[swift-users] Casting function pointers

[Joe Groff]

> On Jan 25, 2017, at 1:23 AM, Rien via swift-users <swift-users at swift.org> wrote:
> 
> I have a case where a callback has the following signature:
> 
> @convention(c) (_ ssl: OpaquePointer?, _ num: UnsafeMutablePointer<Int32>?, _ arg: UnsafeMutableRawPointer?) -> Int32
> 
> But to install this callback I have to use a c-function with this signature:
> 
> SSL_CTX_callback_ctrl (OpaquePointer!, Int32, (() -> Void)! )
> 
> Is it possible to cast the first function to the second? If so, how?

I think your best bet would be to write the shim closure yourself. Is `num` a pointer to the value you're supposed to receive from SSL_CTX_callback_ctrl? Is the `arg` really a function pointer? In that case, something like this might work:

func callback(_ ssl: OpaquePointer?, _ num: UnsafeMutablePointer<Int32>?, _ arg: UnsafeMutableRawPointer?) -> Int32

let shimCallback: SSL_CTX_callback_ctrl = { ctx, cmd, fp in
  var cmdNum = cmd
  callback(ctx, &cmdNum, unsafeBitCast(fp, to: UnsafeMutableRawPointer?.self))
}

-Joe