How to get bytes out of an UnsafeMutableRawPointer?

swift unsafe-pointers
hotpaw2 picture hotpaw2 · Aug 16, 2016 · Viewed 17.5k times · Source

How does one access bytes (or Int16's, floats, etc.) out of memory pointed to by an UnsafeMutableRawPointer (new in Swift 3) handed to a Swift function by a C API (Core Audio, etc.)

Answer

Martin R picture Martin R · Aug 16, 2016

load<T> reads raw bytes from memory and constructs a value of type T:

let ptr = ... // Unsafe[Mutable]RawPointer
let i16 = ptr.load(as: UInt16.self)

optionally at a byte offset:

let i16 = ptr.load(fromByteOffset: 4, as: UInt16.self)

There is also assumingMemoryBound() which converts from a Unsafe[Mutable]RawPointer to a Unsafe[Mutable]Pointer<T>, assuming that the pointed-to memory contains a value of type T:

let i16 = ptr.assumingMemoryBound(to: UInt16.self).pointee

For an array of values you can create a "buffer pointer":

let i16bufptr = UnsafeBufferPointer(start: ptr.assumingMemoryBound(to: UInt16.self), count: count)

A buffer pointer might already be sufficient for your purpose, it is subscriptable and can be enumerated similarly to an array. If necessary, create an array from the buffer pointer:

let i16array = Array(i16bufptr)

As @Hamish said, more information and details can be found at

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