Swift | Get pointer of c char array inside struct

Question

I'm trying to convert some old ObjC code to Swift, I haven't done a lot with Swift regarding Pointers.

Original ObjC/C code:

unsigned char myId[6];
memcpy(myId, packet.header->m1, 6);

Original C Struct:

typedef struct {
    unsigned char m1[6];
    unsigned char m2[6];
} __attribute__((__packed__)) HeaderStruct;

My tried Swift code, not working:

var myId = [CUnsignedChar](repeating: 0, count: 6)
var headerStruct: UnsafePointer<HeaderStruct> = packet!.header()
memcpy(&myId, headerStruct.pointee.m1, 6)

The error regarding headerStruct.pointee.m1

Cannot convert value of type '(UInt8, UInt8, UInt8, UInt8, UInt8, UInt8)' to expected argument type 'UnsafeRawPointer?'

I assume I need the base address (headerStruct) and add the offset for the m1, but how do I do that?

Possibly helpful: https://stackoverflow.com/q/27455773/1187415. — Martin R, Jul 19 '19 at 21:19
@MartinR you mean something like this: `memcpy(&myId, withUnsafePointer(to: packet?.header().pointee.m1){$0}, 6)` ? — John Smith, Jul 19 '19 at 21:28

score 3 · Accepted Answer · answered Jul 19 '19 at 21:36

3

A C array is imported to Swift as a tuple. But the memory layout is preserved, therefore you can obtain a pointer to the storage of the tuple and “bind” it to a pointer to UInt8 values:

let myId = withUnsafeBytes(of: headerStruct.pointee.m1) {
    Array($0.bindMemory(to: UInt8.self))
}

answered Jul 19 '19 at 21:36

Martin R

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I think it works (yay!, thanks for that), but wouldn't memcpy work too? – John Smith Jul 19 '19 at 21:43
1

That would be `withUnsafePointer(to: headerStruct.pointee.m1) { memcpy(&myId, $0, 6) }` – Martin R Jul 19 '19 at 21:47

Swift | Get pointer of c char array inside struct

1 Answers1