To expand on what to3m said. The code was transformed into something like this:

    int launched = 0;
    start_thread(...);
    if (!launched) {
      while (true) {}
    }

That's a perfectly legal optimization. The integer here isn't atomic and if another thread modifies the variable, that would be a data race and those have undefined behavior. Therefore, the compiler can assume that there is no race and move the comparison out of the loop.

Changing it to be "std::atomic<int> launched;" fixes that problem.