C++ atomic_flag query state

c++ multithreading c++11 atomic
polapts picture polapts · Jun 19, 2012 · Viewed 9.5k times · Source

I am using C++ std::atomic_flag as an atomic Boolean flag. Setting the flag to true or false is not a problem but how to query the current state of flag without setting it to some value? I know that there are methods 'atomic_flag_clear' and 'atomic_flag_set'. They do give back the previous state but also modify the current state. Is there any way to query flag state without modifying it or do I have to use full fledged 'std::atomic<bool>'.

Answer

Anthony Williams picture Anthony Williams · Jun 19, 2012

You cannot read the value of a std::atomic_flag without setting it to true. This is by design. It is not a boolean variable (we have std::atomic<bool> for that), but a minimal flag that is guaranteed lock free on all architectures that support C++11.

On some platforms the only atomic instructions are exchange instructions. On such platforms, std::atomic_flag::test_and_set() can be implemented with exchange var,1 and clear() with exchange var,0, but there is no atomic instruction for reading the value.

So, if you want to read the value without changing it, then you need std::atomic<bool>.

Related questions

What exactly is std::atomic?

I understand that std::atomic<> is an atomic object. But atomic to what extent? To my understanding an operation can be atomic. What exactly is meant by making an object atomic? For example if there are two threads …

Read More
How to implement multithread safe singleton in C++11 without using <mutex>

Now that C++11 has multithreading I was wondering what is the correct way to implement lazy initialized singleton without using mutexes(for perf reasons). I came up with this, but tbh Im not really good at writing lockfree code, so …

Read More
How to use std::atomic<>

I have a class that I want to use in different threads and I think I may be able to use std::atomic this way: class A { int x; public: A() { x=0; } void Add() { x++; } void Sub() { x--; } }; and in …

Read More