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?

@MartinR you mean something like this: memcpy(&myId, withUnsafePointer(to: packet?.header().pointee.m1){$0}, 6) ? — John Smith
– John Smith, Commented Jul 19, 2019 at 21:28

Martin R · Accepted Answer · 2019-07-19 21:36:33Z

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, 2019 at 21:36

Martin R

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2 Comments

John Smith Over a year ago

I think it works (yay!, thanks for that), but wouldn't memcpy work too?

Martin R Over a year ago

That would be withUnsafePointer(to: headerStruct.pointee.m1) { memcpy(&myId, $0, 6) }

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