Swift 3 does not, in general, allow you to convert pointers of one type to pointers of a different type. The rules of the road here are outlined in SE-0107 UnsafeRawPointer API, which puts Swift’s type aliasing rules on a firm footing. I strongly recommend you read the UnsafeRawPointer Migration doc before going further.
Alas, I can’t give you specific advice here because I’m not familiar with the API you’re using.
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Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
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Have you accomplish your attempt on using FFMPEG with Swift?
I tried the same attempt recently. But I found it extremely difficult in converting various pointers.