33

In my application, I have an int and a bool variable, which are accessed (multiple write/read) by multiple threads. Currently, I am using two mutexes, one for int and one for bool to protect those variables.

I heard about using atomic variables and operators to write lock-free multi-thread program. My questions are

  1. What's the definition of atomic variables and operators?
  2. What's the main difference between std::atomic and boost/atomic.hpp? Which one is more standard or popular?
  3. Are these libraries platform-dependent? I am using gnu gcc 4.6 on Linux at the moment, but ideally it shall be cross-platform. I heard that the definition of "atomic" actually depends on the hardware as well. Can anyone explain that as well?
  4. What's the best way to share a bool variable among multiple threads? I would prefer not to use the "volatile" keyword.

Are these code thread-safe?

double double_m; // double_m is only accessed by current thread.
std::atomic<bool> atomic_bool_x;
atomic_bool_x = true && (double_m > 12.5);

int int_n; // int_n is only accessed by current thread.
std::atomic<int> atomic_int_x;
std::atomic<int> atomic_int_y;
atomic_int_y = atomic_int_x * int_n;
Improve this question
1
  • 1
    That's a lot of questions to be asking all at once. Also, are you asking if the C++ standard library is... standard? And platform-dependent? Commented Mar 4, 2012 at 2:21

4 Answers 4

Reset to default
19

I'm not an expert or anything, but here's what I know:

  1. std::atomic simply says that calling load and store (and a few other operations) concurrently is well-defined. An atomic operation is indivisible - nothing can happen 'in-between'.
  2. I assume std::atomic is based off of boost::atomic. If you can, use std, otherwise use boost.
  3. They are both portable, with the std being completely so, however your compiler will need to support C++11
  4. Likely std::atomic_bool. You should not need to use volatile.

Also, I believe load/store differs from operator=/operator T only load/store are atomic.

Nevermind. I checked the standard and it appears that the operators are defined in terms of load/store/etc, however they may return different things.

Further reading:

Improve this answer
Sign up to request clarification or add additional context in comments.

2 Comments

Actually Boost.Atomic is based on the standard atomic library.
Also, load and store do more than just be indivisible. They also define memory ordering requirements. Only if you specify memory_order_relaxed are they merely indivisible.
7

Volatile is orthogonal to what you use to implement atomics. In C++ it tells the compiler that certain it is not safe to perform optimizations with that variable. Herb Sutters lays it out:

To safely write lock-free code that communicates between threads without using locks, prefer to use ordered atomic variables: Java/.NET volatile, C++0x atomic, and C-compatible atomic_T.

To safely communicate with special hardware or other memory that has unusual semantics, use unoptimizable variables: ISO C/C++ volatile. Remember that reads and writes of these variables are not necessarily atomic, however.

Finally, to express a variable that both has unusual semantics and has any or all of the atomicity and/or ordering guarantees needed for lock-free coding, only the ISO C++0x draft Standard provides a direct way to spell it: volatile atomic.

(from http://drdobbs.com/article/print?articleId=212701484&siteSectionName=parallel)

Improve this answer

Comments

2
  1. See std::atomic class template
  2. std::atomic is standard since C++11, and the Boost stuff is older. But since it is standard now, I would prefer std::atomic.
  3. ?? You can use std::atomic with each C++11 compiler on each platform you want.

  4. Without any further information...

    std::atomic;

Improve this answer

Comments

1

I believe std::atomic (C++11) and boost.atomic are equivalent. If std::atomic is not supported by your compiler yet, use boost::atomic.

Improve this answer

Comments

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.