How can I get a page-aligned address from an UnsafeMutableRawPointer?

Question

I may be going about this the wrong way because I have not done much pointer arithmetic in the past (and it may not even be necessary).

However, given a pointer in Swift (of type UnsafeMutableRawPointer), I would like to get the nearest (rounding down) page-aligned address to it. This is in order to synchronise some memory mapped data using msync

Basically, I am trying to migrate some Objective C (well, basically C) code which looks like this:

int pageSize = getpagesize();
void *address = [some bytes];
size_t pageIndex = (size_t)address / pageSize;
void *pageAlignedAddress = (void *)(pageIndex * pageSize);

so far in Swift I have something like:

let pageSize: Int = Int(getpagesize())
let bytes: UnsafeMutableRawPointer = [some bytes]
let pageIndex: Int = Int(bytes) / pageSize
let pageAlignedAddress: UnsafeMutableRawPointer = UnsafeMutableRawPointer(pageIndex * pageSize)

Obviously the last two lines of my Swift code do not compile because I am mixing types and using non-existent initialisers. But I cannot see how to shift my existing UnsafeMutableRawPointer to be page-aligned, nor can I see any methods on the type which might assist me in this.

Perhaps this is not even necessary for the Swift types and they are always page aligned…

Answer 1

Seems you just need to know what initializers Int and UnsafeMutableRawPointer have.

Int

init(bitPattern: UnsafeMutableRawPointer?)

UnsafeMutableRawPointer

init?(bitPattern: Int)

Int has an initializer taking UnsafeMutableRawPointer?, and UnsafeMutableRawPointer has an initializer taking Int. Why don't you use them?

let pageIndex: Int = Int(bitPattern: bytes) / pageSize
let pageAlignedAddress: UnsafeMutableRawPointer? = UnsafeMutableRawPointer(bitPattern: pageIndex * pageSize)