Function-Local Static variable For Unique Id Generation [duplicate]

This question already has answers here: Closed 12 years ago.

Possible Duplicate:

ID generator with local st开发者_JAVA百科atic variable - thread-safe?

Would the following function return a unique number every time it is called? What about in a multi-threaded scenario?

int returnUniqueNumber()
{
  static int i = 0;
  ++i;
  return i;
}

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It depends on your platform as to ehwther you'll get multithreading issues. On windows you'd be best off doing the following:

int returnUniqueNumber()
{
  volatile static long l = 0;
  return (int)InterlockedIncrement( &l );
}

The Interlocked* functions are guaranteed atomic.

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If your application were single-threaded, the code you posted would be in fact the correct, standards-compliant C/C++ way of generating unique numbers. However, since you're looking for a thread-safe solution, you must dive into platform-specific solutions. Goz has a good Windows solution. An equivalent on Mac OS X or iOS would be:

int returnUniqueNumber_APPLE(void) {
    static volatile int32_t i = 0;
    return (int)OSAtomicIncrement32Barrier(&i);
}

On any system where you compile with a recent-ish GCC, you also have GCC's intrinsics:

int returnUniqueNumber_GCC(void) {
    static volatile int i = 0;
    return __sync_add_and_fetch(&i, 1);
}

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Following on from Goz's answer regarding windows, for Linux/GCC see this question/answer:

Equivalent of InterlockedIncrement in Linux/gcc

in which case you'll want __sync_add_and_fetch()