Questions tagged [stdatomic]

std::atomic is a class template in the C++11 Standard Library which provides atomic operations.

std::atomic is a class template in the C++11 Standard Library, defined in the <atomic> header. An atomic object of type std::atomic<X> provides member functions to perform atomic operations on its member of type X.

Like mutexes, atomic objects can be used for synchronization in multi-threaded C++ programs. Unlike mutexes or other locks, atomics can be used to build lock-free algorithms that allow concurrent readers and writers.

Atomics can even be used to build custom locking primitives (but that's usually a bad idea on systems with OS-supported locking functions that yield the CPU on contention).

Resources:

std::atomic documentation at cppreference
An Introduction to Lock-Free Programming, by Jeff Preshing, and other articles linked from it.
Memory Reordering Caught in the Act: an experimental test of memory reordering on x86, and how memory_order_seq_cst avoids it.
Acquire and Release Semantics
Memory Barriers Are Like Source Control Operations: LoadStore vs. StoreLoad vs. StoreStore vs. LoadLoad reordering explained.
Can num++ be atomic for 'int num'?: No, not on a multi-core machine, even if it compiles to a single assembly instruction (which isn't guaranteed). Discusses the difference between C++ and asm, and how atomicity works internally on a CPU.

Also points out that a compiler can merge num++; num++; into a single num+=2, if allowed by the as-if rules. See http://wg21.link/p0062 and http://wg21.link/n4455.

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When do I really need to use atomic instead of bool?

Isn't atomic redundant because bool is atomic by nature? I don't think it's possible to have a partially modified bool value. When do I really need to use atomic instead of bool?

asked May 01 '13 at 15:12

user955249

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5 answers

c++, std::atomic, what is std::memory_order and how to use them?

Can anyone explain what is std::memory_order in plain English, and how to use them with std::atomic<>? I found the reference and few examples here, but don't understand at all. http://en.cppreference.com/w/cpp/atomic/memory_order

c++ multithreading atomic memory-model stdatomic

asked Mar 04 '12 at 08:54

2607

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Where is the lock for a std::atomic?

If a data structure has multiple elements in it, the atomic version of it cannot (always) be lock-free. I was told that this is true for larger types because the CPU can not atomically change the data without using some sort of lock. for…

c++ c++11 x86 atomic stdatomic

asked May 11 '18 at 18:38

curiousguy12

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Must I call atomic load/store explicitly?

C++11 introduced the std::atomic<> template library. The standard specifies the store() and load() operations to atomically set / get a variable shared by more than one thread. My question is are assignment and access operations also atomic?…

c++ c++11 atomic stdatomic

asked Sep 17 '13 at 13:07

bavaza

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What does the [[carries_dependency]] attribute mean?

Can someone explain it in a language that mere mortals understand?

c++ multithreading memory-model stdatomic carries-dependency

asked Jun 20 '11 at 12:39

Yakov Galka

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Why don't compilers merge redundant std::atomic writes?

I'm wondering why no compilers are prepared to merge consecutive writes of the same value to a single atomic variable, e.g.: #include std::atomic y(0); void f() { auto order = std::memory_order_relaxed; y.store(1, order); …

c++ multithreading c++11 compiler-optimization stdatomic

asked Aug 30 '17 at 12:25

PeteC

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Does std::atomic work appropriately?

I am reading through Anthony Williams' "C++ Concurrency in Action" and in Chapter 5, which talks about the new multithreading-aware memory model and atomic operations, and he states: In order to use std::atomic for some user-defined UDT, this…

c++ atomic stdstring stdatomic

asked Jun 01 '13 at 19:25

Thomas Russell

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How do memory_order_seq_cst and memory_order_acq_rel differ?

Stores are release operations and loads are acquire operations for both. I know that memory_order_seq_cst is meant to impose an additional total ordering for all operations, but I'm failing to build an example where it isn't the case if all the…

c++ c++11 memory-model stdatomic

asked Sep 09 '12 at 16:26

AProgrammer

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Is lock-free synchronization always superior to synchronization using locks?

In C++, there is one atomic type std::atomic. This atomic type may be lock-free or maybe not depending on the type T and on the current platform. If a lock-free implementation for a type is available in a platform for a type T, then most…

c++ synchronization lock-free stdatomic

asked Feb 28 '23 at 21:47

Sourav Kannantha B

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Acquire/Release versus Sequentially Consistent memory order

For any std::atomic where T is a primitive type: If I use std::memory_order_acq_rel for fetch_xxx operations, and std::memory_order_acquire for load operation and std::memory_order_release for store operation blindly (I mean just like resetting…

c++ concurrency atomic stdatomic memory-model

asked Feb 13 '13 at 19:50

zahir

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"Use of deleted function" error with std::atomic_int

I want to use an std::atomic_int variable. In my code, I have: #include std::atomic_int stop = 0; int main() { // Do something } And this gives me a compile error: use of deleted function…

c++ c++11 stdatomic

asked Dec 05 '14 at 11:06

Karnivaurus

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Do I have to use atomic for "exit" bool variable?

I need to set a flag for another thread to exit. That other thread checks the exit flag from time to time. Do I have to use atomic for the flag or just a plain bool is enough and why (with an example of what exactly may go wrong if I use plain…

c++ c++11 atomic stdatomic

asked Apr 19 '13 at 18:55

PowerGamer

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How to initialize a static std::atomic data member

I would like to generate identifiers for a class named order in a thread-safe manner. The code below does not compile. I know that the atomic types do not have copy constructors, and I assume that explains why this code does not work. Does anybody…

c++ c++11 constants atomic stdatomic

asked Dec 08 '13 at 11:50

Teisman

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C++11 memory_order_acquire and memory_order_release semantics?

http://en.cppreference.com/w/cpp/atomic/memory_order, and other C++11 online references, define memory_order_acquire and memory_order_release as: Acquire operation: no reads in the current thread can be reordered before this load. Release…

c++ mutex atomic memory-model stdatomic

asked Apr 23 '13 at 22:02

Cedomir Segulja

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2 answers

Why isn't atomic double fully implemented

My question is quite simple. Why isn't std::atomic implemented completely? I know it has to do with atomic RMW (read-modify-write) access. But I really don't see, why this shouldn't be possible on a double. It's specified that any trivially…

c++ floating-point atomic compare-and-swap stdatomic

asked May 05 '15 at 09:02

laurisvr

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