[swift-users] C Pointers and Memory

[Chris McIntyre]

Hi guys, 

I’m having issues with Swift pointers. I feel like the Interactive With C APis document only gets you half way there. 

For example, look at this from the docs

If you have declared a function like this one:

func takesAMutablePointer(x: UnsafeMutablePointer<Float>) {
    // ...
}
You can call it in any of the following ways:

var x: Float = 0.0
var p: UnsafeMutablePointer<Float> = nil
var a: [Float] = [1.0, 2.0, 3.0]
takesAMutablePointer(nil)
takesAMutablePointer(p)
takesAMutablePointer(&x)
takesAMutablePointer(&a)
Seem simple enough. But then I was trying to figure out Core Data validation, which takes an AutoreleasingUnsafeMutablePointer<AnyObject?> and I can’t figure out what to pass to it. 

I tried to create a simple test in a Playground:

func takesAPointer(p: AutoreleasingUnsafeMutablePointer<AnyObject?>){
    return
}

var myString = "Hello"

takesAPointer(p: &myString)

Then I get an error stating 'Cannot pass immutable type “AnyObject?” as inout argument’. 

Everything seems to match the example from the docs. I have a var (so it should be mutable) and I’m using the ampersand, but still I’m getting an error. 

Another problem. I have a specific byte pattern I want to create. For arguments sake, lets call it 0x123ABC, and I have it as an Int. I want to access the individual bytes (i.e. 12, 3A, BC). 

The struct reference for UnsafePointer<T> doesn’t talk much about initializing it. Most of the initializers take a pointer. I tried the init(_ bitPattern:) initializer, and was able to create a pointer, but it seemed to point to the address 0x123ABC rather than the address *of* 0x123ABC. I tried creating a buffer with malloc, and it gives me an UnsafeMutablePointer  but now I can’t figure out how to copy my bytes to this buffer.

So clearly there’s something I’m just not grocking about Swift pointers. Does anyone know of a more remedial tutorial that is updated for Swift 3? I’d like to continue to work in pure Swift, but it just isn’t clicking.

--
Chris McIntyre




> On Jul 29, 2016, at 6:11 AM, James Campbell via swift-users <swift-users at swift.org> wrote:
> 
> No I haven't thats a big help thank you !
> 
> ___________________________________
> 
> James⎥Head of Trolls
> 
> james at supmenow.com <mailto:james at supmenow.com>⎥supmenow.com <http://supmenow.com/>
> Sup
> 
> Runway East
> 
> 
> 10 Finsbury Square
> 
> London
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> 
> EC2A 1AF 
> 
> 
> On 29 July 2016 at 10:40, Zhao Xin <owenzx at gmail.com <mailto:owenzx at gmail.com>> wrote:
> Have you read https://developer.apple.com/library/tvos/documentation/Swift/Conceptual/BuildingCocoaApps/InteractingWithCAPIs.html#//apple_ref/doc/uid/TP40014216-CH8-ID17 <https://developer.apple.com/library/tvos/documentation/Swift/Conceptual/BuildingCocoaApps/InteractingWithCAPIs.html#//apple_ref/doc/uid/TP40014216-CH8-ID17> ?
> 
> Zhaoxin
> 
> On Fri, Jul 29, 2016 at 4:55 PM, James Campbell via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
> ​Do you know of any resources to brush up on the pointer aspect of swift ? ​
> 
> ___________________________________
> 
> James⎥Head of Trolls
> 
> james at supmenow.com <mailto:james at supmenow.com>⎥supmenow.com <http://supmenow.com/>
> Sup
> 
> Runway East
> 
> 
> 10 Finsbury Square
> 
> London
> 
> 
> EC2A 1AF 
> 
> 
> On 29 July 2016 at 09:10, Dmitri Gribenko <gribozavr at gmail.com <mailto:gribozavr at gmail.com>> wrote:
> On Fri, Jul 29, 2016 at 12:55 AM, James Campbell <james at supmenow.com <mailto:james at supmenow.com>> wrote:
> > So this:
> >
> > if let data = someArrayGeneratingFunction() {
> >   cFunction(UnsafeMutablePointer(data))
> > }
> >
> > Has issues with the array passed to c getting corrupted, but this doesn't:
> >
> > let data = someArrayGeneratingFunction()
> >
> > if let data = data {
> >   cFunction(UnsafeMutablePointer(data))
> > }
> 
> Neither piece of code is guaranteed to work.  (You are just getting
> lucky that the second one happens to work.)  Array-to-pointer
> conversion only extends the lifetime of the array until the immediate
> function call returns.  So after UnsafeMutablePointer(data) returns,
> the array can be freed.
> 
> Use someArrayGeneratingFunction.withUnsafeMutableBuffer { ... } instead.
> 
> Dmitri
> 
> --
> main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
> (j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com <mailto:gribozavr at gmail.com>>*/
> 
> 
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