Question

I have been learning thread programming in Java, where there are are sophisticated APIs for thread management. I recently came across this. I am curious to know if these are used now. Is the POSIX thread obsolete or is it the standard used now for threading in C++. I am not familiar with Threading in any other language apart from Java.

Answer 1

phtreads are the current standard POSIX threading library. They are missing some important new things, and I hope they will be updated to accomodate them. And the C++1x standard will also have some threading primitives built in.

pthreads is mostly missing atomic value operations. For example there are no thread safe primitive counter operations that are expected to be compiled to 1-5 machine instructions.

These are needed because while the semantics of the volatile keyword seem to suggest that you might be able to use it for some of these things, this is not the case. Modern CPUs manage their L1, L2 and L3 caches in a way that frequently results in writes reads and writes being seen in different order by different CPUs. And current optimizing compilers can significantly re-ordering operations so the order in which they happen no longer bears a lot of resemblance to the order they appear in the source code.

Mutexes, even the modern Linux version that avoids any system calls unless there is contention, are too heavyweight for something like a reference count.

C and C++ could be changed so the language made these guarantees happen all the time. But that would be contrary to their spirit of being ‘high-level assembly’.