[swift-users] Still having trouble with C interop (passing buffers)

[Rick Mann]

Yup, that seems to be it. I would have thought that init(capacity:) should work, too (it could defer allocation until, say, the call withUnsafeMutableBytes()), and just not zero the contents (the init(count:) call zeroes the contents).

> On Apr 25, 2017, at 15:20 , Philippe Hausler <phausler at apple.com> wrote:
> 
> 
>> On Apr 25, 2017, at 2:57 PM, Rick Mann via swift-users <swift-users at swift.org> wrote:
>> 
>> I'm trying to pass a Data of allocated size to a C function for it to fill in:
>> 
>> 
>> lib_example_call(_ params: UnsafePointer<lib_call_params_t>!, _ data: UnsafeMutableRawPointer!)
>> 
>> ...
>> {
>>   self.dataBuffer = Data(capacity: BufferSizeConstant)
> 
> 
> If you want to create a buffer with a given count you should use Data(count: Int), the method you are using just reserves a given capacity. (from what I can tell from your code that is likely what you really want)
> 
>> 
>>   var params = lib_call_params_t();
>>   params.data_capacity = BufferSizeConstant;
>> 
>>   self.dataBuffer?.withUnsafeMutableBytes
>>   { (inBuffer) -> Void in
>>       lib_example_call(&params, inBuffer)
>>   }
>> }
>> 
>> I later get called back by the library with a size value of the actual data it got. self.dataBuffer is a var. I set self.dataBuffer?.count = result size, which is a reasonable value.
>> 
>> Unfortunately, the resulting buffer is all zeros. The data generated by the call is definitely not all zero, and a C example program using the same library works correctly.
>> 
>> So, I think there's something wrong in the way I'm making the call.
>> 
>> Can anyone please enlighten me? Thanks!
>> 
>> -- 
>> Rick Mann
>> rmann at latencyzero.com
>> 
>> 
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> 


-- 
Rick Mann
rmann at latencyzero.com