Bad Access error when calling Swift closure from C api in Cocoa

Question

I'm trying to implement a "file copy with progress" in Swift on macOS. After lots of searching I just found rustle's implement in Objective-C. It works pretty fine. But I would like it "swifty". I tried it with some simplified code:

import Cocoa

@NSApplicationMain
class AppDelegate: NSObject, NSApplicationDelegate {

    var copyfileCallback: copyfile_callback_t = {(what, stage, state, sourcePath, destPath, context) -> Int32 in
        return COPYFILE_CONTINUE
    }

    @IBOutlet weak var window: NSWindow!

    func applicationDidFinishLaunching(_ aNotification: Notification) {

        let src = NSURL(fileURLWithPath: "Source_File_Path").fileSystemRepresentation
        let dst = NSURL(fileURLWithPath: "Destination_File_Path").fileSystemRepresentation
        let flag: copyfile_flags_t = UInt32(COPYFILE_ALL)

        let state = copyfile_state_alloc()

        // If I implement this, the copyfile() method will complain "EXC_BAD_ACCESS(code=2..." error
        copyfile_state_set(state, UInt32(COPYFILE_STATE_STATUS_CB), &copyfileCallback)

        copyfile(src, dst, state, flag)
    }
}

The basic function copyfile() works fine. But if I implement the callback function by providing a pointer of copyfileCallback closure to copyfile_state_set(), then copyfile() just complains "Bad_Access...". I guess maybe the closure was released before the C api tries to access it. But I have no idea how to solve this problem... Any clue will be so appreciated.

Answer 1

The error is here:

copyfile_state_set(state, UInt32(COPYFILE_STATE_STATUS_CB), &copyfileCallback)

because is passes the address of the copyfileCallback variable to the function, not the function pointer itself. In C you can pass an arbitrary function as a void * argument. In Swift you have to cast the function to a pointer explicitly:

let state = copyfile_state_alloc()
copyfile_state_set(state, UInt32(COPYFILE_STATE_STATUS_CB),
                   unsafeBitCast(copyfileCallback, to: UnsafeRawPointer.self))

And don't forget to release the memory eventually, after the copy operation:

copyfile_state_free(state)

Remark: In Swift it is recommended to use the (value overlay type) URL instead of NSURL:

let srcURL = URL(fileURLWithPath: "Source_File_Path")
let destURL = URL(fileURLWithPath: "Destination_File_Path")

let result = srcURL.withUnsafeFileSystemRepresentation { srcFile in
    destURL.withUnsafeFileSystemRepresentation { destFile in
        copyfile(srcFile, destFile, state, flag)
    }
}