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#include <signal.h>
#include <threads.h>
 
volatile sig_atomic_t flag = 0;

void handler(int signum) {
  flag = 1;
}

/* Runs until user sends SIGUSR1. */
int func(void *data) {
  while (!flag) {
    /* ... */
  }
  return 0;
}

int main(void) {
  signal(SIGUSR1, handler); /* Undefined! */
  int result;
  thrd_t tid;
  
  if ((resultthrd_success != thrd_create(&tid, func, NULL))
        != thrd_success) {
    /* Handle Error */
  }

  /* ... */

  return 0;
}

#include <stdatomic.h>
#include <threads.h>
 
atomic_flag flag = ATOMIC_VAR_INIT(0);

int func(void *data) {
  while (!flag) {
    /* ... */
  }
  return 0;
}

int main(void) {
  int result;
  thrd_t tid;
  
  if ((resultthrd_success != thrd_create(&tid, func, NULL))
        != thrd_success) {
    /* Handle Error */
  }

  /* ... */

  /* Set flag when done. */
  while (!atomic_flag_test_and_set(&flag))
    ; /* Continue attempts. */

  return 0;
}