Converting an UnsafePointer with length to a Swift Array type
Solution 1
You can simply initialize a Swift Array
from an UnsafeBufferPointer
:
func convert(length: Int, data: UnsafePointer<Int8>) -> [Int8] {
let buffer = UnsafeBufferPointer(start: data, count: length);
return Array(buffer)
}
This creates an array of the needed size and copies the data.
Or as a generic function:
func convert<T>(count: Int, data: UnsafePointer<T>) -> [T] {
let buffer = UnsafeBufferPointer(start: data, count: count);
return Array(buffer)
}
where length
is the number of items that the pointer points to.
If you have a UInt8
pointer but want to create an [T]
array from
the pointed-to data, then this is a possible solution:
// Swift 2:
func convert<T>(length: Int, data: UnsafePointer<UInt8>, _: T.Type) -> [T] {
let buffer = UnsafeBufferPointer<T>(start: UnsafePointer(data), count: length/strideof(T));
return Array(buffer)
}
// Swift 3:
func convert<T>(length: Int, data: UnsafePointer<UInt8>, _: T.Type) -> [T] {
let numItems = length/MemoryLayout<T>.stride
let buffer = data.withMemoryRebound(to: T.self, capacity: numItems) {
UnsafeBufferPointer(start: $0, count: numItems)
}
return Array(buffer)
}
where length
now is the number of bytes. Example:
let arr = convert(12, data: ptr, Float.self)
would create an array of 3 Float
s from the 12 bytes pointed to by ptr
.
Solution 2
extension NSData {
public func convertToBytes() -> [UInt8] {
let count = self.length / sizeof(UInt8)
var bytesArray = [UInt8](count: count, repeatedValue: 0)
self.getBytes(&bytesArray, length:count * sizeof(UInt8))
return bytesArray
}
}
You can convert row data to byts (Uint8)
Copy Extension and use it..
Ephemera
Updated on June 02, 2022Comments
-
Ephemera over 1 year
I'm looking for the simplest ways to achieve reasonable C interoperability in Swift, and my current block is converting an
UnsafePointer<Int8>
(which was aconst char *
), into an[Int8]
array.Currently, I have a naïve algorithm that can take an
UnsafePointer
and a number of bytes and converts it to an array, element by element:func convert(length: Int, data: UnsafePointer<Int8>) { let buffer = UnsafeBufferPointer(start: data, count: length); var arr: [Int8] = [Int8]() for (var i = 0; i < length; i++) { arr.append(buffer[i]) } }
The loop itself can be sped up by using
arr.reserveCapacity(length)
, however that does not remove the issue of the loop itself.I'm aware of this SO question which covers how to convert
UnsafePointer<Int8>
toString
, howeverString
is a different beast entirely to[T]
. Is there a convenient Swift way of copying length bytes from anUnsafePointer<T>
into a[T]
? I'd prefer pure Swift methods, without passing throughNSData
or similar. If the above algorithm is really the only way to do it, I'm happy to stick with that. -
Alexander over 7 yearsDoes this cause a memory leak? I can't find any indication that
UnsafeBufferPointer
will free its memory -
Martin R over 7 years@AMomchilov:
UnsafeBufferPointer
is just a pointer, it does not allocate memory, so there is nothing to free. TheArray
is then managed by Swift. -
Alexander over 7 yearsYes, but the pointer passed into
convert
has to be freed, right? That should be made explicit, IMO -
Martin R over 7 years@AMomchilov: You are right, if the memory has been allocated then it must be freed eventually. But we don't know where that pointer comes from, it could point to static memory. As I see it, that is independent of the
convert
function. -
kabiroberai about 7 yearsIn Swift 3, the
let buffer
line should belet buffer = data.withMemoryRebound(to: T.self, capacity: 1) {UnsafeBufferPointer(start: $0, count: length/MemoryLayout<T>.stride)}
-
Martin R about 7 years@kabiroberai: Thanks, updated! (The capacity should be the number of target items as well.)
-
kabiroberai about 7 years@MartinR welcome 😀 may I ask though, why does
capacity
need to be the number of items? It seems to work even when capacity is set to 1 (since$0
only refers to the start and not the entire data). -
Martin R about 7 years@kabiroberai: Well, that is how I understand the description "Rebinds memory at self to type T with capacity to hold count adjacent T values while executing the body closure. ". I must admit that I don't know what the consequences of a different value would be.
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dugla almost 7 yearsThis is insanely helpful. My C <-> Swift interop anxiety is just melting away.
-
tBug almost 6 yearsNice to hear that ^^