[swift-users] Why does withUnsafePointer(to:) require a var argument?

[Rick Mann]

Yeah, okay. So: how do I do this in a way that is safe?

-- 
Rick Mann
rmann at latencyzero.com

> On Apr 27, 2017, at 23:00, Rien <Rien at Balancingrock.nl> wrote:
> 
> To address your question:
> 
> https://developer.apple.com/reference/foundation/data/1779823-withunsafemutablebytes
> 
> "Warning
> The byte pointer argument should not be stored and used outside of the lifetime of the call to the closure."
> 
> Which is exactly what you are doing, hence the code is unsafe (no pun intended).
> 
> Regards,
> Rien
> 
> Site: http://balancingrock.nl
> Blog: http://swiftrien.blogspot.com
> Github: http://github.com/Balancingrock
> Project: http://swiftfire.nl - A server for websites build in Swift
> 
> 
> 
> 
> 
> 
>> On 28 Apr 2017, at 01:38, Rick Mann <rmann at latencyzero.com> wrote:
>> 
>> 
>>> On Apr 27, 2017, at 01:48 , Alex Blewitt <alblue at 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(&params, inBuffer)
>>           }
>> 
>>       if self.request == nil
>>       {
>>           //  Error
>>       }
>>   }
>> 
>>   var             dataBuffer:     Data?
>> }
>> 
>> 
>> -- 
>> Rick Mann
>> rmann at latencyzero.com
>> 
>> 
>