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struct multi_threaded_flags {
  unsigned char flag1;
  unsigned char flag2;
};
 
struct multi_threaded_flags flags;
 
voidint *thread1(void *arg) {
  flags.flag1 = 1;
  return 0;
}

voidint *thread2(void *arg) {
  flags.flag2 = 2;
  return 0;
}

struct multi_threaded_flags {
  unsigned int flag1 : 2;
  unsigned int flag2 : 2;
};

struct multi_threaded_flags flags;

voidint *thread1(void *arg) {
  flags.flag1 = 1;
  return 0;
}

void*int thread2(void *arg) {
  flags.flag2 = 2;
  return 0;
}

#include <threads.h>
#include <assert.h>
 
struct multi_threaded_flags {
  unsigned int flag1 : 2;
  unsigned int flag2 : 2;
};

union mtf_protect {
  struct multi_threaded_flags s;
  long padding;
};
struct mtf_mutex {
  union mtf_protect u;
  mtx_t mutex;
};

struct mtf_mutex flags;

void chk_flags(void) {
  static_assert(sizeof(long) >=
              sizeof(struct multi_threaded_flags));
}

voidint *thread1(void *arg) {
  if (thrd_success != mtx_lock(&flags.mutex)) {
    /* Handle error */
  }
  flags.u.s.flag1 = 1;
  if (thrd_success != mtx_unlock(&flags.mutex)) {
    /* Handle error */
  }
  return 0;
}
 
voidint *thread2(void *arg) {
  if (thrd_success != mtx_lock(&flags.mutex)) {
    /* Handle error */
  }
  flags.u.s.flag2 = 2;
  if (thrd_success != mtx_unlock(&flags.mutex)) {
    /* Handle error */
  }
  return 0;
}