Sample code for https://openradar.appspot.com/radar?id=5030997057863680

main.m

//
//  main.m
//  signaltest
//
//  Created by Dave MacLachlan on 11/12/18.
//  Copyright © 2018 Dave MacLachlan. All rights reserved.
//

#import <Foundation/Foundation.h>
#import <pthread.h>

int theSignal = SIGUSR1;
const char *theSignalName = "SIGUSR1";

void print_sigs(const char *source) {
  sigset_t sigmask;
  sigemptyset(&sigmask);
  pthread_sigmask(SIG_BLOCK, NULL, &sigmask);
  pthread_t thread = pthread_self();
  struct sigaction action;
  sigaction(theSignal, NULL, &action);
  printf("%s: %p %s thread: %s proc: %s\n", source, thread, theSignalName,
         sigismember(&sigmask, theSignal) ? "BLOCKED" : "UNBLOCKED",
         action.sa_handler == SIG_IGN ? "Ignored" : (action.sa_handler == SIG_DFL ? "Default" : "Other"));
}

int main(int argc, const char * argv[]) {
  @autoreleasepool {
    signal(theSignal, SIG_IGN);
    dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
    dispatch_queue_t mainQueue = dispatch_get_main_queue();
    dispatch_source_t signalSource = dispatch_source_create(DISPATCH_SOURCE_TYPE_SIGNAL, theSignal, 0, mainQueue);
    dispatch_source_set_event_handler(signalSource, ^(void) {
      printf("Received Signal\n");
      print_sigs("Signal");
    });
    dispatch_source_set_registration_handler(signalSource, ^{
      printf("Source Registered\n");

    });
    dispatch_source_set_cancel_handler(signalSource, ^{
      printf("Source Cancelled\n");
    });
    dispatch_resume(signalSource);
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 1000000000), globalQueue, ^{

      printf("Raising From Global Queue\n");
      print_sigs("globalQueue");
      raise(theSignal);
      printf("Raised From Global Queue\n");
    });
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 2000000000), mainQueue, ^{
      printf("Raising From Main Queue\n");
      print_sigs("mainQueue");
      raise(theSignal);
      printf("Raised From Main Queue\n"); });
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 6000000000), mainQueue, ^{
      dispatch_cancel(signalSource);
      exit(0);
    });
    print_sigs("CFRunLoopRun"); CFRunLoopRun();
    //print_sigs("dispatch_main"); dispatch_main();
  }
  return 0;
}