateInstanceLayerProperties)
> (define len (vkEnumerateInstanceLayerProperties/private 0 #f))
> (define props (malloc _VkLayerProperties len))
> (define new-len (vkEnumerateInstanceLayerProperties/private len props))
> (values new-len props))
> At Sun, 20 Oct 2019 03:47:52 +0000, Sage Gerard wrote:
>
> > Consider the following C++ that calls a function once to learn how much it
> > should allocate for a vector, and again to populate said vector.
> > uint32_t count;
> > vkEnumerateInstanceLayerProperties(&count, NULL);
> > std::vector<VkLayerProperties> properties(count);
> > vkEnumerateInstanceLayerProperties(&count, properties.data());
> > Below is my generated signature so far. The docs for
> > [_list](https://docs.racket-lang.org/foreign/foreign_procedures.html?q=fun#(
> > form.%28%28lib._ffi%2Funsafe..rkt%29.__list%29%29) shows similar examples,but they assume either the length of the vector is known in advance.
> > How would I make a custom function type to manage both o1 and o0 values with
> > the garbage collector, call the function twice as described, and return the
> > value for o1 in a single call?
> > (define-vulkan vkEnumerateInstanceLayerProperties
> > (_fun (o0 : (_ptr o _uint32_t))
> > (o1 : (_ptr o _VkLayerProperties))
> > -> (r : _VkResult)
> > -> (begin (check-vkResult r (quote vkEnumerateInstanceLayerProperties))
> > (values o0 o1))))
> > ~slg
> >
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