#define _GNU_SOURCE 1

#include <pthread.h>
#include <signal.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>

#define sys(code) if ((code) < 0) { perror(#code); exit(1); }

pthread_t mainThread;
pthread_t childThread;

unsigned int receipts = 0;

void handler(int signo, siginfo_t* siginfo, void* context) {
  if (pthread_equal(pthread_self(), mainThread)) {
    printf("received signal in main thread\n");
    __atomic_fetch_or(&receipts, 1, __ATOMIC_RELAXED);
  } else if (pthread_equal(pthread_self(), childThread)) {
    printf("received signal in child thread\n");
    __atomic_fetch_or(&receipts, 2, __ATOMIC_RELAXED);
  } else {
    printf("received signal in other thread (?)\n");
  }
}

void* threadMain(void* param) {
  // Send SIGUSR1 to main thread.
  pthread_kill(mainThread, SIGUSR1);

  // Give main thread time to send us SIGUSR1.
  usleep(100000);

  // Unblock SIGUSR1 to see if it gets delivered.
  sigset_t mask;
  sys(sigemptyset(&mask));
  sys(sigaddset(&mask, SIGUSR1));
  sys(pthread_sigmask(SIG_UNBLOCK, &mask, NULL));

  return NULL;
}

int main() {
  // Register a signal handler for SIGUSR1.
  struct sigaction action;
  memset(&action, 0, sizeof(action));
  action.sa_sigaction = &handler;
  action.sa_flags = SA_SIGINFO;
  sys(sigaction(SIGUSR1, &action, NULL));

  // Block SIGUSR1.
  sigset_t mask;
  sys(sigemptyset(&mask));
  sys(sigaddset(&mask, SIGUSR1));
  sys(pthread_sigmask(SIG_BLOCK, &mask, NULL));

  // Start child thread.
  mainThread = pthread_self();
  if (pthread_create(&childThread, NULL, &threadMain, NULL) < 0) {
    fprintf(stderr, "pthread_create failed\n");
    exit(1);
  }

  // Send SIGUSR1 to child thread.
  pthread_kill(childThread, SIGUSR1);

  // Unblock SIGUSR1 to see if it gets delivered to this thread.
  sys(sigemptyset(&mask));
  sys(sigaddset(&mask, SIGUSR1));
  sys(pthread_sigmask(SIG_UNBLOCK, &mask, NULL));

  pthread_join(childThread, NULL);

  if (receipts == 3) {
    printf("both threads received signal (CORRECT)\n");
  } else if (receipts == 1 || receipts == 2) {
    printf("only one thread received signal (WRONG)\n");
  } else {
    printf("signal not delivered at all (WRONG)\n");
  }
}