Convert unsinged char array from objective-c to UnsafeMutablePointer<UInt8> swift

Question

I have an unsigned char array in .h file. and, also I have a function that takes unsigned char array as input. When I try to call this function in swift, it gets UnsafeMutablePointer<UInt8>. So, automatic objective-c to swift converter thinks unsigned char should be converted as UnsafeMutablePointer<UInt8>. But, I can not pass my variable to this function directly. Because I get,

Cannot convert value of type '(UInt8, UInt8, UInt8, UInt8, UInt8)' to expected argument type 'UnsafeMutablePointer<UInt8>!'

Length of this array is static. So, I can create

let key = UnsafeMutablePointer<UInt8>.allocate(capacity: 64);

and, also I can get value by KEY.0, KEY.1 ... etc. So, all I need is to create a for loop to read value from KEY and assign it to key

 for index in 1...64 {
     key[index] = KEY[index];
 }

but, this gives compilation error as:

 Value of tuple type '(UInt8, UInt8, UInt8, UInt8, UInt8)' has no member 'index'

so, first question, how I can get any value from KEY by index. Second is, is there any easier way to convert a unsigned char array to UnsafeMutablePointer<UInt8>

Martin R · Accepted Answer · 2016-11-12 08:36:05Z

A C array

unsigned char ckey[64];

is imported as a tuple in Swift:

public var ckey: (UInt8, ..., UInt8)

and one cannot access the tuple elements by subscripting with an index. (Well, you can when using Mirror).

However:

The address of a C array is the same as the address of the first array element, and
for structures imported from C, Swift preserves the memory layout.

The latter was confirmed by Apple engineer Joe Groff:

... You can leave the struct defined in C and import it into Swift. Swift will respect C's layout.

Therefore you can pass the address of the first tuple element to the function:

f(key: &ckey.0)

Of course – as in C – the information about the size of the array is lost, so the array should be NUL-terminated, or f should know about the array size. Alternatively, pass the size as an additional argument to the function:

foo(key: &ckey.0, length: MemoryLayout.size(ofValue: ckey))

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