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UNDER CONSTRUCTION

When accessing a bit-field, a thread may inadvertently access a separate bit-field in adjacent memory. This is because compilers are required to store multiple adjacent bit-fields in one storage unit whenever they fit. Consequently, data races may exist not just on a bit-field accessed by multiple threads but also on other bit-fields sharing the same byte or word.  The problem is difficult to diagnose because it may not be obvious that the same memory location is being modified by multiple threads.

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#include <mutex>
 
struct multi_threaded_flags {
  unsigned int flag1 : 2;
  unsigned int flag2 : 2;
};

struct mtf_mutex {
  struct multi_threaded_flags s;
  std::mutex mutex;
};

struct mtf_mutex flags;

int thread1(void *arg) {
  std::lock_guard<std::mutex> guard(&flags.mutex);
  flags.s.flag1 = 1;
  return 0;
}
 
int thread2(void *arg) {
  std::lock_guard<std::mutex> guard(&flags.mutex);
  flags.s.flag2 = 2;
  return 0;
}

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