C：如何使此函数具有线程安全性？

Question

Usually when I want to make functions thread-safe I lock the shared data, then release when I'm finished doing work.

Here I'm working with a global variable. Simply locking and unlocking is not going to ensure that the function returns a unique value. How do I modify the following function to be thread-safe and ensure it always returns a unique integer?

int count = 0;
int GetUnique()
  {
    count = count + 1;
    return count;
  }

Some people have mentioned making a local static variable inside the function? How and why would this work (if true)?

Answer 1

You want to in this case use an atomic increment, no need for locking or mutexes if you use GCC intrinsics. __sync_add_and_fetch is what you are looking for here, or for VC InterlockedIncrement will perform the same.

You could make this code portable with the following:

#ifdef _WIN32
#define SYNC_ADD_AND_FETCH(x) InterlockedIncrement(&(x))
#else
#define SYNC_ADD_AND_FETCH(x) __sync_add_and_fetch(&(x), 1)
#endif

int main(int argc, char *argv[])
{
  int v = 0;
  SYNC_ADD_AND_FETCH(v);
  printf("%d\n", v);
}

Making it static is not enough to protect the variable from other threads modifying it on the fly, in fact it does not help in any way.