Question

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In g++ is C++ 11 thread model using pthreads in the background?

I have read from somewhere that OpenMP is implemented using PThreads in Linux systems although they seem quite different to me. Considering the (relative) similarities between C++11 threads and PThreads I was wondering,

Does anyone know if C++11 threads implemented using PThreads or any other multithreading library in gcc or clang?

Answer 1

There are multiple parts of C++ 2011 multi-threading:

1. Higher-level abstractions like std::thread, std::mutex, std::condition_variable, etc. These abstractions are implemented in terms of pthreads for both libc++ (clang’s native library) and libstdc++ (gcc’s native library). libstdc++ uses an indirection (gthr.h) which can be used to, e.g., stub things out for a single threaded implementation. This is quite obvious from the source of the different synchronization classes.
2. The lower-level synchronization facilities, i.e., atomics and the various memory visibility controls, are not available from pthreads. It seems both gcc and clang implement these using compiler build-ins which probably create suitable instructions. I haven’t tracked down the actual code for either of these, however.
3. It isn’t sufficient to implement things in the library: the compiler needs to be prevented from reordering instruction across synchronization primitives and it needs to make values visible in appropriate locations.