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  • thread::id with libstdc++

    From Bonita Montero@Bonita.Montero@gmail.com to comp.lang.c++ on Wed Apr 2 11:57:44 2025
    From Newsgroup: comp.lang.c++

    C++ recognizes a thread ID, which is represented by the class thread
    ::id. Internally, this class has only one data member in libstdc++,
    and that is a variable of type pthread_t. However, thread::id recognizes
    a default constructed state that does not refer to any thread. I asked
    myself how this works, because all possible values ​​of pthread_t can
    refer to a thread. This is what the default constructor of thread::id
    looks like:

    id() noexcept : _M_thread() { }

    This means that thread::id::id assumes the default-constructed state of pthread_t as not belonging to any thread. This is actually outside the specification.
    Then I was interested in how two thread::id are compared for equality:

    /// @relates std::thread::id
    inline bool
    operator==(thread::id __x, thread::id __y) noexcept
    {
    // pthread_equal is undefined if either thread ID is not valid, so we
    // can't safely use __gthread_equal on default-constructed values (nor
    // the non-zero value returned by this_thread::get_id() for
    // single-threaded programs using GNU libc). Assume EqualityComparable.
    return __x._M_thread == __y._M_thread;
    }

    This means that pthread_equal is not used as it should be.
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