On 2016-06-09 21:37, Joe Groff wrote:
On Jun 9, 2016, at 8:39 PM, Saagar Jha via swift-users
<swift-users@swift.org> wrote:
Nevermind, I lied. Swift does allow direct pointer arithmetic:
import Foundation
var source = [UInt8](repeating: 0x1f, count: 32)
var destination = [UInt8](repeating: 0, count: 64)
memcpy(&destination, source, 32) // the C function
memcpy(&destination + 3, source, 13) // the + operator works to offset
Arrays can indeed be used as pointer parameters, but the second one
only works by accident. The pointer bridging has the same semantics as
an 'inout' parameter, so the pointer is only valid for the duration of
the immediate call, and since operators in Swift are also function
calls, the pointer expires after the '+' operation. If you're doing
anything with an array other than passing it off to a single C
function, you should use withUnsafeMutableBufferPointer instead:
destination.withUnsafeMutableBufferPointer { p in
memcpy(p.baseAddress, source, 32)
memcpy(p.baseAddress + 3, source, 13)
}
In addition to not having undefined behavior, this will also probably
be faster, since it'll only need to pin the array for pointer access
once instead of twice.
-Joe
Thanks for the good answers, both Saager and Joe. I like the way you
both developed solutions and then improved them, and for good reasons.
I do have a situation in the ugly C code I am porting to Swift where a
pointer gets passed down to a second function, so Joe's solution looks
like the right approach.
Thanks again.
--
Ken Burgett
Principal Software Engineer
Email: k...@iotone.io
Office: 530.693.4449
Mobile: 831.332.6846
URL: www.iotone.co
_______________________________________________
swift-users mailing list
swift-users@swift.org
https://lists.swift.org/mailman/listinfo/swift-users
