memory fences/barriers in C++: does boost or other libraries have them?

Question

I am reading these days about memory fences and barriers as a way to synchronize multithreaded code and avoid code reordering.

I usually develop in C++ under Linux OS and I use boost libs massively but I am not able to find any class related to it. Do you know if memory barrier of fences are present in boost or if there is a way to achieve the same concept? If not what good library can I have a look to?

Answer 1

simple barrier: http://www.boost.org/doc/libs/1_41_0/doc/html/thread/synchronization.html#thread.synchronization.barriers

also, look here: http://lyx.sourcearchive.com/documentation/1.6.3/spinlock__w32_8hpp-source.html

glibc has atomic functions: http://stackoverflow.com/questions/2302674/functions-for-performing-atomic-operations

Answer 2

There is a boost::barrier class/concept but it's a bit high level. Speaking of which, why do you need a low level barrier? Synchronization primitives should be enough, shouldn't they? And they should be using memory barriers where necessary, either directly or indirectly through other, lower level primitives.

If you still think you need a low-level implementation, I know of no classes or libraries that implement barriers, but there's some implementation-specific code in the Linux kernel. Search for mb(), rmb() or wmb() in include/asm-{arch}/system.h.

Answer 3

There are no low-level memory barriers in boost yet, but there is a proposed boost.atomic library that provides them.

Compilers provide their own either as intrinsics or library functions, such as gcc's __sync_synchronize() or _mm_mfence() for Visual Studio.

The C++0x library provides atomic operations including memory fences in the form of std::atomic_thread_fence. Though gcc has supplied various forms of the C++0x atomics since V4.4, neither V4.4 or V4.5 include this form of fence. My (commercial) just::thread library provides a full implementation of C++0x atomics, including fences for g++ 4.3 and 4.4, and Microsoft Visual Studio 2005, 2008 and 2010.