[swift-users] Passing Data to a f(void *) function

[Daniel Dunbar]

> On Jun 30, 2017, at 7:40 AM, Martin R via swift-users <swift-users at swift.org> wrote:
> 
> I have a C function 
> 
>    void myfunc(const void *ptr);
> 
> which is imported to Swift as
> 
>    func myfunc(_ ptr: UnsafeRawPointer!)
> 
> This compiles and runs without problems:
> 
>    let data = Data(bytes: [1, 2, 3, 4])
>    data.withUnsafeBytes { (ptr) in myfunc(ptr) } // (A)
> 
> and the type of `ptr` is inferred as `UnsafePointer<Void>`. But adding an explicit type
> annotation produces a compiler warning:

How do you know the inferred type is `UnsafePointer<Void>`? I think it is more likely it is `UnsafePointer<UInt8>`, and the following compiles:
```
let data = Data(bytes: [1, 2, 3, 4])
data.withUnsafeBytes { (ptr: UnsafePointer<UInt8>) in
    myfunc(ptr)
}
```

 - Daniel

> 
>    data.withUnsafeBytes { (ptr: UnsafePointer<Void>) in myfunc(ptr) } // (B)
>    // warning: UnsafePointer<Void> has been replaced by UnsafeRawPointer
> 
> which is understandable in the view of "SE-0107 UnsafeRawPointer API".
> 
> The "Fix-it" replaces `UnsafePointer<Void>` by `UnsafeRawPointer`, and that does not
> compile anymore:
> 
>    data.withUnsafeBytes { (ptr: UnsafeRawPointer) in myfunc(ptr) } // (C)
>    // error: cannot convert value of type 'Void' to closure result type '_'
> 
> because there is no `withUnsafeBytes()` method taking a `(UnsafeRawPointer)->ResultType`
> closure.
> 
> 
> My questions are:
> 
> 1. Why are (A) and (B) treated differently?
> 
> 2. Is (A) "legal", or should one use some non-void pointer
> 
>    data.withUnsafeBytes { (ptr: UnsafePointer<Int8>) in myfunc(ptr) } // (D)
> 
>   (which feels wrong to me because it is converted back to a void pointer when
>   calling the function).
> 
> 3. Or should there be a `withUnsafeRawPointer()` method which makes (C) compile as
> 
>      data.withUnsafeRawBytes { (ptr: UnsafeRawPointer) in myfunc(ptr) }
> 
>   This would also allow to access the data at byte offsets more easily, e.g.
> 
>       data.withUnsafeRawBytes { ptr in
>           let u16 = ptr.load(fromByteOffset: 4, as: UInt16.self)
>       }
> 
>   Does that makes sense?
> 
> Regards, Martin
> 
> 
> 
> 
> 
> 
> 
> 
> 
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