[SR-1390] UIApplicationMain has wrong signature

[mattneub]

Previous ID	SR-1390
Radar	rdar://26062787
Original Reporter	@mattneub
Type	Bug
Status	Closed
Resolution	Done

Environment

Xcode 7.3, Swift toolchain of April 25 (https://swift.org/builds/development/xcode/swift-DEVELOPMENT-SNAPSHOT-2016-04-25-a/swift-DEVELOPMENT-SNAPSHOT-2016-04-25-a-osx.pkg)

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: 9103fd117383d156f0aeb5a09ed4e03a

Issue Description:

There is still a problem with the signature of UIApplicationMain, and it needs to change. When it does, I presume @UIApplicationMain will have to change with it.

We used to write main.swift by calling UIApplicationMain like this:

UIApplicationMain(
Process.argc, Process.unsafeArgv, nil, NSStringFromClass(AppDelegate)
)

But now that won't compile, because there is a type mismatch between Process.argv, which is typed like this:

UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>

and the second parameter to UIApplicationMain, which is typed like this:

UnsafeMutablePointer<UnsafeMutablePointer<Int8>>!

As you can see, they are subtly different. I am able to work around the problem temporarily by writing this:

withUnsafeMutablePointer(&Process.unsafeArgv.pointee!) {
UIApplicationMain(
Process.argc, $0, nil, NSStringFromClass(AppDelegate)
)
}

But whether that is safe or correct, I have no idea.

It is suggested by Quinn "The Eskimo!" that this problem is a consequence of https://github.com/apple/swift-evolution/blob/master/proposals/0055-optional-unsafe-pointers.md.

[belkadan]

Rather than withUnsafeMutablePointer, you should just use UnsafeMutablePointer's converting initializer:

UIApplicationMain(Process.argc, UnsafeMutablePointer<UnsafeMutablePointer<CChar>>(Process.argv), nil, NSStringFromClass(AppDelegate.self))

[mattneub]

Will do, thanks, @belkadan! I'll report this back on bugreport and the forum. (When I can, that is; at present I can't log in to either of those.)

[belkadan]

This is really something UIKit should fix (and I duped your Radar to an existing one there), but we might be able to have the Swift compiler handle it if they don't get to it, so keeping the SR around for now.

[mattneub]

@belkadan UnsafeMutablePointer no longer has a converting initializer, so I am completely at a loss for how to call UIApplicationMain now. (I did figure out that Process is now called CommandLine.)

I came up with this; it compiles and runs, but I have no idea whether it is right or safe or even what it means; if ever there was a man who has absolutely idea what he is doing, I am that man:

let umrp = UnsafeMutableRawPointer(CommandLine.unsafeArgv)
let ump = umrp.bindMemory(to: UnsafeMutablePointer<Int8>.self, capacity: 1)
UIApplicationMain(
    CommandLine.argc, ump, nil, NSStringFromClass(AppDelegate.self)
)

[belkadan]

Yuck. @atrick?

[atrick]

Oh man, things get really bad when imported pointer types are incorrect. The only thing wrong with your fix is that `capacity` should be `CommandLine.argc` not 1. There is no good way to do this, but I'll claim that this is the most idiomatic:

CommandLine.unsafeArgv.withMemoryRebound(
to: UnsafeMutablePointer<Int8>.self, capacity: Int(CommandLine.argc)) {
    argv in
    UIApplicationMain(
        CommandLine.argc, argv, nil, NSStringFromClass(AppDelegate.self)
    )
}

[0xpablo]

I just tested this and it seems to be fixed in Xcode 10b1 (Swift 4.2)

[mattneub]

@0xpablo Thank you, yes! I can now write:

UIApplicationMain(
    CommandLine.argc, CommandLine.unsafeArgv, nil, NSStringFromClass(AppDelegate.self)
)

[mattneub]

@belkadan This can be closed, and thanks.