How to use atomic_fetch_add_explicit to replace OSAtomicIncrement32?

Question

I have these two lines of code in my app

volatile __block int32_t counter = 0;

and later in a loop...

 OSAtomicIncrement32(&counter);

But now OSAtomicIncrement32 is deprecated and Xcode is showing me this error message.

'OSAtomicIncrement32' is deprecated: first deprecated in iOS 10.0 - Use atomic_fetch_add_explicit(memory_order_relaxed) from instead

See this error message, there is one parameter to atomic_fetch_add_explicit, right?

So I try

atomic_fetch_add_explicit(&counter)

and I see this message

Too few arguments to function call, expected 3, have 1

I love the crappy messages Xcode dumps.

How do I use this?

Answer 1

Read the documentation... It takes the pointer to the target variable, the number to add (in your case probably 1) and the memory order, suggested by the compiler as memory_order_relaxed, probably to match the existing behavior of OSAtomicIncrement32.

atomic_fetch_add_explicit(&counter, 1, memory_order_relaxed);

If you don't know what a memory order is, probably you'd be better served by a plain atomic_fetch_add with the "safest" sequential ordering.

atomic_fetch_add(&counter, 1);

---

That being said, if you are actually working in C++ and that variable is used only by your code, you can just use an std::atomic_int (or std::atomic<std::int32_t> if you want guaranteed 32 bit range) and the plain ++ operator.

std::atomic_int counter{0};

//...

++counter;