[LAD] Atomic Operations

Recently on this list, Paul referred to an "atomic integer swap" and an "atomic pointer swap." This was a new concept to me (and possibly others), and this e-mail shares what I've learned. If you access a variable from multiple threads -- even a built-in variable like 'int', it is important to control access to the variable with a mutex, semaphore, or an atomic operation. Alexander Sandler, on his blog, wrote a couple of good articles on the subject: "Do you need a mutex to protect an int?" http://www.alexonlinux.com/do-you-need-mutex-to-protect-int "Multithreaded simple data type access and atomic variables" http://www.alexonlinux.com/multithreaded-simple-data-type-access-and-atomic… The first article contains code that calculates a wrong answer on multiprocessor machines. I've attached a similar example that will even fail on a single-processor machine. There is a wealth of reading material on using Mutexes and Semaphores. However, information on atomic operations appears to be sparse and hard-to-follow. So, here's what I've found: + At the moment, there is no built-in support in C/C++ for atomic operations. You will need to use a library, compiler extension, or write your own in assembly code. + The GCC compiler has the built-in __sync_*() functions[1] that provide atomic operations. Note that the attached example is using this. + glib provides the g_atomic_*() functions[2]. + Qt 4 has the q_atomic_*() functions.[3] While they are accessible, they are /not/ a part of their stable, public API. + The next version of ISO C++ (code name c++0x) is expected to have support for atomic operations (E.g. the std::atomic<T> template) and memory barriers. It may even require that all built-in types be atomic. + In the x86 instruction set, these are usually implemented using the 'LOCK' instruction prefix.[5] When using atomic operations, perhaps the best advice I found is near the end of Sandler's second article: "When using atomic variables, some extra precautions have to be taken.... There is nothing that prevents you from incrementing value of the atomic variable with __sync_fetch_and_add() as I just demonstrated and later in the code doing same thing with regular ++ operator. "To address this problem, I strongly suggest wrapping around atomic functions and variables with either ADT in C or C++ class."[4] Peace, Gabriel [1] http://gcc.gnu.org/onlinedocs/gcc-4.1.0/gcc/Atomic-Builtins.html [2] http://www.gtk.org/api/2.6/glib/glib-Atomic-Operations.html [3] http://doc.trolltech.com/4.3/atomic-operations.html See also the Qt header file QtCore/qatomic_i386.h, and its brothers. [4] http://www.alexonlinux.com/multithreaded-simple-data-type-access-and-atomic… [5] http://siyobik.info/index.php?module=x86&id=159