Memory model spec in pthreads

Question

Are there any guarantees on when a memory write in one thread becomes visible in other threads using pthreads?

Comparing to Java, the Java language spe

Answer 1

I am not aware that POSIX threads give such guarantees. They don't have a model for atomic access to thread-shared objects. If it'd be for POSIX threads, the only guarantees that you can have for visibility of modifications is using some kind of lock.

Modern C, C11, (and probably also C++11) has a model for this kind of questions. It has threads and atomics (fences and all that stuff) that give you exact rules when you may assume that a modification done by one thread is visible by another.

The thread interface of C11 is a cooked-down version of POSIX threads, with less functionality. Unfortunately, the specification for the semantics of that thread interface is yet much to loose, basically the semantics are missing in many places. But a combination of C11 interfaces and POSIX thread semantics can give you a good view of how things work in modern systems.

Edit: So if you want to have guarantees for memory synchronization use either the lock interfaces that POSIX provides or go for atomic operations. All modern compilers have extensions that provide these, gcc and family (icc, opencc, clang) have e.g the series of __sync... builtins. Clang it its newest version also already has support of the new C11 _Atomic feature. There are also wrappers available that give you interfaces for the other compilers that come close to _Atomic.