[swift-users] Swift DispatchSource not working

[Darren Mo]

I am trying to catch SIGWINCH (terminal window size changed) in Swift 3 (Xcode 8). The following code should exit with a status of 1 when it receives SIGWINCH. But it never exits.

```swift
// compile with `xcrun -sdk macosx swiftc sigwinch.swift`

import Darwin
import Dispatch

let source = DispatchSource.signal(signal: SIGWINCH, queue: DispatchQueue.main)
source.setEventHandler {
   exit(1)
}
source.resume()

dispatchMain()
```

The equivalent Objective-C code works as expected.

```objc
// compile with `xcrun -sdk macosx clang sigwinch.m`

#import <dispatch/dispatch.h>
#import <signal.h>
#import <stdlib.h>

int main() {
   dispatch_source_t source = dispatch_source_create(DISPATCH_SOURCE_TYPE_SIGNAL,
                                                     SIGWINCH,
                                                     0,
                                                     dispatch_get_main_queue());
   dispatch_source_set_event_handler(source, ^{
      exit(1);
   });
   dispatch_resume(source);

   dispatch_main();
}
```

What am I doing wrong?
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