So, the dataBuffer has a maximum initial size, then I resize it down to the
actual resulting size (the data comes from a sensor and real-world vagaries
change the resulting volume of data). Then it eventually gets passed to
existing Objective-C++ code, and on to C++ code that wants a void* and length.
Because this runs on iOS, I'd like to reduce the amount of time I hang on to
memory as much as possible (there's a lot of parallel processing happening in
our pipeline). I'd also like to avoid copying the data.
The resizing was the main reason I chose to use Data.
> On Apr 27, 2017, at 18:56 , Hooman Mehr <hoo...@mac.com> wrote:
>
> You should be able to type your `dataBuffer ` as [Int8] (Byte array). Then
> you won’t need `withUnsafeMutableBytes`. You can simply call it like this:
>
> self.request = c_library_call(¶ms, dataBuffer) // Call as if it is a C
> array
>
> It works because of C interoperability compiler magic.
>
> As long as the instance holding `dataBuffer` is not deallocated and you have
> not resized the array, the pointer should remain valid.
>
>> On Apr 27, 2017, at 4:38 PM, Rick Mann via swift-users
>> <swift-users@swift.org> wrote:
>>
>>
>>> On Apr 27, 2017, at 01:48 , Alex Blewitt <alb...@apple.com> wrote:
>>>
>> ...
>>
>>> The let constant may not even be stored in a single place; if it's known to
>>> be constant it can be in-lined at the point of use and potentially unpacked
>>> and dead code elimination throw away the unused members, for example.
>>>
>>> If you want to pass in a let constant into the pointer, you can create a
>>> copy of it locally in a local variable and then use that instead. However
>>> this will be in the local scope, so the pointer isn't valid after it
>>> returns.
>>
>> Ah, so this brings up another issue, then. Many of the calls in the C
>> library take a pointer to some memory and hang on to it, filling it in at a
>> later point (they make network requests). I've been doing it like this, and
>> it's been working, but I wonder if this is fragile:
>>
>> class
>> MyClass
>> {
>> func
>> execute()
>> {
>> self.dataBuffer = Data(count: kLGSImageDataSize)
>> precondition(self.dataBuffer != nil, "Unable to allocate image buffer
>> (\(kLGSImageDataSize) bytes)")
>>
>> var params = c_library_params_t()
>> params.data_capacity = self.dataBuffer!.count
>>
>> self.dataBuffer?.withUnsafeMutableBytes
>> { (inBuffer) -> Void in
>> // This call returns immediately, but assumes
>> // it can write to inBuffer later…
>>
>> self.request = c_library_call(¶ms, inBuffer)
>> }
>>
>> if self.request == nil
>> {
>> // Error
>> }
>> }
>>
>> var dataBuffer: Data?
>> }
>>
>>
>> --
>> Rick Mann
>> rm...@latencyzero.com
>>
>>
>> _______________________________________________
>> swift-users mailing list
>> swift-users@swift.org
>> https://lists.swift.org/mailman/listinfo/swift-users
>
--
Rick Mann
rm...@latencyzero.com
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